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Pediatricians and Emergency Physicians


Capnography in Pediatrics

Bhavani Shankar Kodali MD

For physics and physiology of capnography, refer to appropriate sections.

In the last two decades, measurement of carbon dioxide (CO2) in the expired air (capnography) has become increasingly popular in the operating room to monitor patients during anesthesia.1 Its use is strongly recommended in every patient requiring endotracheal intubation,2,3 because capnography can instantaneously identify potentially life threatening conditions such as failed intubation, failed ventilation, failed circulation, and failed circuits before irreversible damage is done to the patients.1,4,5 While the anesthesiologists have appreciated the value of capnography in the last decade, physicians in other specialties are beginning to appreciate its value as a reliable diagnostic, and monitoring aid.6,7

Pediatric critical care medicine has matured dramatically over the last two decades. The American Academy of Pediatrics (Committee on hospital care, 1991-92) has issued 'minimum guidelines and levels of care' required for pediatric intensive care units as a means of ensuring proper patient care and professional creditability.8 The guidelines require CO2 and oxygen (O2) monitoring be performed on all patients receiving care in level I and II pediatric critical care centers. In the past, the only method to quantify the adequacy of ventilation and oxygenation was by assessment of arterial blood gas (ABG). While ABG's remain the gold standard, limitations also exist. ABG's require either a painful, time consuming procedure or an invasive arterial line to obtain a specimen for evaluation. Further, ABG's have inherent fallacies such as the amount of heparin, the amount of time before analysis, and hyperventilation due to pain or breath holding if the child cries during percutaneous sampling. ABG's provide intermittent, not continuous data, which limits its use in documenting transient events. Therefore noninvasive monitors such as pulse oximetry to determine oxygenation, and transcutaneous CO2 (PctCO2) monitoring and end-tidal CO2 monitoring (capnography) to monitor the CO2 status of critically ill infants and children have become increasingly popular.9 They have lessened the need for invasive monitoring with indwelling catheters, with the subsequent reduction in complications due to transfusions, infections and vascular events. However, unlike pulse oximetry sensors, the heated electrodes of transcutaneous CO2 sensors are associated with complications such as burns in the neonate, damages to the skin by adhesive, excessive drift of electrodes, erratic behavior in the presence of acidosis, long calibration and stabilization intervals and the need to change the sensor every 2-4 h.10-14 These effects are more pronounced in infants with decreasing gestational age. In contrast, capnography is not associated with any deleterious effects. It provides continuous surveillance of arterial CO2 tensions and provides information which is not obtainable from ABG's or PtCO2's alone. For example, it serves as a reliable and instantaneous apnoea monitor. It also generates valuable information regarding the mechanical and gas exchanging functions of the child's lungs. Further, in conjunction with ABG analysis, capnography can provide information about ventilation/perfusion (V/Q) disturbances in the lung. Therefore, the value of capnography has recently been recognized in pediatric care, and its use is progressively increasing in pediatric and neonatal intensive care units,15-23 However, the potential diagnostic and therapeutic abilities of capnography are not often recognized, and thus capnography is not used as often as would seem indicated.18 This may be due to two reasons. First, capnography in pediatric patients is viewed skeptically due to limitations in obtaining accurate measurements of CO2 in expired air. In the past five years, though, newer techniques were developed that allowed the accurate measurement of CO2 in expired air even in neonates.16 One such example is the development of Microstream technology which could measure expiratory CO2 accurately in infants and children.24,25 Secondly, the information on capnography in pediatric practice is scattered in the pediatric literature, and a comprehensive paper describing applications and limitations of capnography in pediatric practice is currently lacking. The objective of this web based presentation is to provide a brief background of physiology of CO2 monitoring in expired air, enumerate its potential benefits in healthy children as well as a children with cardiopulmonary abnormalities at various levels of pediatric care, and consider the current limitations of capnography as a diagnostic and monitoring tool as applicable to infants and children.

The clinical usefulness of capnography in pediatric practice is best recognized if its usefulness is considered at various clinical circumstances encountered at different levels of care such as at community hospitals (secondary care centers), during transport from community hospitals to tertiary care centers and at the tertiary care centers.

Community hospitals: There are four important uses of capnography in secondary care pediatric centers:

1. Noninvasive predictor of PaCO2:

By far the most important use of capnography is its noninvasive ability to provide an instantaneous clue to the level of CO2 in the arterial blood. In infants and children breathing spontaneously, the PETCO2 values range from 36-40 mmHg.9 Normally PETCO2, as sampled from the nasal cavity in neonates, infants and children with healthy lungs breathing spontaneously is a good estimate of PaCO2.22,23 In infants and children the (a-ET)PCO2 gradient can vary from - 0.65 mmHg to 2.4 mmHg.22 In preterm infants the gradient may be 3.5 mmHg.22 Alveolar hypoventilation increases PaCO2 as well as PETCO2. Therefore PETCO2 monitoring serves as a important non-invasive monitor of PaCO2 and avoids repeated ABG's. Several factors though, affect the (a-ET)PCO2 difference and the predictability of PaCO2 from PETCO2 particularly in neonates and infants. This is highlighted in the next section. Hence, it is prudent that abnormal PETCO2 values should be confirmed by an immediate ABG.


2. Apnea monitor:

Apnea is a serious threat to the health of infants. it is defined as the cessation of respiration originating from the central nervous system or obstruction of the airway.19 Transthoracic impedance pneumography, which is generally used to monitor chest wall movements, can only detect central apnea and shows false-positive responses in the presence of shallow breathing. Further, impedance devices do not detect the presence of obstructive apnea as chest wall movements persist during obstructive apnea. Detection of obstructive apnea necessitates the evaluation of oral-nasal airflow. It is estimated that up to 60% of observed apnea in preterm infants may be obstructive.19 Accurate information about the rate and rhythm of respiration can be obtained by sampling CO2 from respired gases using nasal adaptors. During apnea of either type, the CO2 concentration at the sampling site falls rapidly and can be instantaneously detected by capnography. Therefore CO2 monitoring serves as a reliable apnea monitor in neonates, infants and children. Further, capnography can be used as reliable monitor to detect sleep apnea syndromes.26

3.Airway Obstruction:

In severe airway obstruction such as bronchial asthma and laryngotracheobronchitis, the shape of the capnogram can be altered, with a prolongation or slanting of phase II and increased slope of phase III, the expiratory plateau. With adequate treatment, the capnogram reverts to normal.

Bronchial asthma After treatment
bronch
nromalbroncho

Therefore the effectiveness of bronchodilator treatments in a child with asthma, and racemic epinephrine treatments in the child with stridor can be assessed from the capnography.18

4. (a-ET)PCO2 gradient as an indicator of pulmonary disease:

As stated in the physiology section, the (a-ET)PCO2 is an indicator of V/Q mismatching resulting from pulmonary disease. In neonates with respiratory disease, the (a-ET)PCO2 difference becomes wider, as for example, in infants with bronchopulmonary dysplasia, where the gradient may be as much as 9mm Hg.22

5. Prehospital stabilization before transport to tertiary care centers.

Critically ill children often require endotracheal intubation before interhospital transportation. Unrecognized esophageal intubation may be catastrophic and can occur even in the hands of the most experienced personnel.20 When used with the standard techniques of chest asculstation, CO2 monitoring is probably the best way to detect esophageal intubation and displacement of endotracheal tube at a later stage. X-rays can only confirm the ETT placement at the time of the X-ray. The tube can be displaced at anytime following the X ray. Although CO2 may be present in the stomach, it is rapidly flushed out during ventilation of the stomach and PETCO2 would decrease, resulting in a flat capnogram. Recently, PETCO2 detectors, which change color on exposure to 4% CO2 have been used successfully to confirm ETT placement in children.27-29

During transport from secondary to tertiary care centers:

Because of the nature of transport, inadvertent extubation may occur at any point enroute. The noisy environments of the ambulance or helicopter makes evaluation of ETT position difficult. Continuous use of portable CO2 monitors during transport would provide an effective visual check of ETT position and effectively reassure team members. Further, it indirectly confirms ventilation and circulation. Although oximetry would give an indication of hypoxia, presence of CO2 confirms ETT position and would assist in the differential diagnosis.20

At the tertiary care pediatric centers:

In addition to the applications stated above, capnography plays a significant role in the ventilatory management of neonates and children in intensive care units at tertiary care centers.

1.Non-invasive monitor of adequacy of mechanical ventilation:

Capnography is not only a reliable non-invasive monitor to predict PaCO2 in awake infants and children who are breathing spontaneously, but it also serves a as useful device to monitor PaCO2 during mechanical ventilation of intubated children in intensive care units. In intubated neonates and infants with normal respiratory and cardiovascular physiology, PETCO2 values approximate PaCO2 values. In older children though, PETCO2 values are lower than PaCO2 by 2-5 mmHg.29-34 Changes in PETCO2 can often be regarded as indicative of changes in PaCO2. Several factors effect the relationship of PETCO2 and PaCO2 which are discussed in the physiology section. It is prudent to establish the relationship of PETCO2 to PaCO2 initially by blood gas analysis. Thereafter, changes in PaCO2 may be assumed to occur in parallel with those in PETCO2 thus avoiding repeated ABG's. During mechanical ventilation, children are frequently repositioned in their cribs or beds. These positional changes can greatly affect the delivered tidal volume. These alterations in tidal volume result in changes in PETCO2 which can be detected with capnography thus enabling the physician to institute corrective measures.

2. Integrity of Ventilation:

Capnography can identify disconnections in the ventilatory circuit instantaneously before O2 and CO2 levels change in the blood. During the course of IPPV in children with no spontaneous breathing, PETCO2 falls to zero instantaneously following the disconnections in the circuit and sounds an alarm. Corrective measures can be instituted immediately before irreversible damage is caused by prolonged hypoxia. However, in children breathing spontaneously, circuit disconnections distal to CO2 sampling site (towards the child) can be identified instantaneously as CO2 concentration falls to zero in the sampling adaptor, whereas, circuit disconnection proximal to the sampling site may not be detected instantaneously as PETCO2 values depends on the adequacy of spontaneous breathing. If spontaneous breathing is adequate, the PETCO2 values remains normal, whereas, PETCO2 values may rise gradually if spontaneous breathing is inadequate, and thereby alert the physician. Capnography is also useful in giving an early warning of CO2 retention caused by faulty ventilators and misconnections.

3. Occlusion and displacement of endotracheal tube:

In addition to the value of end-tidal CO2 monitoring to confirm the endotracheal tube placement in the trachea, capnography can detect a total occlusion or accidental extubation. Total occlusion or displacement of ETT produces loss of CO2 waveform in capnography. Ventilation through partially kinked or obstructed tube produces distortions in CO2 waveform (prolonged phase II and steeper phase III, and irregular height of the CO2 tracings.35 This would enable capnography to identify a partially kinked or obstructed tube. Therefore, capnography is a useful monitor to detect accidental kinking or displacement of ETT during positioning the child while bathing, cleaning or changing bed covers. In a busy ICU, the need for suctioning of children who are on ventilators is not always noticed immediately. The high pressure alarm often alerts the nurse to a problem. With continuous CO2 monitoring, the need for suctioning can usually be detected before high pressure alarm is activated as partial obstruction of ETT results in inadequate ventilation and CO2 waveform distortions.35

4. Weaning:

Capnography along with pulse oximetry can be used to monitor alveolar ventilation during weaning from mechanical ventilation. Using ABG's alone may not be an adequate guide to decide on weaning because occasionally pain from blood sampling will cause patients to hyperventilate and thus decrease the PCO2 level in the blood, which may not be a true indicator of patients ventilatory status. Capnography can be used to evaluate the trend of PaCO2, breathing pattern, and importantly the consistency of breathing before extubation.36 Ventilator rates can be gradually decreased to the lowest point at which the patient can comfortably breathe and maintain adequate alveolar ventilation. If the child becomes distressed and increases the work of breathing, or PETCO2 rises, ventilatory rates can be returned to the previously acceptable settings. Evaluation of CO2 waveforms produced by spontaneous ventilation during weaning gives information about the depth and consistency of spontaneous ventilation. The stability of PETCO2 and increasing similarity of capnograms between the patient and the ventilator breaths indicate the patient's readiness to wean from the mechanical ventilation.18

5.PETCO2 as a non-invasive monitor of pulmonary blood flow.

A reduction in the cardiac output (pulmonary blood flow) produces high V/Q ratio (increased alveolar dead space) resulting in lower PETCO2 and an increased (a-ET)PCO2 gradients. As pulmonary blood flow increases, thereby improving V/Q ratio, the PETCO2 increases and (a-ET)PCO2 gradient becomes small. Thus PETCO2 is a function of cardiac output for a given ventilation.37 Hence PETCO2 is a noninvasive monitor of pulmonary blood flow. Utilizing this principle, PETCO2 monitoring can be used to monitor the effectiveness of cardiopulmonary resuscitation. During cardiac arrest, circulation ceases and PETCO2 gradually disappears, reappearing only when circulation is restored either by effective cardiopulmonary resuscitation or cardiac function. PETCO2 monitoring enables the physician to change the technique of pediatric advanced cardiac life support to produce effective pulmonary circulation.38-40 Further, the PETCO2 monitoring may have a prognostic significance. It has been observed that non-survivors had lower PETCO2 than survivors and no patient with PETCO2 < 10 mmHg could be successfully resuscitated.41

6. CO2 production:

Under normal respiratory conditions, changes in CO2 production are usually accompanied by changes in minute ventilation. Thus PETCO2 levels should remain constant If a child is unable to alter minute ventilation sufficiently, increased CO2 production will be manifested by an increase in PETCO2 whereas decrease CO2 production will be manifested by decreased PETCO2. Malignant hyperpyrexia, sepsis, thyrotoxicosis, seizures, shivering, bicarbonate injection and parenteral nutrition increases CO2 production and increase PETCO2, whereas, hypothermia leads to a decrease in PETCO2. Hence, increasing PETCO2 may, therefore, be an early warning sign of an impending hypermetabolic crisis.42


7. Monitoring the course of Pulmonary Disease:

Progress of pulmonary disease can be monitored by improved oxygenation with reduced oxygen requirements, X-ray improvement of lung fields and by serial (a-ET)PCO2. As the pulmonary disease improves (eg., resolving RDS) the initial wider (a-ET)PCO2 gradient is progressively reduced due to improvements in V/Q status of the lung. The (a-ET)PCO2 gradient has been used to assess the effectiveness of diuretic therapy in the improvement in V/Q status of the lung in infants with chronic lung disease.43 The gradient may also be used to assess the improvement in lung function following surfactant therapy in newborns with RDS. Therefore serial (a-ET)PCO2 gradient can be used as a trend monitor to assess the progress of pulmonary disease.44 Further the shape of capnogram also gives information about V/Q status of the lung. Increased V/Q mismatch is suggested by an increase in the slope of phase III. In the presence of abnormal V/Q ratios in lungs, for example during bronchospasm, the emptying pattern of alveoli with various time constants produces a characteristic capnogram with prolonged phase II and a steeper phase III. As bronchospasm improves with therapy, the capnogram reverts to normal as V/Q ratios normalize.18,36

References:

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

2 Standards for basic intraoperative monitoring. In: American Society of Anesthesiologists 1993 Directory of Members. Park Ridge, IL: American Society of Anesthesiologists, pp 709-10.

3 Guidelines to the practice of anaesthesia as recommended by the Canadian Anaesthetist's Society.Toronto, 1989.

4 Weingarten M. Prioritization of monitors for the detection of mishaps. Seminars in Anesthesia 1989;3:1-12

5 Eichhorn JH. Prevention of intraoperative anesthesia accidents and related severe injury through safety monitoring. Anesthesiology 1989;70:572-7.

6 Chopin C, Fesarad P, Mangalaboyi J, et al. Use of capnography in diagnosis of pulmonary embolism during acute respiratory failure of chronic obstructive pulmonary disease. Crit Care Med 1990;18:353-7.

7 Chambers JB, Kiff PJ, Gardner WN, Jackson G, Bass C. Value of measuring end-tidal partial pressure of carbon dioxide as an adjunct to treadmill exercise testing.BMJ 1988;296:1281-5.

8 Guidelines and levels of care for pediatric intensive care units (American academy of pediatrics: Committee on hospital care). Critical Care Medicine 1993;7:1077-86.

9. Curley MAQ, Thompson JE. End-tidal CO2 monitoring in critically ill infants and children. Pediatric Nursing 1990; July-August:16:397-403.

10 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.

11 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.

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

13 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.

14 Phan CQ, Tremper KK, Lee SE, Barker SJ. Noninvasive monitoring of carbon dioxide: A comparison of the partial pressure of transcutaneous and end-tidal carbon dioxide with the partial pressure of arterial carbon dioxide. J Clin Monit 1987;3:149-54.

15 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.

16 Badgwell JM. Respriatory gas monitoring in the pediatric patient. In: International Anesthesiology clinics 1992;30;131-46.

17 Nobel JL. Carbon dioxide monitors: Exhaled gas (capnographs, capnometers, end-tidal CO2 monitors). Pediatric Emergency Care 1993;9:244-6.

18 Nuzzo PF, Anton WR. Practical applications of capnography. Respiratory Therapy 1986;Nov/Dec:12-17.

19 Toubas PL, Duke JC, Sekar KC, McCaffree MA. Microphonic versus end-tidal carbon dioxide nasal airflow detection in neonates with apnoea. Pediatrics 1990;6:950-4.

20 Bhende MS, Thompson AE, Orr RA. Utility of end-tidal carbon dioxide detector during stabilization and transport of critically ill children. Pediatrics 1992;6;1042-4.

21 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.

22 Dumpit FEM, Brady JP. A simple technique for measuring alveolar CO2 in infants. J Appl Physiol 1978;45:648-50.

23 Meredith KS, Monaco FJ. Evaluation of a mainstream capnometer and end-tidal carbon dioxide monitoring in mechanically ventilated infants. Pediatric Pulmonology 1990;9:254-9.

24 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.

25. Colman Y, Krauss B. Microstream Capnography Technology: A New Approach to an Old Problem. Journal of Clinical Monitoring 1999;15:403-409.

26. Magnan A, Philip-Joet F, Rey M, Reynaud M, Porri F, Arnaud A. End-tidal CO2 analysis in sleep apnea syndromes. Conditions for use. Chest 1993;103:129-31.

27. Kelly JS, Wilhoit RD, Brown RE, James R. Efficacy of the FEF colorimetric end-tidal carbon dioxide detector in children. Anesth Analg 1992;75(1):45-50.

28. Higgins D, Forrest ET, Lloyd-Thomas A. Colorimetric end-tidal carbon dioxide monitoring during transfer of intubated children. Intensive Care Medicine 1991;17:63-4.

29. Bhende MS, Thompson AE, Cook DR, Saville AL. The validity of a disposable end-tidal CO2 detector in verifying endotracheal tube placement in infants and children. Annals of Emergency Medicine 1992;21(2):142-5.

30. Burrows FA. Physiologic deadspace, venous admixture, and the arterial to end-tidal carbon dioxide difference in infants and children undergoing cardiac surgery. Anesthesiology 1989;70:219-25.

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

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

33 Stokes MA, Hughes OG, Hutton P. Capnography in small subjects. Br J Anaesth 1986;58:814P.

34 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.

35. Cote CJ, Liu LMP, Szyfelbein SK, et al. Intraoperative events diagnosed by expired carbon dioxide monitoring in children. Can Aaesth Soc J 1986;33:315-20.

36. Nuzzo PF. Capnography in infants and children. Pediatric Nursing 1978;May-June:30-8.

37. Leigh MD, Jones JC, Mottley HL. The expired carbon dioxide as continuous guide of the pulmonary and circulatory systems during anaesthesia and surgery. J Thoracic and Cardiovasc Surg 1961;41:597-610.

38. Falk JL, Rackow EC, Weil MH. End-tidal carbon dioxide concentration during cardiopulmonary resuscitation. N Engl J Med 1988;318:607-11.

39 Treveno RP, Bisera J, Weil MH, Rackow EC, Grundler WG. End-tidal CO2 as a guide to successful cardiopulmonary resuscitation. A preliminary report. Crit Care Med 1985;13:910-11.

40. Weil MH, Besera J, Trevino RP, Rackow EC. Cardiac output and end-tidal carbon dioxide. Crit Care Med 1985;13:907-9.

41. Sanders AB, Kern KB, Otto CW, Milander MM, Ewy GA. End-tidal carbon dioxide monitoring during cardiopulmonary resuscitation. A prognostic indicator of survival. JAMA 1989;262:1347-51.

42. Baudendistel L, Goudsouzian N, Cote C, Strafford M. End-tidal CO2 monitoring: its use in the diagnosis and management of malignant hyperpyrexia. Anaesthesia 1984;39:100-3.

43. McCann EM, Lewis K, Deming DD, Donovan MJ, Brady JP. Controlled trial of furosemide therapy in infants with chronic lung disease. The Journal of Pediatrics 1985;106:957-62.

44. Meny RG, Bhat AM, Heavner JE, May WS, Goldthorn JF, Lerman J. Mass spirometer monitoring of expired carbon dioxide in critically ill neonates. Crit Care Med 1985;13:1064-6.



Proceed to next section for a review of various factors affecting capnography in infants and children

figure


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Capnograms recorded in infants and children using side-stream capnographs. Distortion in CO2 waveforms as a result of dispersion of gases in the sampling line. Phase II and the descending limb can be prolonged. Base line is elevated and phase III can have increased slope.

References

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

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

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

Anesthesoiologists

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.

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