Sidestream Capnometer Research Articles

Technical validation of the EMMA capnometer under hyperbaric conditions.

End-tidal carbon dioxide (ETCO2) monitoring is essential for monitoring intubated critical care patients, yet its use in hyperbaric environments can be problematic. We postulated that the EMMA mainstream capnometer may function accurately under hyperbaric conditions. Stage 1. The EMMA mainstream capnometer was tested at 101 kPa against a reference side-stream capnometer, Philips IntelliVue M3015B microstream, using 10 customised reference gases of various carbon dioxide (CO₂) concentrations (2.47%-8.09%, or 18.5-60.7 mmHg at 101 kPa) in either air or oxygen. Stage 2. The functionality and accuracy of the EMMA capnometer was tested under hyperbaric conditions, 121-281 kPa, using the same test gases. At 101 kPa, the EMMA capnometer measured CO₂ at levels lower than expected (mean of differences = -2.5 mmHg (95% CI -2.1 to -2.9, P < 0.001)). The Philips capnometer measured CO₂ more closely to expected CO₂ (mean of differences = -1.1 mmHg (95% CI -0.69 to -1.4, P < 0.001). Both devices demonstrated a significant linear relationship with expected CO₂. The EMMA capnometer functioned up to the maximum test pressure (281 kPa). The device over-read CO₂ measurements at pressures > 141 kPa. Although variance increased at pressures in the therapeutic range for hyperbaric treatments, a significant linear relationship between expected and EMMA measured CO₂ was demonstrated. The EMMA capnometer tolerated pressures to 281 kPa, but its display was limited to CO₂ < 99 mmHg. This study validated EMMA capnometer function to 281 kPa in the hyperbaric environment. The device over-read CO₂ measurements at pressures >141 kPa, however there was a linear relationship between expected and measured CO₂. The EMMA capnometer may be clinically useful for monitoring expired CO₂ in patients undergoing hyperbaric oxygen treatment.

Comparison of mainstream (Capnostat 5) and a low-flow sidestream capnometer (Capnostream) in mechanically-ventilated rabbits anesthetized with a Bain coaxial breathing system.

Introduction: Measurement of end-tidal carbon dioxide (PE'CO2) provides continuous non-invasive estimation of arterial carbon dioxide partial pressure (PaCO2). Agreement can be affected by the high respiratory rates and shallow tidal volumes of rabbits, especially when anesthetizing with a non-rebreathing system and high fresh gas flows. Capnometer function may be improved by increasing tidal volume and decreasing respiratory rate with mechanical lung ventilation.

Comparison between mainstream (Capnostat 5) and a low-flow sidestream capnometer (Capnostream) in mechanically ventilated, sevoflurane-anesthetized rabbits using a Bain coaxial delivery system

ObjectiveTo evaluate agreement between end-tidal carbon dioxide (Pe′CO2) and PaCO2 with sidestream and mainstream capnometers in mechanically ventilated anesthetized rabbits, with two ventilatory strategies. Study designProspective experimental study. AnimalsA total of 10 New Zealand White rabbits weighing 3.6 ± 0.3 kg (mean ± standard deviation). MethodsRabbits anesthetized with sevoflurane were intubated with an uncuffed endotracheal tube (3.0 mm internal diameter) and adequate seal. For Pe′CO2, the sidestream capnometer sampling adapter or the mainstream capnometer was placed between the endotracheal tube and Bain breathing system (1.5 L minute–1 oxygen). PaCO2 was obtained from arterial blood collected every 5 minutes. A time-cycled ventilator delivered an inspiratory time of 1 second and 12 or 20 breaths minute–1. Peak inspiratory pressure was initially set to achieve Pe′CO2 normocapnia of 35–45 mmHg (4.6–6.0 kPa). A total of five paired Pe′CO2 and PaCO2 measurements were obtained with each ventilation mode for each capnometer. Anesthetic episodes were separated by 7 days. Agreement was assessed using Bland-Altman analysis and linear mixed models; p < 0.05. ResultsThere were 90 and 83 pairs for the mainstream and sidestream capnometers, respectively. The mainstream capnometer underestimated PaCO2 by 12.6 ± 2.9 mmHg (proportional bias 0.44 ± 0.06 mmHg per 1 mmHg PaCO2 increase). With the sidestream capnometer, ventilation mode had a significant effect on Pe′CO2. At 12 breaths minute–1, Pe′CO2 underestimated PaCO2 by 23.9 ± 8.2 mmHg (proportional bias: 0.81 ± 0.18 mmHg per 1 mmHg PaCO2 increase). At 20 breaths minute–1, Pe′CO2 underestimated PaCO2 by 38.8 ± 5.0 mmHg (proportional bias 1.13 ± 0.10 mmHg per 1 mmHg PaCO2 increase). Conclusions and clinical relevanceBoth capnometers underestimated PaCO2. The sidestream capnometer underestimated PaCO2 more than the mainstream capnometer, and was affected by ventilation mode.

Monitoring of carbon dioxide in ventilated neonates: a prospective observational study

ObjectiveTo assess the reliability, accuracy and precision of distal end-tidal capnography (detCO2) in neonates compared with transcutaneous (tcCO2) carbon dioxide measurements.DesignObservational, prospective clinical study.SettingNeonatal intensive care unit at Medical University...

Chest compressions induce errors in end-tidal carbon dioxide measurement

BackgroundReal-time measurement of end-tidal carbon dioxide (ETCO2) is used as a non-invasive estimate of cardiac output and perfusion during cardiopulmonary resuscitation (CPR). However, capnograms are often distorted by chest compressions (CCs) and this may affect ETCO2 measurement. The aim of the study was to quantify the effect of CC-artefact on the accuracy of ETCO2 measurements obtained during out-of-hospital manual CPR. MethodsWe retrospectively analysed monitor-defibrillator recordings collected by two advanced life support agencies during out-of-hospital cardiac arrest. These two agencies, represented as A and B used different side-stream capnometers and monitor-defibrillators. One-minute capnogram segments were reviewed. Each ventilation within each segment was identified using the transthoracic impedance signal and the capnogram. ETCO2 values per ventilation were manually annotated and compared to the corresponding capnometry values stored in the monitor-defibrillator. Ventilations were classified as distorted or non-distorted by CC-artefact. ResultsA total of 407 1-min capnogram segments from 65 patients were analysed. Overall, 4095 ventilations were annotated, 2170 (32.4% distorted) and 1925 (31.8% distorted) for agency A and B, respectively. Median (IQR) unsigned error in ETCO2 measurement increased from 1.5 (0.6–3.1)% for non-distorted to 5.5 (1.8–14.1)% for distorted ventilations; from 0.7 (0.3–1.2)% to 3.7 (1.0–9.9)% in agency A and from 2.3 (1.2–3.9)% to 8.3 (3.9–19.5)% in agency B (p < 0.001). Errors were higher than 10 mmHg in 9% and higher than 15 mmHg in 5% of the distorted ventilations. ConclusionCC-artefact causes ETCO2 measurement errors in the two studied devices. This suggests that capnometer algorithms may need to be adapted to reliably perform in the presence of CC-artefact during CPR.

Comparison of mainstream (Capnostat 5) and two low-flow sidestream capnometers (VM-2500-S and Capnostream) in spontaneously breathing rabbits anesthetized with a Bain coaxial breathing system

ObjectiveTo evaluate agreement with PaCO2 of two low sampling rate sidestream capnometers and a mainstream capnometer in rabbits and the effect of using high fresh gas flow from a Bain coaxial breathing system. Study designProspective, crossover study. AnimalsA total of 10 New Zealand White rabbits weighing 3.4 ± 0.3 kg [mean ± standard deviation (SD)]. MethodsTwo sidestream analyzers (Viamed VM-2500-S and Capnostream 35) with a sampling rate of 50 mL minute–1 and a mainstream capnometer (Capnostat 5) were tested. All capnometers used infrared spectroscopy and advanced microprocessor technology. Rabbits were anesthetized and intubated with noncuffed endotracheal tubes of 3 mm internal diameter and adequate seal. A sidestream sampling adapter or the mainstream capnometer was attached to the endotracheal tube and connected to a Bain coaxial breathing system. Oxygen (1.5 L minute–1) delivered sevoflurane to maintain anesthesia. An auricular artery catheter allowed blood sampling for PaCO2 analysis corrected to rectal temperature. Inspired and end-tidal carbon dioxide (Pe′CO2) measurements were recorded during blood sample withdrawal. From each rabbit, 10 paired PaCO2/Pe′CO2 measurements were obtained. Each rabbit was recovered from anesthesia and was anesthetized again with an alternate capnometer after 1 week. Data were analyzed using Bland–Altman and two-way anova for repeated measures. ResultsAnalysis included 100 paired samples. Negative bias reflects underestimation of PaCO2. Bland–Altman mean (±1.95 SD) was –16.7 (–35.2 to 1.8) mmHg for Capnostat 5, –27.9 (–48.6 to –7.2) mmHg for Viamed, and –18.1 (–34.3 to –1.9) mmHg for Capnostream. Viamed PaCO2–Pe′CO2 gradient was greater than other two capnometers. ConclusionsAll three capnometers underestimated PaCO2. Capnostat 5 and Capnostream performed similarly. Clinical relevanceThese capnometers underestimated PaCO2 in spontaneously breathing rabbits anesthetized using a Bain coaxial breathing system with high fresh gas flows.

Utility of end-tidal carbon dioxide monitoring in critically ill children.

Carbon dioxide is produced in the tissues as a byproduct of aerobic metabolism, then transported to the lungs by the venous circulation and is finally eliminated during the expiratory phase of ventilation. End-tidal CO2(ETCO2) is defined as the peak CO2 value during expiration and is dependent on adequate pulmonary blood flow to the ventilating areas of the lung. ETCO2 in healthy subjects differs normally by <5 mmHg from the arterial CO2(PaCO2). An increased ETCO2-PaCO2 gap can be found in association with a decreased pulmonary blood flow and/or increased dead-space. Capnometry is the measurement of CO2 concentration of tidal gas at the airway opening. Capnography is the graphic display of the measured CO2. Capnography is an important non-invasive technique that can monitor CO2 production, pulmonary perfusion and alveolar ventilation as well as respiratory patterns.[1] Capnography has been shown to be effective as a tool for early detection of complications during anesthesia including, hypoventilation, esophageal intubation and circuit disconnection. Tinker et al.[2] concluded after a closed claim analysis for the American Society of Anesthesiology that the application of capnography and pulse oximetry together could have helped prevent 93% of avoidable anesthesia mishaps identified in their study. Capnometry is now a standard of care during administration of general anesthesia, for confirmation of placement of endotracheal tube after intubation and during transportation of an intubated patient. It has been increasingly used in intubated ventilated patients in intensive care units to safely wean patients, decrease the number of arterial blood gases, detect endotracheal displacement and prevent hypercarbia. Nasal ETCO2 monitoring is useful during procedural sedation and post-operative monitoring in the recovery room. In addition, volumetric capnography can be used as a non-invasive method to measure cardiac output. In this journal, Mehta and colleagues presented their data showing good correlation between ETCO2 and PaCO2 in neonates and children.[3] As expected correlation between ETCO2 and PaCO2 was weaker when P/F ratio was 200. Although, the mean bias with the Bland-Altman plot was low in neonates as well as children the limits of agreement is large suggesting variability of PaCO2-ETCO2 gap in their sample. Their findings confirm previously published data in neonates. The gradient between PaCO2 and ETCO2 depends upon the amount of dead space during ventilation.[4] Many variables influence the percentage of dead space to total minute ventilation. Decreased pulmonary blood flow, pulmonary disease or hyperinflation of lungs can lead to increased dead space and consequently high PaCO2-ETCO2 gap. An important limitation of using ETCO2 as a surrogate for PaCO2 measurement is that the PaCO2-ETCO2 gap changes from patient to patient based on underlying pulmonary and cardiovascular conditions and ventilation strategies and within the same patient with significant changes in the cardiopulmonary condition. In a stable patient ETCO2 monitoring can be used as trending tool during weaning or after the change in ventilator setting. Periodic measurement of PaCO2 is necessary to make sure the PaCO2-ETCO2 gap is not significantly altered. Mehta et al.[3] confirmed previous reports of ETCO2 monitoring as a valid tool in sick, mechanically ventilated neonates and children. In addition to monitoring ventilation status, continuous ETCO2 is useful in early detection of dislodgement of endotracheal tube. Waveform of capnography may be useful in detecting certain type of pulmonary pathology, such as obstructive airway disease. ETCO2 monitoring is a valuable tool during cardiopulmonary resuscitation (CPR). ETCO2 levels fall to low levels at the onset of cardiac arrest, increases with effective CPR and returns to normal levels at return of spontaneous circulation. Although there are no absolute contraindications of use of ETCO2 monitoring there few limitations for its use.[5] Large main stream capnometers can add dead space and may cause kinking of endotracheal tubes. Fortunately, most modern capnometers add a small amount dead space and are of light weight. Gas sampling rate from a side stream capnometer can lead to auto triggering and sometimes inadequate tidal volume delivery in very small neonates. Secretions and condensation in the tubing and leaks in the ventilator circuit can affect the accuracy of ETCO2. End-tidal carbon dioxide monitoring is a useful tool in the management of acutely ill and is becoming a standard of care in many clinical situations. In ventilated patients, ETCO2 monitoring is best used as a trending device for ventilation in addition to detecting endotracheal position.

Atypical upswing at initial part of phase III in capnograms due to technical cause

We sporadically experienced three paediatric cases of atypical upswing at the initial part of phase III in the capnograms via side-stream capnometer immediately following endotracheal intubation and mechanical ventilation. No fault was found in the monitor or anaesthetic system including breathing circuits, carbon dioxide (CO2 ) sampling tube, water trap, and unidirectional valves. The upsurge of CO2 disappeared with increasing the respiratory rate; however, it reappeared with decreasing the respiratory rate and vice versa. We experimentally reproduced the phenomenon of overshooting CO2 measurement after the luer lock connection of the sample gas line at the water trap had been unscrewed a little bit.

Accuracy of Postoperative End-tidal Pco2Measurements with Mainstream and Sidestream Capnography in Non-obese Patients and in Obese Patients with and without Obstructive Sleep Apnea

Obtaining accurate end-tidal carbon dioxide pressure measurements via nasal cannula poses difficulties in postanesthesia patients who are mouth breathers, including those who are obese and those with obstructive sleep apnea (OSA); a nasal cannula with an oral guide may improve measurement accuracy in these patients. The authors evaluated the accuracy of a mainstream capnometer with an oral guide nasal cannula and a sidestream capnometer with a nasal cannula that did or did not incorporate an oral guide in spontaneously breathing non-obese patients and obese patients with and without OSA during recovery from general anesthesia. The study enrolled 20 non-obese patients (body mass index less than 30 kg/m) without OSA, 20 obese patients (body mass index greater than 35 kg/m) without OSA, and 20 obese patients with OSA. End-tidal carbon dioxide pressure was measured by using three capnometer/cannula combinations (oxygen at 4 l/min): (1) a mainstream capnometer with oral guide nasal cannula, (2) a sidestream capnometer with a nasal cannula that included an oral guide, and (3) a sidestream capnometer with a standard nasal cannula. Arterial carbon dioxide partial pressure was determined simultaneously. The major outcome was the arterial-to-end-tidal partial pressure difference with each combination. In non-obese patients, arterial-to-end-tidal pressure difference was 3.0 +/- 2.6 (mean +/- SD) mmHg with the mainstream capnometer, 4.9 +/- 2.3 mmHg with the sidestream capnometer and oral guide cannula, and 7.1 +/- 3.5 mmHg with the sidestream capnometer and a standard cannula (P < 0.05). In obese non-OSA patients, it was 3.9 +/- 2.6 mmHg, 6.4 +/- 3.1 mmHg, and 8.1 +/- 5.0 mmHg, respectively (P < 0.05). In obese OSA patients, it was 4.0 +/- 3.1 mmHg, 6.3 +/- 3.2 mmHg, and 8.3 +/- 4.6 mmHg, respectively (P < 0.05). Mainstream capnometry performed best, and an oral guide improved the performance of sidestream capnometry. Accuracy in non-obese and obese patients, with and without OSA, was similar.

Systemic recirculation assessed in apnoeic anaesthetized patients using carbon dioxide concentration measurements during stepwise expiration

Mixed venous partial pressure of carbon dioxide has often been estimated in anaesthesia, usually by rebreathing techniques. This assumes equilibrium between respired gas and mixed venous blood before significant recirculation can occur, and requires vigorous rebreathing and precise identification of equilibrium. Modern clinical sidestream capnometers do not measure gas composition as rapidly as the devices used in previous studies, and cannot measure end-tidal values adequately during vigorous rebreathing. In contrast, a single-staged exhalation provides a stable sample for more accurate measurement of gas composition, and clinical measurements then allow examination of the evidence that equilibration occurs before recirculation. Theoretically, this assumption is optimistic, although it forms the basis of a method for non-invasive measurement of cardiac output. We used measurements of staged exhalation to study the evidence for equilibrium more closely. We studied 19 patients during general anaesthesia. The lungs were inflated with mixed exhaled gas. Stepwise expiration allowed exhaled gas to be analysed over approximately 1 min. The rate of increase in exhaled carbon dioxide fraction was related to the duration of expiration. Carbon dioxide concentration continued to change throughout the study period. The lack of equilibrium of carbon dioxide concentration over this time supports simulation studies which predict recirculation from well-perfused body compartments within 20 s, and that the subsequent increase of carbon dioxide reflects the wash in of multiple body compartments. This method allows adequate time for full response of sidestream analysers. Recirculation is an important early feature affecting breath-hold estimates of mixed venous carbon dioxide. Carbon dioxide accumulation during apnoea or rebreathing will have prompt effects on arterial carbon dioxide values and a constant mixed venous composition cannot be assumed when methods based on partial rebreathing are used over this time period.

Flow-Through Versus Sidestream Capnometry for Detection of End Tidal Carbon Dioxide in the Sedated Patient

End tidal carbon dioxide (ETCO(2)) in non-intubated patients can be monitored using either sidestream or flow-through capnometry [Yamamori et al., J Clin Monit Comput 22(3):209-220, 2008]. The hypothesis of this validation study is that, flow-through capnometry will yield a more accurate estimate of ETCO(2) than sidestream capnometry when evaluated in a bench study during low tidal volumes and high oxygen administration via nasal cannula. Secondarily, when ETCO(2) from each is compared to arterial CO(2) (PaCO(2)) during a study in which healthy, non-intubated volunteers are tested under normocapnic, hypocapnic and hypercapnic conditions, the flow-through capnometer will resemble PaCO(2) more closely than the sidestream capnometer. This will be especially true during periods of lower minute ventilation and high oxygen flow rates via mask in non-intubated, remifentanil sedated, healthy volunteers whose physiologic deadspace is small. The performance of a flow-through (cap-ONE, Nihon Kohden, Tokyo, Japan) and a sidestream (Microcap Smart CapnoLine Plus, Oridion Inc., Needham, MA) capnometer were compared in a bench study and a volunteer trial. A bench study evaluated ETCO(2) accuracy using waveforms generated via mechanical lungs during low tidal volumes and high oxygen flow rates. A volunteer study compared the ETCO(2) for each capnometer against PaCO(2) during sedation in which 8 l O(2) was delivered via mask rather than the nasal cannula. In the bench study, the flow-through capnometer gave slightly higher values of ETCO(2) during high-flow oxygen and no discernable differences during variable tidal volumes. Bland and Altman plots comparing ETCO(2) to PaCO(2) showed essentially equal performance between the two capnometers in the volunteers. Within a wide limit of agreement between the volunteer and bench study, flow-through and sidestream capnometry performed equally well during bench testing and in non-intubated, sedated patients.

A Flow-Through Capnometer for Obstructive Sleep Apnea

Capnogram is often distorted due to aspiration of expired gas when a sidestream capnometer is used for non-intubated, spontaneously breathing condition. The purpose of this study was to make a flow-through capnometer without aspiration and to check if this capnometer precisely detected apnea during obstructive sleep apnea (OSA). (1) Flow-through capnometer The capnometer consisted of a flow-through etCO(2) sensor, cap-ONE, and an accompanying capnometer. The size of cap-ONE was small enough to be fitted under the nose where gas expired from the nose and the mouth passed through. Thus, the expired gas to be measured is directly blown into the cap-ONE. (2) The cap-ONE using a spontaneously breathing model Capnograms obtained by the cap-ONE and sidestream capnometers during nasal and oral breathing under normal and reduced ventilation were compared with a reference capnogram. (3) Clinical study with OSA patients With nineteen OSA patients capnograms during apnea events diagnosed as OSA by polysomnography were examined using the cap-ONE. (4) Simulation study with an OSA model Apnea in which inspiratory flow was zero and small expiratory flows repeated was produced. Capnograms and apnea detection were compared between the cap-ONE and sidestream capnometers. In the spontaneouly breathing model capnograms and etCO(2) of the cap-ONE during nasal and oral breathing were almost identical with the reference capnogram but those of sidetream capnometers during oral breathing were significantly reduced. In the clinical study 41% of total OSA events showed capnograms with prolonged and elevated phase with small ripples. In a simulation study reduction of CO(2) tension during no-inspiration was small and apnea was successfully detected with the cap-ONE. However, with sidestream capnometers the reduction of CO(2) tension was large and apnea was not detected. We concluded that the cap-ONE can record capnograms with minimum distortion and detect apnea reliably during OSA.

Atypical "tails-up" capnograph due to breach in the sampling tube of side-stream capnometer.

An atypical "tails-up" capnograph pattern was noticed in a patient during the use of an accidentally crushed sampling tube with a slit-like hole. We investigated the mechanics involved in the observed capnograph pattern. Forty consenting ASA I patients of both sexes presenting for tonsillectomy were included in this study. After intravenous induction of anaesthesia, intermittent positive pressure ventilation (IPPV) using a mechanical ventilator was maintained for 20 min and the capnograph trace, ETCO2, and inspiratory/expiratory sevoflurane were compared using a breached sampling tube and then an intact sampling tube. Similar comparisons were made during spontaneous breathing. During IPPV, an atypical "tails-up" capnograph was noted using the breached sampling tubing. At similar inspiratory sevoflurane (2.0 +/- 0.03) levels, expiratory levels (0.9 +/- 0.03) were significantly lower when using the breached sampling tube than the intact tube (1.7 +/- 0.03). ETCO2 with the breached sampling tube (26.8 +/- 0.30 mmHg) showed significantly lower values than with the intact sampling tube (37 +/- 0.3). During spontaneous breathing, the capnograph was normal in shape with both sampling tubes, but ETCO2 and both insp./exp. sevoflurane levels were lower with the breached sampling tube. During IPPV, pressure in the breathing circuit is lower during exhalation, thus allowing air to enter through the slit-like hole in the sampling tube causing erroneously low ETCO2 and expiratory sevoflurane. With inspiration, positive pressure in the breathing circuit, transmitted to the sampling tube, prevents air admixture and the upsurge in CO2 is displayed giving the capnograph an atypical "tails-up" appearance. During spontaneous breathing, since pressure in the breathing circuit barely becomes positive during exhalation and is negative during inspiration, air mixes with the sampled gas during both phases and so the capnograph shape was normal but with lower values for ETCO2, insp./exp. sevoflurane, and nitrous oxide levels. If undiagnosed, this defect in the sampling tube can lead to significant errors in the measurement of inspired and expired gas concentrations.

Carbon dioxide levels during pre-hospital active compression–decompression versus standard cardiopulmonary resuscitation

In a prospective randomised study we investigated end-tidal carbon dioxide levels during standard versus active compression–decompression (ACD) cardiopulmonary resuscitation (CPR) assuming that the end-tital carbon dioxide reflects cardiac output during resuscitation. In each group 60 patients with out-of-hospital cardiac arrest were treated either with the standard or the ACD method. End-tidal CO 2 ( p etCO 2, mmHg) was assessed with a side-stream capnometer following intubation and then every 2 min up to 10 min or restoration of spontaneous circulation (ROSC). There was no difference in p etCO 2 between both patient groups. However, CO 2 was significantly higher in patients who were admitted to hospital as compared to patients declared dead at the scene. All of the admitted patients had a p etCO 2 of at least 15 mmHg no later than 2 min following intubation, none of the dead patients ever exceeded 15.5 mmHg. From these data we conclude that capnometry adds valuable information to the estimation of a patient's prognosis in the field (threshold, 15 mmHg), but we could not detect any difference in p etCO 2 between ACD and standard CPR.

Checking the precision of capnometers

Capnometry, i.e. measurement of CO2 concentration (cCO2, vol%) and calculation of the respective CO2 partial pressure (pCO2, mmHg), is simple to apply, but the user must understand its principles of operation in order to appreciate its power and its limitations. However, sidestream capnometers use to dry the humidified respired gas (37 degrees C, pH2O 47 mmHg) for methodological reasons, thus increasing pCO2 by ca. 6% which, in turn, requires correction. With infrared spectroscopy, overestimation of pCO2 in presence of N2O and underreporting of pCO2 in presence of O2 also require to be corrected. These require either knowledge of the respective gas concentrations (mainstream analyzers) or adequate (i.e. correct) measurement (some sidestream capnometers). Changes in barometric pressure (pB) must be also either known or be measured automatically. Evaluation of the precision of capnometers must therefore focus on, (1) the possible pH2O correction, (2) the possible effects exercised by O2 and N2O, and (3) the possible dependence on barometric pressure. Two mainstream (Capnolog D/Dräger; Sirecust 404-1/Siemens) and eight sidestream capnometers (AGM 1304/Brüel & Kjaer; SARAcap A. G. and SARA-trans/Biomedical Systems/Hellige; Capnomac and Normocap 200/Datex/Hoyer; CO2 Monitor/Dräger; Nellcor N-1000/Nellcor/Dräger; Multinex/Datascope) were investigated. Dry and humidified gases (37 degrees C) of defined composition in respect of CO2, O2, N2, N2O, and H2O, were used for evaluation. The results prove reproducibility of ca. =/- 1 mmHg for the 10 capnometers within the 30-50 mmHg pCO2 range.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Accuracy of end-tidal PCO2 measurements using a sidestream capnometer in infants and children ventilated with the Sechrist infant ventilator

To determine the accuracy of end-tidal PCO2 (PETCO2) measurements analyzed with a sidestream capnometer in infants and children whose lungs were ventilated with a Sechrist infant ventilator and an Ayre's t-piece, we compared PETCO2 measurements obtained from the proximal (PETCO2-p) and distal (PETCO2-d) ends of the tracheal tube to arterial PCO2 (PaCO2) in 37 healthy infants and children between 1.3 and 24.5 kg. Both PETCO2-p and PETCO2-d accurately approximated PaCO2, however, the mean (+/- SD) arterial to end-tidal PCO2 difference (delta(a-ET)PCO2) was significantly greater with proximal (1.27 +/- 1.54 mmHg) than with distal sampling (0.64 +/- 1.64 mmHg) (P less than 0.01). In the subgroup of patients who weighted less than 12 kg, the delta(a-ET)PCO2 using proximal gas sampling (1.94 +/- 1.29 mmHg) was also significantly greater than it was using distal sampling (0.74 +/- 1.31 mmHg) (P less than 0.001). We conclude that although statistically different, both proximal and distal estimates of PETCO2 provide acceptable estimates of PaCO2 in healthy infants and children who are ventilated with a Sechrist infant ventilator and an Ayre's t-piece system.

Mechanical influences on the capnogram

We found that altering aspirating flow rate (ASR) and sample tubing length (STL) affected the response time and capnogram of 2 different side-stream capnometers. Increased flow rate shortened rise time, transit time, and total delay time (TDT), whereas increased STL increased transit and TDTs but not rise time. Increasing rise time pushed the ascending limb of the capnogram to the right. Peak and baseline PCO2 values were accurate only when TDT did not exceed respiratory cycle time (RCT). Awareness of the potential for artifactual changes in the capnogram will avoid their misinterpretation as physiologic events.