An evaluation of a noninvasive cardiac output measurement using partial carbon dioxide rebreathing in children

Cardiac output (CO) is an important hemodynamic measure that helps to guide the therapy of critically ill patients. Invasive CO assessment in infants and children is often avoided because of the inherent risks. A noninvasive CO monitor that uses partial rebreathing has been recently developed to determine CO via the Fick principle for carbon dioxide. There have been no clinical studies confirming its accuracy in pediatric patients. This is a prospective observational study of 37 children <12 yr of age who underwent cardiac catheterization. Under general anesthesia via an endotracheal tube without a leak, we made multiple CO measurements using thermodilution and compared them with noninvasively determined CO measurements. Paired measurements were analyzed for bias, precision, and correlation via Bland-Altman plot and linear regression. Noninvasive measurements showed a linear correlation with thermodilution CO assessment with an r value of 0.83 (P < 0.03). Bland-Altman analysis yielded a bias of -0.27 L/min and a precision +/-1.49 L/min. Cardiac index measurements demonstrated a decreased r value of 0.67 (P = 0.15) and a bias of -0.18 L . min(-1) . m(-2) and precision of +/-2.13 L . min(-1) . m(-2). Differences between partial rebreathing measurements and thermodilution measurements were largest in children with a body surface area of 0.6 m(2) body surface area and >300 mL tidal volume.     

Evaluation of partial carbon dioxide rebreathing cardiac output measurement during thoracic surgery

OBJECTIVE: Noninvasive partial CO2 rebreathing (NICO; Novametrix Medical Systems, Inc, Wallingford, CT) is a relatively new alternative to thermodilution (TDCO) for measurement of cardiac output. This study compares the 2 methods during thoracic surgery and one-lung ventilation. DESIGN: A prospective, observational study. SETTING: A tertiary hospital. PARTICIPANTS: Twelve adult patients undergoing elective thoracotomy and one-lung ventilation in the lateral decubitus position. INTERVENTIONS: Paired measurements of cardiac output were performed during (1) 2-lung ventilation in the supine position (postinduction of anesthesia), (2) 10 minutes after initiation of one-lung ventilation in the lateral decubitus position with the nondependent chest open, and (3) after 30 minutes on one-lung ventilation. An average of 3 consecutive (10 mL 20 degrees C saline) TDCO measurements made during end-expiration was compared with corresponding NICO measurements. MEASUREMENTS AND MAIN RESULTS: The NICO showed a tendency to underestimate cardiac output compared with TDCO at all measurement times. Overall, bias was -0.29 L/min and limits of agreement -1.69 to 1.43 L/min. CONCLUSIONS: There was a moderate agreement between cardiac output measurements obtained with the NICO and TDCO. The present data suggest that the NICO technique may be useful during thoracic surgery.     

Non-invasive measurement of cardiac output by a single breath constant expiratory technique

A new single breath test has been developed that measures pulmonary blood flow (Qc) and pulmonary tissue volume by using the fact that Qc is proportional to the relationship between the absorption rate of acetylene (C2H2) from the alveolar gas and the rate of change of lung volume during constant expiratory flow. To make these measurements a bag in bottle system with a rolling seal spirometer, a mass spectrometer, and a minicomputer with analogue to digital conversion have been used. Qc was compared with cardiac output measured by the thermodilution technique in 20 patients with cardiac disease; some also had mild chronic obstructive pulmonary disease. The mean (SD) resting Qc for the group was 5.27 (1.22) l/min and the cardiac output measured by thermodilution was 5.30 (1.31) l/min. The mean difference between the two estimations of cardiac output was 0.03 l and the standard deviation of this difference was 0.76 l. The Qc technique was not successful in patients with an FEV1/FVC less than 60%, but seemed to be accurate in those with higher FEV1/FVC values. Correction of Qc for the effect of venous admixture in 14 patients resulted in an average 19% overestimation of cardiac output (6.01 (2.52) l/min v 5.05 (1.64) l/min). It is concluded that cardiac output can be accurately measured in patients with cardiac or mild pulmonary disease. No correction for venous admixture due to ventilation-perfusion mismatch was necessary in these patients, presumably because the large breath used by the technique overcomes most mild ventilation-perfusion maldistribution. These findings, in addition to the non-invasive nature of the technique, suggest potential value for the measurement of cardiac output in various clinical conditions.     

Evaluation of a Doppler catheter probe to measure cardiac output

Although the present study demonstrated that a nondirectional Doppler catheter probe accurately measures unidirectional pulsatile flow velocity, the in vivo evaluation reveals several significant discrepancies. When positioned in the ascending aorta of dogs, the Doppler transducer underestimated changes in blood flow velocity and the degree of underestimation varied significantly between animals. Under these conditions the Doppler-recorded signal indicated nearly continuous aortic flow and a zero-flow reference level was not present in late diastole. The extent of apparent diastolic flow was greater at higher flow rates. These findings indicate that the nondirectional Doppler catheter tip velocity transducer in its present form is not suitable for accurately measuring changes in aortic blood flow velocity.     

Comparison of cardiac output measurement using the CardioQP oesophageal Doppler with cardiac output measurement using thermodilution technique in children during heart catheterisation

The minimally invasive CardioQP oesophageal Doppler probe estimates cardiac output by measuring blood flow velocity in the descending aorta. Individual variables to enter are patient's age, weight and height. We measured cardiac output simultaneously with CardioQP and pulmonary artery catheter thermodilution techniques during heart catheterisation in 40 paediatric patients with congenital heart defects. Median [range] age was 8.2 years [0.5-16.7 years], cardiac output values measured by thermodilution and CardioQP were 3.6 l.min(-1) [1.2-7.1 l.min(-1)] and 3.0 l.min(-1) [0.7-6.7 l.min(-1)], respectively. These values showed only moderate correlation (r = 0.809; p < 0.0001). Bias and precision were 0.66 l.min(-1) and 1.79 l.min(-1) (95% limits of agreement: -1.13 to +2.45 l.min(-1)). Based on our preliminary experience, cardiac output values measured by CardioQP in children do not reliably represent cardiac output values compared with the thermodilution technique. We suggest measurement of individual aortic diameter to improve performance of the CardioQP.     

Noninvasive cardiac output measurement using partial carbon dioxide rebreathing is less accurate at settings of reduced minute ventilation and when spontaneous breathing is present

BACKGROUND: Although evaluation of cardiac output by the partial carbon dioxide rebreathing technique is as accurate as thermodilution techniques under controlled mechanical ventilation, it is less accurate at low tidal volume. It is not clear whether reduced accuracy is due to low tidal volume or low minute ventilation. The effect of spontaneous breathing on the accuracy of partial carbon dioxide rebreathing measurement has not been fully investigated. The objectives of the current study were to investigate whether tidal volume or minute ventilation is the dominant factor for the accuracy, and the accuracy of the technique when spontaneous breathing effort is present. METHODS: The authors enrolled 25 post-cardiac surgery patients in two serial protocols. First, the authors applied three settings of controlled mechanical ventilation in random order: large tidal volume (12 ml/kg), the same minute ventilation with a small tidal volume (6 ml/kg), and 50% decreased minute ventilation with a small tidal volume (6 ml/kg). Second, when the patient recovered spontaneous breathing, the authors applied three conditions of partial ventilatory support in random order: synchronized intermittent mandatory ventilation-pressure support ventilation, pressure support ventilation with an appropriately adjusted rebreathing loop, and pressure support ventilation with the shortest available loop. After establishing steady state conditions, the authors measured cardiac output using both partial carbon dioxide rebreathing and thermodilution methods. The correlation between the data yielded by the two methods was determined by Bland-Altman analysis and linear regression. RESULTS: Cardiac output with the carbon dioxide rebreathing technique correlated moderately with that measured by thermodilution when minute ventilation was set to maintain normocapnia, regardless of tidal volumes. However, when minute ventilation was set low, the carbon dioxide rebreathing technique underreported cardiac output (y = 0.70x; correlation coefficient, 0.34; bias, -1.73 l/min; precision, 1.27 l/min; limits of agreement, -4.27 to +0.81 l/min). When there was spontaneous breathing, the correlation between the two cardiac output measurements became worse. Carbon dioxide rebreathing increased spontaneous tidal volume and respiratory rate (20% and 30%, respectively, during pressure support ventilation) when the rebreathing loop was adjusted for large tidal volume. CONCLUSIONS: During controlled mechanical ventilation, minute ventilation rather than tidal volume affected the accuracy of cardiac output measurement using the partial carbon dioxide rebreathing technique. When spontaneous breathing is present, the carbon dioxide rebreathing technique is less accurate and increases spontaneous tidal volume and respiratory rate.     

Positive end-expiratory pressure has little effect on carbon dioxide elimination after cardiac surgery

We investigated the effects of positive end-expiratory pressure (PEEP) on carbon dioxide (CO2) elimination in a cross-over study of 14 patients whose lungs were ventilated after cardiac surgery. They initially received either 7.5 cm H2O PEEP or zero end-expiratory pressure and were then changed over to the other mode. We measured CO2 minute elimination (Vco2) and "efficiency," a quantification of the shape of CO2 single-breath test (SBT-CO2), the plot of expired CO2 against expired volume. Vco2 and efficiency (and therefore the shape of SBT-CO2) were not significantly affected by PEEP. These results agree with findings in patients with acute lung injury, but are in contrast with those in an open-chest dog model, in which 7.5 cm H2O PEEP caused a 19% decrease in Vco2 and significant changes in SBT-CO2. IMPLICATIONS: During artificial ventilation, applying a positive pressure in expiration expands the lung and improves the uptake of oxygen, but there is a theoretical risk of reduced carbon dioxide elimination. We applied positive end-expiratory pressure to patients immediately after heart surgery and found that it has no effect on carbon dioxide elimination.     

Monitoring cardiac output trends with end-tidal carbon dioxide pressures in off-pump coronary bypass

The optimum method of intraoperative monitoring in patients undergoing off-pump coronary bypass remains debatable. Using capnography and end-tidal carbon dioxide pressure may be a helpful method of monitoring cardiac output trends, especially when grafting posterior and lateral vessels. After repositioning the heart, End-tidal carbon dioxide pressures that continue to trend down usually indicate decreased cardiac output and precede hemodynamic and electrical instability. The advantages of this method include simplicity, universal availability, and a short response time to changes in cardiac output.     

Do oxygen consumption and carbon dioxide production affect cardiac output after cardiopulmonary bypass?

This study examines the oxygen consumption (VO2) and carbon dioxide production (VCO2) occurring before, during, and after cardiopulmonary bypass (CPB) and whether they correlate with changes in cardiac output. Twenty-three patients undergoing open heart surgery were studied. Group 1 (N = 11) received fentanyl citrate, 50 micrograms/kg, intravenously during the induction of anesthesia. Group 2 (N = 12) received 100 micrograms/kg of fentanyl citrate intravenously. We measured VO2, VCO2, as well as hemodynamic and biochemical factors. Initial statistical analyses failed to show any differences in the VO2, VCO2, hemodynamic, or biochemical factors between groups 1 and 2. Therefore, the data from both groups were combined. In comparing the average (for all data) of the post-CPB with the pre-CPB periods in both groups for the metabolic factors, there were 9.0%, 11.5%, and 2.4% increases in the VO2, VCO2, and respiratory quotient, respectively. There was an 80% increase in total serum lactate levels seen in the post-CPB periods when compared with the pre-CPB periods. Serum triglyceride and free fatty acid levels measured in the post-CPB period decreased 39% and 25%, respectively, when compared with the pre-CPB periods. Although there were no changes in the cardiac outputs following CPB, the post-CPB periods showed a 37% increase in central venous pressure when compared with the pre-CPB periods. These data suggest that although there are significant metabolic and biochemical sequelae to CPB, the modest increases in post-CPB VO2, and VCO2 did not affect cardiac output following cardiovascular surgery. Increasing doses of narcotic do not have an effect on those relationships.     

超声心输出量监测仪用于心输出量测定的研究进展和评价

心输出量(cardiac output,CO)是反映心脏功能最直接的指标之一,准确监测CO及相关的血流动力学指标对指导临床治疗具有重要意义.超声心输出量监测仪(ultrasonic cardiac output monitor,USCOM)是目前较新的一种CO临测仪,具有无创、快速、准确等特点,近年来刚刚投入临床试用,对此技术的优越性还有待于在临床来进一步检验和改进.现就此对国内外相关文献进行综述,以便了解和评定USCOM的临床应用价值.     

Non-invasive cardiac output determination: comparison of a new partial-rebreathing technique with thermodilution

This study compares a derivative Fick technique using carbon dioxide (CO2) with the thermodilution pulmonary artery catheter (PAC), for determination of cardiac output (CO). Subjects were sedated, mechanically ventilated adults following elective cardiac surgery Microprocessor controlled deadspace activation and side-stream capnography in a ventilator circuit enabled calculation of CO (CO(CO2)) every four minutes. Thermodilution CO (CO(TD)) was performed as clinically indicated and at 20-minute intervals. Simultaneous CO(TD)/CO(CO2) pairs were recorded from time of admission to ICU for a minimum period of two hours for each patient. There were 358 CO(TD)/CO(CO2) pairs recorded from 41 patients. Cardiac output measurements ranged from 2. 7 to 10.6 l/min. The bias (Bland-Altman) was 0.050 l/min (95% CI -0.024 to 0.125 l/min). The 95% limits of agreement were -1.354 to 1.455 l/min. This simple, non-invasive partial-rebreathing technique is a valid alternative to thermodilution for cardiac output determination in sedated, mechanically ventilated patients. There are significant implications for improved safety, reduced complexity and reduced cost in anaesthesia and intensive care.     

Transpulmonary thermodilution cardiac output measurement using the axillary artery in critically ill patients

STUDY OBJECTIVE: To compare cardiac output (CO) as measured by the arterial thermodilution technique using only a central venous catheter and an arterial catheter inserted into the axillary artery, with conventional CO measurement with thermodilution using a pulmonary artery (PA) catheter (PAC). DESIGN: Prospective clinical study in which each patient served as his/her own control. SETTING: General ICU of a large tertiary-care teaching hospital. PATIENTS: 22 patients who required invasive hemodynamic monitoring in the ICU. INTERVENTIONS AND MEASUREMENTS: CO measurements made using the PAC (COpa) were compared to bolus arterial thermodilution measurements (COax). The significance of acute changes in the continuous CO measurements during acute hemodynamic episodes was observed. MAIN RESULTS : The correlation between the two techniques (COpa and COax) was R(2) = 0.82. There was a tendency for 5% overestimation of COpa by the COax. The SEM% (SEM/average CO) for COax and COpa was 2.6% and 3.2%, respectively. The bias between measurements was 0.27 +/- 0.67 L/min, and the limits of agreement (mean difference +/- 2 SD) from minus 1.07 L/min to 1.63 L/min. CONCLUSIONS: In critically ill patients, in whom the measurement of CO is required, arterial thermodilution, using a central vein and the axillary artery is accurate and reproducible.     

Measurement of oxygen uptake and carbon dioxide elimination using the bymixer: validation in a metabolic lung simulator

BACKGROUND: The authors have developed a new clinical bymixer that bypasses a constant fraction of gas flow through a mixing arm. A separate bymixer was interposed in the expiratory and inspiratory limbs of the ventilation circuit to measure mixed gas fractions. By utilizing nitrogen conservation, the clinical bymixer allows the determination of airway carbon dioxide elimination (VCO2) and oxygen uptake (VO2), whenever basic expired flow and gas monitoring measurements are used for the patient. Neither an expiratory exhaust gas collection bag nor expensive, complex equipment are needed. This study tested the accuracy of airway bymixer-flow measurements of VCO2 and VO2 in a new bench apparatus. METHODS: The authors compared airway bymixer-flow measurements of VCO2 and VO2 over a range of reference values generated by ethanol combustion in a new metabolic lung simulator, which was ventilated by a volume-cycled respirator. An airway humidity and temperature sensor permitted standard temperature and pressure, dry, correction of airway VCO2 and VO2. RESULTS: Bymixer-flow airway measurements of VCO2 and VO2 correlated closely (R2 = 0.999 and 0.998, respectively) with the stoichiometric values generated by ethanol combustion. Limits of agreement for VCO2 and VO2 were 0.1 +/- 4.7 and 1.1 +/- 5.7%, respectively. The average (+/- SD) percent error for airway VCO2 (compared with the stoichiometric value) was 0.1 +/- 2.4%. The same error for airway VO2 was 1.1 +/- 2.9%. CONCLUSIONS: The new clinical bymixer, plus basic expired flow and gas fraction measurements, generated clinically accurate determinations of VCO2 and VO2. These measurements are helpful in the assessment of metabolic gas exchange in the critical care unit. In contrast to using the gas collection bag or complex metabolic monitor, the bymixer should measure mixed gas concentrations in the inspired or expired limb of the common anesthesia circle ventilation circuit.     

Continuous measurement of cardiac output with a catheter blood flow probe

It is concluded that an accurate and reliable measure of cardiac output can be achieved by the intravascular probe positioned in the pulmonary artery. Heparin coating of the intravascular probe is essential to its long-term use.