Comparison of a sidestream and mainstream capnometer in infants

Closed-Loop ventilation using sidestream versus mainstream capnography for automated adjustments of minute ventilation-A randomized clinical trial in cardiac surgery patients

BACKGROUND

INTELLiVENT-Adaptive Support Ventilation (ASV) is a closed-loop ventilation mode that uses capnography to adjust tidal volume (VT) and respiratory rate according to a user-set end-tidal CO2 (etCO2) target range. We compared sidestream versus mainstream capnography with this ventilation mode with respect to the quality of breathing in patients after cardiac surgery.

METHODS

Single-center, single-blinded, non-inferiority, randomized clinical trial in adult patients scheduled for elective cardiac surgery that were expected to receive at least two hours of postoperative ventilation in the ICU. Patients were randomized 1:1 to closed-loop ventilation with sidestream or mainstream capnography. Each breath was classified into a zone based on the measured VT, maximum airway pressure, etCO2 and pulse oximetry. The primary outcome was the proportion of breaths spent in a predefined 'optimal' zone of ventilation during the first three hours of postoperative ventilation, with a non-inferiority margin for the difference in the proportions set at -20%. Secondary endpoints included the proportion of breaths in predefined 'acceptable' and 'critical' zones of ventilation, and the proportion of breaths with hypoxemia.

RESULTS

Of 80 randomized subjects, 78 were included in the intention-to-treat analysis. We could not confirm the non-inferiority of closed-loop ventilation using sidestream with respect to the proportion of breaths in the 'optimal' zone (mean ratio 0.87 [0.77 to ∞]; P = 0.116 for non-inferiority). The proportion of breaths with hypoxemia was higher in the sidestream capnography group versus the mainstream capnography group.

CONCLUSIONS

We could not confirm that INTELLiVENT-ASV using sidestream capnography is non-inferior to INTELLiVENT-ASV using mainstream capnography with respect to the quality of breathing in subjects receiving postoperative ventilation after cardiac surgery.

TRIAL REGISTRATION

NCT04599491 (clinicaltrials.gov).

Comparing the novel microstream and the traditional mainstream method of end-tidal CO2 monitoring with respect to PaCO2 as gold standard in intubated critically ill children

The objective of this study was to evaluate a novel microstream method by comparison with PaCO₂ and the more standard mainstream capnometer in intubated pediatric patients. We hypothesized that the novel microstream method would superior compared to the traditional mainstream method in predicting PaCO₂. This was a prospective single-center comparative study. The study was carried out on 174 subjects with a total of 1338 values for each method. Data were collected prospectively from mainstream and microstream capnometer simultaneously and compared with PaCO₂ results. Although both mainstream PetCO₂ (mainPetCO₂) and microstream PetCO₂ (microPetCO₂) were moderately correlated (r = 0.63 and r = 0.68, respectively) with PaCO₂ values, mainPetCO₂ was in better agreement with PaCO₂ in all subjects (bias ± precision values of 3.8 ± 8.9 and 7.3 ± 8.2 mmHg, respectively). In those with severe pulmonary disease, the mainPetCO₂ and microPetCO₂ methods were highly correlated with PaCO₂ (r = 0.80 and r = 0.81, respectively); however, the biases of both methods increased (14.8 ± 9.1 mmHg and 16.2 ± 9.0 mmHg, respectively). In cases with increased physiologic dead space ventilation, the agreement levels of mainPetCO₂ and microPetCO₂ methods became distorted (bias ± precision values of 20.9 ± 11.2 and 25.0 ± 11.8 mm Hg, respectively) even though mainPetCO₂ and microPetCO₂ were highly correlated (r = 0.78 and r = 0.78, respectively). It was found that the novel microstream capnometer method for PetCO₂ measurements provided no superiority to the traditional mainstream method. Both capnometer methods may be useful in predicting the trend of PaCO₂ due to significant correlations with the gold standard measurement in cases with severe pulmonary disease or increased physiological dead space –despite reduced accuracy.

Capnogram slope and ventilation dead space parameters: comparison of mainstream and sidestream techniques

BACKGROUND

Capnography may provide useful non-invasive bedside information concerning heterogeneity in lung ventilation, ventilation-perfusion mismatching and metabolic status. Although the capnogram may be recorded by mainstream and sidestream techniques, the capnogram indices furnished by these approaches have not previously been compared systematically.

METHODS

Simultaneous mainstream and sidestream time and volumetric capnography was performed in anaesthetized, mechanically ventilated patients undergoing elective heart surgery. Time capnography was used to assess the phase II (SII,T) and III slopes (SIII,T). The volumetric method was applied to estimate phase II (SII,V) and III slopes (SIII,V), together with the dead space values according to the Fowler (VDF), Bohr (VDB), and Enghoff (VDE) methods and the volume of CO2 eliminated per breath ([Formula: see text]). The partial pressure of end-tidal CO2 ([Formula: see text]) was registered.

RESULTS

Excellent correlation and good agreement were observed in SIII,T measured by the mainstream and sidestream techniques [ratio=1.05 (sem 0.16), R(2)=0.92, P<0.0001]. Although the sidestream technique significantly underestimated [Formula: see text] and overestimated SIII,V [1.32 (0.28), R(2)=0.93, P<0.0001], VDF, VDB, and VDE, the agreement between the mainstream and sidestream techniques in the difference between VDE and VDB, reflecting the intrapulmonary shunt, was excellent [0.97 (0.004), R(2)=0.92, P<0.0001]. The [Formula: see text] exhibited good correlation and mild differences between the mainstream and sidestream approaches [0.025 (0.005) kPa].

CONCLUSIONS

Sidestream capnography provides adequate quantitative bedside information about uneven alveolar emptying and ventilation-perfusion mismatching, because it allows reliable assessments of the phase III slope, [Formula: see text] and intrapulmonary shunt. Reliable measurement of volumetric parameters (phase II slope, dead spaces, and eliminated CO2 volumes) requires the application of a mainstream device.

Optimal management of apparatus dead space in the anesthetized infant

Mechanical ventilation of the anesthetized infant requires careful attention to equipment and ventilator settings to assure optimal gas exchange and minimize the potential for lung injury. Apparatus dead space, defined as dead space resulting from devices placed between the endotracheal tube and the Y-piece of the breathing circuit, is the primary source of dead space controlled by the clinician. Due to the small tidal volumes required by infants and neonates, it is easy to create excessive apparatus dead space resulting in unintended hypercarbia or increased minute ventilation in an effort to achieve a desirable PCO₂ . The goal of this review was to evaluate the apparatus that are commonly added to the breathing circuit during anesthesia care, and develop recommendations to guide the clinician in selecting apparatus that are best matched to the clinical goals and the patient's size. We include specific recommendations for apparatus that are best suited for different size pediatric patients, with a particular focus on patients <5 kg.

Quantitative end-tidal carbon dioxide monitoring in the delivery room: a randomized controlled trial

OBJECTIVES

To investigate the utility of continuous quantitative end-tidal CO2 (Etco2) monitoring for managing assisted ventilation in the delivery room (DR).

STUDY DESIGN

This is a prospective randomized controlled trial. Infants who received positive pressure ventilation (PPV) were randomized to a control versus a monitored arm. In the monitored arm, the resuscitating team adjusted PPV based on Etco2 values. In the control arm, the team provided PPV based on clinical assessment. Etco2 levels after resuscitation and admission Pco2 levels were compared between groups.

RESULTS

Fifty infants were enrolled, and 48 subjects had primary outcome data. Of those, 7 infants were intubated in the DR, 41 infants received PPV via mask until respiratory support could be maintained on continuous positive airway pressure alone. The median Etco2 levels at the end of resuscitation were 44 mm Hg (16-66 mm Hg) in the control arm and 43 mm Hg (29-59 mm Hg) in the monitored arm. The proportion of Etco2 levels outside of the prespecified range was 52.6% (control) and 33.3% (monitored) (P = .236). The median admission Pco2 levels were 57 mm Hg (36-110 mm Hg) in the control arm and 55 mm Hg (40-93 mm Hg) in the monitored arm. The proportion of admission Pco2 levels outside of the prespecified range was 33.3% (control) versus 37.5% (monitored) (P = .763).

CONCLUSIONS

Etco2 monitoring in the DR did not reduce the proportion of admission Pco2 levels outside of the prespecified range in a population of infants supported mostly with noninvasive ventilation.

Transcutaneous monitor approximates PaCO(2) but not PaO(2) in anesthetized rabbits

OBJECTIVE

To compare the accuracy of transcutaneous (tc) to arterial partial pressure of carbon dioxide (PaCO(2) ) and partial pressure of oxygen (PaO(2) ) in anesthetized rabbits.

STUDY DESIGN

Prospective, randomized, experimental study.

ANIMALS

Eight healthy adult female New Zealand white rabbits weighing 4.05± 0.30 kg.

METHODS

Isoflurane anesthetized rabbits received six treatments in random order; PaCO(2) <35, 35-45, and >45 mmHg and PaO(2)  < 80, 100-200, >200 mmHg. Arterial and transcutaneous measurements were taken after 15 minutes of stabilization at each condition. Linear regression, correlation and Bland-Altman analysis were performed to compare PtcCO(2) to PaCO(2) and PtcO(2) to PaO(2) .

RESULTS

Over a range of measured PaCO(2) values from 21 to 67 mmHg (n=24) mean bias for PtcCO(2) was -1 mmHg and the 95% limits of agreement were -7 to 5 mmHg. The correlation between PtcCO(2) and PaCO(2) was strong with R(2) value of 0.9454. Over the entire range of measured PaO(2) values (46-508 mmHg) mean bias for PtcO(2) was -61 mmHg and the 95% limits of agreement were -226 to 104 mmHg. Correlation was poor with R(2) =0.5969. Comparing PtcO(2) to PaO(2) over a narrower range [PaO(2) < 150 mmHg (n=13)] improved the correlation, with an R(2) value of 0.8518, mean bias of -7 mmHg and 95% limits of agreement from -33 to 19 mmHg.

CONCLUSIONS AND CLINICAL RELEVANCE

In healthy anesthetized rabbits, PtcCO(2) closely approximated PaCO(2) . In contrast PtcO(2) underestimated PaO(2) , particularly at high values. The PtcCO(2) sensor may be a useful noninvasive way to assess adequacy of ventilation in anesthetized rabbits.

The impact of shunt size on lung function in infants with univentricular heart physiology

OBJECTIVES

To assess the relationship that shunt size, blood gases, and radiologic findings has on respiratory function in infants with univentricular heart physiology.

SETTING

Cardiac catheter laboratory at Rikshospitalet University Hospital, Norway.

PATIENTS

Fifteen infants with univentricular heart physiology admitted for cardiac catheterization.

MEASUREMENTS

Lung function was measured by a fixed-orifice differential pressure flow sensor and mainstream volumetric capnography in 15 infants with univentricular heart arrangements during routine invasive assessment before the bidirectional cavopulmonary connection. Blood gases were measured from the indwelling catheters. Chest radiographs were assessed for heart size and pulmonary vasculature. Shunt size was assessed angiographically.

MAIN RESULTS

Respiratory compliance was reduced in patients with a large surgical systemic-pulmonary arterial shunt (r = -0.67, r = 0.45, p = 0.03). Respiratory resistance was higher with increased heart size (r = 0.72, r = 0.52, p = 0.004). There was no association between arterial and end-tidal CO2 values. The arterial to end-tidal CO2 difference had an inverse relationship with the pulmonary to systemic shunt ratio (r = -0.38, r = 0.14, p = 0.015).

CONCLUSION

A large surgical shunt size is related to stiffer lungs and a large heart is associated with a higher respiratory resistance. During mechanical ventilation of patients with univentricular heart physiology the end-tidal CO2 may be an unreliable substitute for arterial CO2 before the bidirectional cavopulmonary connection. We found a relationship between a decreased pulmonary to systemic shunt ratio and an increased arterial to end-tidal CO2 difference. This may indicate that a reason for the unreliability of end-tidal CO2 is an impaired gas exchange partially due to pulmonary hypoperfusion.

A novel method of distal end-tidal CO2 capnography in intubated infants: comparison with arterial CO2 and with proximal mainstream end-tidal CO2

OBJECTIVE

The objective of this study was to evaluate a novel method of distal end-tidal CO2 capnography by comparison with PaCO2 and with the more standard method that measures mainstream proximal end-tidal CO2 in intubated infants.

METHODS

Included in the study were all infants who were ventilated with conventional mechanical ventilation and intubated with a double-lumen endotracheal tube in our NICU during the study period. Data were collected prospectively from 2 capnographs simultaneously and compared with PaCO2. Sidestream distal end-tidal CO2 was measured by a Microstream capnograph via the extra port of a double-lumen endotracheal tube. Mainstream proximal end-tidal CO2 was measured via capnograph connected to the endotracheal tube.

RESULTS

Twenty-seven infants (median [range] birth-weight: 1835 [490-4790] g; gestational age: 32.5 [24.8-40.8] weeks) participated in the study. We used for analysis 222 and 212 measurements of distal end-tidal CO2 and proximal end-tidal CO2, respectively. Distal compared with proximal end-tidal CO2 had a better correlation with PaCO2 and a better agreement with PaCO2. The accuracy of distal end-tidal CO2 decreased, but it remained a useful measure of PaCO2 in the high range of PaCO2 (>or=60 mmHg) or in conditions of severe lung disease. A subanalysis for infants who weighed<1500 g (13 infants, 84 observations) revealed a good correlation and agreement between distal end-tidal CO2 and PaCO2 and poor correlation and agreement for proximal end-tidal CO2.

CONCLUSIONS

Distal end-tidal CO2 measured via a double-lumen endotracheal tube was found to have good correlation and agreement with PaCO2, remained reliable in conditions of severe lung disease, and was more accurate than the standard mainstream proximal end-tidal CO2.

Good estimation of arterial carbon dioxide by end-tidal carbon dioxide monitoring in the neonatal intensive care unit

End-tidal carbon dioxide pressure (PetCO(2)) was measured in the neonatal intensive care unit (NICU) to assess its reliability and accuracy in predicting arterial partial pressure of carbon dioxide (PaCO(2)). Arterial blood was drawn for gas analysis and compared with exhaled CO(2) measured by mainstream capnography. In total, 130 PetCO(2)/PaCO(2) comparisons were obtained from 61 patients (20 term and 41 preterm infants). PetCO(2) was significantly different from PaCO(2) (PetCO(2) = 42.3 +/- 10.5 mmHg vs. PaCO(2) = 45.8 +/- 12.3 mmHg, P < 0.001, mean +/- SD). The overall PetCO(2) bias (mean +/- SD) was 3.5 +/- 7.1 mmHg. There was a positive correlation between PetCO(2) and PaCO(2) (n = 130, r = 0.818, P < 0.001) in both term (n = 44, r = 0.779, P < 0.001) and preterm infants (n = 86, r = 0.849, P < 0.001). The PetCO(2) biases (95% CI) were 3.5 +/- 9.0 mmHg (0.8-6.2) in the term group and 3.4 +/- 6.0 mmHg (2.2-4.7) in the preterm group. Therefore, PetCO(2) was a valid and reliable method for monitoring PaCO(2) in neonates, especially preterm infants. This method decreases blood loss and prevents complications associated with arterial catheters. In conclusion, we recommend using mainstream capnography to monitor PetCO(2) instead of measuring PaCO(2) in the NICU.

Comparison of a sidestream capnograph and a mainstream capnograph in mechanically ventilated dogs

OBJECTIVE

To compare the ability of a sidestream capnograph and a mainstream capnograph to measure end-tidal CO2 (ETCO2) and provide accurate estimates of PaCO2 in mechanically ventilated dogs.

DESIGN

Randomized, double Latin square.

ANIMALS

6 healthy adult dogs.

PROCEDURE

Anesthesia was induced and neuromuscular blockade achieved by IV administration of pancuronium bromide. Mechanical ventilation was used to induce conditions of standard ventilation, hyperventilation, and hypoventilation. While tidal volume was held constant, changes in minute volume ventilation and PaCO2 were made by changing the respiratory rate. Arterial blood gas analysis was performed and ETCO2 measurements were obtained by use of either a mainstream or a sidestream capnographic analyzer.

RESULTS

A linear regression model and bias analysis were used to compare PaCO2 and ETCO2 measurements; ETCO2 measurements obtained by both capnographs correlated well with PaCO2. Compared with PaCO2, mainstream ETCO2 values differed by 3.15 +/- 4.89 mm Hg (mean bias +/- SD), whereas the bias observed with the sidestream ETCO2 system was significantly higher (5.65 +/- 5.57 mm Hg). Regardless of the device used to measure ETCO2, bias increased as PaCO2 exceeded 60 mm Hg.

CONCLUSIONS AND CLINICAL RELEVANCE

RelevancehAlthough the mainstream cas slightly more accurate, both methods of ETCO2 measurement correlated well with PaCO2 and reflected changes in the ventilatory status. However, ETCO2 values > 45 mm Hg may inaccurately reflect the severity of hypoventilation as PaCO2 may be underestimated during conditions of hypercapnia (PaCO2 > 60 mm Hg).

Capnography in spontaneously breathing preterm and term infants

OBJECTIVE

To investigate sidestream EtCO2 wave patterns as related to prematurity.

DESIGN

The EtCO2 wave pattern was analyzed longitudinally in 20 sequential preterm, 32-37 weeks of gestation and 39 fullterm controls. Infants with a cardiorespiratory disease, neurological deficit or a metabolic disorder were not included in the study. Sidestream EtCO2 was employed. Wave patterns were identified and baseline expiratory/inspiratory length and wave amplitude were measured.

RESULTS

Two predominant (about 75%) wave patterns were identified: (i) (with plateau) significantly more prevalent among infants born at term as compared with preterm infants across their postconceptional ages (PCA) (P=0.005-0.04), (ii) (plateau free) significantly more prevalent among the youngest preterm infants as compared with the fullterm controls. Expiratory length was significantly correlated with respiratory rate (RR) across ages (P=0.01-0.001) whereas inspiratory length was correlated with RR among the two youngest groups of infants only (P=0.002 and 0.004). Wave patterns were not found to be affected by environmental temperatures, blood pressure, body weight, haemoglobin level, aminophylline or O2 supplementation.

CONCLUSION

These findings suggest that EtCO2 wave pattern distribution among preterm infants is distinctly different from that of term controls, regardless of PCA, while inspiration is related to the degree of maturity. Alveolar pathology could probably be missed by sidestream capnography.

Carbon dioxide kinetics and capnography during critical care

Greater understanding of the pathophysiology of carbon dioxide kinetics during steady and nonsteady state should improve, we believe, clinical care during intensive care treatment. Capnography and the measurement of end-tidal partial pressure of carbon dioxide (PETCO2) will gradually be augmented by relatively new measurement methodology, including the volume of carbon dioxide exhaled per breath (VCO2,br) and average alveolar expired PCO2. Future directions include the study of oxygen kinetics.

End-tidal carbon dioxide for monitoring primary closure of gastroschisis

Previous criteria for primary reduction of the herniated viscera in newborn infants with gastroschisis included intraoperative respiratory rate, cardiac indices, degree of viscero-abdominal disproportion, size of defect, and lower extremity turgor. From 1976 through 1993, 129 neonates with gastroschisis were treated at Children's Hospital of Oklahoma. Intraoperative end-tidal carbon dioxide (ETCO2) monitoring was standard therapy beginning in 1985. The authors evaluated the effect of abdominal closure on ETCO2 to determine if there was a particular ETCO2 level at which closure was not feasible. There was no difference in overall mortality, birth weight, or postoperative ventilation requirements between children who had closure before 1985 (ie, without ETCO2 monitoring) and those who had repair after 1985. However, more cases in the 1985-1993 group had primary closure, and none of these required conversion to a staged procedure. An ETCO2 of > or = 50 suggests that primary closure may be unsafe. These data suggest that infants with gastroschisis can have primary closure based on intraoperative ETCO2 monitoring; no additional invasive monitoring would be necessary to assess closure.

Terminology and the current limitations of time capnography: a brief review

The carbon dioxide (CO2) trace versus time (time capnography) is convenient and adequate for clinical use. This is the method most commonly utilized in capnography. However, the current terminology in time capnography has not yet been standardized and is, therefore, a potential source of confusion. Standard terminology that is based on convention and logic to represent the various phases of a time capnogram is essential. The time capnogram should be considered as two segments: an inspiratory segment and an expiratory segment. The inspiratory segment is termed as phase ); the expiratory segment is divided into phases I, II, III, and, occasionally, IV. Phase I represents the CO2-free gas from the airways (anatomical dead space); phase II consists of a rapid S-shaped upswing on the tracing due to mixing of dead space gas with alveolar gas; and phase III, the alveolar plateau, represents CO2-rich gas from the alveoli. The physiologic basis of phase IV, the terminal upswing at the end of phase III, which is observed in capnograms recorded under certain circumstances (such as in pregnant subjects and obese subjects) is discussed in detail. The clinical implications of the alpha angle, which is the angle between phases II and III, and the beta angle, which is the angle between phases III and the descending limb of phase 0, are outlined. The subtle but important limitations of time capnography are reviewed; its current status as well as its future potential are explored.

The use of capnography for recognition of esophageal intubation in the neonatal intensive care unit

Failure to recognize esophageal intubation can result in severe hypoxia and permanent neurologic injury. Capnography is a standard monitoring modality in the operating room but has not been utilized fully in other environments. We used capnography at the time of endotracheal intubation in the neonatal intensive care unit (NICU) to determine whether capnography could more quickly and accurately identify endotracheal tube position than other clinical indicators of endotracheal tube position. One hundred intubation episodes were studied in 55 neonates. Capnograms were obtained 15 and 120 sec following tube placement. Intubating personnel were blinded to the capnographic data and determined endotracheal tube location (trachea vs. esophagus) by clinical criteria only. The sensitivity and specificity of capnography and clinical examination for identification of tube position were analyzed, and the time required for establishing by clinical confirmation whether the tube was in the trachea or not was compared to that required for capnography. Forty of 100 intubation attempts resulted in esophageal intubation. Capnography correctly identified these errant tube placements in 39 of 40 instances and did so in 1.6 sec (SD +/- 2.4). Capnography failed to identify successful endotracheal intubation on only one occasion. Clinical indicators of tube position required 97.1 sec (SD +/- 92.6) to identify an esophageal intubation and failed to identify successful endotracheal intubation in 5 of 60 cases. We conclude that capnography is a valuable adjunct to clinical examination to demonstrate whether an endotracheal tube is placed correctly in the trachea of neonates in the NICU.

Apparatus to measure the step and frequency responses of gas analysis instruments

The dynamic characteristics of gas analysers are often assessed by measuring the step response. It is difficult to generate a verifiable instantaneous step change in gas composition. We constructed a 0.06 ml measurement chamber connected via high-speed valves (0.5 ms response time) to two 31 reservoirs pressurized to 50 kPa with gases containing different concentrations of CO2. An electronic system opens the valves alternately depending on the polarity of a control voltage Vc. Two walls of the chamber contain narrow-band infra-red filters centered at 4.24 microns (50% transmission points at 4.16 and 4.32 microns) where CO2 absorption is high. A photoconductive infra-red sensor and an infra-red source are positioned on either side of the chamber. The output of the sensor is amplified by an instrumentation amplifier. Signal averaging of the sensor output in either the time or frequency domain was used to overcome the noise of the infra-red sensor. Step changes in Vc yielded exponentially changing outputs with a time constant of 1.1 ms. A quadrupole mass spectrometer's response to step changes in CO2 concentration generated in the measurement chamber fitted single exponential curves well with a maximum time constant of 37.7 ms and transport delay of 194 ms. The frequency response of the infra-red system, from Vc to the sensor output, fell by 0.7 dB with a phase lag of 30 degrees between 1 and 50 Hz. Using the infra-red system to measure the true input to the mass spectrometer, the frequency response of the mass spectrometer was found to fall by 35 dB with a phase lag of over 3000 degrees between 0.2 and 50 Hz. A first-order model with delay fitted to the step response predicted the mass spectrometer frequency response well below 10 Hz but overestimated the response above 10 Hz. A third-order model with delay fitted to the frequency response predicted the step response very well. Our results suggest that low-order models cannot predict the high-frequency performance of a mass spectrometer.

Capnometry and anaesthesia

In the last decade, capnography has developed from a research instrument into a monitoring device considered to be essential during anaesthesia to ensure patient safety. Hence, a comprehensive understanding of capnography has become mandatory for the anaesthetist in charge of patients in the operating room and in the intensive care unit. This review of capnography includes the methods available to determine carbon dioxide in expired air, and an analysis of the physiology of capnograms, which are followed by a description of the applications of capnography in clinical practice. The theoretical backgrounds of the effect of barometric pressure, water vapour, nitrous oxide and other factors introducing errors in the accuracy of CO2 determination by the infra-red technique, currently the most popular method in use, are detailed. Physiological factors leading to changes in end-tidal carbon dioxide are discussed together with the clinical uses of this measurement to assess pulmonary blood flow indirectly, carbon dioxide production and adequacy of alveolar ventilation. The importance of understanding the shape of the capnogram as well as end-tidal carbon dioxide measurements is emphasized and its use in the early diagnosis of adverse events such as circuit disconnections, oesophageal intubation, defective breathing systems and hypoventilation is highlighted. Finally, the precautions required in the use and interpretation of capnography are presented with the caveat that although no instrument will replace the continuous presence of the attentive physician, end-tidal carbon dioxide monitoring can be effective in the early detection of anaesthesia-related intraoperative accidents.

Technical and clinical aspects of capnography in neonates

This article reviews the current literature on the uses of capnometry and capnography as applied to neonates. The first part addresses the technical aspects and principles of the measurements, including definitions. The features of available carbon dioxide analysers are discussed and factors known to influence their accuracy are highlighted. In the second part of this paper, in vivo studies in neonates are reviewed, with particular emphasis on understanding why the accuracy of end-tidal CO2 measurements differs among studies. This is attributable to various factors: aspiration flow rate, the sampling site (whether distal or proximal) and the type of capnometer. The critical limitation of their overall accuracy in the presence of lung disease is discussed. Potential applications are considered, as are the current limitations of transcutaneous monitoring. We conclude that capnometry with capnography is a potentially useful tool to arterial CO2 tension (PaCO2) monitor infants with normal lungs.

End-tidal carbon dioxide measurements in critically ill neonates: a comparison of side-stream and mainstream capnometers

To determine whether end-tidal PCO2 (PETCO2) measurements obtained with two infrared capnometers accurately approximates the arterial PCO2 (PaCO2) in critically ill neonates, simultaneous measurements of PETCO2 were obtained from the distal and proximal ends of the tracheal tube with a sidestream capnometer (Puritan Bennett/Datex--BP/D) and from the proximal end with a mainstream capnometer (Hewlett-Packard-HP) in 20 intubated neonates. Distal sidestream PETCO2 and mainstream PETCO2 correlated with the PaCO2 (r2 = 0.66 and 0.61, respectively) within the range of 26-57 mmHg PaCO2. However, proximal PETCO2 with the sidestream capnometer correlated very poorly (r2 = 0.09) with PaCO2. The slope of the least square regression line for the distal sidestream capnometer, 0.67, was significantly less than that for the mainstream capnometer, 0.78 but both were significantly greater than that for the proximal sidestream capnometer, 0.39 (P less than 0.05). The slope of the regression for the proximal sidestream capnometer did not differ significantly from horizontal. Insertion of the mainstream sensor for the HP capnometer significantly increased the transcutaneous CO2 when compared with preinsertion values. We conclude that both distal sidestream and mainstream capnometry provide accurate estimates of the PaCO2 in critically ill neonates.