Capnography in Pediatrics

Bhavani Shankar Kodali MD

CAPNOGRAPHY IN INFANTS AND SMALL CHILDREN

Highlights

1. Interaction between physical and physiological factors results in normal variants in a capnogram (figure).

2. Main stream capnometers are more accurate than side stream capnometers for proximal site PETCO₂ measurements,7 however main stream capnometers may cause unacceptable rebreathing.2,16

3. Distal PETCO₂ should be used if the weight of the child is < 12 kg, and during the use of partial rebreathing circuits.1,4,5,15 Further, the use of rebreathing circuits produce distorted capnograms either with no alveolar plateau or a flat alveolar plateau due the dilution of end-tidal gas by the fresh gas flows.6

4. Microstream technology (introduced by Oridion, Oridion Capnostream 20) with an aspiration flow rate of 30 ml.min-1 but with rapid response time obviates several disadvantages of conventional capnographs. It provides accurate PETCO₂ measurements, minimizes distortions in CO₂ waveforms and reduces the chances of secretions being drawn into the system.

The basic physical, physiological and clinical principles remain valid in infants and children. Several factors, however pertaining to infants and children, influence the CO₂ measurements, thus PETCO₂ values, (a-ET)PCO₂ gradient as well as the shape of the capnogram.

These factors are;

Physical:

Response time of the instrument to measure CO₂

Type of the capnometer – side-stream or main-stream

Site of monitoring of CO₂ – proximal or distal site in the endotracheal tube

Weight of the child

Type of the ventilator and circuit used during ventilation

Presence of water condensation and secretions in the CO₂ sampling device.

Physiological:

Relatively higher respiratory rates and smaller tidal volumes encountered in children; Low volumes of CO₂ produced in neonates and infants.

Pathological:

Presence of pulmonary disease

Acyanotic and cyanotic heart diseases.

The interaction between physical and physiological factors results in normal variants in a capnogram (figure). It is essential to understand these variations in order to minimize abnormal interpretations of the capnogram.

Spontaneous or controlled ventilation:

In adults and older children, the shape of the capnogram may be similar during spontaneous and controlled ventilation. In smaller children and neonates however, a ‘sinewave’ type of capnogram can occur during spontaneous ventilation where there is no clear alveolar plateau (figure). Despite the absence of alveolar plateau, there is a good correlation of PETCO₂ with PaCO₂.1,2 Often sinewave capnogram can also occur during controlled ventilation in neonates. Lack of alveolar plateau is due to several reasons such as sampling flow too high for volume of CO₂ produced, turbulence produced by extraction of the gas sample, too long sampling tube, longer response time of the analyzer and gas sample extracted from an unsuitable site. These are discussed further below.3-9

Response time of the analyzer:

For accuracy, a capnometer should have rapid response time. Slow response time can alter the profile of the capnogram, particularly at lower tidal volumes and faster respiratory rates as encountered in neonates and infants. The response time has two components – Transit time and rise time. Transit time is the time taken for the sample to move from the point of sampling to the point of measurement (in side-stream capnometers). Rise time (T70) is the time taken to change from 10% to 70% of the final value. A prolonged response time results in errors in PETCO₂ measurements and distortions in CO₂ waveforms such as prolonged phase II with a steeper phase III (resembles capnogram recorded during asthma5 and elevation of the baseline (resembles capnogram obtained during rebreathing).7 The distortions increase with faster respiratory rates.10 However, the distortion of the capnogram can be minimized with the use of higher sampling flow rates of 150 ml/min, use of main-stream capnometers (which are superior to side-stream capnometers in accuracy of CO₂ measurements since they analyze the CO₂ inside the airway), faster CO₂ analyzers (T70 of 80 ms to measure PETCO₂ in children with 5% accuracy at 100 breaths/min and I:E ratios less than 2:1), and minimizing the volume of sampling chamber and tubes.7,8,10-12 As a general rule, the response time of the analyzer should be less than the respiratory cycle time of the infant in order to achieve predictable PETCO₂ values and CO₂ waveform.13

Recently developed Microstream technology which utilizes low aspiration sampling flows but with rapid response time is being tested in pediatric patients. Results are encouraging as it minimizes distortions in CO₂ waveforms and thus preserving the accuracy of end-tidal PCO₂ measurements as well as normal shape of capnograms in infants and children. The details are included at the end of the section.

Site of measurement CO₂ in intubated children:

Sampling of CO₂ can be performed at the proximal (between the endotracheal tube and the circuit) or distal part (by inserting a catheter or a needle into the tracheal tube) of the endotracheal tube. Distal PCO₂ measurements produce a normal capnogram whereas the proximal measurements result in a distorted capnogram due to dilution of end-tidal gas by the fresh gas flows.6 Further, distal PETCO₂ measurements are higher than proximal PETCO₂ measurements and accurately approximate PaCO₂ values in intubated infants and children.4,5,14 Distal site sampling, however is associated with difficulties which include: an increased risk of disconnection at the site of insertion of catheter or needle into the tracheal tube, and blockage of the catheter by secretions. Proximal sampling, though, is easy and carries minimal risk of blockage or contamination by secretions and is generally preferred by most clinicians.14 In addition to the type and sampling characteristics of capnometer, several factors such as weight of the child, type of ventilator, circuit and fresh gas flows affect the proximal and distal CO₂ measurements and their correlation with PaCO₂ values.14 The following generalizations can be drawn from the literature depending upon the size of the child, type of the capnometer and type of ventilator/circuit in use.15

1. Main stream capnometers are more accurate than side stream capnometers for proximal site PETCO₂ measurements,7 however main stream capnometer may cause unacceptable rebreathing.2,16

2. Distal PETCO₂ should be used if the weight of the child is < 12 kg, and during the use of partial rebreathing circuits.1,4,5,15 Further, the use of rebreathing circuits produce distorted capnograms either with no alveolar plateau or a flat alveolar plateau due the dilution of end-tidal gas by the fresh gas flows.6

Presence of secretions:

Water droplets and patient secretions may result in an accumulation of water and secretions in breathing hoses. The contaminant may enter the sampling tubes of the side-stream capnometers and increase flow resistance in the tubing thus affecting accuracy of the CO₂ measurement. The sampling tube may also be occluded. Some units either increase the sampling flow or reverse the flow (purge) when a drop in pressure from a flow restriction is sensed. This will help to clear the secretions from the tube. If the occlusion is not cleared the sampling tube must be replaced. Occasionally, liquids enter the main unit of the analyzer despite the presence of water traps. This can degrade the performance of the CO₂ monitor producing abnormal wave patterns by interfering with the sampling flow or fouling the sensor, in which case the chamber will require cleaning. It should be noted that positioning the sampling tube upwards away from the patient decreases the frequency with which liquids are drawn into the tubes. When abnormal measurements are encountered, it should be ensured that they are not a result of faulty system. A quick, but less accurate, method is to record a normal CO₂ tracing(eg., one’s own). The typical CO₂ waveform with PETCO₂ readings between 38-42 mmHg, confirms the proper functioning of the capnometer.

Critically ill neonates:

(i) In neonates with mild to moderate lung disease (FIO2 < 0.3) and respiratory frequency < 70/min), the distal sampling of CO₂ measurements are preferred to proximal measurements as the former reflect PaCO₂ more accurately than the later.1,15 However, in children with severe lung disease even the distal PETCO₂ may not be good predictor of PaCO₂ because (a-ET)PCO₂ gradients varies with changing V/Q relationship of the sick neonate thus making PETCO₂ measurements less reliable.17 Under these circumstances, PtcCO₂ is more accurate estimate of PaCO₂.1 The emphasis here is on more ABG’s until V/Q mismatch improves and a more constant and consistent (a-ET)PCO₂ relationship is established.

Cyanotic heart diseases:

In infants and children with acyanotic heart disease (left to right shunt), PETCO₂ is closer to PaCO₂ and (a-ET)PCO₂ gradient is not significantly different from children with normal circulation.18 Further PETCO₂ is a reliable estimate of PaCO₂.19,20 However in children with cyanotic heart diseases, PETCO₂ underestimates PaCO₂ and the (a-ET)PCO₂ gradient is increased up to 15 mmHg due a combination of venous admixture and low pulmonary perfusion.21,22 Under these circumstances, (a-ET)PCO₂ is linearly correlated with arterial oxygen saturation (SPO2).22 With a decrease in SPO2 by 10% caused by right to left shunt, the (a-ET)PCO₂ gradient can be expected to increase by 3 mm Hg.22

Microstream Technology:

Traditional high flow side-stream capnometers (150 ml.min-1) result in erroneous PETCO₂ measurements and distorted waveforms in neonates and infants with small tidal volumes and high respiratory rates as discussed above. In addition, high flow capnometers also likely to aspirate water and secretions into the sampling tubes resulting either erroneous PETCO₂ values or in total occlusion of sampling tube. To obviate these disadvantages, recently, Microstream capnometers such as NBP-75®, Nellcor Puritan Bennett, Plesanton, CA,USA), with an aspiration flow rate of 30 ml.min-1, have been developed. It uses a highly CO₂-specific infrared source where the IR emission exactly matches the absorption spectrum of the CO₂ molecules. This facilitates the sample cell to utilize a much smaller volume (15 µml) that permits a low flow rate without compromising response rate or accuracy. Low aspiration flow rates also minimize dispersion of gases in the sampling tube and prevent mixing of the small inspiratory and expiratory volumes observed in newborns. Rapid response time is preserved by laminar gas flow throughout the breathing circuit

End-tidal PCO₂ values obtained by this monitor provide a sufficiently accurate estimation of PaCO₂ in intubated mechanically ventilated as well as non-intubated, spontaneously breathing adults.23 Studies are underway to demonstrate the superiority of this technology over conventional technology in infants and children.

References:

1. Mcevady BAB, Mcleod ME, Mulera M, Kirpalani H, Lerman J. End-tidal, transcutaneous, and arterial PCO₂ measurements in critically ill neonates. A comparitive study. Anesthesiology 1988;69:112-16.

2. Mcevedy BAB, Mcleod ME, Kirpalani H, Volgyesi GA, Lerman J. End-tidal carbon dioxide measurements in critically ill neonates: a comparison of side-stream and main-stream capnometers. Can J Anaesth 1990;37:322-6.

3. Schena J, Thompson J, Crone RK. Mechanical influences on the capnogram. Crit Care Med 1984;12:672-4.

4. Rich GF, Sconzo JM. Continuous end-tidal CO₂ sampling within the Proximal endo-tracheal tube estimates arterial CO₂ tension in infants. Can J Anaesth 1991;38:201-3.

5. Badgewell JM, Mcleod ME, Lerman J, Creighton RE. End-tidal PCO₂ measurements sampled at the distal and proximal ends of the endotracheal tubes in infants and children. Anesth Analg 1987;66:959-64.

6. Badgwell JM, Heavner JE, May WS, Goldthorn JF, Lerman J. End-tidal PCO₂ monitoring in infants and children ventilated with either a partial rebreathing or a non rebreathing circuit. Anesthesiology 1987;66:405-10.

7. Pasucci RC, Schena JA, Thompson JE. Comparison of a sidestream and mainstream capnometers in infants. Crit Care Med 1989;17:560-2.

8. Badgewell JM, Kleinman SE, Heavner JE. Respiratory frequency and artifact affect the capnographic baseline in infants. Anesth Analg 1993;77:708-11.

9. Nuzzo PF. Capnography in infants and children. Pediatric Nursing 1978;May-June:30-48.

10. From RP, Scamman FL. Ventilatory frequency influences accuracy of end-tidal CO₂ measurements: analysis of seven capnometers. Anesth Analg 1988;67:884-6.

11. Bhavani-shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and anaesthesia. Review article. Can J Anaesth 1992;39:617-32.

12. Brenner JX, Westenskow DR. How the rise time of carbon dioxide analysers influence the accuracy of carbon dioxide measurements. Br J Anaesth 1988;61:628-38.

13. Mogue LR, Rantala B. Capnometer. J Clin Monit 1988;4:115-21.

14. Hillier SC, Badgwell JM, Mcleod ME, Creighton RE, Lerman J. Accuracy of end-tidal PCO₂ measurements using a sidestream capnometer in infants and children ventilated with the Sechrist infant ventilator. Can J Anaesth 1990;37:318-21.

15. Kirpalani H, Kechagias S, Lerman J. Technical and clinical aspects of capnography in neonates. Journal of Medical Engineering and Technolology 1991;15:154-61.

16. Epstein MF, Cohen AR, Feldman HA, Raemer DB. Estimation of PaCO₂ by two noninvasive methods in the critically ill newborn infant. Journal of Pediatrics 1985;106:282-6.

17. Hand IL, Sheperd EK, Krauss AN, Auld PAM. Discrepancies between transcutaneous and end-tidal carbon dioxide monitoring in the critically ill neonate with respiratory distress syndrome. Crit Care Med 1989;17:556-9.

18. Fletcher R. The relationship between the arterial and end-tidal PCO₂ difference and hemoglobin saturation in patients with congenital heart disease. Anesthesiology 1991;75:210-6.

19. Fletcher R. Invasive and noninvasive measurement of the respiratory deadspace in anesthetized children with cardiac disease. Anesth Analg 1988;67:442-7.

20. Fletcher R, Niklason L, Defeldt B. Gas exchange during controlled ventilation in children with normal and abnormal pulmonary circulation. Anesth Analg 1986;65:645-52.

21. Sivan Y, Eldadah MK, Cheah TE, Newth CJ. Estimation of arterial carbon dioxide by end-tidal and transcutaneous PCO₂ measurements in ventilated children. Pediatric Pulmonology 1992;12(3):153-7.

22. Fletcher R. The relationship between the arterial and end-tidal PCO₂ difference and hemoglobin saturation in patients with congenital heart disease. Anesthesiology 1991;75:210-6.

23. Casti A, Gallioli G, Scandroglio M, Passaretta R, Borghi B, Torri G. Accuracy of end-tidal carbon dioxide monitoring using the NBP-75 microstream capnometer. A study in intubated ventilated and spontaneously breathing nonintubated patients. European J Anesthesiology 2000;17:622-626.