Neonatal monitoring and equipment

Neonatal monitoring and equipment

D. I. Tudehope, A. D. Shearman

impress other physicians, patients and visitors regarding the sophistication of the ICN, with the tacit assumption that advanced technology equates to quality of care. Too many technologies introduced into neonatal intensive care nurseries have not been validated properly. The onus should be placed on manufacturers of high technology equipment to support a reasonable amount of research into the performance of their instruments in clinical field trials before launching new equipment on ill prepared clinical staff. When new neonatal equipment is being assessed it should be quite clear whether the trial is to market the product, to identify areas requiring improvement or to test its suitability in their own clinical area.

Over the last two decades neonatal medicine has relied on increasingly complex monitoring technology to improve the delivery of patient care. When monitoring technology is used intelligently it assists the work of nurses and clinicians and improves the quality of patient care. However, reliance on electronic monitors may impair the monitoring skills of the clinical nurse at the cotside. The technology must not be misused as nurses and physicians can easily become slaves to technology. The tiny preterm infant is surrounded by a mass of technology which may adversely impact on parental attachment and behaviours as well as the focus of healthcare professionals. The general noise level of the Intensive Care Nursery (ICN) with each monitor having its own alarm adds to the stress level of staff working in this environment and in addition can lead to parental anxiety. Many monitors, electrodes, cables and sensors used in the ICN are scaled down from adult monitoring systems. The monitoring equipment is used for infants of birth weight varying from 500 to 5000 g. Thus, in the smallest babies there is a worse signal to noise ratio demanding better processing facilities than for infants of greater birth weight.1 The sensors for these tiny babies must take into account their size and extreme skin fragility. In the current era of high risk of professional liability, extensive monitoring with trending and hard copy facilities have been seen as means to protect oneself against the eventuality of a lawsuit for medical liability. The intrusion of technology is used to

Specific monitoring 1. Cardio-respiratory Continuous monitoring of heart rate and breathing are considered the minimum requirements for all babies receiving intensive care. Respiratory only monitoring is often used in a recovery or 'growers' nursery and in the home environment for graduates of neonatal intensive care. Cardio-respiratory monitoring is also used as a diagnostic modality with use of pneumograms and event recording. Cardiac monitoring consists of a three lead ECG cable connecting the infant to a monitor. Most monitors are able to select leads I, II or III. Respiration is monitored with the same cable and electrodes using thoracic impedance between right and left arm electrodes. Because impedance measures chest wall movement and not airflow the obstructive component of any apnoea may go undetected. Respiratory monitoring usually involves thoracic impedance but other

D. I. TudehopeMBBS, FRACP, Director of Neonatology, A. D. Shearman BTC, Respiratory Technician, Mater Mothers' Hospital, Raymond Terrace, South Brisbane, Queensland 4101, Australia. Correspondence and requests for offprints to DIT.

Current Paediatrics (1995) 5, 195500 © 1995 Pearson Professional Ltd

195

196 CURRENT PAEDIATRICS

methods available for breath detection include movement sensors, airway flow, airway thermistors and tracheal sounds. Monitors are designed to alarm for bradycardia, tachycardia and apnoea >10, 15 or 20 s. Although ECG signal detection has been a mature technology for many years refinements continue with electrode design and the methods selected for rate averaging, frequency of rate update and duration of alarm delay. Very fine, light and flexible electrode leads have some advantages for babies under 1000 g but they readily become tangled and may make rapid access to the baby awkward and inconvenient. 1 Noise and artefact produced by non-ideal electrodes and cables are less likely to be troublesome with high quality ECG amplifiers and monitors. Many approaches for the attachment of ECG electrodes have been tried, including conventional adhesives, clips, clamps and even needles. Prewired electrodes have advantages over clip-on electrodes because of fewer connections and better skin contact. Recent improvements in electrode design have reduced the complications of skin breakdown, trauma and subsequent scarring. Ideally electrodes for tiny infants should be cheap, reusable, radiolucent, maintain prolonged electrical contact and gentle to the fragile skin. Configuredmonitors and modularsystems: Configured monitors are generally cheaper, lighter and more portable than modular systems. Modular systems produce versatility, up-gradability and flexibility with patient monitoring. Both configured and modular monitors have the capacity for trending and event recording. Modular systems have the capacity to monitor more physiological parameters in various combinations with advantage of a single button to cancel any alarm. During maintenance of separate modules the base system is still useable. Adult intensive care units analyse trending, particularly of ECG, more fre-

quently than neonatal units. Real time computerisation of data enable integration of physiological variables. Networking of monitors and alarms enables optimal utilisation of staff, more effective patient care and interfacing with a central printer and computer. 2. Monitoring of lung function in the intensive care unit Measures of pH, PaO 2 and PaCO 2 are invaluable tools in neonatal intensive care, being the prime determinants indicating the need for intubation, setting of mechanical ventilators and weaning and extubation of patients with respiratory failure. The analysis of arterial blood gases is expensive e.g. syringes, personnel time and laboratory processing, and requires repeat arterial punctures, or placement of indwelling arterial lines with associated risks of complications. Complications of arterial catheterisation are included in Table 1. Intermittent arterial catheter sampling and analysis provides an accurate assay at a single point in time but cannot provide the second to second changes required following suctioning, handling of the baby and ventilator circuit mishaps. Invasive, continuous oxygen monitoring: The continuous monitoring of 02 tension using an umbilical arterial catheter with an 02 electrode on its tip has been available since 1974. The technology was developed by the Medical Physics and Bioengineering Departments of University College Hospital, London and subsequently marketed by Searle Life Support Systems and Biomedical Sensors Ltd and Shiley Inc. Reports from early studies revealed 25% of apparently correctly sited and calibrated catheters malfunctioned before removal due to structural faults, unsatisfactory membranes, faulty wiring of an amplifier, clotting over the tip and siting at the entrance to an iliac artery. 2 Although progressive refinements in catheter and monitor technology have improved function this

Table I Comparison of complications of umbilical and peripheral arterial catheters Umbilical

Peripheral

Frequency

Comment

Frequency

Comment

Thrombosis

++

Usually occult

+

Absent pulse, cool, cyanosis usually acute

Vasospasm

+++

Blanching of limbs, may by transient. If persists remove catheter

++++

Often obstructive, benign and sampling related

Haemorrhage

+

Faulty connections, dislodgment and on removal. Bleeding diathesis

+

Obvious and easily managed

Infection

+++

Usually coagulase negative Staphylococci

+

Potential source

Visceral ischaemia

+

Necrotizing enterocolitis. Renal and spinal cord

Haematoma

+

Rare 'blue belly' syndrome

Nil +++

Common with infiltration and withdrawal

NEONATALMONITORING 197

technique has not been widely accepted into neonatal practice. Catheters are expensive, rigid and only available in 4 and 5 F sizes. Improvements include bipolar design sampling lumen close to the tip, a single calibration sequence and a 9-h battery life. Continuous, invasive monitoring of O 2 saturation using an umbilical arterial catheter was successfully used in neonatal resuscitation and subsequent stabilization in the mid 1970s but further refinements in non-invasive pulse oximetry have halted its progress. However, indwelling continuous arterially recording blood catheters containing 3 optical fibres tipped with 2 different chemical indicators that change colour in proportion to the concentration of H + ions, O 2 and CO z molecules have been commercially available in adult practice since 1992.3 The use of these is limited by their size (20 F) making them too large for newborn infants. Non-invasive continuous gas monitoring: Continuous monitoring can now be provided by non-invasive means including pulse oximetry, transcutaneous pO 2 and pCO 2 monitors and end tidal CO 2 analysis. The availability of non-invasive monitoring has dramatically decreased the dependency on arterial blood gases. (i) Transcutaneous pO 2 and pCO 2 monitoring: This is predominantly used in the ICN because the relatively low skin thickness and metabolism in infancy enhances accuracy. Additionally in critically ill and extremely preterm infants not only hypoxia, but also hyperoxia, hypo- and hypercarbia need to be avoided. The difficulties of cannulation in very small infants make this technique especially attractive. To aid trans -° cutaneous blood gas monitoring the electrode heats the skin to between 43-45°C producing local vasodilatation and arterialization of the capillary bed, making the skin more permeable to blood gases. Although non-invasive this technique is not without risk as the requirement for the skin to be heated presents the possibility of thermal skin injury unless the probe site is changed every 2 4 h. Drawbacks include an initial delay while the skin is being heated together with slow response times. Temperature probes should not be sited near transcutaneous electrodes to prevent erroneous body temperature measurements. Transcutaneous measurements depend on 3 main factors - - capillary temperature, blood flow and skin metabolic rate. In shock conditions, exposure to cold and during treatment with vasoconstrictive drugs such as dopamine the skin blood flow and capillary pO 2 decrease with consequent loss of correlation between transcutaneous and arterial O 2 measurements. Good correlation between transcutaneous and arterial O 2 tensions exists in the first few weeks of life especially in stable preterm infants. Continuous monitoring of CO~ tension can assist clinical care especially management of persistent

pulmonary hypertension, following birth asphyxia, hypoventilation and early detection of clinical deterioration. Transcutaneous pCO 2monitoring is based on the pH determination of a thin film of electrolyte separated from the skin by a hydrophobic membrane permeable to CO 2. When CO2 diffuses through the membrane the pH of the electrolyte changes as CO 2 reacts with water to form hydrogen and bicarbonate ions. Good correlation exists between transcutaneous and arterial pCO~ measurements. The transcutaneous technique involving localized heating of the skin and the production of CO~ by skin cells gives a higher pCO 2 level than the actual arterial value. The differences between the transcutaneous and arterial pCO~ values in any one neonate can be established with direct comparison of these values. Transcutaneous pCO 2 is invariably higher than the corresponding arterial value even in patients with normal skin perfusion due to the heating coefficient of blood and metabolic differences which result from CO~ production by skin cells. Nevertheless, numerous studies have shown good correlation between transcutaneous and arterial measurements. Mean differences between arterial and transcutaneous pCO 2 have ranged from 1.3-26.5 mmHg (0.2-3.5 Kp). 4,5 By combining the reference electrodes and providing a cathode for 02 and a glass pH electrode a combined pO 2 and pCO2 sensor can be made. Transcutaneous gas monitoring is of limited value under circumstances of shock, hypothermia, severe acidosis, sclerema, hydrops, anaemia and particularly with increasing thickness of the epidermal layer with advancing postnatal age. Transcutaneous gas monitoring is unreliable in infants with bronchopulmonary dysplasia. (ii) Pulse oximeters: Pulse oximetry is now widely accepted as a simple, useful and user friendly clinical tool. This ubiquitous piece of equipment relies on complex algorithms to analyse information on the transmittance and absorbence of red and infra-red light through perfused tissue. The two wavelengths 660 and 920-940 nm (Figure) are used to measure oxyhaemoglobin and deoxhaemoglobin to give functional saturation i.e. HbO 2 Hb + HbO 2

x 100

Although only capable of measuring two types of haemoglobin, some oximeters calculate and display fractional 02 saturation, i.e. HbO 2 Hb + HbO2 + HbCO + HbMet

× 100

HbF has no significant effect on the measurement of SpO 2 in the neonate. 5 Certain clinical and technical situations may interfere with the acquisition of

198 CURRENT PAEDIATRICS

RED 660 nm

20,000_

! a I i ! ! i

10,000-C

8

INFRARED 92( n m

5000--

k Hgb 'o 1000--

e. O

500--

I

a

HgbO 100-

O

!

,,

,

|

50t |

10 500

i

I

l[

I I

600

700

I

I

I

800

660

I

i

900

I 1000

920 Wavelength (nm)

Figure--Haemoglobin and light absorption. (Reprinted by permission of Nelkor Incorporated, Pleasanton, California.)

reliable data from the pulse oximeter (see Table 2). Increased motion may result in increased activity of the sensor site creating pulsations that mimic arterial pulsations. These may contribute to unreliable information when SpO 2 values are processed. Under circumstances of low perfusion only very small amounts of arterial blood may flow into the arteriolar bed. Inaccuracies may result from accumulation of carbon monoxide or methaemoglobin. On the rare occasion when venous blood is pulsatile it may interfere with results. Ambient light interference may affect the photodetector. In the sick neonate, movement artefact and/or poor perfusion can make reliable pulse oximetry difficult. As an aid to over come this, some manufacturers recommend ECG synchronisation of their oximeters. A study from this unit has shown that ECG synchro-

nised oximeters record m o r e 0 2 desaturation events than non-ECG synchronised oximeters. 7 The aim of monitoring arterial oxygenation in newborn infants is not only to avoid hypoxaemia but also to detect hyperoxaemia which is still an important risk factor in the development of retinopathy of prematurity. Empirical recommendations for S a O 2 in infants at risk of ROP generally fall into the 90-95% range. Although the pulse oximeter has an obvious application to detection of hyperoxaemia its inherent accuracy of 2-3% in the region of the O2 dissociation curve above SaO 2 79% has been assumed to be too large. Bucher et al8 concluded that pulse oximeters can be highly sensitive in detecting hyperoxaemia provided that type specific alarms limits are set and a low specificity is accepted. The optimal alarm limit, defined as a sensitivity of > 95% with maximal specificity was determined for

Table 2 Causes of error in the measurement of SpO z

Absorbence at the specific wavelengths • • • •

Dyshaemog~obins - methaemoglobin, carboxy-haemoglobin, sulphahaemoglobin. Dyes methylene blue, indigo carmine, indocyanine blue, fluoroscein. Increase in the transmitted light intensity causing a shift in the wavelengths. Low arterial saturation (<70%).

Measurement of non-arterial pulsatile flow • • •

Motion artefact. 50-60 Hz radiant warmers and phototherapy and surgical lights. Cardiac conditions causing pulsatile venous flow.

Reduction in pulsatile flow • • • •

Poor perfusion. Anemia. Sensor applied too tightly Use of vasoconstricting drugs.

NEONATAL MONITORING 199

Table 3 Comparison of pulse oximetryand transcutaneouspO2monitoring Response time Movement Skin preparation Perfusion 1st week Postnatal age Conditions for use

Pulse oximetry

TranscutaneouspO2

Seconds Very dependent Independent Dependent Accurate Not affected Resuscitation,surgery,BPD

15s Less movementat sites used Site changes, dependent on skin or operator Dependent Maximum accuracy Affectedby skinfoldthickness Acute RDS

Nellcor N-100 at 96% SpO 2 (specificity 52%). The combination of pulse oximetry and transcutaneous gas monitoring is ideal for critically ill infants. Comparisons between use of pulse oximetry and transcutaneous pO 2monitoring are displayed in Table 3.

give a more physiological mechanical breath, reduce baro/volume trauma and inadvertent PEER record the effects of medications and therapy, or as an aid in teaching.

(iii) End tidal CO 2 monitoring." End tidal CO 2 monitors suitable for use with neonates have recently been developed and have been proposed as alternatives to YcpO 2 monitors for non-invasive monitoring of CO 2 content in ill preterm infants. End tidal CO 2 maybe a good approximation to PaCO 2 but is not identical as it varies depending on the ventilation pattern and lung function of the subject at the time. End tidal CO 2 measurements have been shown to correlate well with PaCO 2 in subjects without lung disease. 9 End tidal CO 2 is not likely to equal or exceed PaCO 2 at high tidal volumes, high CO 2 outputs and at low respiratory rates. In conclusion, there may well be a place for end tidal CO 2 measurement in the management of preterm infants with normal lung function, e.g. in detection analysis of obstructive apnoea or during anaesthesia. For most preterm infants with respiratory illness, however, this technique cannot be recommended at present.

Blood pressure monitoring

(iv) Respiratory mechanics monitoring." A new innovation in monitoring is the respiratory mechanics monitor designed for use in infants receiving mechanical ventilation. These may be stand alone units or integrated into mechanical ventilators. They consist of a flow sensing device, such as a pneumotachograph or a hot wire anemometer placed at the patient wye, and a pressure sensor measuring close to the distal end of the endotracheal tube. The flow signal can be integrated to give volume. The combination of flow, pressure, time and volume can measure basic parameters such as tidal volume, rate and minute ventilation. More sophisticated monitors may also measure and display more complex parameters including flowvolume and pressure-volume loops, resistance of the airway and dynamic compliance. With the use of time cycled pressure limited ventilators in the neonatal intensive care unit, respiratory monitors allow for the quantification and visual display of mechanical and spontaneous breaths. This data can then be used to,

Intra-arterial blood pressure monitoring." Intra-arterial blood pressure monitoring using an umbilical or peripheral arterial catheter (radial or posterior tibial) is a routine procedure in most neonatal intensive care nurseries. Intra-arterial pressure monitoring via the catheter also provides an important guide to catheter patency. Infants <1500 g or <32 weeks gestation require a small catheter 3.5 or 4 F with a greater chance of catheter lumen blockage due to thrombus formation. The arterial pressure and wave form can be monitored using a narrow gauge cannula or catheters with a modern low flow pressure transducer either connected directly to a Luer fitting of the catheter or some 60 cm distant. The phasic pressure waveform is displayed on a monitor screen to identify the signs of catheter obstruction. The suggestive features are damping of waveform, loss of dicrotic notch and narrowing of pulse pressure. Previously transducers were multiple use or indisposable, but the availability of disposable pressure domes negates the need for sterilisation. Catheter patency is improved by continuous infusion of heparinised saline or use of an 'intraflow' device. Accuracy is affected if pressure transducer is not at the same level as the heart. Non-invasive blood pressure monitoring." The non-invasive arterial pressure measurement is widely practised with a limb encircling cuff and utilisation of the oscillometric principle. Blood pressure measurements can be provided in a semi-continuous fashion. If the cuff is not matched to the size of the baby's limb, systematic errors will be introduced. Measurements of noninvasive blood pressure in tiny infants usually exceed intra-arterial pressure measurements. Interpretation of newborn systolic, diastolic and mean arterial pressures requires knowledge of birth weight, gestational and postnatal ages (Table 4). ~° Research continues on a non-invasive technique for continuous arterial pressure measurement.

200 CURRENT PAEDIATRICS

Table 4 Mean blood pressure, birth weight, and postnatal age. Results (in mmHg) are given as 10th percentile (mean) *m

Birth Weight (g) 500 600 700 800 900 1000 1100 1200 1300 1400 1500

3 23 (35) 24 (35) 24 (36) 25 (36) 25 (37) 26 (38) 27 (38) 27 (39) 28 (39) 28 (40) 29 (40)

12 24 (36) 25 (36) 25 (37) 26 (37) 26 (38) 27 (29) 27 (39) 28 (40) 29 (40) 29 (41) 30 (42)

24 25 (37) 26 (37) 26 (38) 27 (39) 27 (39) 28 (40) 29 (40) 29 (41) 30 (41) 30 (42) 31 (43)

Postnatal age (h) 36 48 60 26 (38) 28 (39) 29 (41) 27 (39) 28 (40) 29 (41) 28 (39) 29 (42) 30 (42) 28 (40) 29 (41) 31 (42) 29 (40) 30 (42) 31 (43) 29 (41) 31 (42) 32 (43) 30 (42) 31 (43) 32 (44) 30 (42) 32 (43) 33 (45) 31 (41) 32 (44) 33 (45) 32 (43) 33 (44) 34 (46) 32 (44) 33 (45) 35 (46)

72 30 (42) 31 (42) 31 (43) 32 (44) 32 (44) 33 (45) 34 (45) 34 (46) 35 (46) 35 (47) 36 (48)

84 31 (43) 32 (44) 32 (44) 33 (45) 34 (45) 34 (46) 35 (46) 35 (47) 36 (48) 36 (48) 37 (49)

96 33 (44) 33 (45) 34 (45) 35 (47) 35 (47) 35 (47) 36 (48) 37 (48) 37 (49) 38 (49) 38 (50)

*Data obtained by birthweight specific regression for mean arterial pressure on postnatal age using data from 131 infants.

Conclusion

Monitoring equipment purchased 2 decades ago had a much longer life expectancy than current equipment. Due to advances in technology current equipment is rapidly becoming superseded. The performance of new equipment may be enhanced by insertion of updated software. Equipment is now generally more reliable with design improvement, less electronic component failures and improved quality control. Modern hospitals with complex equipment need servicing technicians with detailed knowledge of equipment operation but also require back up from agents and manufacturers. Equipment is frequently modular internally as well as externally and faults often require changing modules such as printed circuit boards rather than replacement of component parts. Because of the high costs and short life expectancy of complex equipment innovative funding strategies are frequently negotiated such as leasing, amortisation and capital depreciation replacement. References 1. Harvey D, Cooke RWI, Levitt GA. The baby under 1000 g. Published by Wright 1989:106-119. 2. Pollitzer M J, Soutler LP, Reynolds EOR. Continuous monitoring of arterial oxygen tension in infants: four years of experience with an intravascular oxygen electrode. Pediatrics 1980; 66(1): 31-36. 3. Severinghaus JW, Kelleher JE Recent developments in pulse oximetry. Anesthesiology 1992; 76: 1018-1038.

4. Sivan Y, Eldadah MK, Cheah TH, Newth CJL. Estimation of arterial carbon dioxide by end-tidal and transcutaneous pC O 2 measurements in ventilated children. Pediatr Pulmonol 1992; 12: 153-157. 5. Monaco F, McQultty JC. Transcutaneous measurements of carbon dioxide partial pressure in sick neonates. Crit Care Med 1981; 9: 756-758. 6. Rajadurai VS, Walker AM, Yu V, Oates A. Effect of fetal haemoglobin on the accuracy of pulse oximetry in preterm infants. J Paed Child Health 1992; 28: 43-46. 7. Shearman AD, Steer PA. Comparison of SpO2 measured by pulse oximetry with and without ECG synchronisation. Abstract. 13th Annual Congress of the Australian Perinatal Society and Annual Scientific Meeting of the New Zealand Perinatal Society. 8. Bucher HU, Fanconi S, Baeckert P, Duc G. Hyperoxemia in newborn infants: Detection by pulse oximetry. Pediatrics 1989; 84(2): 226-230. 9. Watkins AMC, Weindling AM. Monitoring of end tidal CO 2 in neonatal intensive care. Arch Dis Child 1987; 62: 837-839. 10. WeindlingAM. Blood pressure monitoring in the newborn. Arch Dis Child 1989; 64: 444447.

Further Reading Harvey D, Cooke RWI, Levitt GA. The baby under 1000 g. Published by Wright 1989. Brans YW (Editor). Newer technologies and the neonate. Clinic Perinatol 1991; 18(3). Numa AH, Newth CJL. Assessment of lung function in the intensive care unit. Pediatr Pulmonol 1995; 19:118-128. Greenough A. Pulse oximetry. Current Paediatrics 1994; 4: 196-199. Joint Committee of the ATS Assembly on Pediatrics and the ERS Paediatrics Assembly. American Thoriac Society/European Respiratory Society. Respiratory mechanics in infants: PhysioloNc evaluation health and disease. Am Rev Resp Dis 1993; 147; 474-496.