Recognition and Management of Respiratory Failure

Symposium on The Chest

Recognition and Management of Respiratory Failure Christopher J. L. Newth, B.Sc., M.B., ChB., F.R.CP. (C)*

Respiratory failure may be defined as the inability of the pulmona7 ry system to meet the metabolic demands of the body. The clinical assessment of respiratory failure is difficult and often unreliable, except when the patient is in extremis. The diagnosis is most easily made by the physician who retains a high index of suspicion and who is aware of the clinical situations in which respiratory failure is likely to be a complication (Tables 1 to 3).41 To document the diagnosis of respiratory failure, quantitate the disease process, and undertake rational management, facilities for the measurement of arterial blood gas and acid-base status are mandatory. The chest roentgenogram, electrocardiogram, and various laboratory tests may provide helpful information in specific circumstances.

INCIDENCE In Canada, respiratory death, the ultimate outcome of progressive respiratory failure, is the second most common cause of death. 57 However, the most common cause of death is congenital malformation (particularly spina bifida and cardiac anomalies), and many of these babies die in respiratory failure. The third and fourth most common causes of death are, respectively, "unspecified" (presumably including symptom complexes such as the sudden infant death syndrome) and "infections." Again, many patients affected by these conditions will have died in respiratory failure. To add further perspective, during the first year of life (excluding the first 28 days of neonatal life) the age-specific death rate from respiratory disease alone is 12.5 per 1000. The incidence decreases during the childhood and adolescent years and is not reached again until the sixth decade when emphysema, chronic bronchitis, and lung cancer are the major respiratory diseases (Fig. 1).56 • Assistant Professor of Pediatrics, University of Toronto; Chest Physician, Intensive Care Unit, and Research Associate, The Hospital for Sick Children, Toronto, Ontario, Canada Pediatric Clinics of North America - Vol. 26, No.3, August 1979

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CHRISTOPHER

Table 1.

J. L.

NEWTH

Causes of Obstructive Respiratory Disease* SPECIFIC DISEASE CONDITIONS

SITE AND TYPE OF DISTURBANCE

Upper Airway Anomalies

Aspiration Infection

Newborn and Early Infancy

Choanal atresia, Pierre-Robin syndrome, flabby epiglottis, laryngeal web, tracheal stenosis, vocal cord paralysis, tracheomalacia, vascular ring, goiter Meconium, mucus, vomitus

Tumors

Hemangioma, cystic hygroma, teratoma

Reflex

Laryngospasm from local irritation, tetany

Lower Airway Anomalies

Aspiration

Infection

Tumors Reflex

Bronchostenosis, bronchomalacia, lobar emphysema, aberrant vessels Amniotic contents, tracheoesophageal fistula, pharyngeal incoordination Pneumonia, pertussis

Late Infancy and Childhood

Tracheal stenosis, vocal cord paralysis, vascular ring, laryngotracheomalacia

Foreign body, vomitus Laryngotracheitis, diphtheria, epiglottitis, peritonsillar or retropharyngeal abscess Papilloma, hemangioma, lymphangioma, teratoma, hypertrophy of tonsils and adenoids Laryngospasm from local irritation (aspiration, intubation, drowning) or tetany

Bronchostenosis, lobar emphysema, aberrant vessels Foreign body, vomitus, pharyngeal incoordination (Riley-Day syndrome), near-drowning Bronchiolitis, pneumonia, tuberculosis (endobronchial, hilar adenopathy), cystic fibrosis Bronchogenic cyst, teratoma, atrial myxoma Bronchospasm (secondary to inhalation of noxious gases or upper respiratory tract infections), asthma

*Adapted from Pagtakhan, R. D., and Chernick, V.: Intensive care of respiratory disorders. In Kendig, E. L., Jr., and Chernick V. (eds.): Disorders of the Respiratory Tract in Children.

Edition 3. Philadelphia, W. B. Saunders Co., 1977, with permission.

Table 2. SITE AND TYPE OF DISTURBANCE

Parenchymal Anomalies

Causes of Restrictive Respiratory Disease* SPECIFIC DISEASE CONDITIONS Newborn and Early Infancy Late Infancy and Childhood

Agenesis, hypoplasia, lobar emphyserna, congenital cystic lung. pulmonary sequestration

Hypoplasia, congenital cystic lung, pulmonary sequestration

Atelectasis Infection

Hyaline membrane disease

Thick secretions (postoperative)

Pneumonia

Pneumonia, cystic fibrosis, bronchiectasis, pleural effusion, pneumatocele

Alveolar rupture

Pneumothorax (spontaneous or iatrogenic) Pulmonary hemorrhage, pulmonary edema, Wilson-Mikity syndrome

Pneumothorax (trauma, asthma)

Diaphragmatic hernia, eventration, myasthenia gravis, fatigue

Amyotonia congenita, poliomyelitis, diaphragmatic hernia, eventration, myasthenia gravis, muscular dystrophy, botulism, repaired exomphalos

Skeletal malformations

Hemivertebrae, absent ribs, thoracic dystrophy

Kyphoscoliosis (> 40 degrees), hemivertebrae, absent ribs

Others

Abdominal distention

Obesity, flail chest, abdominal distention

Others

Chest Wall Muscular

Pulmonary edema, lobectomy, chemical pneumonitis

*Adapted from Pagtakhan, R. D., and Chernick, V.: Intensive care of respiratory disorders. In Kendig, E. L., Jr., and Chernick, V. (eds.): Disorders of the Respiratory Tract in Children.

Edition 3. Philadelphia, W. B. Saunders Co., 1977, with permission.

Table 3. Causes of Inefficient Gas Transfer* SITE AND TYPE OF DISTURBANCE

SPECIFIC DISEASE CONDITIONS

Defect in Pulmonary Diffusion Increased path of diffusion between alveoli and capillaries Decreased alveolocapillary surface area

Pulmonary edema, pulmonary fibrosis, collagen disorders, Pneumocystis carin ii, sarcoidosis Pulmonary embolism, sarcoidosis, pulmonary hypertension, mitral stenosis, fibrosing alveoli tis Inadequate erythrocytes and hemoglobin Anemia, hemorrhage

Depression of Respiratory Center Increased cerebrospinal fluid pressure

Cerebral trauma (birth injuries), intracranial tumors, central nervous system infection (meningitis, encephalitis, sepsis), intracranial hemorrhage

Excess of central nervous system depressant drugs

Maternal oversedation, overdosage with barbiturates or morphine

Excessive chemical changes in arterial blood

Severe asphyxia (hypercapnea, hypoxemia)

Toxicity Prematurity

Tetanus, gastroenteritis Neonatal apnea

*Adapted from Pagtakhan, R. D., and Chernick, V.: Intensive care of respiratory disorders. In Kendig, E. L., Jr., and Chernick, V. (eds.): Disorders of the Respiratory Tract in Children. Edition 3. Philadelphia, W. B. Saunders Co., 1977, with permission.

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CHRISTOPHER

J.

L. NEWTH

RATE 15

5

0

"" __

1 mo.' 1· 1 yr. 4

59

1014

FaJFaJrBlrBl~~11 1519

20· 24

2529

3().

34

AGE

3539

40-

""

4549

5054

55- 6059 64

Figure 1. Age-specific death rates for respiratory disease per population of 1000 in Canada in 1974.

FACTORS PREDISPOSING PEDIATRIC PATIENTS TO RESPIRATORY FAILURE About two-thirds of the cases of respiratory failure in children occur in the first year of life j of these, approximately 50 per cent occur in the newborn period. This remarkably high incidence in infancy can be attributed largely to structural immaturity (and, in the case of neonates, to congenitally abnormal development as well) of the various organs of the ventilatory pathway (Table 4). Some of the reasons why infants are less able to cope with a given stress to the ventilatory pathway are discussed below. Table 4. Ventilatory Pathway MAJOR STRUCTURES

CONTRIBUTING SYSTEMS SUSCEPTIBLE TO DisEASE

Ventilatory afferent information

Central and peripheral chemoreceptors, vagal afferents

1 1

Respiratory center

Cerebrum, medulla

Ventilatory motor nerves

Vagal efferents, sympathetic nervous system, phrenic and intercostal nerves

1 1 Thoracic' cage 1 Large airways 1 Lung structures

Neuromuscular junctions

Cholinergic neurotransmitters Diaphragm, intercostal muscles, ribs Larynx, trachea, and major bronchi Small airways, blood vessels and lymphatics

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621

The Thoracic Cage The sternum is soft and affords an unstable base for the ribs. Additionally, the highly compliant ribs are horizontally placed and the intercostal muscles are poorly developed, so the bucket-handle motion upon which thoracic respiration depends is eliminated. The diaphragm is the most important ventilatory muscle, and it produces the most forceful ventilation when it is longer and more curved. A disease or physiologic process which impairs diaphragmatic function will predispose the individual to respiratory failure. Such impairment may result from problems as diverse as: gastric or abdominal distention from fluid or air; abdominal surgery involving the diaphragm; hyperinflation of the lungs, producing a shorter, flatter diaphragm which is at a mechanical disadvantage and may seriously compromise the ability of the infant to achieve an adequate tidal volume; increased work of breathing which leads to exhaustion of the energy-producing enzymes in the relatively few type I fibers in the diaphragmatic muscle of the premature baby;31 and rapid eye movement (REM) sleep during which the ventilatory movements of the rib cage become uncoordinated and out of phase with the diaphragm. The proportion of respiration performed by the diaphragm increases dramatically in REM sleep, but unfortunately, much of the diaphragmatic effort is wasted in distorting the rib cage. Since the infant spends most of his time asleep, and the predominant sleep state is REM,22 there are considerable periods when ventilation is less than optimal in the normal situation. Stress will only impair performance further. Airways The airways of a child are large in comparison with those of an adult. The trachea of a newborn infant is one-third and the bronchioles are one-half the diameter of those of an adult 20 times his size!S However, because of the fourth power relationship of the radius of the airway to resistance, a 1 mm increase in the thickness of the mucosa at the subglottic level will cause a 75 per cent reduction in the cross-sectional area in the neonate, but only a 20 per cent reduction in an adult. 17 Small increases in the mucosal thickness of the bronchioles of infants will cause even greater increases in the resistance to air flow, thereby adversely affecting the work of breathing. This is further magnified by the fact that these peripheral airways make a major contribution to the total airway resistance, even in normal children under the age of five years,27 whereas their contribution in adults is very small. Thus, laryngotracheobronchitis (croup) and bronchiolitis are serious diseases in the young infant or child, but are of little importance in adults. Alveoli The elastic tissue in the septae of the alveoli surrounding the conducting airways provides the elastic recoil which enables the airways to remain open. Early in life, there are few, relatively large alveoli that

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CHRISTOPHER

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NEWTH

provide little support for the airways, which are thus able to collapse easily. Alveolar addition continues throughout childhood and puberty by septal division, thus providing more elastic recoil and greater support for the airways, resulting in a decreased tendency for collapse. 7 With age, alveolar size also increases, causing the peripheral conducting airways to enlarge in diameter, thereby reducing their relatively high contribution to airway resistance. In addition, immaturity may lead to a deficiency in the development of pulmonary surfactant, thereby causing an inability to inflate (or maintain inflated) alveoli, with further loss of elastic recoil, collapse of airways, atelectasis,and imbalance of air and blood matching in the lungs.

Collateral Ventilation In the adult lung, the collateral pathways of ventilation are well developed so that it is easy to ventilate the parenchyma beyond obstructed airways, thus maintaining gas exchange. In infants, the intra-alveolar pores of Kohn and broncho-alveolar canals of Lambert are small in number and size (or absent),5.6 but increase with age. Thus, ventilation of the lung beyond obstructed units is more difficult the younger the child, and gas exchange must suffer. Central and Peripheral Nervous Systems The ventilatory pathway (see Table 4) has the respiratory center of the brain as its principal data processor. Afferent information flows to it from peripheral and central chemoreceptors, diaphragmatic muscle spindles, airway irritant and stretch receptors, and so on. After rapid interpretation, suitably integrated commands are sent outward via the ventilatory motor nerves (vagus, phrenic, and intercostal) to the airways, pulmonary blood vessels, and muscles of the thoracic cage. Interference with one or a group of these ventilatory pathway components may cause respiratory failure. Examples of such interference include immaturity (a lack of neurotransmitters, presence of strong inhibitory ventilatory reflexes), an altered level of consciousness (sleep state, drugs, convulsions, meningitis), and impaired neural connections (surgically severed phrenic nerves, expanding syrinx). Anatomical Anomalies Anatomical anomalies may affect any part of the respiratory system or its associated organs. Cardiac lesions such as ventricular septal defect and patent ductus arteriosus which result in pulmonary edema are especially frequent. Pulmonary hypoplasia may occur with a diaphragmatic hernia. Esophageal atresia and tracheoesophageal fistulae are associated with aspiration pneumonia and tracheomalacia both before and after repair. Upper airway obstruction occurs in association with vascular rings, cysts, or as a result of choanal atresia in the young infant (an obligatory nose-breather until four months of age). Socioeconomic Factors Socioeconomic conditions greatly influence the incidence or effect of diseases leading to respiratory failure.

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BREAST FEEDING. Breast milk supplies immunoglobulins, macrophages, lymphocytes, antiviral and antibacterial agents, and clearly protects the infant from lower respiratory tract and enteric infections in homes with poor hygiene. 23 This protection is also seen in modern industrialized circumstances with good sanitation. II Thus the social acceptance of breast feeding in a community may influence the incidence of respiratory failure there. PREMATURITY. Prematurity is associated with an increased incidence and severity of lung disorders and other diseases, particularly those caused by bacterial infections. This may be a result of the lack of protection from breast feeding or hospital exposure to virulent organisms among other things. The quality of prenatal care therefore will influence the incidence of respiratory failure. AIR POLLUTION. Parental smoking is associated with an increased risk of respiratory disease in children. 32 The London smog ~n December of 1952 further demonstrated that atmospheric conditions can acutely affect the respiratory tract, as evidenced by the excessive mortality seen in children under the age of one year. 33 The mechanism by which air pollution can influence the incidence of respiratory illness in infants is not known, but impairment of lung defense mechanisms by depression of mucociliary transport is a possibility. 38 Again, an increased incidence of respiratory failure results from excessive air pollution.

CLINICAL CONSIDERATIONS Unfortunately, respiratory failure fits no well defined clinical description. It may have an abrupt onset or it may occur insidiously with gradual and progressive deterioration of pulmonary function. Insufficient alveolar ventilation from any cause invariably results in hypoxemia and hypercapnea, which may contribute to further depression of ventilation, culminating in frank respiratory failure and death. Comroe has shown that the clinical assessment of cyanosis, and hence arterial P0 2 , is unreliable. 9 Similarly, Mithoefer has shown that the clinical assessment of alveolar ventilation, and hence arterial PC0 2 , is also unreliable. 36 In addition, severe respiratory distress is not always associated with retention of carbon dioxide and hypoxemia may be present in the absence of clinically detectable cyanosis, especially if the infant is anemic for any reason. Thus, precise assessment of ventilatory adequacy must be based on both clinical and laboratory studies, but much emphasis has to be placed on arterial blood gases for help with both the diagnosis and management. The clinical signs and laboratory findings associated with impending or frank respiratory failure are listed in Table 5. Any patient who has unexplained signs such as confusion, cyanosis, and an obtunded or comatose state, or symptoms such as dyspnea, anxiety, or restlessness may have respiratory failure and arterial blood gases should be measured. If there is a good reason for such signs or symptoms other than respiratory failure, an arterial blood gas may not be necessary.

624

CHRISTOPHER

Table 5.

J. L.

NEWTH

Criteria for Respiratory Failure

CLINICAL SIGNS

General Fatigue, sweating Respiratory WheeZing Expiratory grunting Decreased or absent breath sounds Flaring of alae nasi Retractions of chest wall Tachypnea, bradypnea or apnea Cyanosis

LABORATORY SIGNS

Hypoxemia (acute or chronic) Hypercapnia (acute or chronic) Acidosis (respiratory and/or metabolic) Vital capacity < 30 per cent predicted Vital capacity < 3 x tidal volume Maximum inspiratory pressure> -20 cm H 2 0

Cardiac Bradycardia or excessive tachycardia Hypotension or hypertension Pulsus paradoxus > 12 mm Hg Cardiac arrest Cerebral Restlessness Irritability Headache Mental confusion Convulsions Coma

Measurements of arterial blood gases (Pa02 and PaC0 2) and pH, although a necessary part of the assessment and long-term management, are unimportant when compared with skillful and rapid clinical judgment in an emergency. However, once the life-threatening situation is dealt with satisfactorily, the patient with respiratory failure must be transferred to an area in which facilities for arterial blood gas studies, radiographic studies, and other ancillary services are available on a 24 hour basis, along with skilled nursing personnel. Those responsible for transport should aim to transfer the patient in the shortest possible time with the least deterioration in condition. This goal necessitates adequate clothing to prevent cooling, facilities for removal of pharyngeal secretions, adequate oxygen supplies, intravenous infusion of drugs and fluids, and, sometimes, protection of the airway and assisted ventilation.

TYPES OF RESPIRATORY FAILURE Since respiratory failure is the inability of the pulmonary system to meet the metabolic demands of the body, the arterial blood will show an abnormally low P0 2 and/or an abnormally high PC02. The actual values used to represent "high" and "low" are somewhat arbitrary, but in most clinical situations, in patients of any age, breathing room air at sea level, an arterial P02 less than 50 torr (in the absence of an intra-

RECOGNITION AND MANAGEMENT OF RESPIRATORY FAILURE

625

cardiac shunt) and an arterial PC0 2 greater than 50 torr, or both, indicate respiratory failure. Two types of respiratory failure are possible: type I is manifested by a low Pa0 2 and a normal or low PaC0 2 ; type II is indicated by a low Pa0 2 and a high PaC0 2 • All patients with respiratory failure are, to some extent, hypoxemic, but not all patients with respiratory failure exhibit retention of carbon dioxide. Both types I and II may occur in the course of a single illness, and are often caused by the same physiologic process - a mismatch of alveolar gas and pulmonary blood or, stated in other terms, an imbalance in ventilation to perfusion.

PHYSIOLOGIC MANIFESTATIONS OF RESPIRATORY FAILURE The physiologic symbols used to discuss the characteristics of respiratory failure are detailed in Table 6. Oxygenation A decreased arterial P0 2 is present in all patients with respiratory failure. However, some patients may have profound respiratory failure with a "normal" P aOz when inspiring high concentrations of oxygen. Hence, all arterial blood gases must be interpreted knowing the concentration of inspired oxygen. A low P a 0 2 constitutes a respiratory problem in all but two cases: when the oxygen concentration inspired is low, as it is at high altitudes; or when an intracardiac shunt is present as in tetralogy of Fallot Table 6. PA O 2 P I0 2 F I0 2

Physiologic Symbols

partial pressure of alveolar oxygen partial pressure of inspired oxygen = fraction of inspired oxygen (0.21 = room air) PH = barometric pressure (approximately 760 torr) PH,o = water vapor pressure at body temperature (47 torr) P A CO2 = partial pressure of alveolar carbon dioxide P,02 = partial pressure of arterial oxygen (normal: infants and older children, 85 to 95 torr; neonates, > 60 torr) P a C0 2 = partial pressure of arterial carbon dioxide (normal: all ages, 37 ± 3 torr) P A-a02 = alveolar-arterial oxygen difference (normal: infants and older children, 5 to 15 torr; neonates, < 60 torr) Veo , = production of carbon dioxide Vo, = consumption of oxygen R = respiratory quotient (the ratio of Vco, : Vo,) Vo = total dead space (normal: all ages, 2 ml per kg of body weight) Vo = total dead space ventilated per minute VT = tidal volume (normal: for all ages, 6 ml per kg of body weight) VO/VT = the ratio of dead space to tidal volume ("wasted ventilation") HCO,,- = bicarbonate ion concentration VE = total volume of air breathed per minute VA/Q = ratio of alveolar ventilation to pulmonary perfusion VA = volume of alveolar ventilation per minute = =

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CHRISTOPHER

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or transposition of the great arteries. With these two exceptions, a low Pa0 2 must be the result of either hypoventilation (type II respiratory failure) or the result of a ventilation-perfusion mismatch, impairment of diffusion, or intrapulmonary shunting. These latter are virtually impossible physiologically, and unnecessary clinically, to distinguish as causes of hypoxemia. The mean alveolar P0 2 is given by the alveolar air equation. The abbreviated form, which is suitable for clinical purposes states:

where

The clinical usefulness of this equation stems from the fact that, in most situations, R can be assumed to be 0.8 and alveolar PC02 is generally equal to arterial PC0 2 which is easily measured. The equation then can be readily solved for P AO 2. A useful nomographic representation is given in Figure 2. The difference between P A0 2 (calculated) and P a 0 2 (measured) is the alveolar-arterial oxygen difference which is normally 5 to 15 torr, but may be as high as 60 in neonates. This normal gradient is caused by slight physiologic shunting. The equation is important for two reasons. First, it demonstrates the fundamental principle that, for a given P 10 2, as the P aC02 increases the P a0 2 decreases. Second, in evaluating patients with respiratory failure, when hypoxemia is caused by V/Q imbalance, intracardiac or intrapulmonary shunting, or impairment of diffusion, the P A.a0 2 is increased. It is only when hypoxemia is caused solely by hypoventilation that the P A.a02 is normal. CASE 1. An 18 month old infant accidentally poisoned herself by eating her mother's barbiturate sleeping tablets. When seen in the emergency department, her breathing was noted to be shallow, and there was vomitus at the mouth. The initial blood gases taken while the patient was breathing 40 per cent oxygen showed a P a C0 2 of 56 torr, a P a 0 2 of 88 torr, and a pH of 7.29. A chest x-ray film showed diffuse infiltrates throughout the lungs, particularly in the upper lobes. This patient had type II respiratory failure with a "normal" P a0 2 • The high P a C0 2 was attributed to central nervous system drug depression, which led to decreased ventilation. However, the P A.a02 was also increased, indicating type I failure as well; assuming a barometric pressure of 760 and a respiratory quotient of 0.8, it was approximately 127 torr. Thus, there were two physiologic reasons for the respiratory failure. The first was hypoventilation which led to an increased PaCOz and a decreased P a0 2 , the latter being masked by the use of 40 per cent oxygen. In other words, if her PaCO, was a normal 37, with a PA.aO, of 127, her PaO, would have been 112 rather than 88. The second reason for the respiratory failure was also the cause of the increased P A.a O2 ; that is, aspiration of vomitus led to atelectasis and pneumonia with subsequent imbalance of ventilation and perfusion in the lungs.

The most common physiologic cause of respiratory failure is ventilation-perfusion mismatch in the lung. With a thorough clinical

RECOGNITION AND MANAGEMENT OF RESPIRATORY FAILURE

627

Alveolar Air Equation Nomogram Expected PA02

Pa02

mmHg mmHg F102

PaC02

400

370

(Yo)

mmHg

070

120

060

100

350

330 290

300 250

050

80

60 40

3 2

040

028 024 ---21

20

o

200

210 170

j::: --- ~100 1::-'1 ::

035

. . . . . . . . 1..

250

150

o

~1O

Figure 2. This represents the graphic solution of the alveolar air equation, assuming sea level and R = O.S. To determine P AO" locate PaCO, to the left, and FlO, on the middle left. Join the points with a ruler and read P AO, at the middle right. At the far right are approximate values of PaO" assuming normal values for the P A.aO, gradient. If the observed PaO, is lower than the PaO, on the far right, the P A-aO, gradient is increased. The dashed line represents an example:

PaC0 2

40

Expected PaO, 0.21

102

92

© by the American Thoracic Society (reproduced with permission).

history, a lack of cardiac murmurs on physical exrunination, and a chest x-ray film that shows no cardiac enlargement, it is usually not difficult to exclude right-to-Ieft cardiac shunting as a cause of hypoxemia. However, the etiology may remain doubtful, particularly in the very young infant. A hyperoxic test in which 100 per cent oxygen administered for 15 to 20 minutes with a repeat arterial blood gas at the end is useful in this situation because ventilation-perfusion imbalance will be at least partially corrected and P a 0 2 will increase; absolute (intracardiac) shunting will not improve and P a 0 2 will remain much the srune.

Ventilation The arterial PC0 2 is the measurement of alveolar ventilation and is synonymous with hypo ventilation when raised, and hyperventilation

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CHRISTOPHER

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NEWTH

when lowered. This relationship is demonstrated by the following equation: P A CO.,

=

-

0.863

X

VC02

VA

where 0.863 is a conversion factor. If VCO is assumed to be constant and P A C02 is the same as P a C0 2 (which is true in most clinical circumstances), P a C02 is related only to the inverse of alveolar ventilation. Therefore, elevation of the P a C0 2 indicates that there is insufficient alveolar ventilation for the carbon dioxide produced, but it does not tell why VA is reduced. There are two mechanisms for hypoventilation given in the relationship:

The dead space is any part of the pulmonary system which does not take part in gas exchange and includes "anatomic" (trachea and bronchi) and "physiologic" (alveoli which are ventilated but inadequately perfused) portions. When lungs become diseased and additional physiologic dead space is created by altered ventilation-perfusion relationships, VA will decrease and P a C0 2 will increase. This is the more common cause of hypoventilation and it occurs when VA cannot be increased to compensate for the increased V D, as in severe bronchiolitis, asthma, or cystic fibrosis. 58 The V D/V T ratio, or wasted ventilation, can be measured on the basis of the Bohr equation:

The implications of the VJV T ratio are important, though the measurements are not commonly done in young children and infants because of the difficulty of collection. In order to collect and analyze the expired alveolar carbon dioxide, children are required to breathe on a respiratory valve, which requires cooperation, or they must be intubated with no air leak which requires the use of a cuffed endotracheal tube, a type rarely used in the pediatric age group. The normal VD/V T ratio is approximately 0.3; a value greater than 0.6 usually signifies that the ratio of dead space to tidal volume is too high for the patient to sustain his own respiratory effort. Two options are available when managing a patient who has an increase in the V D/V T ratio. The disease process underlying the respiratory failure largely determines the option chosen. In a patient with a chronic obstructive pulmonary disease such as cystic fibrosis, the most reasonable approach is to decrease physiologic dead space, and hence VD/V T , by the same techniques used to increase oxygenation. That is, secretions are thinned and removed by a combination of antibiotics,

RECOGNITION AND MANAGEMENT OF RESPIRATORY FAILURE

629

humidification, and postural drainage. Decreasing the physiologic dead space improves V/Q relationships which affect retention of carbon dioxide and hypoxemia. Alternatively, in a patient with right heart failure, the end result of numerous pulmonary disorders, improving the pulmonary blood flow with vasodilators such as oxygen and intravenous isoproterenol or dopamine will increase perfusion, reduce the physiologic dead space, and redress V/Q imbalance. The second therapeutic option is instituted in patients with restrictive lung disease or central nervous system depression. In these conditions, VD/V T is increased because of decreased tidal volume (with a normal dead space). Although even this small dead space can be further reduced by endotracheal intubation or tracheotomy, it is better to manipulate tidal volume and frequency, if possible, whether it be by mechanical ventilation or arrest of the underlying disease and its complications (such as progressive scoliosis and pneumonia). Increasing the tidal volume (deeper breaths) is more efficient than increasing the frequency of breathing. The less common mechanism of hypoventilation is simply a decreased total ventilation (VE ) which occurs with central nervous system depression, and with weakness of ventilatory muscles, whether it is secondary to myasthenia gravis, muscular dystrophy, or accidental phrenic nerve trauma during intrathoracic surgery. In such cases, the VD does not change and hence VA is decreased. Theoretically, there is a third mechanism possible for hypoventilation in situations in which the metabolic rate and work of breathing are both markedly increased, such as in a febrile infant with bronchiolitis. In these circumstances, more carbon dioxide is produced than can be eliminated by the already maximal ventilation. Such patients also have increased dead space secondary to ventilation-perfusion imbalance. A subsequent lack of energy reserve (fatigue) along with admihistration of sodium bicarbonate for metabolic acidosis (more carbon dioxide) would rapidly contribute to further respiratory failure.

Acid-Base Balance Assessment of acid-base balance requires at least two of three variables, as described by the Henderson-Hasselbalch equation: [HCO a-] pH = pK + log 0.03 x Pa C0 2

which can be thought of very simply as: kidney lungs The pH and P aC02 are easily measured in arterial blood; from them the HC0 3 is subsequently derived. The numbers can then be plotted on an acid-base graph (a nomographic solution of the Henderson-Hasselbalch equation), upon which are outlined 95 per cent confidence limits for the primary, in vivo, acid-base disorder. The nomogram2 allows a visual

630

CHRISTOPHER J. L. NEWTH

Figure 3. Acid-base nomogram. (From Arbus, G. S.: An in vivo acid-base nomogram for clinical use. Canad. Med. Assoc. J., 109:291-293, 1973, with permission.)

interpretation of the acid-base status, and is particularly useful in determining whether retention of carbon dioxide is causing acute (less than 6 to 12 hours) or chronic respiratory acidosis (24 to 48 hours' duration, during which time the kidneys have had a chance to compensate by retaining bicarbonate), and whether there is a complicating metabolic problem (Fig. 3). The nomogram has three uses: to confirm the presence of a primary acid-base disorder; to exclude a primary disorder as the sole cause of the acid-base disturbance; and to guide management of a disorder. The nomogram does not diagnose disease; it confirms and quantitates the sequelae of disease. An isolated arterial blood gas determination should be viewed on the nomogram as one point that can be arrived at by various pathways and hence is not diagnostic of any particular acid-base disorder. The diagnosis of the disease requires full clinical evaluation and laboratory information, including serial blood gas determinations. CASE 2. A 10 year old boy was admitted for an acute asthmatic attack which had worsened over the previous 12 hours at home despite therapy. On admission, arterial blood gases in room air were: a pH of 7.48, a PC0 2 of 28 torr, a P0 2 of 50 torr, and an HC0 3 - of 21 mEq per liter. Peak expiratory flow was 110 liters per minute. The patient was given intravenous aminophylline, steroids, and bronchodilator therapy but an hour later seemed much more tired

RECOGNITION AND MANAGEMENT OF RESPIRATORY FAILURE

631

and the peak flow had decreased to 80 liters per minute. A repeat arterial blood gas study while the patient was on 40 per cent humidified oxygen showed a pH of 7.15, a PC0 2 of 60 torr, a P0 2 of 60 torr, and an HC0 3- of 21 mEq per liter. At this point, the patient was intubated and mechanically ventilated, and intravenous isoproterenol was infused. The patient initially demonstrated acute respiratory alkalosis, presumably secondary to several hours of hyperventilation. However, he then hypoventilated acutely, which placed his blood gases between the bands of acute respiratory acidosis and metabolic acidosis. Clearly, the clinical picture showed that the problems were respiratory in nature and therapy was first directed at restoring alveolar ventilation rather than at correcting the metabolic process. Later; it may have been appropriate to give sodium bicarbonate to raise the pH above 7.20, if the respiratory maneuvers were not sufficient, in order to allow the bronchodilators a suitable medium in which to work. One risk would be that if the bronchospasm was not relieved and alveolar ventilation increased, the net effect of the sodium bicarbonate in this "closed" ventilatory system would have been to raise the P a C0 2 further.

MANAGEMENT OF THE PATIENT WITH RESPIRATORY FAILURE A systematic approach to the patient with respiratory failure is outlined in Table 7. In the majority of cases, respiratory failure proTable 7. Systematic Approach to the Patient with Respiratory Failure Suspect

I

Confirm

Diagnosis +-<- - - - - - - - - - + . Cardiopulmonary resuscitation if needed Diagnosis +-<- - - - - - - - - - + . Arterial blood gases ~

1

Type I or II respiratory failure

Complete Evaluation

<

•

Arterial blood gases (serial) Clinical assessment Laboratory data (complete blood count, electrolytes, blood sugar, BUN, serum calcium) Electrocardiogram Examination of sputum Bedside pulmonary function tests (peak flow, vital capacity) Other procedures Gung or lumbar puncture, drug screen)

Clinical Diagnosis and Status

Acid-base Oxygenation Ventilation

Management Oxygen Humidification ± aerosol therapy Pharyngeal suction Physiotherapy Fluids Treatment of specific conditions General nursing care, observation, and monitoring Ventilation

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gresses over hours or even days, which allows ample time for diagnosis and institution of specific therapy if available. However, a proportion of patients will present with respiratory or cardiac arrest or severe asphyxia with cardiorespiratory arrest imminent. This may occur because respiratory failure was not suspected as a possible complication of the disease, the patient was a late referral, or the illness progressed more rapidly than expected. Management should be directed immediately toward restoration of adequate alveolar ventilation and oxygenation, and treatment of the causes and complications. 48 Adequate oxygen and suction facilities must be available as well as a laryngoscope and a variety of blades for infants or older children. In order to manage emergencies, a selection of endotracheal tubes of various sizes and lengths should be available. A bag, mask, and endotracheal tube connector assembly is necessary, preferably one with a device that limits the inspiratory pressure to 35 to 40 cm of water, thus avoiding iatrogenic pneumothorax by the unskilled operator. Drugs required for resuscitation may include adrenalin, sodium bicarbonate, dextrose, and calcium gluconate. Equipment for defibrillation may be needed. Once the patient has been stabilized, the systematic approach to diagnosis and management can be applied as outlined. A suspected diagnosis of respiratory failure must be confirmed by analysis of arterial blood gases for the reasons discussed previously. A full clinical evaluation is essential, with particular attention being given to the mental and neurologic status of the patient, the findings on chest x-ray films, gram stain and culture of sputum, and simple pulmonary function tests when appropriate (such as peak expiratory flow before and after inhaled bronchodilators in asthmatics, and vital capacity and tidal volume in patients with muscular dystrophy). By this method, a clinical diagnosis as well as the status of acid-base balance, oxygenation, and ventilation may be readily ascertained and acted upon. The details of management vary with the nature and severity of the illness, but the same factors must be considered in the management of every patient with respiratory failure. Large amounts of data will be generated, making a flow sheet essential from the beginning. This flow sheet should contain all pertinent physiologic variables such as respiratory and heart rates, temperature, systemic and pulmonary artery pressures, concentrations of inspired oxygen, level of consciousness, arterial blood gases, peak inflating pressures, biochemical values, fluid balance, and so on.

GENERAL MANAGEMENT

Oxygenation Severe hypoxemia will kill humans of any age. However, while this and other effects of less severe hypoxia are common to all ages, other sequelae may develop in neonates and infants.3

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The newborn infant can withstand hypoxia better than the adult because of an increased capacity for anaerobic metabolism, especially glycolysis. This may, however, result in a large increase in the concentration of blood lactate with a subsequent profound metabolic acidosis. The increased tolerance has other disadvantages, though. Hypoxic cerebral lesions occur frequently, often causing convulsions or apneic episodes, especially in premature infants. Further, hypoxia may be partly responsible for massive cerebral hemorrhage,25 or it may be associated with pulmonary hemorrhage,44 hyperkalemia and hypoglycemia,54 and hypoventilation with bradycardia, causing a subsequent drop in cardiac output and oxygenation of tissues. 12, 43 Hypoxia and acidosis cause pulmonary vasoconstriction with further hypoxemia and, in the first few days of life, may result in the circulation reverting to the fetal pattern, with a right-to-Ieft shunt through the patent foramen ovale and patent ductus arteriosus. This in turn leads to further hypoxia and acidosis and may be very difficult to reverse. Acidosis also considerably reduces the oxygen saturation of the blood at low oxygen tension,55 while a pH of 7.25 or less may cause significant impairment of myocardial contractility. Oxygen is best administered by placing the entire patient or just his head in an enclosure through which the gas is administered. For the neonate, it is relatively easy to achieve a 40 per cent concentration by delivering oxygen directly into the incubator; this can be increased up to 70 per cent if all orifices are closed. However, such levels are difficult to maintain because the air is being circulated and considerable dilution with room air occurs, especially if a porthole is opened. Perspex (Lucite) head boxes are suitable for infants who are quiet or restrained. These devices allow high flows of nearly 100 per cent to be obtained, though intermittently, moisture "raining-out" on the walls may obscure the patient's head from easy observation. Oxygen tents 42 are useful for infants when concentrations of about 60 per cent are desired. Other measures such as intranasal catheters and masks are more suitable for older children who can understand their use and cooperate. The concentrations of oxygen required are variable, but generally patients with extrathoracic diseases rarely require a concentration of more than 30 per cent unless intubation or other assistance is required. Those individuals with intrathoracic diseases often require a greater concentration. 50 An arterial oxygen saturation of 95 per cent or greater is usually acceptable; this is achieved by making adjustments according to measurements of arterial blood gas. An indwelling catheter from which samples can be drawn or a mass spectrometer sample continuously facilitates this, as does an indwelling electrode capable of determining arterial oxygen tension29 or saturation. 59 Transcutaneous oxygen electrodes may also be helpfuPo but are contraindicated in patients in shock in whom decreased skin perfusion is a complication. The electrode must be moved every four to eight hours to prevent local heat damage to the skin. Another technique for oxygen saturation is an ear oximeter such

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as the Hewlett-Packard 47201A, but this has limited pediatric application as the earpiece is cumbersome and designed only for adolescents or adults. Though infrequently available, measurements of arterialized capillary blood gases following warming of an extremity are another alternative. Under special circumstances, excessive oxygen therapy may be dangerous, as in the following instances. Retrolental fibroplasia, with rare exceptions, occurs only in neonates of less than 36 weeks' gestation who have had a P a D 2 of 150 torr or more for at least four hours. 51 Many premature infants have significantly increased P A-aD2 gradients for weeks, requiring increased concentrations of inspired oxygen. Careful monitoring is essential, and at least some arterial blood gases must be measured from the right radial or temporal arteries (preductal blood) to make certain that a large right-to-Ieft shunt through a patent ductus arteriosus is not deceiving the physician into prescribing an excessive amount of inspired oxygen. Pulmonary oxygen toxicity may occur at any age. 40 Although elevated concentrations of inspired oxygen of greater than 60 per cent are implicated in the etiology of pulmonary oxygen toxicity, the underlying disease process and mechanical ventilation with high inflating pressures are also important. Pulmonary oxygen toxicity is relatively uncommon, its development is slow, and its effects are often reversible. Consequently, a fear of producing toxicity must never be allowed to militate against the need to treat hypoxia. To quote an unknown cynic: "The brain always goes soft before the lungs become hard." Patients with chronic obstructive pulmonary disease (e.g., cystic fibrosis) may have retention of carbon dioxide, and high concentrations of inspired oxygen may remove their hypoxic drive, thus killing them through further hypoventilation. Low flow oxygen (24 to 28 per cent by Venturi face mask or 2 to 3 liters per minute by nasal cannulae) can raise oxygen saturation to safe levels without cutting off the hypoxic drive. 52 Before dismissing the subject of oxygenation, catheterization of the right heart by use of a Swan-Ganz catheter (flow-directed) should be mentioned. This technique is infrequently used in most pediatric centers, but should be employed more often in patients with acute respiratory failure considered to be secondary to widespread pulmonary disease. By measuring pulmonary wedge pressure, it can exclude left heart failure as a contributing factor, and provide a method of measuring mixed venous oxygen tension, cardiac output, and pulmonary shunt, thus greatly facilitating the management of these patients. 49 Humidification and Aerosol Therapy The humidifying mechanisms of the upper airway are usually very efficient, even in an infant. However, their efficiency may be impaired in the presence of systemic dehydration, inspiration of dry gas mixtures (such as oxygen), rapid respirations, and mouth breathing, thus bypassing the nasal airways which contribute a great deal to humidification and heat exchange. If humidification is inadequate, secretions

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become viscid and more difficult to expel by coughing. (The protective cough reflex may also be depressed in the presence of disease.) When the secretions are thick or purulent, a heated or ultrasonic nebulizer should be used to provide a supersaturated atmosphere. Although only relatively few droplets will reach the trachea,60 their deposition on the nasal and pharyngeal mucosa will ensure that the inspired gases are at a relatively humidity of 100 per cent. When the secretions are less copious and sticky, a hot water bath will probably provide adequate humidification, even though the relative humidity may be only 70 per cent. Aerosol therapy is designed to prevent or treat respiratory disease by the intermittent inhalation and subsequent deposition of air-borne particles of bronchodilating, mucolytic, or antimicrobial agents. The particles are produced by a nebulizer and are delivered by a mask or endotracheal tube using continuous flow or intermittent positive pressure breathing. The nebulized material is presumed to be distributed in' accordance with gas flow patterns within the lungs. Thus, underventilated or atelectatic regions receive little or no aerosol (as in patients with obstructive lung disease). There is a large amount of clinical evidence, much of it testimonial, that aerosol therapy is useful. Undoubtedly, it is beneficial for the delivery of bronchodilating agents in asthma. 24 Aerosolized racemic epinephrine, delivered by an intermittent positive pressure breathing apparatus,l has been advocated for the treatment of croup and postintubation laryngeal edema, though Taussig et al. 53 have warned that its benefits in croup do not include a change in the natural history of the illness. Nocturnal aerosol therapy by mist tent is no longer recommended for cystic fibrosis. 8 The application of aerosolized antimicrobial therapy for prophylaxis for pulmonary disease has not been successful, and is associated with deleterious effects on pulmonary function 14 and increased mortality.20 However, it may be beneficial in the treatment of Pseudomonas pneumonia. 20 Intermittent inhalations of aerosols of N-acetylcysteine have been widely used for their mucolytic action. Considerable testimonial evidence supports its beneficial action, but studies have failed to show objective improvement. 4. 26

Pharyngeal Suction Gentle pharyngeal suction performed at frequent intervals not only removes secretions which tend to pool in the pharynx in the very ill (particularly those with severe intracranial disease), but also stimulates coughing and the expulsion of tracheobronchial secretions. This therapeutic measure may avoid intubation. Occasionally, a patient with retained secretions will benefit from periodic endotracheal suction without prolonged intubation. Alternatively, pharyngeal suction in some patients with significant retention of sputum is contraindicated (such as patients with croup) for fear of precipitating complete upper airway obstruction by stimulating already inflamed vocal cords into spasm. In these patients, elective in-

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tubation for 24 to 48 hours in severe cases is the preferred treatment for both relief of obstruction and removal of secretions. 45 If pooling of pharyngeal secretions is a particular problem in an infant or child with an unprotected airway, a small suction catheter with an additionalline for delivering oxygen (Replogle tube) can be passed nasally into the oropharynx for continuous aspiration of fluid without risk of hypoxemia. This device is particularly useful in conditions as diverse as the Arnold-Chiari malformation syndrome or esophageal atresia. Physiotherapy Physiotherapy involves chest clapping and vibrating, hyperinflation, and body positioning for postural drainage. The results are thought to improve if the techniques are combined with inhalations before treatment and coughing afterward. There is a widespread clinical impression that pulmonary physiotherapy is useful in pediatric patients especially in acute (postoperative) retention of sputum and mucous plugging of bronchi. However, objective demonstration of its efficacy in either the acute or chronic situation occurs only rarely. 35 Recently, Etches and Scott showed an increase in the amount of upper airways secretions that were removed by suctioning after chest physiotherapy in six neonates with various chronic lung disorders. 19 Other workers have shown either an increase in P a02 following postural drainage with chest percussion in infants,21 or a decrease in P a02' 28 In older children with chronic obstructive pulmonary disease, advocacy of pulmonary physiotherapy and postural drainage is reinforced by reports that this regimen leads to production of a greater volume of sputum and a longer subsequent period during which patients are relatively free of spontaneous cough. This view is supported by two studies on patients with cystic fibrosis ,13. 34 and another recent report 20a shows improved gas flow at low lung volumes after postural drainage in cystic flbrosis and chronic bronchitis.

Fluids The intake of oral fluids should be suspended when dyspnea is present, and nasogastric feeding should be substituted. If dyspnea becomes severe enough and impairment of gastrointestinal absorption ensues, or if neurologic or other diseases cause palatopharyngeal incoordination, tube feeding should be ceased because of the risk of choking, vomiting, and aspiration. In addition, if gastric dilatation is allowed to occur, respiratory failure can be aggravated by mechanical embarrassment of the diaphragm. Instead, an intravenous infusion should be used to enable fluids, electrolytes, calories, and drugs to be given, and dehydration, metabolic acidosis, and other biochemical disturbances to be corrected. If the illness is likely to be prolonged, full intravenous alimentation should be undertaken early. Two particular points should be noted, especially with regard to the management of small infants. The humidification of inspired gases not only prevents respiratory fluid losses, but it may also achieve a

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positive net delivery of water to the respiratory tract, thereby causing overhydration. Further, full maintenance fluid on a mechanical ventilator is about 50 per cent of the volume when breathing spontaneouslyY

Treatment of Specific Conditions The therapeutic possibilities are too numerous to review in this article. They may include antibiotics for pneumonia or meningitis, narcotic or curare antagonists for postanesthetic central nervous system depression or neuromuscular blockade, respectively, bronchodilators for asthma, and aminophylline for neonatal apnea. General Nursing Care, Observation, and Monitoring Skilled nurses must be available for constant supervision of the patient. The patient should be nursed in a slightly head-up tilted position (more tilted if cerebral edema is present), and in a semi-sitting position if gastroesophageal reflux is suspected. Handling of the patient should be gentle and procedures should be performed rapidly with little disturbance. General observations should be directed at the level of consciousness, and the presence or absence of hypoxia, adequate ventilation, circulatory embarrassment, and fatigue. Monitoring (electronic or otherwise) of the various physiologic variables should be undertaken as discussed earlier, and a flow sheet should be used to record this data. Ventilation The major questions that are generally raised about ventilation are when to intubate and when to assist ventilation mechanically. The indications for assistance are modified by knowledge of the natural history of the various disease entities involved and assessment of the pathophysiologic disturbance in each child (Table 8). Mechanical assistance increases the complexity of management, and has its own set of complications,15 which have to be weighed against those that one avoids by its use. It is, therefore, usually reserved for those patients who are unlikely to survive without it, but who have a disease process which is reversible within a few days or weeks. There are some general rules to be followed when intubation is Table 8.

Indications for Intubation

With Mechanical Ventilation: PaCO, riSing, especially if accompanied by a change in mental status PaO, falling, despite alternative methods of providing oxygenation Fatigue Postoperative patients, particularly after major intrathoracic, intraabdominal or intracranial surgery With or Without Mechanical Ventilation: Upper airway obstruction accompanied by fatigue Patients in whom life-threatening aspiration may occur (comatose patients or those whose ninth and tenth cranial nerves are damaged).

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undertaken. The formula for choosing the proper size of endotracheal tube (mm of internal diameter) is: Age (years)

+4

4

The exception to this formula is when upper airway obstruction is present and a smaller tube is indicated. (See the article on Current Management of Croup and Epiglottitis in this symposium.) All but the most premature newborn infants will accept a 3.0 mm. endotracheal tube, which is the smallest through which suctioning can be performed. The tube should be precut to the appropriate length for each individual patient, if there is time. The nasotracheal route is preferred, though if there is urgency the airway should be quickly controlled with oro tracheal intubation, with or without the use of a stylet. The tube can later be changed to a nasal one at leisure. After intubation, there should be an air leak. The tube should be fastened securely after checking the position by listening for equal air entry in both lung fields during inflation. Care should be exercised while securing the tube to prevent pressure necrosis of the nasal labium. After the procedure is completed, a chest x-ray film should be taken to ensure that the end of the tube is at the level of the third thoracic vertebra. If it is too high, it can be easily coughed or pulled out; if it is too low, it will enter one of the main bronchi.

CLINICAL SITUATIONS OFTEN REQUIRING MECHANICAL ASSISTANCE OF VENTILATION

Upper Respiratory Tract Obstruction In many cases, patients with obstruction of the upper respiratory tract present with dyspnea, fatigue, and hypoxemia, but with little retention of carbon dioxide 39 until very late in the course of the disease. Marked fatigue, especially when accompanied by evidence of retention of sputum and carbon dioxide, should be used as an indication for intubation. This will relieve the obstruction and allow improved pulmonary care with better oxygenation and ventilation. Pulmonary Disease In the neonatal period, indications for assistance in hyaline membrane disease and other pulmonary diseases may be based upon clinical observations or arterial blood gas determinations. The parameters of clinical assessment include central cyanosis in 100 per cent oxygen and the occurrence of one or two episodes of apnea not responding rapidly to inflation by bag and mask. 51 Mortality is greater than 90 per cent when the P a 0 2 falls below 50 torr in 100 per cent oxygen,47 so this is a logical level at which to begin assistance. Infants with bronchiolitis generally manage extremely well if proper attention is directed toward providing calories, adequate hu-

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midification, oxygenation, and physiotherapy. However, the work of breathing is heavy, and if the P a C0 2 rises above 60 to 65 torr and the infant becomes fatigued, nasotracheal intubation and controlled ventilation will be necessary.46 Sedation or neuromuscular blockade may also be needed. Of special concern is the premature baby in whom bronchiolitis develops, reopening the ductus arteriosus with a left-toright shunt, and increasing the blood flow to the lungs. The high pulmonary blood flow may lead to a further decrease in lung compliance, and pulmonary edema, thus precipitating respiratory failure. Aggressive treatment of patients with asthma with intravenous aminophylline, steroids, and aerosolized or intravenous bronchodilators has made the need for mechanical assistance an uncommon event at The Hospital for Sick Children in Toronto, Canada. However, if the clinical picture and arterial blood gases worsen (P a C02 greater than 60 to 65 torr) despite all measures including intravenous isoproterenol or salbutamol, we paralyze and mechanically ventilate the patient. ' Neurologic Disease The three most common causes for ventilatory assistance in patients with neurologic disorders are: apnea which is recurrent or does not respond to stimulation; the inability to swallow or cough up accumulated pharyngeal secretions because of cranial or spinal nerve disease (prevention of aspiration); and an increase in cerebral edema secondary to a rise in the arterial tension of carbon dioxide in diseases such as postanoxic brain injury, meningitis, and Reye's syndrome (prevention of hypoventilation). Postoperative Assistance Mter major surgery, there may be residual effects on ventilation and oxygenation from: anesthetic agents and muscle relaxants, the area of surgery (brain, thorax, abdomen), pain, inadequate blood replacement, hypothermia, and acidosis. Prophylactic assistance is useful in selected neonatal patients following major surgery.'O Ventilation should be used prophylactically before and after repair of a diaphragmatic hernia, and after esophageal atresia with a tracheoesophageal fistula. Acute respiratory failure following cardiovascular surgery is well recognized and mechanical support is used. This has been discussed in detail by Downes et al. '6 Pulmonary Edema In patients with a high pulmonary blood flow secondary to a left-toright shunt (such as in an endocardial cushion defect, a ventricular septal defect, or a patent ductus arteriosus), left ventricular failure is usually adequately controlled by medical measures. However, a respiratory tract infection may increase the tracheobronchial secretions as well as the pulmonary blood flow and left ventricular strain by increasing the metabolic rate. The resulting increase in pulmonary edema and sputum can interfere sufficiently with oxygenation and ventilation to precipitate acute respiratory failure. A similar result may follow pulmonary

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edema secondary to anomalous pulmonary venous return or left ventricular outflow tract obstruction. Assisted ventilation is often required in these circumstances, and sometimes it is impossible to wean an infant from the respirator until palliative or corrective surgery is undertaken.

METHODS OF MECHANICAL VENTILATORY ASSISTANCE

This topic is beyond the scope of this paper but has been reviewed in detail by Stocks SO and Downes. ls The techniques available include: an artificial airway created by intubation, tracheotomy, or nasal cannula (neonate); intermittent positive pressure breathing by a ventilator; constant positive airway pressure involving constant pressure applied to the airway continuously during spontaneous breathing; continuous positive pressure breathing which consists of intermittent positive pressure breathing combined with a positive end-expiratory pressure; and intermittent or continuous negative pressure respiration utilizing a body-enclosing box.

Weaning from the Ventilator and Extubation Three steps are involved in the weaning process. First, the patient has to be in a stable condition with improvement in the underlying disease and its complications which precipitated the respiratory failure. Second, the patient has to be weaned or removed from the ventilator and his nasotracheal or tracheotomy tube connected to a "T" piece or other type of adaptor that delivers extra oxygen but allows the patient to ventilate on his own. Several physiologic variables can pre-

Table 9. Weaning from the Ventilator I. Stable Condition Underlying cause for respiratory failure reversed. Pain, starvation, electrolyte and acidbase abnormalities corrected. II. A. Physiologic Method (older, cooperative children) Vital capacity > 10-15 ml per kg of body weight Peak inspiratory pressure < -20 to -30 cm water P A •a 0 2 on 100 per cent oxygen < 300 to 350 torr P a C02 < 50 torr Ratio of dead space to tidal volume <0.60 B. Trial and Error Method (younger, uncooperative infants and children) In the following order, frequently monitor respiratory rate, effort, and blood gases. Decrease concentration of inspired oxygen Decrease positive end-expiratory pressure Decrease ventilator rate (intermittent mandatory ventilation) C. Connect patient to a "T" piece with 40 per cent humidified oxygen and repeat blood gases at 30 minutes and just prior to extubation several hours later. III. Extubate and deliver humidified oxygen via mask or head box.

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dict when a patient is ready for this step. However, infants and young children cannot cooperate so trial and error methods must be used (Table 9). With the patient ventilating and oxygenating well on his "T" piece, the third step, extubation, is undertaken. The whole process should be monitored by serial measurements of arterial blood gases.

SUMMARY Respiratory failure is common in infants and young children who are peculiarly disadvantaged compared with adults. An illness that is minor in an adult may be life-threatening in an infant because of the numerous factors which predispose the pediatric patient to respiratory failure. Respiratory failure, therefore, is a common complication of many disease processes, and should always be readily suspected, especially if there are unexplained signs or symptoms. ' The emphasis in diagnosis and management must be on measurements of arterial blood gases. If good clinical judgment and a thorough knowledge of the natural history of the disease can be combined with the intelligent assessment of oxygenation, ventilation, and acidbase status as afforded by an understanding of arterial blood gases and the physiologic principles involved, the management of many of these small patients can be exceptionally rewarding.

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