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Background: The aims of this study were to determine the agreement between pulmonary artery thermodilution (PA-TD), transpulmonary thermodilution (TP-TD) and the pulse contour method, and to test the ability of the pulse contour method to track changes in cardiac output.
Methods: Cardiac output was determined twice before cardiac surgery with both PA-TD and TP-TD. The precision (two standard deviations of the difference between repeated measurements) and agreement of the two methods were calculated. Post-operatively, cardiac output was determined with the PA-TD and pulse contour methods, and the bias and limits of agreement were again calculated. Finally, in patients with heart rates below 60 beats/min or a cardiac index of less than 2.5 l/min/m2, atrial pacing was started and the haemodynamic consequences were monitored with the PA-TD and pulse contour methods.
Results: Twenty-five patients were included. The precisions of PA-TD and TP-TD were 0.41 l/min [95% confidence interval (CI), ± 0.07] and 0.48 l/min (95% CI, ± 0.08), respectively. The bias and limits of agreement between PA-TD and TP-TD were – 0.46 l/min (95% CI, ± 0.11) and ± 1.10 l/min (95% CI, ± 0.19), respectively. Post-operatively, the bias and limits of agreement between the PA-TD and pulse contour methods were 0.07 l/min and ± 2.20 l/min, respectively. The changes in cardiac output with atrial pacing were in the same direction and of the same magnitude in 15 of the 16 patients.
Conclusion: The precision of cardiac output measurements with PA-TD and TP-TD was very similar. The transpulmonary method, however, overestimated the cardiac output by 0.46 l/min. Post-operatively, cardiac output measurements with the PA-TD and pulse contour methods did not agree, but the pulse contour method reliably tracked pacing-induced changes in cardiac output.
The estimation of cardiac output by the pulse contour method was re-introduced more than two decades ago by Wesseling et al. (1).
In the mid-1990s, Pulsion Medical Systems developed an algorithm and a monitor (PiCCO) that estimated the left ventricular stroke volume, beat-to-beat, from the pressure curve of the aorta. The PiCCOplus, however, requires a patient-specific in vivo calibration to compensate for the unknown compliance of the aorta. For this purpose, Pulsion Medical Systems elected to use the bolus transpulmonary thermodilution (TP-TD) cardiac output. Iced saline (15 ml) is injected into a central vein and, from the temperature change in the aorta, the cardiac output can be calculated and the system can be calibrated.
Most studies have found, a posteriori, ‘acceptable’ agreement between TP-TD, the pulse contour method and the de facto clinical standard – pulmonary artery thermodilution (PA-TD) (2–9). In none of the studies, however, has the precision of the methods been determined a priori, and therefore the notion that the methods agree and can be used interchangeably is not securely founded. Moreover, the ability of the PiCCO algorithm to track changes, induced or spontaneous, in cardiac output has not been determined unequivocally.
The issues to be presented and discussed in this paper are as follows: (i) the precision of PA-TD and TP-TD cardiac output measurements; (ii) the agreement between PA-TD and TP-TD cardiac output measurements; (iii) the agreement between PA-TD and pulse contour cardiac output measurements; and (iv) a comparison of the changes in cardiac output determined with the PA-TD and pulse contour methods.
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The ethics committee of Copenhagen County approved the study, and all patients gave written informed consent.
Twenty-five patients without heart valve pathology and in sinus rhythm, scheduled for adult cardiac surgery [coronary artery bypass grafting (CABG) or off-pump coronary artery bypass (OPCAB)], were included in the study. The patients were anaesthesized with induction doses of fentanyl 5–10 μg/kg, midazolam 2–5 mg and pancuronium 0.1 mg/kg, and anaesthesia was continued with enflurane 0.8–1.2% or sevoflurane 1–2% inspired in oxygen. During extracorporeal circulation, anaesthesia was maintained with propofol 200–300 mg/h. All patients were normothermic and normoventilated (PaCO2 between 4.5 and 6.0 kPa) during the study.
A 7.5F PA-TD catheter (Paceport®, Edwards, Copenhagen, Denmark) was inserted via an internal or external jugular vein and advanced until a typical pulmonary artery pressure contour, measured from the tip of the catheter, was evident. The PA-TD cardiac output was determined by the Siemens Sirecust 1281 computer (Siemens, Ballerup, Denmark).
A 20-cm 5F thermistor-tipped arterial catheter (Pulsion Medical Systems, Munich, Germany) was advanced, at the same time, into the abdominal aorta via one of the femoral arteries. The catheter was connected to a pressure transducer from which the pressure signal was transferred to the PiCCO monitor (Pulsion Medical Systems).
After the induction of anaesthesia and haemodynamic stabilization, the cardiac output was determined twice with both PA-TD and TP-TD to determine the precision of the methods. The results from TP-TD were also used for a patient-specific calibration of the PiCCO monitor.
Post-operatively, the cardiac output was determined simultaneously using the pulse contour method and PA-TD. The four PiCCO readings matching the four thermal indicator injections for PA-TD were averaged to determine the pulse contour cardiac output. In patients with a cardiac index of less than 2.5 l/min/m2 or a heart rate of less than 60 beats/min, epicardial pacing (heart rate, 80 beats/min) from the right atrium was started and the cardiac output was again determined using PA-TD and the pulse contour method. In the present investigation, we have defined an increase/decrease in cardiac output as a change of more than one standard deviation (0.2 l/min) of the difference in replicate cardiac output determinations with PA-TD.
The thermodilution results are the average of four thermal indicator injections. Pre-operatively, 15 ml of iced saline was used to simultaneously calibrate the PiCCO system and to assess cardiac output with PA-TD. As the Sirecust 1281 computer does not have a constant for 15 ml, the results were multiplied by 1.5 to obtain the actual cardiac output. Post-operatively, the volume of thermal indicator injections was 10 ml.
The injections were by hand and always completed within 3 s (10 ml) and 5 s (15 ml). All injections were started as soon as the cardiac output computer indicated that the pulmonary and peripheral artery temperatures were stable (± 0.05 °C), without considering the relationship between the timing of the injection and the respiratory cycle. All curves of changes in temperature in the pulmonary artery or the aorta were inspected for irregularities and accepted/rejected before results were displayed on the monitor. Aborted attempts were replaced by a new injection.
From the data on the pre-operative cardiac output measurements with PA-TD and TP-TD, the precision (two standard deviations of the difference between replicate measurements) of the two methods was determined. Subsequently, PA-TD and TP-TD cardiac outputs were compared with bias and limits of agreement analysis according to Bland and Altman (10).
Post-operative cardiac output determinations with the PA-TD and pulse contour methods were likewise compared according to Bland and Altman (10).
Changes in cardiac output (ΔCO) after the initiation of pacing were analysed using a paired t-test. ΔCO values measured with the PA-TD and pulse contour methods were compared using correlation analysis.