- Email: [email protected]
Multiple stab wounds: a short-term respiratory case study
Multiple stab wounds: a shortterm respnratory case study Nicola A. Burchmore Respiratory management is a role that is of paramount importance to intensive care nurses. Proactive nursing allows a nurse to have a positive effect on a patient's condition by actively improving the patient's status and setting priorities of care, avoiding deterioration wherever possible. In this account of the respiratory management of a patient who has sustained bilateral pneumothoraces following multiple stab wounds to the chest and back, the physiological assessment of the patient is described and a proactive respiratory plan of care is formed. Anatomy, physiology and pathophysiology in relation to pneumothoraces, metabolic acidosis and magnesium links with functional residual capacity are addressed. The potential risk of Adult Respiratory Distress Syndrome (ARDS) following direct and indirect lung injury is discussed. Recent research into ventilation modes and into areas of nursing practice relating to respiratory management are also considered.
B
INTRODUCTION
Nicola A. Burchmore RN, DipH, ENB 100, Staff Nurse, St. Bartholomew's Hospital, ITU, London, UK
(Requests for offprints to NAB) Manuscript accepted I December 1997
Respiratory care management is an integral role within the duties of an intensive care nurse. The nurse plays an active role in improving the patient's condition based on research and evidence. Holistic physiological assessment is necessary to enable the nurse to consider other factors that may have an effect on the patient's ventilation. Metabolic acidosis can be compensated by hyperventilation causing a reduction in Partial Pressure of arterial carbon dioxide
Intensive and Critical Care Nursing (I 997) 13, 341-350
© 1997Harcourt Braceand Co. Ltd
(PaCO2). A reduction in oxygen saturation (SaO2) , haemoglobin or cardiac index will cause a reduction in the delivery of oxygen (DO2) to cells. Patient position can affect Partial Pressure of arterial oxygen (PaO2) and the matching o f ventilation to perfusion. Management of ventilation is a complex role of intensive care nurses in which they can optimize patients' status through holistic assessment, planning and implementation of care. The nurses' role in respiratory management is expanding, and in some units nurses can initiate weaning of patients from ventilators and make appropriate changes to ventilation, e.g. change Fraction of inspired oxygen (FiO2) or pressure support. Nurses are now able to initiate extubation of cardiac fast-track patients if the medical and nursing staff have predetermined extubation criteria. Contemporary medical practices also have a direct effect on the extent of clinical decisionmaking undertaken by nurses (Endacott 1996). Currently, it is preferred that patients are ventilated using support ventilation rather than fully controlled ventilation, whenever possible. Endacott (1996) suggests that this may have implications for the nurses' role. Patient-triggered modes may require a more intense and ongoing assessment in relation to the patients' ability to cope both physically and psychologically. The nurses' role in clinical decision-making is also increased by the trend to have patients lightly sedated rather than fully sedated and paralyzed. Continuous assessment is required to ensure that the patient is not under or over-sedated. Titration of some drugs and fluids is at the nurses' discretion in an intensive care unit and is required to meet desired physiological parameters, e.g. mean arterial pressures above 701nmHg and urinary output above 0.5 ml/kg/h. The nurses' role in this area again needs to be holistic and clinical decisions made should be based on a complete overview of the patient. The aim in this case study is to highlight the importance of the nurses' role in respiratory management whilst expanding readers' knowledge on respiratory anatomy, physiology and pathophysiology, ventilation modes and relevant recent research into respiratory management. To maintain confidentiality a false name, Peter Smith, will be used throughout this article.
PATIENT SCENARIO Peter Smith, a 24-year-old male, was admitted into an Intensive Therapy Unit (ITU) following an emergency laparotomy after sustaining multiple stab wounds to his chest and back. N o
342 Intensive and Critical Care Nursing previous medical history was known. Peter had presented with a right tension pneumothorax and left spontaneous pneumothorax for which bilateral chest drains had been inserted in the Accident and Emergency Department. The laparotomy revealed free blood in the peritoneum, which was washed out. A small hole in the diaphragm, right posterior suprahepatic region, was repaired. Peter received a massive transfusion for hypovolaemic shock. This consisted of 12 units of blood, 6 units of fresh frozen plasma and 3.5 litres of gelofusine before admission to ITU. Upon admission into I T U he was cold centrally and haemodynamically unstable. Peter was initially ventilated on 12 synchronized intermittent mandatory ventilation (SIMV) breaths per minute, pressure support 2 6 c m H 2 0 , 5-cm H 2 0 positive end-expiratory pressure (PEEP) and FiO 2 0.6. O n day two after admission into the ITU, a computed tomography (CT) scan revealed bilateral lung consolidation, no head injuries and fluid lying principally within the pelvis. The author nursed Peter on his fourth day in ITU, and this formed the basis for this paper. A physiological system approach employed in the I T U to plan and evaluate care was used to make an assessment of the patient's condition.
_
•
.
....
_
.....
"
........
•
.....
•
•
•
.
PHYSIOLOGICAL ASSESSMENT • Respiratory: Physically Peter looked well perfused, his colour was good, oxygen saturations (SAO2) were 95%, partial pressure of arterial oxygen (PaO2) 12.5 kPa. Chest examination showed chest movement was bilateral and equal with an approximate inspiratory and expiratory ratio of 1:2. Auscultation revealed decreased breath sounds at the bases of both lungs. The chest X-ray showed bilateral consolidation o f both lungs, and that the pneumothoraces were diminishing. Peter was receiving warm humidified oxygen (02) via a concha to loosen any consolidation and minimal white secretions were obtained from his chest by endotracheal suction (ETS). A concha is a device which can be used in conjunction with a ventilator to humidify/inspired oxygen. Peter had metabolic acidosis with respiratory compensation. He was breathing spontaneously although he remained dependent on the ventilator with pressure support o f 18 cm H 2 0 , PEEP 5 cm H 2 0 and FiO 2 0.35, despite a high dose of sedation. SIMV breaths and pressure support had been gradually reduced (Peter was being 'weaned' from ventilation support) during the previous day. Peter's breathing appeared
•
to be comfortable. Ventilatory observations were all within normal limits at 0900 hrs. The bilateral chest drains that had been inserted upon admission were still in situ. There had been no bubbling of air from the drains or movement of fluid levels in the tubes with respiration ('swinging') for approximately 18 hours. Cardiac: The patient was cardiovascularly stable with a normotensive blood pressure and sinus rhythm. Peter's central venous pressure was 4 cm H 2 0 when the transducers was re-zeroed. Peter was peripherally cool (30.3°C) and centrally warm (37.4°C). Neurological: Peter was sedated with 30 mg midazalom and 10 mg morphine per hour. Sedation score according to Ramsey (1974) was 6, i.e. patient asleep with no response to stimuli. Pupils were equal and reacting briskly to light. Peter had a cough reflex on ETS. O n the second day of admission Peter had a urine screen for drugs which detected cocaine. Fluid and electrolytes: Peter's fluid balance had been positive due to the massive blood transfusion. Generalized oedema was noted. Urinary output was good, above 0.5 ml/kg/h, demonstrating good perfusion to kidneys. Magnesium levels were low, 0.65 mmol/1. All other electrolyte levels were normal. Blood sugar levels were normal. Gastro-intestinal: Peter was receiving osmolite feed nasogastrically, continuously with a break at night, at the rate of 2 litres per day. There had been minimal aspirate from the stomach. The patient's bowels had opened since admission. Psychological: All procedures were explained to Peter. A rest period for 2 hours daily was adhered to, to reduce sensory overload and allow the patient to have an opportunity to have rapid eye movement (REM) sleep. Peter's wife visited regularly and was kept abreast of progress by the nursing and medical staff. Mrs Smith played an active role in Peter's mouth and eye care and this was encouraged.
After the morning assessment of the patient, the author made a respiratory plan of action for the day as summarized in Table 1.
ANATOMY, PHYSIOLOGY A N D PATHOPHYSIOLOGY Pneumothoraces Chest trauma such as Peter sustained will allow air to be entrained into the pleural space. An
Multiplestab wounds
343
Problem
Plan of action
Goal
Consolidation and atelectasis of both lungs
=> ETS as required Humified oxygen Regular repositioning
• Oxygen saturations (SaOz) > 98% • Remove chest secretions • Improve air entry to lung bases
Chest drains have stopped bubbling and swinging
Clamp chest drains Observe for 1" airway pressures, ,l, tidal volumes and 1" respiratory rates If signs of pneumothorax remove clamps Check X-ray in 12 hours to assesswhether the drains can be removed
• Patient not to develop a pneumothorax
Risk of Adult Respiratory Distress Syndrome (ARDS)
Optimize gaseousexchange by regular repositioning Minimize colloid infusion, give Elohaes if colloid required or blood if haemoglobin low Maintain mean arterial pressure (MAP) above 70 mmHg Aim for negative fluid balance of I litre in 24 hours, give diuretics if required Daily chest X-ray
• • • • • •
Patient is receiving pressure support via mechanical ventilation
Liaise with doctors regarding the reduction of pressure support => Reducesedation
accumulation of air in the pleural cavity separates the visceral and thoracic wall layers of the pleura and the elastic pull of the lung on the visceral pleura causes the lung to collapse (Welch 1993). This is known as a pneumothorax. Figure 1 (a) illustrates the normal lung anatomy and Fig~l (b), a lung affected by a pneumothorax. A tension pneumothorax is a very serious condition causing respiratory distress and circulatory collapse. W e s t (1992) describes a tension pneumothorax as a consequence of the communication between the lung and the pleural space functioning as a check valve. Air therefore enters this space during inspiration but cannot escape during expiration. The intrathoracic pressure may considerably exceed atmospheric pressure and thus interfere with venous return to the thorax. Peter had a right tension pneumothorax and a left spontaneous pneumothorax. This may have been the result of the pressure in the pleural space on the right side of the chest building up to a point where the mediastinum is shifted into the opposite lung causing it to collapse (Welch 1993). Peter's pneumothoraces were treated with the insertion of bilateral chest drains. The chest drains were underwater-seal drainage systems and were not attached to the suction but were open to air. H o u g h (1996) explains that suction may help to oppose the pleural surfaces and seal
Prevent ARDS PaO2 greater than I 0 kPa Saturations > 98% Achieve negative fluid balance MAP greater than 70 mmHg CVP4-Scm H20
• To wean patient to pressure support of 16 cm H20 • Patient to be comfortable but responsive and to prevent respiratory depression
the leak more quickly but sudden re-inflation of the lung should be avoided to prevent reexpansion pulmonary oedema.
Ventilation/Perfusion (V/Q) The pleural rupture causing a pneumothorax may become sealed by the patient lying on the affected side (Zidulka et al 1982). This position may cause desaturation with oxygen if there is a large pneumothorax because of a V / Q mismatch. Optimal oxygenation depends on the match of (V) to (Q). Ventilation is the process by which gases are exchanged between the atmosphere and the lung alveoli (Tortora & Grabowski 1996). Ventilation is regulated by neural and chemical control systems that include centres in the brain stem, peripheral and central chemoreceptors, and mechanoreceptors in skeletal muscle and joints. Perfusion relates to the pulmonary blood flow. Normally V / Q ratio is between 0.8 and 1.2 (Fig. 2(a)). West (1995) explains gravitational forces and the relationship of alveolar pressure to pulmonary vascular pressures and capillary blood flow in three zones (Fig. 3). The maximum exchange of carbon dioxide for oxygen in the alveoli occurs when the best ventilated areas are the best perfused. Zone III has good matching of ventilated alveoli to perfused capillaries. This is because the mean pulmonary arterial pressure
344 Intensiveand Critical Care Nursing
Parietal
pleura~
~--~Trachea
// Plcural
cavity~ I
"C ~ ]/
Intrapleural pressure is negative
Visceralpleura
Chest
wall~ ' ~ Diagphragm
Fig. I (a) The pleurae are held tight againstthe chestwall by the negative intrapleural pressure. Adapted from Welch (1993).
Reduced lung
size
/Tracheal
deviation
S: ~
'~Air in pleural cavity (pneumothorax) as a result of a stab wound
Negatwe space
I (b) Airis introduced into thepleuralcavity as a resultof the stab wound, causing a pneumothorax.Surfacetension and recoil of elastic fibres cause the lung on that side to collapse. Adapted from Welch (1993).
dependent (uppermost) areas through what is termed the preferential distribution of perfusion (Yeaw 1996). V / Q mismatch occurs when ventilation is in excess of perfusion and is described as deadspace-producing, with a V / Q >0.8 normally causing a raised partial pressure of carbon dioxide, (PaCO2) (Fig. 2(b)). If perfusion exceeds ventilation the disorder is shunt-producing with a V / Q <0.8. This normally causes a low PaCO2 and low PaO 2 (Fig. 2(c)) (Thomas 1997). Peter had atelectasis caused by the pneumothorax and bilateral consolidation. Fig. 2(b) shows diagrammatically an example of an alveolus in this state with the reflex constriction of vasculature that adjusts the perfusion to ventilation by shunting blood away from the under-ventilated area. This illustrates ,I,V--9,1, PaO2--> 4, Q. Peter's PaO2 was 12.5kPa despite bilateral consohdation of both lungs. Oxygen saturations were 95%.
Metabolic acidosis Oxygen saturation represents the percentage of haemoglobin saturated with oxygen (Hudak 1994). Abnormal pH, PaCO2 and temperature have a Bohr effect on the oxyhaemoglobin dissociation curve, shifting it to the right or left (refer to Fig. 4). Peter's arterial blood pH was 7.4, which is normal, but he had a metabolic acidosis because his base excess was -5.4 and bicarbonate 17.8. The metabolic acidosis was respiratory-compensated because the CO2 was low at 3.4 kPa and pH normal, whereas previously pH was low and PaCO 2 normal. Peter's axillary temperature was slightly elevated at 37.5°C and this had spiked during the night to 38.5°C. Acidosis and pyrexia can cause the oxygen dissociation curve to shift to the right and this would mean a decrease in the affinity of haemoglobin for oxygen. The respiratory compensation would cause a shift of the curve to the left and increases the affinity of haemoglobin for oxygen. More oxygen can be picked up in the lungs, but oxygen is less readily released in the capillaries. This could cause tissue hypoxia even with an adequate PaO 2 (Hudak & Gallo 1994).
Fig.
is about 23 m m H g at the lung bases compared to 3 m m H g at the apex (Darovic 1995). The alveoli in the base of the lung are also smaller and more numerous in comparison to those in the apex (Yeaw 1992). Gravity and hydrostatic pressure cause the dependent lung to be better ventilated and better perfused than the non-
Magnesium Peter's magnesium levels were below normal, 0.65 mmol/1. It is thought that magnesium sulphate may induce bronchodilation through opposing calcium-mediated bronchoconstriction and inhibiting parasympathetic acetycholine (Oh 1996). Research has also associated magnesium dietary intake of the population with lung function. A study by Britton et al
Multiple stab wounds
345
to have also made a comparison of the serum magnesium levels in relation t o both dietary magnesium intake and lung function. If Peter had a poor diet before trauma, coupled with the fact that he had a drug abuse problem this may have predisposed him to lung function problems. Hypomagnesium can be caused with large infusions of saline solutions such as gelofusine (Goldhill 1997) or diuretics (Oh 1996) both of which Peter had incurred.
.o,,/\ (a) Optimal oxygenation: Depends on the match of ventilation (V) and perfmion. Normal V/Q= 0.8 - 1.2. Pulmonary capillary
Reflex Bronchiolar constriction
Adult Respiratory Distress Syndrome
(b) Dead space producing: Ventilation exceeds perfusion because of reduction in blood supply. V/Q > 0.8. This causesa constriction of bronchioles. $ blood flow
~(~/~ ~
Embolism ~
(c) Shunt producing: Perfusion exceeds ,], ventilation ventilation because the alveoli is partially collapsed. V/Q < 0.8. Blood is shunted away fromthe underventilated alveoli. bloodflow
2
Reflex arteriole constriction Atelectasis
Fig.
Ventilationand perfusion relationships.Adapted from Hudak (1994).
Peter was at risk of Adult Respiratory Distress Syndrome (ARDS). ARDS sets in motion an inflammatory response which has devastating effects on the individual's respiratory physiology which may not only succumb to severe respiratory failure but also multiple organ dysfunction syndrome (Cutler 1996). The cascade of events is illustrated diagrammatically in Fig. 5 In ARDS there is increased pulmonary capillary permeability resulting in noncardiogenic pulmonary oedema (Hudak & Gallo 1996). Peter was at high risk because of direct and indirect lung injury. Predisposing factors included severe thoracic injury, prolonged hypovolaemic shock and a massive blood transfusion (Oh 1996). Clinical problems that can occur are hypoxemia, hypercarbia, barotrauma and V / Q mismatch (Thomas 1997).
Zone:l Potentially no bloodflow:PA>P~L~Pv.Mean Pulmonaryarterialpressureat apex= approx.3mmHg.
Intermittentbloodflow:Pa>PA>Pv Zone: 3
ConstantbloodflowPa>Pv>PA.Meanpulmonary arterialbloodpressureat baseapprox.= 23mmHg.
Fig. Representation of West Zones Model of the lung demonstrating the uneven distribution of blood flow in the lung caused by the differences in pressures affecting the capillaries (a: Pressure Arterial, A: Pressure Alveolar, PV: Pressure Venous). Adapted from West (1992) and Darovic (I 995).
(1994) of 2633 adults concluded that low magnesium intake is associated with wheezing, airway hyperactivity and low functional residual capacity (laP,C). Estimation of dietary intake is open to error because of the variation of intake from day to day. It would have been valuable
VENTILATION MODES Peter was receiving ventilation support in the form of pressure support and PEEP. Pressure support ventilation (PSV) augments or assists spontaneous breathing efforts up to a point where the pre-set pressure is reached (Hudak & Gallo 1994). In PSV pressure rather than volume is set. The volume delivered for any particular inflation pressure will vary with lung comphance and airways resistance, as well as with patient comfort (Beale 1994). PEEP is an airway pressure above atmospheric pressure at the end of a ventilator cycle, during which spontaneous breathing is absent (Oh 1996). A study by Brochard et al (1994) concluded that the duration of weaning from a ventilator was significantly shorter and the total length of stay in hospital shorter with PSV than with SIMV or T-piece trials. The study had a significant sample of 109 patients with multiple diagnosis and took into account all aspects of the patient and the weaning process. PSV is used in weaning because it helps with the inspiratory work by decreasing the work of airway resistance, increasing compliance and decreasing tissue work (Pierce et al 1993). It is critical that
346
Intensive and Critical Care Nursing
80 70 ,E 60
so
~o 3O 2O 10 0
zo,
~0
i 30
i 4o
i so pO= (ramHg)
, so
i 70
9to
sO
100
Fig. 4 Oxygen-haemoglobin dissociation curve showing the relationship between haemoglobin saturation and POaO 2. Increased blood pH, low PaCO> and low temperature all cause the oxygen-haemoglobin to shift to the left causing a higher affinity with which haemoglobin binds with oxygen. Decreased blood pH, high blood PaCO2 and high temperature all cause the oxygenhaemoglobin to shift to the right causing a lower affinity with which haemoglobin binds with oxygen.
Initial event of injury to
I
I
+ Damage to alveolar
/. alveolocapillarymembrane |
*
epithelium i
Dysfunction of Type II i
i Activation of camp ement, coagulatranand klnm systems
; i
pneumocytes
i
Damage to capillary endothelium
Platelet
I
I
activation
i I Chemotaxis of i ]Cytokine release (eg:
agg~aton Ilneutrophi,s II~F&,'-t~ ' I Neutrophit aggregation & release - oxygen
of~
radicals
- proteolytic enzymes - arachydonic acid metabolites
I Haemorrhage
- PAF E
I Increased capillary I membranepermeabI ty I
Surfactant deficit & dysfunction
Reduced lung comp ance & [ atelectasis
~ ~1
JL r I
Alveolar cedema
/
I
i Non-selective il vascoconstriction &
I
Movement of inflamatory ceils into interstitium
Hyaline membrane Iformaton & fibrosis
/I
I Right to left shunt'ng & V/Q m smatch ]
L
I Fulminant respiratory failure
i
Fig. 5 A simplified model: of the pathogenesis of ARDS (PAF: Platelet activating factor, TNF: Tumour necrosis factor, IL-I: interleukin I). Reproduced by kind permission of Cutler, Intensive and Critical Care Nursing 1996:12:316-326.
weaning from a ventilator be initiated as soon as possible in order to prevent the risk ofnosocomial pneumonia (Clement & Buck 1996). When mechanically ventilated patients are Fully supported, between 72 and 96 hours respiratory muscle atrophy begins (MacIntyre 1988). Weaning from pressure support can be performed by gradually reducing the support the patient is receiving as his condition allows. There has been much research into when and how to wean a patient successfully from a ventilator but there is little conclusive evidence and weaning practices differ from hospital to hospital. A study by Burns & Burns (1994) attempted to compare five different weaning indices, designed to predict weaning potential of 37 patients. The researchers compared the Burns Weaning Assessment Program (BWAP), weaning index; frequency tidal volume ratio; compliance, resistance, oxygenation and pressure index; and negative inspiratory pressure. The BWAP was developed from a general and respiratory checklist, which provided a score depending on the percentage of positive answers. The researchers suggested using the BWAP weaning index but the author would be wary of this advice because the researchers developed that index and the study Failed to demonstrate the superiority of'any of'the weaning indices. No weaning index was employed in the I T U in which Peter was nursed. The initiation of weaning is normally under the control of the anaesthetist although the nurse has a role in indicating when the patient is ready and in the management of'weaning. PEEP encourages alveolar recruitment (aeration of additional alveoli) which would be advantageous to Peter because of'his atelectasis. PEEP increases the area available For gaseous exchange, reduces shunt fraction and increases the PaO 2 (Mackenzie 1992). PEEP has the advantage of increasing the Functional residual capacity (FRC) and, at normal levels, reducing the work of breathing (Beale 1994). PEEP is helpful in patients with low FRC and high intrapulmonary shunt fraction. PEEP has effects on the cardiovascular system and can cause a decrease in cardiac output by increasing the intrathoracic pressure and thus reducing venous return. Hyperinflation and barotrauma can also be caused by the use of PEEP. Peter had only a small amount of PEEP but a larger amount might have resulted in adverse effects. Increasing PEEP beyond a critical level could cause over-distension of the lungs leading to a reduction in tidal exchange (Naik et al 1996). A study by Naik et al (1996) demonstrated that compliance and PaO 2 in patients with lung volume loss due to lobar collapse and/or consolidation, as was evident in Peter, could be increased with PEEP. In this quantitative study
Multiple stabwounds 347 arterial blood gases were measured in between increasing PEEP levels from a base line of 2-3 cm H 2 0 to 5 - 6 c m H 2 0 with children with no abnormal lung function compared to those with different lung problems, diagnosed by chest X-ray. Compliance was measured by dividing tidal volume by the difference between the peak inspiratory pressure and PEEP. It could be argued that findings from the study could not be applied to adults because of age-related differences in lung mechanics. Also twenty of the sample were receiving neuromuscular blocking agents and this may affect the validity of the results.
DISCUSSION AND I M P L E M E N T A T I O N OF RESPIRATORY CARE Humidification was used to decrease consolidation in Peter's lungs by loosening secretions to make them easy to remove by ETS. In normal respiration people humidify inspired air during its passage to the lungs, with moisture from the mucus secreted by the goblet cells o f the mucous membranes which line airways (Ballard et al 1991). The upper respiratory tract is bypassed when a patient has an endotracheal tube in situ. Infection associated with use of humidifiers has frequently been reported. Pseudomonas is the most c o m m o n organism found (Oh 1996). Research by Kirton et al (1997) on 200 patients found that the rate of ventilator-associated pneumonia (VAP) with heat moisture exchanging filters (HMEF) was half in comparison to heated water humidifiers. Kirton et al (1997) acknowledged that there are still some questions regarding the possible role oftranslocation o f systemic bacteria, seeding the lungs and causing VAP, which could affect interpretation of the research findings. The researchers failed to identify the type o f heated water humidifier and simply described it as a heated wire humidifier. Whether the H M E F would humidify adequately in comparison to a heated water humidifier needs to be studied, although the increased likelihood of infection could be detrimental to critically ill patients and this is a very valid point to take into consideration when choosing a humidification method. Peter was regularly repositioned to aid in the drainage of chest secretions, Pearson & Parr (1993) advocate this. In the afternoon Peter was placed on his left side and very quickly his oxygen saturation (SaO2) fell to 87%. He was turned to a supine position and his saturation increased to within normal limits. The reason for this could be twofold. Peter's left pneu-
mothorax may not have resolved, causing a V / Q mismatch or more specifically a shunt effect as explained earlier. Another reason may have been that the consolidation was worse in his left lung than the right, also causing a shunt. ETS is an aseptic technique used to clear secretions and ensure the airway is clear. Prior to ETS the author would explain the procedure to Peter because it can be a frightening experience. A size 12 catheter was used, as this catheter was half the diameter of his endotracheal tube so air and oxygen could still flow down towards the lungs while the catheter was being introduced (Hough 1996). Without this inflow the negative pressure caused by suctioning can cause air to be sucked out of the alveoli causing them to collapse (Fiorientini 1992). ETS can cause hypoxia and may also cause vagal stimulation leading to bradycardia. Aspiration of nasogastric feed can be a problem especially with continuous feeding, as Peter was receiving, rather than intermittent bolus feeds (Fiorientini 1992). The author did not use sodium chloride 0.9% (NaC1) before ETS, but the physiotherapist had used this solution. NaC1 instillation (NSI) is still quite c o m m o n practice and theoretically it is presumed that NSI increases secretion removal when secretion is viscid due to inadequate humidity in the airway, and also encourages a cough. R.esearch into NSI by Bostick & Wendelgass (1985) found no statistical difference in the weight of samples with no normal saline compared to 5-10 ml NSI. It was not possible to measure the percentage o f NaC1 comprised in the aspirate which was recovered. The difference among the post-treatment aspirate weights therefore may not necessarily have reflected the effects of treatment in terms of secretion removal. A more recent quasi-experimental study by Ackerman (1993) showed patients' 8aO 2 worsened over a period of 2, 3, 4, 5 minutes with NSI. But the researcher did not comment on the patients' position, and full blood gas analysis might have provided a more sensitive measurement than SaO 2. Hagler et al (1994) found that instillation of NaC1 solution actually increases bacteria in the lower airway. However, this quantitative research was based on only 10 patients and therefore the author would question the general significance of the research. Further study of NSI may find it beneficial to a certain group o f patients, but the research currently available is definite about it being a procedure with negative effects. Peter's bilateral chest drains had stopped bubbling and swinging for 18 hours and were clamped prior to removal to ensure that the pleural leaks had healed. Bubbling indicates that there is a hole in the visceral pleura, allowing air to escape from the lung. The swinging
348
Intensive and Critical Care Nursing
which is seen is caused by variation in intrapleural pressure during respiration, and is no longer evident when the lung is fully expanded and the tube occluded. Chest tube clamping is potentially dangerous if pleural rupture causing the pneumothorax has not stopped draining air and may convert a pneumothorax into a tension pneumothorax (Welch 1993). Refer to Table 2 for signs o f a pneumothorax. It had been planned after the morning assessment to reduce Peter's PSV to 16cm H 2 0 . Before Peter's drains were clamped he became very tachypnoeic and tidal volumes fell. O n auscultation there was no indication o f secretion retention and the water traps of his humidification equipment were empty. The reason for the changes may have been that Peter had become exhausted and his work of breathing increased with the reduction of ventilary support in the past 24 hours. O h (1996) comments that a hole in the diaphragm may result in difficulty in weaning especially if bilateral nerve palsy has been caused. The doctors were consulted and it was agreed to increase the pressure support to 20 cm H 2 0 . After a short period of time Peter's breathing was more comfortable. In the afternoon the author sought to reduce the extremely high dose of midazalom and morphine that Peter was receiving because these drugs can cause respiratory centre dysfunction. A large amount o f sedation may have been required because Peter had a history o f cocaine abuse. Flumazenil may have helped to reduce the effects of morphine and midazalom. A study by Pepperman (1990), cited by Carroll (1993), o f patients receiving flumazenil in a continuous infusion to reverse the effects of morphine and midazalom concluded that these patients were weaned earlier than those receiving a placebo. Flumazenil has a short half-life and poses a risk of re-sedation because the duration of midazalom and morphine may exceed the effect of flumazenil. Pepperman (1990) did not note any occurrence ofresedation in his study. In the evening Peter was wakening and was able to obey commands. A background lower
Signs of a pneumothorax • • • • • • • • • •
,!, breath sounds on the affected side $ tidal volumes "1"airway pressures Asymmetrical chest movement Tachyponoea Tracheal deviation Cyanosis Chest pain on affected side Tachycardia or bradycardia $ cardiac output (tension pneumothorax)
dose of midazalom and morphine was continued to maintain patient comfort. Initially Peter's Ramsey sedation score was 6 and in the evening he was at the awake level of 2. In the evening it was possible to reduce Peter's pressure support to 18cm H 2 0 without any adverse effects. This may have been because he had had time to recover from earlier exhaustion and the reduction of sedation improved his respiratory centre function. Verbal and non-verbal communication with Peter was vital. Sedation would have prevented him communicating much with the author but he still needed reassurance, to be informed o f procedures and to receive reality orientation. Throughout the day as Peter awoke, the author invited his wife to reassure him and this was helpful because she was a familiar voice in an unfamiliar environment. Phenomenological research, using Colazzi's method, by Hafsteindottir (1996) on patients' experiences of communication during the respiratory assistance period revealed the frustration and panic of not being able to communicate verbally. The study was well presented and clear but the research was undertaken in Iceland and cultural differences may mean the results might have been different in the UK, although a study by Johnson and Sexton (1990) supports Hafsteindottir's findings. An experimental study by Hennman (1989) on 26 patients being weaned from a T-piece studied the differences of heart rate, mean arterial pressure (MAP), and respiratory rate between two groups to measure patients' stress response when being weaned. One group received touch and verbal c o m m u nication whereas the control group did not. Surprisingly, there was no difference between the groups. The author feels that because the participants were prepared for this study and qualitative research into the patients' emotional feelings were not incorporated during this research the results should not be used to direct practice. Admission to I T U is not only traumatic for patients but also for their significant others. So in an attempt to alleviate anxiety for Mrs Smith she was enabled to participate in Peter's care. A qualitative study by Jamerson et al (1996) used data from unstructured interviews in an attempt to describe the experience of family members with relatives in ITU. A group interpretative process along with triangulation was used to analyse the results into four categories: Hovering, Information seeking, Tracking and Garnering resources. The study sample comprised 18 w o m e n and 2 men and this could suggest that the research findings may only be relevant with female members of the family. Mrs Smith appeared to be in the 'information seeking' stage, described by Jamerson as 'an
Multiple stab wounds
active process of gathering information about the patient'. Peter's progress was explained to Mrs Smith and arrangements made for her to talk with the doctor.
EVALUATION Changes in a patient's condition can sometimes happen very rapidly in ITU despite planning care, implementing it and making what may seem realistic goals. Peter's condition at the end of the shift was slightly improved and it seemed that the progress in his condition would help in allowing him to be weaned quicker from the ventilator in the following days. An evaluation at the end of the shift revealed that air entry to the bases of both lungs had improved on auscultation and a chest X-ray in the morning would be able to confirm this. Oxygen saturations were now above 98% despite an earlier drop in saturations when Peter was placed on his left side. White secretions were removed f r o m P e t e r ' s chest a n d t h e increase i n t h e i r v o l u m e m a y h a v e b e e n d u e to successful physiotherapy and humidification. Peter had d e v e l o p e d n o signs o f a f u r t h e r p n e u m o t h o r a x or A R D S w i t h t h e p l a n n e d a c t i o n taken, as confirmed by X-ray the following morning. P S V was n o t r e d u c e d to the goal o f 1 6 c m H 2 0 , as it m i g h t h a v e b e e n w i t h earlier p l a n n i n g o f t h e r e d u c t i o n o f sedation. Peter's P S V h a d to b e increased because h e was distressed, b u t was r e d u c e d b a c k to 18 c m H 2 0 w i t h o u t ill-effect w h e n t h e s e d a t i o n was decreased a n d P e t e r h a d a P,.amsey score o f 2. P e t e r was e x t u b a t e d a few days after the a u t h o r h a d l o o k e d after h i m . A l t h o u g h h e was w e a k h e was r e c o v e r i n g v e r y w e l l , c o n s i d e r i n g
his injuries and the large blood transfusion received for hypovolaemic shock.
CONCLUSION T h e nurses' role is c o n t i n u a l l y e x p a n d i n g w i t h i n t h e I T U e n v i r o n m e n t a n d nurses can m a k e a difference to patients' o u t c o m e b y a d m i n i s t e r i n g holistic care based o n s o u n d a n d w h e n possible r e s e a r c h - b a s e d k n o w l e d g e . Respiratory management should be proactive a n d e v a l u a t i o n o f care reflective to c o m m i t o u r profession to excellence.
REFERENCES Ackerman E 1993 The effect of saline lavage prior to suctioning. American Journal of Criticai Care 3: 444-447
349
Ballard K, Cheeseman W, Rapner T 1992 Humidification for ventilated patients. Intensive and Critical Care Nursing 8 (1): 92-99 Beale 1K 1994 Weaning from mechanical ventilation. Brifshdournal of Intensive Care 5:168-175 BostickJ, Wendelgass S 1987 Normal saline instillation as part of the suctioning procedure: Effects on PaO2 and amount of secretions. Heart and Lung 16: 532-537 BrittonJ, Richards K, Wisniewski A 1994 Dietary magnesium, lung function, wheezing and airway hyperactivity in a random adult population sample. Lancet 344:352-357 Brochard L, Reuiss A, Benito S 1994 Comparison of three methods of gradual withdrawal from ventilatory support during weaning from mechanical ventilation. American Journal Respiratory Critical Care Medicine 150:896-903 Burns S M, BurnsJ E 1994 Comparison of five clinical weaning indices. AmericanJournal of Critical Care 3: 342-352 Carroll K 1993 The role of analgesics and sedatives in the management of pain and agitation during weaning from mechanical ventilation. Critical Care Nurse 15 (4): 68-77 Clement j, Buck E 1996 Weaning from mechanical ventilatory support. Dimensions of Critical Care 15: 114-129 Cutler L 1996 Acute Respiratory Distress Syndrome: an overview. Intensive and Critical Care Nursing 12: 316-326 Darovic G 1995 Haemodynamic Monitoring and Non invasive Clinical Application. 2nd Edn. W. B. Saunders Endacott t~ 1996 Staffing intensive care units: a consideration of contemporary issues. Intensive and Critical Care Nursing 12:193-199 Fiorentini A 1992 Potential hazards oftracheaobronchial suctioning. Intensive and Critical Care Nursing 217-226 Goldhill D 1997 Calcium and magnesium. Care of the Critically Ill 13:112-115 Hafsteindotdr/3 1996 Patients experiences of communication during the respiratory treatment period. Intensive and Critical Care Nursing 12: 261-27I Hagler D, Traver G 1994. Endotracheal Saline and Suction Catheters: Sources of Lower Airway Contamination. AmerJ of Crit Care 3 (6): 444-447 Hennman E 1989 Effect of nursing contact on the stress response of patients being weaned from mechanical ventilation. Heart and Lung 18:483-489 Hough A 1996 Respiratory physiology - the essentials. 5th Edn. Wilkins & Wilkins, London Hudak C, Gallo B 1994 Critical Care Nursing. A Holistic Approach. 6th Edn. Lippincott Company, Washington Jamerson P, Scheibmeir M, Bott M 1996 The experiences of families with a relative in the intensive care unit. Heart and Lung 25 (6): 467-473 Johnson M, Sexton D 1990 Distress during mechanical ventilation patients' perception. Critical Care Nurse 10 (7): 48-51 Kirton O, De Haven B, MorganJ 1997 Rates of nosocomial pneumonia associated with HME/bacterial filter and heated wire humidifiers: a prospective randomised trial. InternationalJournal of Intensive Care, Spring, 6-12 Mackenzie S 1992 Clinical use of PEEP, indications, effects and practicalities. BritishJonrnal of Intensive Care 10:335-340 Maclntyre N 1988 Weaning from mechanical ventilatory support: volume assisting intermittent breaths versus
350
Intensive and Critical Care Nursing
pressure assisting every breath. Respiratory Care 33 (2): 121-125 Naik S, Greenough A, Griffen F et al 1996 The effect of changes in PEEP on gas exchange. British Journal of Intensive Care 3:82-88 Oh T E 1996 Intensive Care Manual. 4th Edn. Butterworth-Hennemann, Hong Kong Pearson S, Parr S 1993 Physiotherapy in the critically ill patient. Care of the Critically Ill 9 (3): 128-131 Peppemlan M L 1990 Double-blind study of the reversal of midazalom induced sedation in the intensive care unit with flumazeni: effect on weaning from ventilation, Anaethesia and Intensive Care 18:38-44 Pierce J, Wiggins S, Plaskon C 1993 Pressure support ventilation: reducing the work of breathing during weaning. Dimensions of Critical Care Nursing 12 (6): 282-290 Ramsey M A, Savage J M, Simpson B IK et a119974 Controlled sedation with Alphaxalone-Alphadolone. British Medical Journal 2:656-659 Thomas C 1997 Use of the prone position: The
ventilation/perfusion relationship in ARDS. Care of the Critically Ill 13(3): 96-100 Tortora G, Grabowski S 1996 Principles of Anatomy and Physiology. 8th Edn. HarperCollins, New York WelchJ 1993 Chest tubes and pleural drainage. Surgical Nurse 6 (5): 7-12 WestJ 1992 Pulmonary Pathophysiology - the essentials. 4th Edn. Williams and Wilkins, Baltimore, USA WestJ 1995 Respiratory Physiology - the essentials. 5th Edn. williams and Wilkins, Baltimore, USA Westcott C 1995 The sedation of patients in intensive care units: a nursing review. Intensive and Critical Care Nursing 11:26--31 Yeaw E 1992 How position affects oxygenation. Good Lung Down? American Journal of Nursing 3:27-29 Yeaw E 1996 The effect of bodies positioning upon maximal oxygenation of patients with unilateral lung pathology. Journal of Advanced Nursing 23:55-61 Zidulka A, Braidy T F, Rizzi N C et al 1982 Position may stop pneumothorax progression in dogs. Am. Rev. Resp. Dis. 126:51-53
B
INTRODUCTION
Nicola A. Burchmore RN, DipH, ENB 100, Staff Nurse, St. Bartholomew's Hospital, ITU, London, UK
(Requests for offprints to NAB) Manuscript accepted I December 1997
Respiratory care management is an integral role within the duties of an intensive care nurse. The nurse plays an active role in improving the patient's condition based on research and evidence. Holistic physiological assessment is necessary to enable the nurse to consider other factors that may have an effect on the patient's ventilation. Metabolic acidosis can be compensated by hyperventilation causing a reduction in Partial Pressure of arterial carbon dioxide
Intensive and Critical Care Nursing (I 997) 13, 341-350
© 1997Harcourt Braceand Co. Ltd
(PaCO2). A reduction in oxygen saturation (SaO2) , haemoglobin or cardiac index will cause a reduction in the delivery of oxygen (DO2) to cells. Patient position can affect Partial Pressure of arterial oxygen (PaO2) and the matching o f ventilation to perfusion. Management of ventilation is a complex role of intensive care nurses in which they can optimize patients' status through holistic assessment, planning and implementation of care. The nurses' role in respiratory management is expanding, and in some units nurses can initiate weaning of patients from ventilators and make appropriate changes to ventilation, e.g. change Fraction of inspired oxygen (FiO2) or pressure support. Nurses are now able to initiate extubation of cardiac fast-track patients if the medical and nursing staff have predetermined extubation criteria. Contemporary medical practices also have a direct effect on the extent of clinical decisionmaking undertaken by nurses (Endacott 1996). Currently, it is preferred that patients are ventilated using support ventilation rather than fully controlled ventilation, whenever possible. Endacott (1996) suggests that this may have implications for the nurses' role. Patient-triggered modes may require a more intense and ongoing assessment in relation to the patients' ability to cope both physically and psychologically. The nurses' role in clinical decision-making is also increased by the trend to have patients lightly sedated rather than fully sedated and paralyzed. Continuous assessment is required to ensure that the patient is not under or over-sedated. Titration of some drugs and fluids is at the nurses' discretion in an intensive care unit and is required to meet desired physiological parameters, e.g. mean arterial pressures above 701nmHg and urinary output above 0.5 ml/kg/h. The nurses' role in this area again needs to be holistic and clinical decisions made should be based on a complete overview of the patient. The aim in this case study is to highlight the importance of the nurses' role in respiratory management whilst expanding readers' knowledge on respiratory anatomy, physiology and pathophysiology, ventilation modes and relevant recent research into respiratory management. To maintain confidentiality a false name, Peter Smith, will be used throughout this article.
PATIENT SCENARIO Peter Smith, a 24-year-old male, was admitted into an Intensive Therapy Unit (ITU) following an emergency laparotomy after sustaining multiple stab wounds to his chest and back. N o
342 Intensive and Critical Care Nursing previous medical history was known. Peter had presented with a right tension pneumothorax and left spontaneous pneumothorax for which bilateral chest drains had been inserted in the Accident and Emergency Department. The laparotomy revealed free blood in the peritoneum, which was washed out. A small hole in the diaphragm, right posterior suprahepatic region, was repaired. Peter received a massive transfusion for hypovolaemic shock. This consisted of 12 units of blood, 6 units of fresh frozen plasma and 3.5 litres of gelofusine before admission to ITU. Upon admission into I T U he was cold centrally and haemodynamically unstable. Peter was initially ventilated on 12 synchronized intermittent mandatory ventilation (SIMV) breaths per minute, pressure support 2 6 c m H 2 0 , 5-cm H 2 0 positive end-expiratory pressure (PEEP) and FiO 2 0.6. O n day two after admission into the ITU, a computed tomography (CT) scan revealed bilateral lung consolidation, no head injuries and fluid lying principally within the pelvis. The author nursed Peter on his fourth day in ITU, and this formed the basis for this paper. A physiological system approach employed in the I T U to plan and evaluate care was used to make an assessment of the patient's condition.
_
•
.
....
_
.....
"
........
•
.....
•
•
•
.
PHYSIOLOGICAL ASSESSMENT • Respiratory: Physically Peter looked well perfused, his colour was good, oxygen saturations (SAO2) were 95%, partial pressure of arterial oxygen (PaO2) 12.5 kPa. Chest examination showed chest movement was bilateral and equal with an approximate inspiratory and expiratory ratio of 1:2. Auscultation revealed decreased breath sounds at the bases of both lungs. The chest X-ray showed bilateral consolidation o f both lungs, and that the pneumothoraces were diminishing. Peter was receiving warm humidified oxygen (02) via a concha to loosen any consolidation and minimal white secretions were obtained from his chest by endotracheal suction (ETS). A concha is a device which can be used in conjunction with a ventilator to humidify/inspired oxygen. Peter had metabolic acidosis with respiratory compensation. He was breathing spontaneously although he remained dependent on the ventilator with pressure support o f 18 cm H 2 0 , PEEP 5 cm H 2 0 and FiO 2 0.35, despite a high dose of sedation. SIMV breaths and pressure support had been gradually reduced (Peter was being 'weaned' from ventilation support) during the previous day. Peter's breathing appeared
•
to be comfortable. Ventilatory observations were all within normal limits at 0900 hrs. The bilateral chest drains that had been inserted upon admission were still in situ. There had been no bubbling of air from the drains or movement of fluid levels in the tubes with respiration ('swinging') for approximately 18 hours. Cardiac: The patient was cardiovascularly stable with a normotensive blood pressure and sinus rhythm. Peter's central venous pressure was 4 cm H 2 0 when the transducers was re-zeroed. Peter was peripherally cool (30.3°C) and centrally warm (37.4°C). Neurological: Peter was sedated with 30 mg midazalom and 10 mg morphine per hour. Sedation score according to Ramsey (1974) was 6, i.e. patient asleep with no response to stimuli. Pupils were equal and reacting briskly to light. Peter had a cough reflex on ETS. O n the second day of admission Peter had a urine screen for drugs which detected cocaine. Fluid and electrolytes: Peter's fluid balance had been positive due to the massive blood transfusion. Generalized oedema was noted. Urinary output was good, above 0.5 ml/kg/h, demonstrating good perfusion to kidneys. Magnesium levels were low, 0.65 mmol/1. All other electrolyte levels were normal. Blood sugar levels were normal. Gastro-intestinal: Peter was receiving osmolite feed nasogastrically, continuously with a break at night, at the rate of 2 litres per day. There had been minimal aspirate from the stomach. The patient's bowels had opened since admission. Psychological: All procedures were explained to Peter. A rest period for 2 hours daily was adhered to, to reduce sensory overload and allow the patient to have an opportunity to have rapid eye movement (REM) sleep. Peter's wife visited regularly and was kept abreast of progress by the nursing and medical staff. Mrs Smith played an active role in Peter's mouth and eye care and this was encouraged.
After the morning assessment of the patient, the author made a respiratory plan of action for the day as summarized in Table 1.
ANATOMY, PHYSIOLOGY A N D PATHOPHYSIOLOGY Pneumothoraces Chest trauma such as Peter sustained will allow air to be entrained into the pleural space. An
Multiplestab wounds
343
Problem
Plan of action
Goal
Consolidation and atelectasis of both lungs
=> ETS as required Humified oxygen Regular repositioning
• Oxygen saturations (SaOz) > 98% • Remove chest secretions • Improve air entry to lung bases
Chest drains have stopped bubbling and swinging
Clamp chest drains Observe for 1" airway pressures, ,l, tidal volumes and 1" respiratory rates If signs of pneumothorax remove clamps Check X-ray in 12 hours to assesswhether the drains can be removed
• Patient not to develop a pneumothorax
Risk of Adult Respiratory Distress Syndrome (ARDS)
Optimize gaseousexchange by regular repositioning Minimize colloid infusion, give Elohaes if colloid required or blood if haemoglobin low Maintain mean arterial pressure (MAP) above 70 mmHg Aim for negative fluid balance of I litre in 24 hours, give diuretics if required Daily chest X-ray
• • • • • •
Patient is receiving pressure support via mechanical ventilation
Liaise with doctors regarding the reduction of pressure support => Reducesedation
accumulation of air in the pleural cavity separates the visceral and thoracic wall layers of the pleura and the elastic pull of the lung on the visceral pleura causes the lung to collapse (Welch 1993). This is known as a pneumothorax. Figure 1 (a) illustrates the normal lung anatomy and Fig~l (b), a lung affected by a pneumothorax. A tension pneumothorax is a very serious condition causing respiratory distress and circulatory collapse. W e s t (1992) describes a tension pneumothorax as a consequence of the communication between the lung and the pleural space functioning as a check valve. Air therefore enters this space during inspiration but cannot escape during expiration. The intrathoracic pressure may considerably exceed atmospheric pressure and thus interfere with venous return to the thorax. Peter had a right tension pneumothorax and a left spontaneous pneumothorax. This may have been the result of the pressure in the pleural space on the right side of the chest building up to a point where the mediastinum is shifted into the opposite lung causing it to collapse (Welch 1993). Peter's pneumothoraces were treated with the insertion of bilateral chest drains. The chest drains were underwater-seal drainage systems and were not attached to the suction but were open to air. H o u g h (1996) explains that suction may help to oppose the pleural surfaces and seal
Prevent ARDS PaO2 greater than I 0 kPa Saturations > 98% Achieve negative fluid balance MAP greater than 70 mmHg CVP4-Scm H20
• To wean patient to pressure support of 16 cm H20 • Patient to be comfortable but responsive and to prevent respiratory depression
the leak more quickly but sudden re-inflation of the lung should be avoided to prevent reexpansion pulmonary oedema.
Ventilation/Perfusion (V/Q) The pleural rupture causing a pneumothorax may become sealed by the patient lying on the affected side (Zidulka et al 1982). This position may cause desaturation with oxygen if there is a large pneumothorax because of a V / Q mismatch. Optimal oxygenation depends on the match of (V) to (Q). Ventilation is the process by which gases are exchanged between the atmosphere and the lung alveoli (Tortora & Grabowski 1996). Ventilation is regulated by neural and chemical control systems that include centres in the brain stem, peripheral and central chemoreceptors, and mechanoreceptors in skeletal muscle and joints. Perfusion relates to the pulmonary blood flow. Normally V / Q ratio is between 0.8 and 1.2 (Fig. 2(a)). West (1995) explains gravitational forces and the relationship of alveolar pressure to pulmonary vascular pressures and capillary blood flow in three zones (Fig. 3). The maximum exchange of carbon dioxide for oxygen in the alveoli occurs when the best ventilated areas are the best perfused. Zone III has good matching of ventilated alveoli to perfused capillaries. This is because the mean pulmonary arterial pressure
344 Intensiveand Critical Care Nursing
Parietal
pleura~
~--~Trachea
// Plcural
cavity~ I
"C ~ ]/
Intrapleural pressure is negative
Visceralpleura
Chest
wall~ ' ~ Diagphragm
Fig. I (a) The pleurae are held tight againstthe chestwall by the negative intrapleural pressure. Adapted from Welch (1993).
Reduced lung
size
/Tracheal
deviation
S: ~
'~Air in pleural cavity (pneumothorax) as a result of a stab wound
Negatwe space
I (b) Airis introduced into thepleuralcavity as a resultof the stab wound, causing a pneumothorax.Surfacetension and recoil of elastic fibres cause the lung on that side to collapse. Adapted from Welch (1993).
dependent (uppermost) areas through what is termed the preferential distribution of perfusion (Yeaw 1996). V / Q mismatch occurs when ventilation is in excess of perfusion and is described as deadspace-producing, with a V / Q >0.8 normally causing a raised partial pressure of carbon dioxide, (PaCO2) (Fig. 2(b)). If perfusion exceeds ventilation the disorder is shunt-producing with a V / Q <0.8. This normally causes a low PaCO2 and low PaO 2 (Fig. 2(c)) (Thomas 1997). Peter had atelectasis caused by the pneumothorax and bilateral consolidation. Fig. 2(b) shows diagrammatically an example of an alveolus in this state with the reflex constriction of vasculature that adjusts the perfusion to ventilation by shunting blood away from the under-ventilated area. This illustrates ,I,V--9,1, PaO2--> 4, Q. Peter's PaO2 was 12.5kPa despite bilateral consohdation of both lungs. Oxygen saturations were 95%.
Metabolic acidosis Oxygen saturation represents the percentage of haemoglobin saturated with oxygen (Hudak 1994). Abnormal pH, PaCO2 and temperature have a Bohr effect on the oxyhaemoglobin dissociation curve, shifting it to the right or left (refer to Fig. 4). Peter's arterial blood pH was 7.4, which is normal, but he had a metabolic acidosis because his base excess was -5.4 and bicarbonate 17.8. The metabolic acidosis was respiratory-compensated because the CO2 was low at 3.4 kPa and pH normal, whereas previously pH was low and PaCO 2 normal. Peter's axillary temperature was slightly elevated at 37.5°C and this had spiked during the night to 38.5°C. Acidosis and pyrexia can cause the oxygen dissociation curve to shift to the right and this would mean a decrease in the affinity of haemoglobin for oxygen. The respiratory compensation would cause a shift of the curve to the left and increases the affinity of haemoglobin for oxygen. More oxygen can be picked up in the lungs, but oxygen is less readily released in the capillaries. This could cause tissue hypoxia even with an adequate PaO 2 (Hudak & Gallo 1994).
Fig.
is about 23 m m H g at the lung bases compared to 3 m m H g at the apex (Darovic 1995). The alveoli in the base of the lung are also smaller and more numerous in comparison to those in the apex (Yeaw 1992). Gravity and hydrostatic pressure cause the dependent lung to be better ventilated and better perfused than the non-
Magnesium Peter's magnesium levels were below normal, 0.65 mmol/1. It is thought that magnesium sulphate may induce bronchodilation through opposing calcium-mediated bronchoconstriction and inhibiting parasympathetic acetycholine (Oh 1996). Research has also associated magnesium dietary intake of the population with lung function. A study by Britton et al
Multiple stab wounds
345
to have also made a comparison of the serum magnesium levels in relation t o both dietary magnesium intake and lung function. If Peter had a poor diet before trauma, coupled with the fact that he had a drug abuse problem this may have predisposed him to lung function problems. Hypomagnesium can be caused with large infusions of saline solutions such as gelofusine (Goldhill 1997) or diuretics (Oh 1996) both of which Peter had incurred.
.o,,/\ (a) Optimal oxygenation: Depends on the match of ventilation (V) and perfmion. Normal V/Q= 0.8 - 1.2. Pulmonary capillary
Reflex Bronchiolar constriction
Adult Respiratory Distress Syndrome
(b) Dead space producing: Ventilation exceeds perfusion because of reduction in blood supply. V/Q > 0.8. This causesa constriction of bronchioles. $ blood flow
~(~/~ ~
Embolism ~
(c) Shunt producing: Perfusion exceeds ,], ventilation ventilation because the alveoli is partially collapsed. V/Q < 0.8. Blood is shunted away fromthe underventilated alveoli. bloodflow
2
Reflex arteriole constriction Atelectasis
Fig.
Ventilationand perfusion relationships.Adapted from Hudak (1994).
Peter was at risk of Adult Respiratory Distress Syndrome (ARDS). ARDS sets in motion an inflammatory response which has devastating effects on the individual's respiratory physiology which may not only succumb to severe respiratory failure but also multiple organ dysfunction syndrome (Cutler 1996). The cascade of events is illustrated diagrammatically in Fig. 5 In ARDS there is increased pulmonary capillary permeability resulting in noncardiogenic pulmonary oedema (Hudak & Gallo 1996). Peter was at high risk because of direct and indirect lung injury. Predisposing factors included severe thoracic injury, prolonged hypovolaemic shock and a massive blood transfusion (Oh 1996). Clinical problems that can occur are hypoxemia, hypercarbia, barotrauma and V / Q mismatch (Thomas 1997).
Zone:l Potentially no bloodflow:PA>P~L~Pv.Mean Pulmonaryarterialpressureat apex= approx.3mmHg.
Intermittentbloodflow:Pa>PA>Pv Zone: 3
ConstantbloodflowPa>Pv>PA.Meanpulmonary arterialbloodpressureat baseapprox.= 23mmHg.
Fig. Representation of West Zones Model of the lung demonstrating the uneven distribution of blood flow in the lung caused by the differences in pressures affecting the capillaries (a: Pressure Arterial, A: Pressure Alveolar, PV: Pressure Venous). Adapted from West (1992) and Darovic (I 995).
(1994) of 2633 adults concluded that low magnesium intake is associated with wheezing, airway hyperactivity and low functional residual capacity (laP,C). Estimation of dietary intake is open to error because of the variation of intake from day to day. It would have been valuable
VENTILATION MODES Peter was receiving ventilation support in the form of pressure support and PEEP. Pressure support ventilation (PSV) augments or assists spontaneous breathing efforts up to a point where the pre-set pressure is reached (Hudak & Gallo 1994). In PSV pressure rather than volume is set. The volume delivered for any particular inflation pressure will vary with lung comphance and airways resistance, as well as with patient comfort (Beale 1994). PEEP is an airway pressure above atmospheric pressure at the end of a ventilator cycle, during which spontaneous breathing is absent (Oh 1996). A study by Brochard et al (1994) concluded that the duration of weaning from a ventilator was significantly shorter and the total length of stay in hospital shorter with PSV than with SIMV or T-piece trials. The study had a significant sample of 109 patients with multiple diagnosis and took into account all aspects of the patient and the weaning process. PSV is used in weaning because it helps with the inspiratory work by decreasing the work of airway resistance, increasing compliance and decreasing tissue work (Pierce et al 1993). It is critical that
346
Intensive and Critical Care Nursing
80 70 ,E 60
so
~o 3O 2O 10 0
zo,
~0
i 30
i 4o
i so pO= (ramHg)
, so
i 70
9to
sO
100
Fig. 4 Oxygen-haemoglobin dissociation curve showing the relationship between haemoglobin saturation and POaO 2. Increased blood pH, low PaCO> and low temperature all cause the oxygen-haemoglobin to shift to the left causing a higher affinity with which haemoglobin binds with oxygen. Decreased blood pH, high blood PaCO2 and high temperature all cause the oxygenhaemoglobin to shift to the right causing a lower affinity with which haemoglobin binds with oxygen.
Initial event of injury to
I
I
+ Damage to alveolar
/. alveolocapillarymembrane |
*
epithelium i
Dysfunction of Type II i
i Activation of camp ement, coagulatranand klnm systems
; i
pneumocytes
i
Damage to capillary endothelium
Platelet
I
I
activation
i I Chemotaxis of i ]Cytokine release (eg:
agg~aton Ilneutrophi,s II~F&,'-t~ ' I Neutrophit aggregation & release - oxygen
of~
radicals
- proteolytic enzymes - arachydonic acid metabolites
I Haemorrhage
- PAF E
I Increased capillary I membranepermeabI ty I
Surfactant deficit & dysfunction
Reduced lung comp ance & [ atelectasis
~ ~1
JL r I
Alveolar cedema
/
I
i Non-selective il vascoconstriction &
I
Movement of inflamatory ceils into interstitium
Hyaline membrane Iformaton & fibrosis
/I
I Right to left shunt'ng & V/Q m smatch ]
L
I Fulminant respiratory failure
i
Fig. 5 A simplified model: of the pathogenesis of ARDS (PAF: Platelet activating factor, TNF: Tumour necrosis factor, IL-I: interleukin I). Reproduced by kind permission of Cutler, Intensive and Critical Care Nursing 1996:12:316-326.
weaning from a ventilator be initiated as soon as possible in order to prevent the risk ofnosocomial pneumonia (Clement & Buck 1996). When mechanically ventilated patients are Fully supported, between 72 and 96 hours respiratory muscle atrophy begins (MacIntyre 1988). Weaning from pressure support can be performed by gradually reducing the support the patient is receiving as his condition allows. There has been much research into when and how to wean a patient successfully from a ventilator but there is little conclusive evidence and weaning practices differ from hospital to hospital. A study by Burns & Burns (1994) attempted to compare five different weaning indices, designed to predict weaning potential of 37 patients. The researchers compared the Burns Weaning Assessment Program (BWAP), weaning index; frequency tidal volume ratio; compliance, resistance, oxygenation and pressure index; and negative inspiratory pressure. The BWAP was developed from a general and respiratory checklist, which provided a score depending on the percentage of positive answers. The researchers suggested using the BWAP weaning index but the author would be wary of this advice because the researchers developed that index and the study Failed to demonstrate the superiority of'any of'the weaning indices. No weaning index was employed in the I T U in which Peter was nursed. The initiation of weaning is normally under the control of the anaesthetist although the nurse has a role in indicating when the patient is ready and in the management of'weaning. PEEP encourages alveolar recruitment (aeration of additional alveoli) which would be advantageous to Peter because of'his atelectasis. PEEP increases the area available For gaseous exchange, reduces shunt fraction and increases the PaO 2 (Mackenzie 1992). PEEP has the advantage of increasing the Functional residual capacity (FRC) and, at normal levels, reducing the work of breathing (Beale 1994). PEEP is helpful in patients with low FRC and high intrapulmonary shunt fraction. PEEP has effects on the cardiovascular system and can cause a decrease in cardiac output by increasing the intrathoracic pressure and thus reducing venous return. Hyperinflation and barotrauma can also be caused by the use of PEEP. Peter had only a small amount of PEEP but a larger amount might have resulted in adverse effects. Increasing PEEP beyond a critical level could cause over-distension of the lungs leading to a reduction in tidal exchange (Naik et al 1996). A study by Naik et al (1996) demonstrated that compliance and PaO 2 in patients with lung volume loss due to lobar collapse and/or consolidation, as was evident in Peter, could be increased with PEEP. In this quantitative study
Multiple stabwounds 347 arterial blood gases were measured in between increasing PEEP levels from a base line of 2-3 cm H 2 0 to 5 - 6 c m H 2 0 with children with no abnormal lung function compared to those with different lung problems, diagnosed by chest X-ray. Compliance was measured by dividing tidal volume by the difference between the peak inspiratory pressure and PEEP. It could be argued that findings from the study could not be applied to adults because of age-related differences in lung mechanics. Also twenty of the sample were receiving neuromuscular blocking agents and this may affect the validity of the results.
DISCUSSION AND I M P L E M E N T A T I O N OF RESPIRATORY CARE Humidification was used to decrease consolidation in Peter's lungs by loosening secretions to make them easy to remove by ETS. In normal respiration people humidify inspired air during its passage to the lungs, with moisture from the mucus secreted by the goblet cells o f the mucous membranes which line airways (Ballard et al 1991). The upper respiratory tract is bypassed when a patient has an endotracheal tube in situ. Infection associated with use of humidifiers has frequently been reported. Pseudomonas is the most c o m m o n organism found (Oh 1996). Research by Kirton et al (1997) on 200 patients found that the rate of ventilator-associated pneumonia (VAP) with heat moisture exchanging filters (HMEF) was half in comparison to heated water humidifiers. Kirton et al (1997) acknowledged that there are still some questions regarding the possible role oftranslocation o f systemic bacteria, seeding the lungs and causing VAP, which could affect interpretation of the research findings. The researchers failed to identify the type o f heated water humidifier and simply described it as a heated wire humidifier. Whether the H M E F would humidify adequately in comparison to a heated water humidifier needs to be studied, although the increased likelihood of infection could be detrimental to critically ill patients and this is a very valid point to take into consideration when choosing a humidification method. Peter was regularly repositioned to aid in the drainage of chest secretions, Pearson & Parr (1993) advocate this. In the afternoon Peter was placed on his left side and very quickly his oxygen saturation (SaO2) fell to 87%. He was turned to a supine position and his saturation increased to within normal limits. The reason for this could be twofold. Peter's left pneu-
mothorax may not have resolved, causing a V / Q mismatch or more specifically a shunt effect as explained earlier. Another reason may have been that the consolidation was worse in his left lung than the right, also causing a shunt. ETS is an aseptic technique used to clear secretions and ensure the airway is clear. Prior to ETS the author would explain the procedure to Peter because it can be a frightening experience. A size 12 catheter was used, as this catheter was half the diameter of his endotracheal tube so air and oxygen could still flow down towards the lungs while the catheter was being introduced (Hough 1996). Without this inflow the negative pressure caused by suctioning can cause air to be sucked out of the alveoli causing them to collapse (Fiorientini 1992). ETS can cause hypoxia and may also cause vagal stimulation leading to bradycardia. Aspiration of nasogastric feed can be a problem especially with continuous feeding, as Peter was receiving, rather than intermittent bolus feeds (Fiorientini 1992). The author did not use sodium chloride 0.9% (NaC1) before ETS, but the physiotherapist had used this solution. NaC1 instillation (NSI) is still quite c o m m o n practice and theoretically it is presumed that NSI increases secretion removal when secretion is viscid due to inadequate humidity in the airway, and also encourages a cough. R.esearch into NSI by Bostick & Wendelgass (1985) found no statistical difference in the weight of samples with no normal saline compared to 5-10 ml NSI. It was not possible to measure the percentage o f NaC1 comprised in the aspirate which was recovered. The difference among the post-treatment aspirate weights therefore may not necessarily have reflected the effects of treatment in terms of secretion removal. A more recent quasi-experimental study by Ackerman (1993) showed patients' 8aO 2 worsened over a period of 2, 3, 4, 5 minutes with NSI. But the researcher did not comment on the patients' position, and full blood gas analysis might have provided a more sensitive measurement than SaO 2. Hagler et al (1994) found that instillation of NaC1 solution actually increases bacteria in the lower airway. However, this quantitative research was based on only 10 patients and therefore the author would question the general significance of the research. Further study of NSI may find it beneficial to a certain group o f patients, but the research currently available is definite about it being a procedure with negative effects. Peter's bilateral chest drains had stopped bubbling and swinging for 18 hours and were clamped prior to removal to ensure that the pleural leaks had healed. Bubbling indicates that there is a hole in the visceral pleura, allowing air to escape from the lung. The swinging
348
Intensive and Critical Care Nursing
which is seen is caused by variation in intrapleural pressure during respiration, and is no longer evident when the lung is fully expanded and the tube occluded. Chest tube clamping is potentially dangerous if pleural rupture causing the pneumothorax has not stopped draining air and may convert a pneumothorax into a tension pneumothorax (Welch 1993). Refer to Table 2 for signs o f a pneumothorax. It had been planned after the morning assessment to reduce Peter's PSV to 16cm H 2 0 . Before Peter's drains were clamped he became very tachypnoeic and tidal volumes fell. O n auscultation there was no indication o f secretion retention and the water traps of his humidification equipment were empty. The reason for the changes may have been that Peter had become exhausted and his work of breathing increased with the reduction of ventilary support in the past 24 hours. O h (1996) comments that a hole in the diaphragm may result in difficulty in weaning especially if bilateral nerve palsy has been caused. The doctors were consulted and it was agreed to increase the pressure support to 20 cm H 2 0 . After a short period of time Peter's breathing was more comfortable. In the afternoon the author sought to reduce the extremely high dose of midazalom and morphine that Peter was receiving because these drugs can cause respiratory centre dysfunction. A large amount o f sedation may have been required because Peter had a history o f cocaine abuse. Flumazenil may have helped to reduce the effects of morphine and midazalom. A study by Pepperman (1990), cited by Carroll (1993), o f patients receiving flumazenil in a continuous infusion to reverse the effects of morphine and midazalom concluded that these patients were weaned earlier than those receiving a placebo. Flumazenil has a short half-life and poses a risk of re-sedation because the duration of midazalom and morphine may exceed the effect of flumazenil. Pepperman (1990) did not note any occurrence ofresedation in his study. In the evening Peter was wakening and was able to obey commands. A background lower
Signs of a pneumothorax • • • • • • • • • •
,!, breath sounds on the affected side $ tidal volumes "1"airway pressures Asymmetrical chest movement Tachyponoea Tracheal deviation Cyanosis Chest pain on affected side Tachycardia or bradycardia $ cardiac output (tension pneumothorax)
dose of midazalom and morphine was continued to maintain patient comfort. Initially Peter's Ramsey sedation score was 6 and in the evening he was at the awake level of 2. In the evening it was possible to reduce Peter's pressure support to 18cm H 2 0 without any adverse effects. This may have been because he had had time to recover from earlier exhaustion and the reduction of sedation improved his respiratory centre function. Verbal and non-verbal communication with Peter was vital. Sedation would have prevented him communicating much with the author but he still needed reassurance, to be informed o f procedures and to receive reality orientation. Throughout the day as Peter awoke, the author invited his wife to reassure him and this was helpful because she was a familiar voice in an unfamiliar environment. Phenomenological research, using Colazzi's method, by Hafsteindottir (1996) on patients' experiences of communication during the respiratory assistance period revealed the frustration and panic of not being able to communicate verbally. The study was well presented and clear but the research was undertaken in Iceland and cultural differences may mean the results might have been different in the UK, although a study by Johnson and Sexton (1990) supports Hafsteindottir's findings. An experimental study by Hennman (1989) on 26 patients being weaned from a T-piece studied the differences of heart rate, mean arterial pressure (MAP), and respiratory rate between two groups to measure patients' stress response when being weaned. One group received touch and verbal c o m m u nication whereas the control group did not. Surprisingly, there was no difference between the groups. The author feels that because the participants were prepared for this study and qualitative research into the patients' emotional feelings were not incorporated during this research the results should not be used to direct practice. Admission to I T U is not only traumatic for patients but also for their significant others. So in an attempt to alleviate anxiety for Mrs Smith she was enabled to participate in Peter's care. A qualitative study by Jamerson et al (1996) used data from unstructured interviews in an attempt to describe the experience of family members with relatives in ITU. A group interpretative process along with triangulation was used to analyse the results into four categories: Hovering, Information seeking, Tracking and Garnering resources. The study sample comprised 18 w o m e n and 2 men and this could suggest that the research findings may only be relevant with female members of the family. Mrs Smith appeared to be in the 'information seeking' stage, described by Jamerson as 'an
Multiple stab wounds
active process of gathering information about the patient'. Peter's progress was explained to Mrs Smith and arrangements made for her to talk with the doctor.
EVALUATION Changes in a patient's condition can sometimes happen very rapidly in ITU despite planning care, implementing it and making what may seem realistic goals. Peter's condition at the end of the shift was slightly improved and it seemed that the progress in his condition would help in allowing him to be weaned quicker from the ventilator in the following days. An evaluation at the end of the shift revealed that air entry to the bases of both lungs had improved on auscultation and a chest X-ray in the morning would be able to confirm this. Oxygen saturations were now above 98% despite an earlier drop in saturations when Peter was placed on his left side. White secretions were removed f r o m P e t e r ' s chest a n d t h e increase i n t h e i r v o l u m e m a y h a v e b e e n d u e to successful physiotherapy and humidification. Peter had d e v e l o p e d n o signs o f a f u r t h e r p n e u m o t h o r a x or A R D S w i t h t h e p l a n n e d a c t i o n taken, as confirmed by X-ray the following morning. P S V was n o t r e d u c e d to the goal o f 1 6 c m H 2 0 , as it m i g h t h a v e b e e n w i t h earlier p l a n n i n g o f t h e r e d u c t i o n o f sedation. Peter's P S V h a d to b e increased because h e was distressed, b u t was r e d u c e d b a c k to 18 c m H 2 0 w i t h o u t ill-effect w h e n t h e s e d a t i o n was decreased a n d P e t e r h a d a P,.amsey score o f 2. P e t e r was e x t u b a t e d a few days after the a u t h o r h a d l o o k e d after h i m . A l t h o u g h h e was w e a k h e was r e c o v e r i n g v e r y w e l l , c o n s i d e r i n g
his injuries and the large blood transfusion received for hypovolaemic shock.
CONCLUSION T h e nurses' role is c o n t i n u a l l y e x p a n d i n g w i t h i n t h e I T U e n v i r o n m e n t a n d nurses can m a k e a difference to patients' o u t c o m e b y a d m i n i s t e r i n g holistic care based o n s o u n d a n d w h e n possible r e s e a r c h - b a s e d k n o w l e d g e . Respiratory management should be proactive a n d e v a l u a t i o n o f care reflective to c o m m i t o u r profession to excellence.
REFERENCES Ackerman E 1993 The effect of saline lavage prior to suctioning. American Journal of Criticai Care 3: 444-447
349
Ballard K, Cheeseman W, Rapner T 1992 Humidification for ventilated patients. Intensive and Critical Care Nursing 8 (1): 92-99 Beale 1K 1994 Weaning from mechanical ventilation. Brifshdournal of Intensive Care 5:168-175 BostickJ, Wendelgass S 1987 Normal saline instillation as part of the suctioning procedure: Effects on PaO2 and amount of secretions. Heart and Lung 16: 532-537 BrittonJ, Richards K, Wisniewski A 1994 Dietary magnesium, lung function, wheezing and airway hyperactivity in a random adult population sample. Lancet 344:352-357 Brochard L, Reuiss A, Benito S 1994 Comparison of three methods of gradual withdrawal from ventilatory support during weaning from mechanical ventilation. American Journal Respiratory Critical Care Medicine 150:896-903 Burns S M, BurnsJ E 1994 Comparison of five clinical weaning indices. AmericanJournal of Critical Care 3: 342-352 Carroll K 1993 The role of analgesics and sedatives in the management of pain and agitation during weaning from mechanical ventilation. Critical Care Nurse 15 (4): 68-77 Clement j, Buck E 1996 Weaning from mechanical ventilatory support. Dimensions of Critical Care 15: 114-129 Cutler L 1996 Acute Respiratory Distress Syndrome: an overview. Intensive and Critical Care Nursing 12: 316-326 Darovic G 1995 Haemodynamic Monitoring and Non invasive Clinical Application. 2nd Edn. W. B. Saunders Endacott t~ 1996 Staffing intensive care units: a consideration of contemporary issues. Intensive and Critical Care Nursing 12:193-199 Fiorentini A 1992 Potential hazards oftracheaobronchial suctioning. Intensive and Critical Care Nursing 217-226 Goldhill D 1997 Calcium and magnesium. Care of the Critically Ill 13:112-115 Hafsteindotdr/3 1996 Patients experiences of communication during the respiratory treatment period. Intensive and Critical Care Nursing 12: 261-27I Hagler D, Traver G 1994. Endotracheal Saline and Suction Catheters: Sources of Lower Airway Contamination. AmerJ of Crit Care 3 (6): 444-447 Hennman E 1989 Effect of nursing contact on the stress response of patients being weaned from mechanical ventilation. Heart and Lung 18:483-489 Hough A 1996 Respiratory physiology - the essentials. 5th Edn. Wilkins & Wilkins, London Hudak C, Gallo B 1994 Critical Care Nursing. A Holistic Approach. 6th Edn. Lippincott Company, Washington Jamerson P, Scheibmeir M, Bott M 1996 The experiences of families with a relative in the intensive care unit. Heart and Lung 25 (6): 467-473 Johnson M, Sexton D 1990 Distress during mechanical ventilation patients' perception. Critical Care Nurse 10 (7): 48-51 Kirton O, De Haven B, MorganJ 1997 Rates of nosocomial pneumonia associated with HME/bacterial filter and heated wire humidifiers: a prospective randomised trial. InternationalJournal of Intensive Care, Spring, 6-12 Mackenzie S 1992 Clinical use of PEEP, indications, effects and practicalities. BritishJonrnal of Intensive Care 10:335-340 Maclntyre N 1988 Weaning from mechanical ventilatory support: volume assisting intermittent breaths versus
350
Intensive and Critical Care Nursing
pressure assisting every breath. Respiratory Care 33 (2): 121-125 Naik S, Greenough A, Griffen F et al 1996 The effect of changes in PEEP on gas exchange. British Journal of Intensive Care 3:82-88 Oh T E 1996 Intensive Care Manual. 4th Edn. Butterworth-Hennemann, Hong Kong Pearson S, Parr S 1993 Physiotherapy in the critically ill patient. Care of the Critically Ill 9 (3): 128-131 Peppemlan M L 1990 Double-blind study of the reversal of midazalom induced sedation in the intensive care unit with flumazeni: effect on weaning from ventilation, Anaethesia and Intensive Care 18:38-44 Pierce J, Wiggins S, Plaskon C 1993 Pressure support ventilation: reducing the work of breathing during weaning. Dimensions of Critical Care Nursing 12 (6): 282-290 Ramsey M A, Savage J M, Simpson B IK et a119974 Controlled sedation with Alphaxalone-Alphadolone. British Medical Journal 2:656-659 Thomas C 1997 Use of the prone position: The
ventilation/perfusion relationship in ARDS. Care of the Critically Ill 13(3): 96-100 Tortora G, Grabowski S 1996 Principles of Anatomy and Physiology. 8th Edn. HarperCollins, New York WelchJ 1993 Chest tubes and pleural drainage. Surgical Nurse 6 (5): 7-12 WestJ 1992 Pulmonary Pathophysiology - the essentials. 4th Edn. Williams and Wilkins, Baltimore, USA WestJ 1995 Respiratory Physiology - the essentials. 5th Edn. williams and Wilkins, Baltimore, USA Westcott C 1995 The sedation of patients in intensive care units: a nursing review. Intensive and Critical Care Nursing 11:26--31 Yeaw E 1992 How position affects oxygenation. Good Lung Down? American Journal of Nursing 3:27-29 Yeaw E 1996 The effect of bodies positioning upon maximal oxygenation of patients with unilateral lung pathology. Journal of Advanced Nursing 23:55-61 Zidulka A, Braidy T F, Rizzi N C et al 1982 Position may stop pneumothorax progression in dogs. Am. Rev. Resp. Dis. 126:51-53