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

This study compares two noninvasive techniques for monitoring the partial pressure of carbon dioxide (Pco2) in 24 anesthetized adult patients. End-tidal PCO₂ (PETCO₂) and transcutaneous Pco₂ (PtcCO₂) were simultaneously monitored and compared with arterial Pco₂ (PaCO₂) determined by intermittent analysis of arterial blood samples. PETCO₂ and PtcCO₂ values were compared with PaCO₂ values corrected to patient body temperature (PaC0_2T) and PaCO₂ values determined at a temperature of 37°C (PaCO₂). Linear regression was performed along with calculations of the correlation coefficient (r), bias, and precision of the four paired variables:PETCO₂ versus PaCO₂ and PaCO_2T (n = 211), and PtcCO₂ versus PaCO₂ and PaCO_2T (n = 233). Bias is defined as the mean difference between paired values, whereas precision is the standard deviation of the difference. The following values were found forr, bias, and ± precision, respectively.PetCO₂ versus PaCO₂: 0.67, ?7.8 mm Hg, ±6.1 mm Hg;PETCO₂ versus PaCO_2T: 0.73, ?5.8 mm Hg, ±5.9 mm Hg;PETCO₂ versus PaCO₂: 0.87, ?1.6 mm Hg, ±4.3 mm Hg; PtcCO₂ versus PaC0_2T: 0.84, +0.7 mm Hg, ±4.8 mm Hg. Although each of thesePCO ₂ variables is physiologically different, there is a significant correlation (P < 0.001) between the noninvasively monitored values and the blood gas values. Temperature correction of the arterial values (PaCO_2T) slightly improved the correlation, with respect toPETCO₂, but it had the opposite effect for PtcCO₂. In this study, the chief distinction between these two noninvasive monitors was thatPETCO₂ had a large negative bias, whereas PtcCO₂ had a small bias. We conclude from these data that PtcCO₂ may be used to estimate PaCO₂ with an accuracy similar to that ofPetCO₂ in anesthetized patients.     

Accurate determination of end-tidal carbon dioxide during administration of oxygen by nasal cannulae

Measurement of end-tidal carbon dioxide tension (PetCO₂) by mass spectrometry or infrared capnometry provides a clinically useful approximation of arterial carbon dioxide tension (PaCO₂) in intubated patients. Although several devices have been proposed to samplePetCO₂ during spontaneous breathing (i.e., unintubated patients receiving supplemental oxygen), thus far no reports have documented their efficacy. This article reports the use of an easily constructed modification of simple nasal cannulae that permits accurate sampling ofPetCO₂ during oxygen administration to unintubated patients. After amputation of the closed tip, a cap from a syringe was inserted via a slit made at the base into one prong of a pair of nasal cannulae. A capnometer was connected to the syringe cap, andPetCO₂ and PaCO₂ were determined simultaneously during the administration of 3 L/min oxygen via nasal cannulae to 21 normocapnic patients. The PaCO₂ —PetCO₂ gradients were calculated and compared with values obtained in the same patients after intubation and mechanical ventilation. No significant difference was found between the calculated gradients with nasal cannulae (2.09±2.18 mm Hg) versus intubation (2.87±2.82 mm Hg). Simultaneous oxygen administration and accurate sampling ofPetCO₂ may be achieved in unintubated patients by using this easily constructed modification of nasal cannulae.Supported in part by PPG Biomedical Systems, Lenexa, KS.The apparatus and method described herein are covered by U.S. Patent Application S.N. 181,814: Method and Apparatus for Inhalation of Treating Gas for Quantitative Analysis. Filed April 15, 1988—in the name of Edwin A. Bowe, et al.     

Accuracy of expiratory carbon dioxide measurements using the coaxial and circle breathing circuits in small subjects

Mass spcctrometry is widely used to measure the end-tidal concentrations of inhalation anesthetics and other gases during surgery in order to estimate their arterial concentrations. When certain breathing circuits are used in newborns, however, fresh gas or ambient air may contaminate the expired sample, introducing a systematic error in the measurement of any end-tidal gas concentration. We estimated this error in newborn piglets using carbon dioxide as an indicator substance of expired gas. The capnograms and the difference between arterial carbon dioxide tension (PaCO₂) and peakexpired carbon dioxide tension (PeCO₂) were compared when either a coaxial (Bain) or circle breathing circuit was used. Gas was sampled from the proximal airway and distal trachea. No combination of circuit and sampling site produced a flat alveolar phase until the circle circuit was modified with diversion valves to reduce gas mixing. The mean PaCO₂-PeCO₂ gradients using the coaxial/proximal sampling, coaxial/distal sampling, and modified circle/proximal sampling circuits were 12.4, 9.2, and 8.8 mm Hg, respectively. The mean PeCO₂ in each of these combinations was significantly different from the corresponding mean PaCO₂ (p<0.05). Using the modified circle circuit with distal sampling, mean PeCO₂ was not significantly different from mean PaCO₂: the mean PaCO₂-PcCO₂ gradient was 2.2 ± 0.2 mm Hg (SEM), range, 0 to 6 mm Hg, with 95% confidence limits ⩽ 8 mm Hg. When a coaxial breathing circuit is used in small subjects, PaCO₂ may be significantly underestimated regardless of sampling site, although the circle breathing circuit with distal tracheal sampling yields accurate results. Supported in part by BRS Grant SO RR05507-20 from the Biomedical Research Support Grant Program, Division of Research Resources, National Institutes of Health, and by the American Heart Association, Lancaster, PA Chapter. The authors thank Robert Hirsch, PhD, for his statistical advice, and Greg Harris and Perkin-Elmer, Inc for loaning the mass spectrometer.     

End-tidal carbon dioxide as a measure of arterial carbon dioxide during intermittent mandatory ventilation

To determine if end-tidal carbon dioxide tension (PetCO₂) is a clinically reliable indicator of arterial carbon dioxide tension (PaCO₂) under conditions of heterogeneous tidal volumes and ventilation-perfusion inequality, we examined the expiratory gases of 25 postcardiotomy patients being weaned from ventilator support with intermittent mandatory ventilation. Using a computerized system that automatically sampled airway flow, pressure, and expired carbon dioxide tension, we were able to distinguish spontaneous ventilatory efforts from mechanical ventilatory efforts. ThePetCO₂ values varied widely from breath to breath, and the arterial to end-tidal carbon dioxide tension gradient was appreciably altered during the course of several hours. About two-thirds of the time, thePetCO₂ of spontaneous breaths was greater than that of ventilator breaths during the same 70-second sample period. The most accurate indicator of PaCO₂ was the maximalPetCO₂ value in each sample period, the correlation coefficient being 0.768 (P < 0.001) and the arterial to end-tidal gradient being 4.24 ± 4.42 mm Hg (P < 0.01 compared with all other measures). When all values from an 8-minute period were averaged, stability was significantly improved without sacrificing accuracy. We conclude that monitoring the maximalPetCO₂, independent of breathing pattern, provides a clinically useful indicator of PaCO₂ in postcardiotomy patients receiving intermittent mandatory ventilation.     

Transcutaneous carbon dioxide and oxygen tension in newborn infants: Reliability of a combined monitor of oxygen tension and carbon dioxide tension

We evaluated a new combined sensor for monitoring transcutaneous carbon dioxide tension (PtcCO₂) and oxygen tension (PtcO₂) in 20 critically ill newborn infants. Arterial oxygen tension (PaO₂) ranged from 16 to 126 torr and arterial carbon dioxide tension (PaCO₂) from 14 to 72 torr. Linear correlation analysis (100 paired values) of PtcO₂ versus PaO₂ showed anr value of 0.75 with a regression equation of PtcO₂=8.59+0.905 (PaO₂), while PtcCO₂ versus PaCO₂ revealed a correlation coefficient ofr=0.89 with an equation of PtcCO₂=2.53+1.06 (PaCO₂). The bias between PaO₂ and PtcO₂ was –2.8 with a precision of ±16.0 torr (range, –87 to +48 torr). The bias between PaCO₂ and PtcCO₂ was –5.1 with a precision of ±7.3 torr (range, –34 to +8 torr). The transcutaneous sensor detected 83% of hypoxia (PaO₂ <45 torr), 75% of hyperoxia (PaO₂ >90 torr), 45% of hypocapnia (PaCO₂ <35 torr), and 96% of hypercapnia (PaCO₂ >45 torr). We conclude that the reliability of the combined transcutaneousPo ₂ andPCo ₂ monitor in sick neonates is good for detecting hypercapnia, fair for hypoxia and hyperoxia, but poor for hypocapnia. It is an improvement in that it spares available skin surface and requires less handling, but it appears to be slightly less accurate than the single electrodes.     

An improved nasal prong apparatus for end-tidal carbon dioxide monitoring in awake,sedated patients

We describe and evaluate a new apparatus that monitors end-tidal carbon dioxide (PetCO₂) and augments the inspired oxygen concentration in awake, sedated patients. The unit was evaluated for its effectiveness as an oxygenation device and its accuracy as a predictor of PaCO₂ through the correlation of PaCO₂ withPetCO₂. Twenty cardiac surgical patients, physical status ASA 2–4, participated in this study. ThePetCO₂ monitoring device consisted of a dual-prong nasal oxygen cannula and a 14-gauge intravenous catheter that was inserted into one limb of the oxygen supply tubing and connected to a Datex gas analyzer (Datex Instrumentation Corp, Helsinki, Finland) to measurePetCO₂. The cross-over passage between the prongs was intentionally blocked with the end of a wooden-core cotton swab. The oxygen flow rates were randomly varied (2, 4, and 6 L/min) every 5 minutes, and values forPetCO₂ as well as arterial blood samples for analysis of PaCO₂ and PaO₂ were obtained at the end of each 5-minute period. The accuracy of the system was assessed by comparing the PaCO₂-PetCO₂ differences (bias) at each oxygen flow rate. The ratios ofPetCO₂ compared with PaCO₂ were 0.98, 0.94, and 0.85, with correlation coefficients ofr=0.81, 0.85, and 0.63, respectively. The PaO₂ values were 114, 154, and 183 mm Hg for the corresponding nasal oxygen flow rates of 2, 4, and 6 L/min, respectively. This study indicates that this modified nasal cannula provides supplemental oxygen adequately and yields a satisfactory reflection of the PaCO₂ depending on the oxygen flow rate delivered.     

The gradient between arterial and end-tidal carbon dioxide predicts in-hospital mortality in post-cardiac arrest patient

Purpose

We investigated the predictive value of the gradient between arterial carbon dioxide (PaCO₂) and end-tidal carbon dioxide (ETCO₂) (Pa-ETCO₂) in post-cardiac arrest patients for in-hospital mortality.

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

Background

End tidal carbon dioxide (ETCO₂) 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 ETCO₂ than sidestream capnometry when evaluated in a bench study during low tidal volumes and high oxygen administration via nasal cannula. Secondarily, when ETCO₂ from each is compared to arterial CO₂ (PaCO₂) during a study in which healthy, non-intubated volunteers are tested under normocapnic, hypocapnic and hypercapnic conditions, the flow-through capnometer will resemble PaCO₂ 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.

Methods

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 ETCO₂ accuracy using waveforms generated via mechanical lungs during low tidal volumes and high oxygen flow rates. A volunteer study compared the ETCO₂ for each capnometer against PaCO₂ during sedation in which 8 l O₂ was delivered via mask rather than the nasal cannula.

Results

In the bench study, the flow-through capnometer gave slightly higher values of ETCO₂ during high-flow oxygen and no discernable differences during variable tidal volumes. Bland and Altman plots comparing ETCO₂ to PaCO₂ showed essentially equal performance between the two capnometers in the volunteers.

Conclusions

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.     

Can mainstream end-tidal carbon dioxide measurement accurately predict the arterial carbon dioxide level of patients with acute dyspnea in ED

Objective

This study was designed to determine whether the mainstream end-tidal carbon dioxide (ETCO₂) measurement can accurately predict the partial arterial carbon dioxide (Paco₂) level of patients presented to emergency department (ED) with acute dyspnea.

Methods

This prospective, observational study was conducted at a university hospital ED, which serves more than 110?000 patients annually. Nonintubated adult patients presented with acute dyspnea who required arterial blood gas analysis were recruited in the study for a 6-month period between January and July 2010. Patients were asked to breathe through an airway adapter attached to the mainstream capnometer. Arterial blood gas samples were obtained simultaneously.

Results

We included 162 patients during the study period. The mean ETCO₂ level was 39.47 ± 10.84 mm Hg (minimum, 19 mm Hg; maximum, 82 mm Hg), and mean Paco₂ level was 38.95 ± 12.27 mm Hg (minimum, 16 mm Hg; maximum, 94 mm Hg). There was a positive, strong, statistically significant correlation between ETCO₂ and Paco₂ (r = 0.911, P < .001). The Bland-Altman plot shows the mean bias ± SD between ETCO₂ and Paco₂ as 0.5 ± 5 mm Hg (95% confidence interval, −1.3165-0.2680) and the limits of agreement as −10.5 and +9.5 mm Hg. Eighty percent (n = 129) of the ETCO₂ measurements were between the range of ±5 mm Hg.

Conclusion

Mainstream ETCO₂ measurement accurately predicts the arterial Paco₂ of patients presented to ED with acute dyspnea. Further studies comparing mainstream and sidestream methods in these patients are required.     

Capnometry for continuous postoperative monitoring of nonintubated,spontaneously breathing patients

We continuously monitored spontaneous respiration after extubation by end-tidal CO₂ tension (PetCO₂) in 19 patients aged 20 to 72 years who had undergone major operations. The respiratory gas was sampled from the nasopharynx via a special nasal catheter and analyzed by a side-stream analyzer. In each case, optimal placement of the nasal catheter was determined by CO₂ waveform and the capnograms were recorded for waveform analysis and trend monitoring.PetCO₂ was compared with arterial CO₂ tension (PaCO₂) two to four times during the 2- to 19-hour observation periods by simultaneous measurements. For 65 simultaneous measurements, meanPetCO₂ was 38.9 ± 5.7 mm Hg (range, 26.3 to 48.3 mm Hg) and mean PaCO₂ was 38.9 ± 5.7 mm Hg (range, 26.8 to 46.0 mm Hg;r=0.82;p<0.01). While the mean values forPetCO₂ and PaCO₂ were similar, several patients had large differences for PaCO₂ toPetCO₂. The differences of the individual patients did not differ significantly between the various times of measurement. We conclude that this form of capnometry is well suited for continuous, noninvasive monitoring of respiration in nonintubated, spontaneously breathing patients.     

Accuracy of end-tidal carbon dioxide tension analyzers

Substantial mean differences between arterial carbon dioxide tension (PaCO₂) and end-tidal carbon dioxide tension (PetCO₂) in anesthesia and intensive care settings have been demonstrated by a number of investigators. We have explored the technical causes of error in the measurement ofPetCO₂ that could contribute to the observed differences. In a clinical setting, the measurement ofPetCO₂ is accomplished with one of three types of instruments, infrared analyzers, mass spectrometers, and Raman spectrometers, whose specified accuracies are typically ±2, ±1.5, and ±0.5 mm Hg, respectively. We examined potential errors inPetCO₂ measurement with respect to the analyzer, sampling system, environment, and instrument. Various analyzer error sources were measured, including stability, warm-up time, interference from nitrous oxide and oxygen, pressure, noise, and response time. Other error sources, including calibration, resistance in the sample catheter, pressure changes, water vapor, liquid water, and end-tidal detection algorithms, were considered and are discussed. On the basis of our measurements and analysis, we estimate the magnitude of the major potential errors for an uncompensated infrared analyzer as: inaccuracy, 2 mm Hg; resolution, 0.5 mm Hg; noise, 2 mm Hg; instability (12 hours), 3 mm Hg; miscalibration, 1 mm Hg; selectivity (70% nitrous oxide), 6.5 mm Hg; selectivity (100% oxygen), −2.5 mm Hg; atmospheric pressure change, <1 mm Hg; airway pressure at 30 cm H₂O, 2 mm Hg; positive end-expiratory pressure or continuous positive airway pressure at 20 cm H₂O, 1.5 mm Hg; sampling system resistance, <1 mm Hg; and water vapor, 2.5 mm Hg. In addition to these errors, other systematic mistakes such as an inaccurate end-tidal detection algorithm, poor calibration technique, or liquid water contamination can lead to gross inaccuracies. In a clinical setting, unless the user is confident that all of the technical error sources have been eliminated and the physiologic factors are known, depending onPetCO₂ to determine PaCO₂ is not advised. An erratum to this article is available at .     

Does End‐tidal Carbon Dioxide Monitoring Detect Respiratory Events Prior to Current Sedation Monitoring Practices?

Objectives: The value of ventilation monitoring with end‐tidal carbon dioxide (ETCO₂) to anticipate acute respiratory events during emergency department (ED) procedural sedation and analgesia (PSA) is unclear. The authors sought to determine if ETCO₂ monitoring would reveal findings indicating an acute respiratory event earlier than indicated by current monitoring practices. Methods: The study included a prospective convenience sample of ED patients undergoing PSA. Clinicians performed ED PSA procedures with generally accepted patient monitoring, including oxygen saturation (SpO₂), and clinical ventilation assessment. A study investigator recorded ETCO₂ levels and respiratory events during each PSA procedure, with clinical providers blinded to ETCO₂ levels. Acute respiratory events were defined as SpO₂≤92%, increases in the amount of supplemental oxygen provided, use of bag‐valve mask or oral/nasal airway for ventilatory assistance, repositioning or airway alignment maneuvers, and use of physical or verbal means to stimulate patients with depressed ventilation or apnea, and reversal agent administration. Results: Enrollment was stopped after independent review of 20 acute respiratory events in 60 patient sedation encounters (33%). Abnormal ETCO₂ findings were documented in 36 patients (60%). Seventeen patients (85%) with acute respiratory events demonstrated ETCO₂ findings indicative of hypoventilation or apnea during PSA. Abnormal ETCO₂ findings were documented before changes in SpO₂ or clinically observed hypoventilation in 14 patients (70%) with acute respiratory events. Conclusions: Abnormal ETCO₂ findings were observed with many acute respiratory events. A majority of patients with acute respiratory events had ETCO₂ abnormalities that occurred before oxygen desaturation or observed hypoventilation.     

Characteristics of transcutaneous carbon dioxide tension monitors in normal adults and critically ill patients

The performance of three electrodes used for transcutaneous carbon dioxide (tcPCO₂) monitoring is compared in 15 healthy volunteers and 26 critically ill adults. All three electrodes showed good correlation between tcPCO₂ and arterial blood PCO₂ (PaCO₂) with a correlation coefficient (r) greater than 0.86. There was little difference in the performance characteristics of the three monitors. They may be usefully employed to estimate PaCO₂ values when used with a modified calibration recommended by the manufacturers.     

Concordance between transcutaneous and arterial measurements of carbon dioxide in an ED

Background

Transcutaneous carbon dioxide pressure (PtcCO₂) has been suggested as a noninvasive surrogate of arterial carbon dioxide pressure (PaCO₂). Our study evaluates the reliability of this method in spontaneously breathing patients in an emergency department.

Patients and methods

A prospective, observational study was performed in nonintubated dyspneic patients who required measurement of arterial blood gases. Simultaneously and blindly to the physicians in charge, PtcCO₂ was measured using a TOSCA 500 monitor (Radiometer, Villeurbanne, France). Agreement between PaCO₂ and PtcCO₂ was assessed using the Bland-Altman method.

Results

Forty-eight patients (mean age, 65 years) were included, and 50 measurements were done. Eleven (23%) had acute heart failure; 10 (21%), pneumonia; 7 (15%), acute asthma; and 7 (15%), exacerbation of chronic obstructive pulmonary disease. Median PaCO₂ was 42 mm Hg (range, 17-109). Mean difference between PaCO₂ and PtcCO₂ was 1 mm Hg with 95% limits of agreement of − 3.4 to + 5.6 mm Hg. All measurement differences were within 5 mm Hg, and 32 (64%) were within 2 mm Hg.

Conclusion

Transcutaneous carbon dioxide pressure accurately predicts PaCO₂ in spontaneously breathing patients.     

Accuracy of end-tidal carbon dioxide tension analyzers

Substantial mean differences between arterial carbon dioxide tension (PaCO₂) and end-tidal carbon dioxide tension (PetCO₂) in anesthesia and intensive care settings have been demonstrated by a number of investigators. We have explored the technical causes of error in the measurement ofPetCO₂ that could contribute to the observed differences. In a clinical setting, the measurement ofPetCO₂ is accomplished with one of three types of instruments, infrared analyzers, mass spectrometers, and Raman spectrometers, whose specified accuracies are typically ±2, ±1.5, and ±0.5 mm Hg, respectively. We examined potential errors inPetCO₂ measurement with respect to the analyzer, sampling system, environment, and instrument. Various analyzer error sources were measured, including stability, warm-up time, interference from nitrous oxide and oxygen, pressure, noise, and response time. Other error sources, including calibration, resistance in the sample catheter, pressure changes, water vapor, liquid water, and end-tidal detection algorithms, were considered and are discussed. On the basis of our measurements and analysis, we estimate the magnitude of the major potential errors for an uncompensated infrared analyzer as: inaccuracy, 2 mm Hg; resolution, 0.5 mm Hg; noise, 2 mm Hg; instability (12 hours), 3 mm Hg; miscalibration, 1 mm Hg; selectivity (70% nitrous oxide), 6.5 mm Hg; selectivity (100% oxygen), –2.5 mm Hg; atmospheric pressure change, <1 mm Hg; airway pressure at 30 cm H₂O, 2 mm Hg; positive end-expiratory pressure or continuous positive airway pressure at 20 cm H₂O, 1.5 mm Hg; sampling system resistance, <1 mm Hg; and water vapor, 2.5 mm Hg. In addition to these errors, other systematic mistakes such as an inaccurate end-tidal detection algorithm, poor calibration technique, or liquid water contamination can lead to gross inaccuracies. In a clinical setting, unless the user is confident that all of the technical error sources have been eliminated and the physiologic factors are known, depending onPetCO₂ to determine PaCO₂ is not advised.     

Transcutaneous Pco2 and Po2: A multicenter study of accuracy

A multicentcr study used 756 samples from 251 patients in 12 institutions to compare arterial (PaO₂, PaCO₂) with transcutaneous (PsO₂, PsCO₂) oxygen and carbon dioxide tensions, measured usually at 44°C. Of these samples, 336 were obtained from 116 neonates, 27 from 25 children with cystic fibrosis, and 140 from 40 patients under general anesthesia. Ninety-one patients were between 4 weeks and 18 years of age, 32 were between 18 and 60 years, and 12 were over 60. The ratio of transcutaneous to arterial P(s/a)CO₂ was 1.01 ±0.11 with PaCO₂ less than 30 mm Hg, increasing to 1.04 ±0.08 at PaCO₂ greater than 40 mm Hg. Mean bias and its standard deviation (PsCO₂ — PaCO₂) were + 1.3 ± 3.9 mm Hg in the entire group, + 1.8 ± 4.2 mm Hg in neonates (NS). Bias was +0.2 ± 2.7 mm Hg when PaCO₂ was less than 30 mm Hg (N = 175, NS), 1.0 ± 3.4 with 30 < PaCO₂ < 40 (n = 329,p < 0.001), and +2.04 ± 4.00 mm Hg with 40 < PaCO₂ < 70 (n = 229,p < 0.001). These data suggest that, using transcutaneous PCO₂ monitors with inbuilt temperature correction of 4.5%/‡C, the skin metabolic offset should be set to 6 mm Hg. The linear regression was PsCO₂ =1.052(PaCO₂)-0.56, Sy·x = 3.92, R = 0.929 (n = 756); and PsCO₂ = 1.09(PaCO₂)-1.57, Sy·x = 4.17, R = 0.928 in neonates (n = 336). The use of vasopressors and vasodilators had no significant effect on bias or its standard deviation or on regression slope and intercept (n = 78). In cystic fibrosis patients, bias and standard deviation were 0.0 ± 1.7 mm Hg (n = 27). Under anesthesia, PsCO₂ = 1.07PaCO₂-1.58, with bias and standard deviation = 0.6 ± 3.5 (n = 140). For oxygen, at PaO₂ ≤ 80 the ratio P(s/a)O₂ = 1.05 ± 0.16 in nconates and 0.93 ± 0.21 in older patients, but when PaO₂ > 80, P(s/a)O₂ fell to 0.88 ± 0.18 in neonates and 0.74 ± 0.21 in older patients. The errors were significantly greater (p < 0.001) in older patients than in neonates above but not below 80 mm Hg, and within both groups errors were significantly greater above than below 80 mm Hg.     

Quantitative relationship between end-tidal carbon dioxide and CPR quality during both in-hospital and out-of-hospital cardiac arrest

ObjectiveCardiopulmonary resuscitation (CPR) guidelines recommend the administration of chest compressions (CC) at a standardized rate and depth without guidance from patient physiologic output. The relationship between CC performance and actual CPR-generated blood flow is poorly understood, limiting the ability to define “optimal” CPR delivery. End-tidal carbon dioxide (ETCO₂) has been proposed as a surrogate measure of blood flow during CPR, and has been suggested as a tool to guide CPR despite a paucity of clinical data. We sought to quantify the relationship between ETCO₂ and CPR characteristics during clinical resuscitation care.MethodsMulticenter cohort study of 583 in- and out-of-hospital cardiac arrests with time-synchronized ETCO₂ and CPR performance data captured between 4/2006 and 5/2013. ETCO₂, ventilation rate, CC rate and depth were averaged over 15-s epochs. A total of 29,028 epochs were processed for analysis using mixed-effects regression techniques.ResultsCC depth was a significant predictor of increased ETCO₂. For every 10 mm increase in depth, ETCO₂ was elevated by 1.4 mmHg (p < .001). For every 10 breaths/min increase in ventilation rate, ETCO₂ was lowered by 3.0 mmHg (p < .001). CC rate was not a predictor of ETCO₂ over the dynamic range of actual CC delivery. Case-averaged ETCO₂ values in patients with return of spontaneous circulation were higher compared to those who did not have a pulse restored (34.5 ± 4.5 vs 23.1 ± 12.9 mmHg, p < .001).ConclusionsETCO₂ values generated during CPR were statistically associated with CC depth and ventilation rate. Further studies are needed to assess ETCO₂ as a potential tool to guide care.     

The prognostic value of end tidal carbon dioxide during cardiac arrest: A systematic review

Introduction

Cardiac arrest is a common presentation to the emergency care system. The decision to terminate CPR is often challenging to heath care providers. An accurate, early predictor of the outcome of resuscitation is needed. The purpose of this systematic review is to evaluate the prognostic value of ETCO₂ during cardiac arrest and to explore whether ETCO₂ values could be utilised as a tool to predict the outcome of resuscitation.

Method

Literature search was performed using Medline and EMBASE databases to identify studies that evaluated the relationship between ETCO₂ during cardiac arrest and outcome. Studies were thoroughly evaluated and appraised. Summary of evidence and conclusions were drawn from this systematic literature review.

Results

23 observational studies were included. The majority of studies showed that ETCO₂ values during CPR were significantly higher in patients who later developed ROSC compared to patients who did not. Several studies suggested that initial ETCO₂ value of more than 1.33 kPa is 100% sensitive for predicting survival making ETCO₂ value below 1.33 kPa a strong predictor of mortality. These studies however had several limitations and the 100% sensitivity for predicting survival was not consistent among all studies.

Conclusion

ETCO₂ values during CPR do correlate with the likelihood of ROSC and survival and therefore have prognostic value. Although certain ETCO₂ cut-off values appears to be a strong predictor of mortality, the utility of ETCO₂ cut-off values during CPR to accurately predict the outcome of resuscitation is not fully established. Therefore, ETCO₂ values cannot be used as a mortality predictor in isolation.     

End-tidal PCO2 monitoring via nasal cannulae in pediatric patients: Accuracy and sources of error

Objective. To assess the correlation and accuracy of end-tidal PCO₂ (PetCO₂) sampled via nasal cannulae in pediatric patients by comparison to the criterion standard PaCO₂, and to identify sources of error during PetCO₂ monitoring via nasal cannulae.Methods. PetCO₂ was monitored continuously by sampling end-tidal gas through nasal cannulae that had been designed and manufactured for this purpose in spontaneously breathing children undergoing conscious or deep sedation during either cardiac catheterization (n = 43) or critical care (n = 54). When both the capnographic wave form and the PetCO₂ value had been stable for at least 10 minutes, the PetCO₂ value was recorded while blood was drawn from an indwelling arterial line for PaCO₂ measurement. The effects of age, weight, respiratory rate, oxygen delivery system, airway obstruction, mouth breathing, and cyanotic heart disease were evaluated by linear regression analysis and calculation of absolute bias (PaCO₂-PetCO₂).Results. Mouth breathing, airway obstruction, oxygen delivery through the ipsilateral nasal cannula, and cyanotic heart disease adversely affected accuracy. In patients without those factors, PetCO₂ correlated well with PaCO₂ (R² = 0.994), and absolute bias was 3.0 ± 1.8 mmHg.Conclusions. Several factors — some controllable and all recognizable — affect the accuracy of PetCO₂ monitored via nasal cannulae in pediatric patients. When these factors are not present, PetCO₂ correlates well with PaCO₂ and appears to be a useful monitor of ventilatory status during conscious or deep sedation.