The non-invasive acetylene rebreathing method for estimation of cardiac output: influence of breath-by-breath variation

The inert gas rebreathing method enables non-invasive estimation of pulmonary capillary blood flow, lung tissue volume, transfer factor and functional residual capacity. In the present study, we have examined the influence of breathing pattern during the rebreathing manoeuvre on the precision of the rebreathing method, both theoretically and experimentally. We examined whether the precision of the method could be improved by the guidance and training of the subjects doing the rebreathing manoeuvre. The results of the theoretical study showed that the precision and accuracy of the rebreathing method are practically insensitive to random variation in the breathing pattern. Simulated breath-by-breath variations up to ±50% of the average 3·0 l V_T resulted in coefficients of variation of about 3% for Q_C and about 5% for V_TC. Simulated breath-by-breath variations indicate that with mean tidal volume or rebreathing bag volume lower than 1·0 l the precision will worsen, and with tidal volume lower than 1·5 l the accuracy will worsen. The experimental results showed no significant improvement in the precision of the rebreathing method by visual guiding and training of the subjects to optimize the breathing pattern during the rebreathing manoeuvre.     

Validation of the acetylene rebreathing method for measurement of cardiac output at rest and during high-intensity exercise

The use of the acetylene rebreathing method to estimate cardiac output (CO) during high-intensity exercise, which may be influenced by recirculation of acetylene, has not been validated. This study was designed to validate the acetylene rebreathing method to measure CO during high-intensity exercise using the direct Fick method. CO was measured at rest and during exercise at 25%, 50%, 75% and 90% of the nine subjects maximum oxygen uptake (VO _2max) by the direct Fick and acetylene rebreathing method. CO measured by the acetylene rebreathing method correlated with work rate (r=0·90, P<0·01) and with oxygen uptake (r=0·94, P<0·01). The correlation coefficient of CO between both methods was r=0·91 (P<0·01). There was no significant difference in CO measured by each method at rest as well as at each work rate. The difference in CO between each method was greater at lower CO than at higher CO. At 90% of VO _2max, the CO measured by acetylene rebreathing was nearly identical to that measured by the Fick method. It can be concluded that acetylene rebreathing for measurement of CO is valid not only at rest but also during exercise, especially during high-intensity exercise.     

Reproducibility of a semiautomated acetylene rebreathing technique for measuring cardiac output in humans at rest

The specific aims of the present study were to determine: (1) the day-to-day reproducibility of a semiautomated acetylene rebreathing technique for measuring cardiac output under resting conditions; (2) the reproducibility of this technique among subjects differing in gender and age; and (3) the number of trials within a session necessary to maximize the day-to-day reproducibility of the technique. To address these aims, cardiac output was measured in 21 healthy men (n = 8) and women (n = 13) between the ages of 25 and 71 years in the supine posture on two separate days. Mean levels of cardiac output at rest were similar on day 1 vs. day 2 in the overall group. Cardiac output measured on day 1 was highly correlated (r = 0·98, P<0·001) with cardiac output measured on day 2. The day 1 to day 2 mean difference in cardiac output for the individual subjects was <4%. The mean levels of heart rate and stroke volume also were similar between day 1 and day 2. The relation between cardiac output measured on day 1 vs. day 2 in the gender and age subgroups was similar to that observed in the overall group. The mean absolute difference among the three rebreathing trials within a day was 360 ml min^?1 in the overall group, with a coefficient of variation of 7%. The variability between rebreathing trials measured on day 1 vs. day 2 in the gender and age subgroups was similar to that observed in the overall group. The reliability of cardiac output measured on different days was excellent with a single rebreathing trial (r = 0.93) and improved significantly up to three trials (r = 0.98). In conclusion, the findings of the present study indicate that the acetylene rebreathing technique can be a highly reproducible method for measuring cardiac output under resting conditions. The reproducibility is consistently strong in healthy humans of varying age and in both genders, and is enhanced by the use of multiple trials.     

惰性气体再呼吸法评价心排血量

在心力衰竭管理和治疗中,评价心排血量是判断疾病严重程度的重要环节。理想的测定心排血量的方法应该是无创的。惰性气体再呼吸法是对通气浓度进行连续分析以测定有效肺血流量的方法。在无肺血流分流的情况下可以等同于心排血量,具有准确、安全、简易的特点。该方法的优势在于其结果反映的是运动心血量,测量结果不受呼吸、心率及瓣膜反流等因素的影响。在临床上和6 min步行试验结合使用可以更全面准确地评价心排血量及心脏储备能力。     

Evaluation of the microsphere-method for determination of cardiac output.

Cardiac output was determined by means of radioactive microspheres, 15 +/- 5 microns in diameter. Blood flow in the ascending aorta was measured by an electromagnetic flowmeter. In eight cats thirty-eight simultaneous measurements were made of cardiac output and aortic flow. The observations correlated well (r = 0.89), with a mean difference of 5.2%, probably corresponding to coronary blood flow. Continuous flow recordings showed no alterations in aortic flow during the injections of microspheres; arterial blood pressure and heart rate remained unchanged even after six injections, each consisting of about 2 x 10(5) spheres per kg body weight. Tests for shunting of microspheres were performed and revealed shunt fractions in the systemic circulation of about 8%, with no significant shunting through the lungs. The microsphere method for determining cardiac output is thought to be an accurate method, suitable for small and medium sized animals.     

Reproducibility of cardiac output measurement by the nitrous oxide rebreathing technique

Techniques for the measurement of cardiac output from soluble gas uptake by the lungs include the rebreathing method using nitrous oxide. The accuracy of this␣technique is well accepted, but its repeatability of measurement (precision) has not been well documented. We assessed the repeatability of measurements of pulmonary blood flow by the Innocor, a device employing the nitrous oxide rebreathing method. Successive paired measurements of pulmonary blood flow were made separated by a 5 min interval by the nitrous oxide rebreathing method, in 8 patients pre- or post cardiac surgery, and in 8 healthy volunteers. The standard deviation of the difference between first and second measurements was 0.84 l/min in the cardiac surgery group, and 1.25 l/min in the healthy volunteers. There was no significant bias in successive paired measurements of pulmonary blood flow in either the cardiac surgery patients (mean [95%CI] = −0.02 l/min [−0.62 to 0.57] or the healthy volunteers (0.00 l/min [−0.88 to 0.88]). Intra-class correlation coefficients for the␣healthy and cardiac patients were 0.77 and 0.64 respectively. Multiple measurements should be made and averaged when using the inert gas rebreathing technique for pulmonary blood flow determination. When comparing agreement with other methods for cardiac output measurement, the internal consistency of both methods should be considered. Peyton PJ, Bailey M, Thompson BR. Reproducibility of cardiac output measurement by the nitrous oxide rebreathing technique.     

Evaluation of a noninvasive method for cardiac output measurement in critical care patients

OBJECTIVE: Thermodilution (TD) is the gold standard to monitor cardiac output (CO) in critical care. However, there is concern about the safety of right-ventricular catheterization. The CO(2) rebreathing technique allows noninvasive CO determination by means of the indirect Fick principle. Our objectives were: (a) to assess the accuracy of a new system of CO measurement using the CO(2) partial rebreathing method (PRCO); (b) to evaluate whether the PRCO itself may induce changes in CO. DESIGN AND SETTING: Prospective study in the intensive care department in a university-affiliated hospital. PATIENTS: Twenty-two mechanically ventilated critically ill patients. INTERVENTIONS: CO measured simultaneously by PRCO and TDCO. MEASUREMENTS AND RESULTS: PRCO and TDCO values were compared by concordance analysis. Stability of cardiac output during PRCO was evaluated by comparing the TDCO measurements before, during, and after the partial rebreathing period using analysis of variance. From a total of 79 valid sets of measurements, bias and precision was calculated at -0.18+/-1.39 l/min. The concordance analysis of lower and intermediate CO values (<7 l/min) yielded a bias and precision calculation of -0.07+/-0.91 l/min. No changes in hemodynamics were observed during the partial rebreathing period. CONCLUSIONS: The noninvasive partial CO(2) rebreathing technique may be an alternative method for CO determination in mechanically ventilated critically ill patients. The rebreathing maneuver alone does not induce changes in CO.     

Partial CO2 rebreathing indirect Fick technique for non-invasive measurement of cardiac output

Objective.Evaluation in animals of a non-invasive and continuous cardiac output monitoring system based on partial carbon-dioxide (CO₂) rebreathing indirect Fick technique. Methods.We have developed a non-invasive cardiac output (NICO) monitoring system, based on the partial rebreathing method. The partial rebreathing technique employs a differential form of the Fick equation for calculating cardiac output (Q_T) using non-invasive measurements. Changes in CO₂ elimination (ΔVCO₂) and partial pressure of end-tidal CO₂ (Δ PETCO₂) in response to a brief period of partial rebreathing are used to measure pulmonary capillary blood flow (Q_PCBF). A non-invasive estimate of anatomic and intrapulmonary shunt fraction (Q_S/Q_T), based on oxygen saturation from pulse oximetry (SpO₂) and inspired oxygen concentration (FIO₂), is added to compute total cardiac output [Q_T=Q_PCBF/(1−Q_S/Q_T)]. The performance of the NICO was compared with iced 5% dextrose bolus thermodilution cardiac output (TDco) measurements in 6 dogs. Cardiac output was varied using dobutamine, and halothane, and by clamping of the inferior vena cava. Two hundred and forty-six (n = 246) paired measurements of TDco and NICO over a range of cardiac outputs (TDco range = 0.60–8.87 l/min) were compared using Bland-Altman analysis and weighted correlation coefficient. Results.The Bland–Altman technique yielded a NICO precision of ± 0.70 l/min (13.8%) with a mean bias of −0.07 l/min (−1.4%) compared to TDco. The weighted correlation coefficient between TDco and NICO values was: r= 0.93 (n= 246). Conclusion.The partial CO₂ rebreathing technique for measurement of cardiac output is non-invasive, automated, and based on the well accepted Fick principle. The limits of agreement between NICO and TDco is within the recommended value for NICO to be a clinically acceptable method for cardiac output measurement. The results of this canine study show that NICO performed as well, and in some cases better, than other currently available non-invasive cardiac output techniques over a wide range of cardiac outputs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of ventilation on cardiac output determined by inert gas rebreathing

One of the most important methodological problems of the foreign gas rebreathing technique is that outcome of the measurements depends on procedural variables such as rebreathing frequency (RF), rebreathing bag volume (V(reb)), lung volume at start of rebreathing and intervals between measurements. Therefore, in 10 healthy males we investigated the effects of changes in ventilation pattern on cardiac output (CO) estimated by an N(2)O-rebreathing technique. Reducing the rebreathing volume (V(reb)) from 1.5 to 1.0 l diminished CO by 0.5 +/- 0.2 l min(-1), whereas an increase in V(reb) from 1.5 to 2.5 l had no effects. CO was 1.0 +/- 0.2 l min(-1) higher when, rebreathing was performed after a forced expiration than following a normal tidal expiration. Serial determinations of CO required a 3-min interval between the measurements to avoid effects of recirculation of N(2)O. Changing RF from 15 to 30 breaths min(-1) or adding serial dead space by up to 600 ml did not affect the determination of CO. In conclusion, the rebreathing procedure for determination of CO at rest should be performed following a normal tidal expiration with a rebreathing bag volume of between 1.5 and 2.5 l and with manoeuvres separated by at least 3-5 min. Variations in RF within the physiological range from 15 to 30 breaths min(-1) do not affect outcome of the measurements.     

Non-invasive measurement of cardiac output in patients with acute lung injury using the carbon dioxide rebreathing method.

STUDY OBJECTIVE: To compare measurement of cardiac output by the CO2 rebreathing method with the thermodilution cardiac output technique in mechanically ventilated patients with acute lung injury. DESIGN: Prospective study comparing two methods of cardiac output measurement in 22 consecutive patients with acute lung injury. SETTING: Intensive care unit of a university hospital. PATIENTS: Twenty-two mechanically ventilated patients with acute lung injury monitored with systemic and pulmonary artery catheters. MEASUREMENTS AND RESULTS: Cardiac output was determined using both the thermodilution technique and an indirect CO2 Fick method. Veno-arterial CO2 content difference was calculated from an estimated mixed venous CO2 tension obtained by an equilibrium CO2 rebreathing method and measured arterial CO2 tension. Carbon dioxide pressure was converted to content using the equation of the CO2 dissociation curve described by McHardy. A wide range of cardiac index was studied from 2.7-5.7 l/min/m2. There was a significant correlation between thermodilution and CO2 rebreathing methods (r2 = 0.82, p < 0.01). The mean difference between the CO2 rebreathing method and thermodilution was 0.05 l/min/m2, with a standard deviation for the bias of 0.38 l/min/m2. CONCLUSION: Our results suggest that the CO2 rebreathing method may be a reliable non-invasive technique to determine cardiac output in mechanically ventilated patients with acute lung injury.     

Cardiac output determined by the CO2 rebreathing method during arm exercise

Summary: Since arm exercise affects the respiratory muscles the CO₂ rebreathing method for determining cardiac output (QR) has to be evaluated during arm exercise. The purpose of this study was (1) to compare three different methods of determining arterial CO₂ tension (PaCO₂) during arm exercise, (2) to verify the linearity of the relationship between QR and oxygen uptake (VO₂) during arm exercise, and (3) to investigate whether the CO₂ rebreathing method according to Collier can determine accurately QR during arm exercise. Sixty male subjects performed arm-cranking exercise at 20%, 40% and 60% of their individual maximal load. Carbon dioxide output (V?O₂) was measured by gas exchange measurement, and mixed venous CO₂ tension (Pv?O₂) was determined from the CO₂ rebreathing plateau at each exercise level. PaCO₂ was estimated in three different ways: (A) by the modified Bohr formula for dead space, (B) by an arterialized blood sample from the hyperaemic ear-lobe, and (C) by the end-expiratory CO₂ tension. A, B, and C were used to calculate QRa, Qb and Qc, respectively. The Pearson's correlation coefficient was high (P < 0–01) among the three different ways of estimating PaCO₂. The QR-VO₂ relationship proved to be linear (P < 0–01). The Q-values showed a good agreement with the direct Fick measurements, and were in the same range compared to other results obtained by dye dilution, electrical impedance cardiography and the exponential CO₂ rebreathing method during arm exercise. In conclusion, the CO₂ rebreathing method appeared to be accurate to determine Q during submaximal arm exercise.,     

肺内分流量对部分CO2重复吸入法测定心输出量的影响

目的探讨部分CO2重复吸入法测定危重病患者心输出量的准确性及肺内分流量对其影响.方法26例机械通气患者接无创心输出量监测仪(NICO监测仪),利用部分CO2重复吸入法测定心脏指数(CINICO),同时置入肺动脉漂浮导管,以热稀释法测定心脏指数(CITD).公式法与NICO监测仪同时计算肺内分流量并进行比较.公式法计算的肺内分流量≤15%的患者为低分流组,＞15%为高分流组.结果低分流组(n=16)NICO监测仪计算的肺内分流量与公式法测定的肺内分流量无显著差异;CINICO与CITD相关性良好(r=0.91,P＜0.01),CINICO为(2.24±0.42)L/(min·m2),CITD为(2.25±0.40)L/(min·m2),两者差异无显著性.高分流组(n=10)NICO监测仪计算的肺内分流量明显低于公式法计算的肺内分流量(P＜0.01);CINICO与CITD亦显著相关(r=0.81,P＜0.01),但CINICO明显低于CITD(P＜0.01),两者分别为(2.56±0.64)L/(min·m2)和(2.87±0.64)L/(min·m2).结论CINICO与CITD相关性良好,肺内分流量是影响CINICO准确性的重要因素.     

A comparative evaluation of electrical velocimetry and inert gas rebreathing for the non-invasive assessment of cardiac output

Background  

When assessing the function of the cardiovascular system, cardiac output (CO) is a substantial parameter. For its determination, numerous non-invasive techniques have been proposed in the recent years including inert gas rebreathing (IGR) and impedance cardiography (ICG). The aim of our study was to evaluate whether a novel ICG algorithm (electrical velocimetry) and IGR can be used interchangeably in the clinical setting.     

Non-invasive measurement of cardiac output in heart failure patients using a new foreign gas rebreathing technique

Values of effective pulmonary blood flow (Q(EP)) and cardiac output, determined by a non-invasive foreign gas rebreathing method (CO(RB)) using a new infrared photoacoustic gas analysing system, were compared with measurements of cardiac output obtained by the direct Fick (CO(FICK)) and thermodilution (CO(TD)) methods in patients with heart failure or pulmonary hypertension. In 11 patients, of which three had shunt flow through areas without significant gas exchange, the mean difference (bias) and limits of agreement (+/- 2 S.D.) were 0.6 +/- 1.2 litre x min(-1) when comparing CO(FICK) and Q(EP), and -0.8 +/- 1.3 litre x min(-1) when comparing CO(FICK) and CO(TD). When correction for intrapulmonary shunt flow was applied (i.e. calculation of CO(RB)) in all 11 patients, the bias between CO(FICK) and CO(RB) was 0.1 +/- 0.9 litre x min(-1), primarily because agreement improved in the three patients with significant shunt flow. In the eight patients without significant shunt flow, the agreement between Q(EP) and CO(FICK) was 0.3 +/- 0.9 litre x min(-1). In conclusion, a foreign gas rebreathing method with a new infrared photoacoustic gas analyser provided at least as reliable a measure of cardiac output as did thermodilution. In the absence of significant shunt flow, measurement of Q(EP) itself provides a reliable estimate of cardiac output in heart failure patients. The infrared photoacoustic gas analyser markedly facilitates clinical use of the rebreathing method in general, which makes the method available to a larger group of clinicians working with patients with cardiovascular diseases.     

Determination of cardiac output with a modified Fick method using estimated instead of measured oxygen consumption

Cardiac output (CO) was determined with a modification of the Fick method using estimated, instead of measured, oxygen consumption values. This avoids several possible sources of error in connection with air sampling and oxygen content analysis, thus a relaxed, steady state is obtained more easily. Using the thermodilution technique a good correlation with CO values was found (r = 0.92, p less than 0.001). We conclude that the modified Fick method is simple, accurate and offers reproducible CO estimates in the majority of patients.     

Evaluation of a thoracic bioimpedance cardiac output monitor during cardiac catheterization

OBJECTIVE: To evaluate the accuracy and precision of an advanced thoracic bioimpedance cardiac output monitor by comparing it with conventional thermodilution. DESIGN: Prospective data collected from 47 patients undergoing routine cardiac catheterization. The new bioimpedance system differs from its predecessors in electrode system configuration, advanced signal processing, use of a modified Kubicek equation, and a reliable estimate of left ventricular ejection time from the time derivative bioimpedance signals. SETTING: A cardiac catheterization laboratory in a university affiliated teaching hospital. PATIENTS: A series of 47 relatively homogenous patients undergoing routine cardiac catheterization for suspected cardiac disease. MEASUREMENTS AND MAIN RESULTS: The data from the first 20 patients was used to determine optimal values for coefficients in the bioimpedance cardiac output equations. The coefficients found were used when the system was tested in the subsequent 27 patients. For the last 27 patients, a total of 80 simultaneous pairs of cardiac output measurements were made by conventional thermodilution and by thoracic bioimpedance. The mean difference between the two methods was -0.31 L/min and the standard deviation of the differences was (0.76 L/min). The correlation coefficient was r2 = .72 (p < .001). CONCLUSIONS: The correlation between conventional thermodilution and thoracic bioimpedance cardiac output estimates was good and the standard deviation of the differences was lower than that reported for commercially available devices. The system can be used in the cardiac catheterization lab for reliable and continuous noninvasive measurement of cardiac output.     

Comparison of a modified Fick method with thermodilution for determining cardiac output in critically ill patients on mechanical ventilation

We compared a modified Fick method for measuring cardiac output against the thermodilution method in 11 critically-ill patients on mechanical ventilation. Oxygen consumption (VO2) was calculated indirectly, by measuring the carbon dioxide elimination (VCO2) during steady state and by assuming an average respiratory quotient of 0.9. For a total of 129 measurements, the mean difference in cardiac output between both methods was 0.03 +/- 1.3 l/min (95% confidence interval, -0.19 to 0.25 l/min, p = 0.7) standard deviation, with the largest differences being measured in the low cardiac output range (less than 5 l/min). No statistically significant difference was found between the cardiac output values obtained with either method. These data showed a good correlation between the two methods and suggest that the modified Fick method may be useful in determining cardiac output in seriously ill patients on mechanical ventilation not requiring pulmonary arterial catheterisation, or where facilities for undertaking metabolic measurements are not available.