Capnography in Pediatrics
Bhavani Shankar Kodali MD
CAPNOGRAPHY IN INFANTS AND SMALL CHILDREN
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;
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.
Relatively higher respiratory rates and smaller tidal volumes encountered in children; Low volumes of CO2 produced in neonates and infants.
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
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.
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