A carbon dioxide monitor that does not show the waveform is worthless

The author suggests that the carbon dioxide waveform should be displayed, as are the electrocardiogram and arterial pressure waveforms. He argues that a carbon dioxide analyzer that does not provide a waveform is not of value, as subtle changes in the carbon dioxide waveform can reflect impending problems. Only when a plateau is present in the capnogram can one be certain that end-tidal gas is being measured, and the author asserts that the presence or absence of this plateau can be detected only by visually inspecting the waveform.     

Sensitivity of a disposable end-tidal carbon dioxide detector

A small disposable carbon dioxide detector that can be used to provide evidence of correct endotracheal tube placement is now commercially available (FEF). The device contains an indicator that changes color when exposed to carbon dioxide. This study measured the lowest concentration of carbon dioxide causing a perceivable color change in the device. Ten volunteers were blinded to the concentrations of carbon dioxide in an airway circuit/lung model, and the minimal concentration of carbon dioxide that caused a perceivable color change was recorded. The mean minimum concentration required for detection of a color change was 0.54% (4.1 mm Hg) and ranged from 0.25 to 0.60% (1.9 to 4.6 mm Hg). We conclude that this device should produce a detectable color change even in patients with low end-tidal carbon dioxide, as might be observed during cardiopulmonary resuscitation.Presented in part at the annual meeting of the American Society of Anesthesiologists, New Orleans, October 1989.     

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.     

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.     

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 .     

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.     

Capnography does not reliably detect double-lumen endotracheal tube malplacement

Two patients are described in whom double-lumen endotracheal tube malplacement and its ventilatory consequences were not detected by infrared capnography. Problems were suspected on auscultation, and the malplacement was diagnosed by means of bronchospirometry. We conclude that bronchospirometry helps detect problems with endotracheal intubation.     

Capnography does not reliably detect double-lumen endotracheal tube malplacement

Two patients are described in whom double-lumen endotracheal tube malplacement and its ventilatory consequences were not detected by infrared capnography. Problems were suspected on auscultation, and the malplacement was diagnosed by means of bronchospirometry. We conclude that bronchospirometry helps detect problems with endotracheal intubation.     

Recognition of cardiogenic artifact in pediatric capnograms

The capnograph is in regular use as a respiratory monitor. From measurements of its trace, calculations of breathing rate and end-tidal carbon dioxide can be made. Unfortunately, the reliability of these calculations depends on the quality of the signal. In the case of cardiogenic oscillations, the averaged results may be grossly inaccurate and, therefore, misleading. This paper describes a method for detecting such cardiogenic oscillations and removing their effect from the final results of capnogram analysis. The algorithm used resulted in a sensitivity of 99.6% and a specificity of 99.2%, when compared with manual scoring. The criteria could easily be incorporated into equipment software and are a step in the direction of “intelligent” monitoring.     

Relationship between arterial carbon dioxide and end-tidal carbon dioxide when a nasal sampling port is used

End-tidal carbon dioxide (ETCO₂) values obtained from awake nonintubated patients may prove to be useful in estimating a patient’s ventilatory status. This study examined the relationship between arterial carbon dioxide tension (PaCO₂) and ETCO₂ during the preoperative period in 20 premedicated patients undergoing various surgical procedures. ETCO₂ was sampled from a 16-gauge intravenous catheter pierced through one of the two nasal oxygen prongs and measured at various oxygen flow rates (2, 4, and 6 L/min) by an on-line ETCO₂ monitor with analog display. Both peak and time-averaged values for ETCO₂ were recorded. The results showed that the peak ETCO₂ values (mean = 38.8 mm Hg) correlated more closely with the PaCO₂ values (mean = 38.8 mm Hg; correlation coefficient r = 0.76) than did the average ETCO₂ values irrespective of the oxygen flow rates. The time-averaged PaCO₂-ETCO₂ difference was significantly greater than the PaCO₂-peak ETCO₂ difference (P < 0.001). Values for subgroups within the patient population were also analyzed, and it was shown that patients with minute respiratory rates greater than 20 but less than 30 and patients age 65 years or older did not differ from the overall studied patient population with regard to PaCO₂-ETCO₂ difference. A small subset of patients with respiratory rates of 30/ min or greater (n = 30) did show a significant increase in the PaCO₂-ETCO₂ difference (P < 0.001). It was concluded that under the conditions of this study, peak ETCO₂ values did correlate with PaCO₂ values and were not significantly affected by oxygen flow rate. However, obtaining peak ETCO₂ values is clinically more difficult, especially when partial air-way obstruction is present.     

End-tidal carbon dioxide concentration,carbon dioxide production,heart rate,and blood pressure as indicators of induced hyperthermia

In 4 spontaneously breathing, barbiturate-anesthetized dogs, hyperthermia was induced with 2,4-dinitrophenol while rectal temperature, heart rate, mean blood pressure, end-tidal carbon dioxide, and carbon dioxide production (milliliters per minute) were measured continuously. The latter was determined with a pneumotachygraph (to obtain respired volume) and an infrared carbon dioxide analyzer that measured inspired and expired carbon dioxide concentration. Of the five physiologic measurements, the increase in carbon dioxide production preceded the increase in rectal temperature by more than 120 seconds. End-tidal carbon dioxide was an unreliable indicator in the spontaneously breathing animal of approaching hyperthermia during spontaneous breathing due to a transient tachypnea, which decreased end-tidal carbon dioxide. The carbon dioxide production (milliliters per minute) increased immediately and reached three to five times the control value. Blood pressure and heart rate were insensitive indicators of approaching hyperthermia.     

Combined use of the esophageal-tracheal Combitube with a colorimetric carbon dioxide detector for emergency intubation/ventilation

Objective. The esophageal-tracheal Combitube (Sheridan, Inc., Argyle, NY) is a unique double lumen tube that has been introduced as an emergency intubation device. Since it is placed blindly, proper use requires determination of which lumen can be successfully used for ventilation. The Easycap (Nellcor, Inc., Pleasanton, CA) is a colorimetric carbon dioxide detector that reacts with exhaled gas to indicate proper tracheal tube location. The purpose of this study was to determine if the Easycap can be used to identify which Combitube lumen is patent to the trachea after blind placement in dogs.Methods and Results. The study was conducted using 8 anesthetized dogs. In each of 15 blind insertions of the Combitube, the Easycap device responded appropriately by changing color from purple to yellow when connected to the lumen communicating with the trachea. When the Easycap device was connected to the alternate lumen, no color change was appropriately observed in 9 out of 15 cases (60%) after 6 breaths; in 4 of the remaining 6 (87%, total), no color change was noted after 12 breaths. In the 2 remaining cases, the color change indicated the need for further verification of the tube location. In separate experiments, 10 direct tracheal and esophageal insertions of the Combitube were correctly verified by the appropriate Easycap color change.Conclusions. Our results suggest that the Easycap device may be useful with the Combitube, although human data are required.     

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.     

Placement of esophageal stethoscope by acoustic criteria does not consistently yield an optimal location for the monitoring of core temperature

The esophageal stethoscope has evolved into a device for both acoustic and core temperature monitoring. To test whether routine placement according to acoustic criteria results in placement of the core temperature sensor in the region of contiguity between the esophagus and the heart, we determined the depth of placement electrocardiographically. All patients were undergoing nonthoracic elective operations requiring general anesthesia and tracheal intubation. First, we established that different observers selected the same esophageal depth within ±1 cm electrocardiographically, using the criterion of a symmetric biphasic P wave of maximal amplitude (7 patients). Then, in 30 more patients, we compared routine acoustic placements with the depths of the maximalamplitude biphasic P wave. Stethoscopes placed according to acoustic criteria were within ±3 cm of P-wave depths in 15 of 30 patients. In the remaining patients, measured discrepancies ranged up to 13.5 cm. We conclude that the prevailing stethoscope design, with a thermistor at the tip, below the acoustic window, does not ensure placement of the thermistor within the optimal region for monitoring of core temperature. A modification in design that would take advantage of the reliability of electrocardiographic positioning is suggested. Supported by National Institutes of Health grant HL-16910.     

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.     

Evaluation of two commercially available carbon dioxide sampling nasal cannulae

Our study compared two commercially available carbon dioxide sampling nasal cannulae for efficacy of oxygenation and relationship of end-tidal carbon dioxide (Petco ₂) to arterial carbon dioxide (Paco₂). The two-prong nasal cannula (2PNC) has one prong dedicated to delivering O₂ via one naris and the second prong dedicated to sampling exhaled gases via the other naris. The four-prong nasal cannula (4PNC) delivers O₂ via a prong in each naris, and samples exhaled gases via another set of prongs in each naris. Forty six patients were divided into three groups, which received either 2 (n = 15), 3 (n = 16), or 4(n = 15) L/min O₂, respectively, and were studied sequentially with standard nasal cannula (SNC), the 2PNC, and then the 4PNC. At each O₂ flow rate, Pao₂ was equivalent regardless of whether the SNC, 2PNC, or 4PNC was used. Seventy-four percent (34/46) of the 2PNC and 0% (0/46) of the 4PNCPetco2 values were within ±4 torr of the Paco₂ value. The authors conclude that the 2PNC and 4PNC are equally effective compared with an SNC in oxygenating patients, but thePetco2 measured by the 2PNC provides a superior quantitative estimate of the Paco₂ than that obtained by the 4PNC.