A pilot assessment of the FloTrac TM cardiac output monitoring system

Transcription

1 Intensive Care Med (2007) 33: DOI /s TECHNICAL NOTE Helen Ingrid Opdam Li Wan Rinaldo Bellomo A pilot assessment of the FloTrac TM cardiac output monitoring system Received: 24 December 2005 Accepted: 14 September 2006 Published online: 25 October 2006 Springer-Verlag 2006 H. I. Opdam ( ) L. Wan R. Bellomo Austin Hospital, Department of Intensive Care, Heidelberg, VIC 3084, Australia helen.opdam@austin.org.au Tel.: Fax: Abstract Objective: To compare measurement of cardiac output (CO) by means of the FloTrac TM CO monitor with the pulmonary artery catheter (PAC). Design: Prospective observational study. Setting: Intensive care unit of a tertiary hospital. Patients: Six post-operative cardiac surgery patients with existing arterial cannulas and PACs. Interventions: Attachment of the FloTrac TM CO monitor and transducer to an existing arterial cannula. Simultaneous measurements of CO, indexed to body surface area (cardiac index, CI) by the FloTrac TM CO monitor and by either a bolus thermodilution or continuous CO PAC. Statistical analysis of observations. Measurements and results: We performed CO measurements in six patients every 1 4 h after cardiac surgery. Comparison of all measurements showed a limited correlation for CI with the two devices (r 2 = , bias = 0.21, 95% limits of agreement 0.81, 1.23). CI measurements obtained with the intermittent bolus PAC had better correlation with the FloTrac TM CI values (r 2 = , bias = , 95% limits of agreement , ) than did those obtained with the continuous CO PAC (r 2 = , bias = , 95% limits of agreement , ). When analysed according to heart rhythm, CI values measured during atrial pacing showed the best correlation (r 2 = 0.377, bias = , 95% limits of agreement , ). Conclusions: CO measurements obtained using the FloTrac TM CO monitor show a limited correlation with those acquired using the PAC, relatively wide limits of agreement but no clear bias. Further evaluation is required before this device can be recommended for use in the clinical setting. Keywords Cardiac output Cardiac index Arterial pressure Pulse pressure Pulmonary artery catheter Haemodynamic monitoring Introduction Knowledge of the cardiac output (CO) is frequently useful in deciding the management of critically ill patients. Historically the most common device used has been the pulmonary artery catheter (PAC) [1, 2]. Much effort has gone into developing alternative less invasive devices, but many of these are complex and require calibration [3, 4, 5, 6, 7]. A new device uses arterial pulse pressure waveform analysis to determine the cardiac output (FloTrac TM / Vigileo TM, Edwards Lifesciences, Irvine, CA, USA). It is less invasive than current devices and relatively simple in that it merely uses a monitor (Vigileo TM ) that attaches to an existing arterial cannula via a special transducer (FloTrac TM ). To test the performance of this device, we compared CO values determined using the FloTrac TM CO device with those obtained from the PAC used routinely in patients after cardiac surgery in the intensive care unit (ICU).

2 345 Table 1 Patient demographic data (CABGS, coronary artery bypass graft surgery; AVR, aortic valve replacement; MVR, mitral valve replacement; NA, noradrenaline) Patient Gender M M M M M F Age (years) Height (cm) Weight (kg) Body surface area (m 2 ) Operation CABGS CABGS CABGS AVR & CABGS CABGS MVR, AVR & CABGS Type of PAC IBCO IBCO IBCO CCO CCO CCO Arterial cannula site Femoral Femoral Femoral Brachial Radial Brachial Number of measurements Study duration (h) Inotropes NA None Dobutamine Milrinone & NA Milrinone & NA Milrinone & NA Fig. 1 Bland Altman correlation (a) and scattergram (b) comparing all PAC-derived CI measurements (PAC CI) with those obtained with the FloTrac TM device (FloTrac CI). Bland Altman plots comparing CI measurements using the FloTrac TM device with intermittent bolus CO PAC (IB CI) (c) and CCO PAC (C CI) (d) measurements

3 346 Fig. 2 Bland Altman correlation between conventional PACderived CI (PAC CI) and FloTrac TM -derived CI (FloTrac CI). Bias and limits of agreement are demonstrated in each graph as mean ± 1.96 SD. Measurements are grouped according to patient rhythm: sinus rhythm, dual chamber (DDD) pacing, atrial (AAI) pacing and atrial fibrillation Materials and methods Six patients with arterial lines and PACs in situ as part of their routine care were studied after elective cardiac surgery. On return to the ICU, the FloTrac TM CO device was attached to the existing arterial cannula. Concomitant readings for CO, indexed to body surface area (cardiac index, CI), were obtained from the FloTrac TM device and PAC. Additional information collected included patient demographic details, heart rate and rhythm. Device characteristics The FloTrac TM device uses arterial pressure waveform analysis to determine the CO, without the need for prior calibration. It consists of a special transducer that attaches to an existing arterial cannula and then connects to a processing/display unit (Vigileo TM ). CO is calculated from an arterial pressure-based algorithm that utilises the relationship between pulse pressure and stroke volume. The arterial pulse waveform is assessed at 100 Hz and the standard deviation (SD) of the pulse pressure is determined over a 20-s window. The algorithm takes two additional factors into account: (1) vessel compliance (influenced by age, gender, height and weight) and (2) peripheral resistance effects (determined from arterial waveform characteristics). Intermittent bolus (IBCI) or continuous (CCI) thermodilution cardiac index was measured with either a Swan Ganz PAC or a Swan Ganz CCOmbo PAC with Vigilance monitor respectively, both from Edwards

4 347 Fig. 3 Bland Altman correlation between conventional PACderived CI (PAC CI) and FloTrac TM -derived cardiac index (FloTrac CI). Bias and limits of agreement are demonstrated in each graph as mean ± 1.96 SD. Measurements are grouped according to patient arterial line insertion site, including femoral artery, brachial artery and radial artery Lifesciences. A bolus CI was calculated as the average of three consecutive measurements performed randomly in the respiratory cycle over several minutes, each using 10 ml of 5% dextrose at room temperature. These were compared with the FloTrac-CI values displayed half way through performing the CO measurements. For those patients with continuous thermodilution PACs, CCI values (representing averaged data obtained in the preceding 3 6 min) were compared with simultaneously displayed FloTrac-CI values. CI measurements were performed and documented at least every 4 h during mechanical ventilation and after extubation. Statistical analysis Bland Altman analysis using was used to compare the CO techniques. It was performed for all patients irrespective of type of PAC (FloTrac-CI versus PAC-CI) and for patients grouped according to type of PAC (FloTrac-CI vs IBCI and FloTrac-CI vs CCI). Measurements obtained by the two devices (FloTrac-CI vs PAC-CI) during different heart rhythms were also compared. As the data were not normally distributed, correlation using non-parametric Spearman correlation was performed for individual patient and group measurements. The hospital human research ethics committee approved the study. The authors have no conflict of interest and received no financial support for this research from industry. Results Demographic information is displayed in Table 1. The results for all measurements, displayed in Fig. 1, indicate a limited correlation for CI obtained with the two devices (r 2 = , bias = 0.21, 95% limits of agreement 0.81, 1.23). Comparisons were also made for measurements grouped according to type of PAC device (intermittent bolus or continuous cardiac output) (Fig. 1). CI measurements obtained with the intermittent bolus PAC (IBCI) had better correlation with the FloTrac-CI (r 2 = , bias = , 95% limits of agreement , ) than did those obtained with the continuous cardiac output (CCO) PAC (r 2 = , bias = , 95% limits of agreement , ).

5 348 Plots were constructed for CI measurements grouped according to patient heart rhythm (Fig. 2). Correlation was best but limited for measurements performed during atrial pacing (r 2 = , bias = , 95% limits of agreement , ). Analyses were also performed according to patients arterial line insertion site (Fig. 3). Correlation was best in the group with femoral arterial lines (r 2 = , bias = , 95% limits of agreement , ). Discussion We conducted a pilot assessment of the FloTrac TM CO device by comparing the cardiac output values obtained with it with those obtained using PAC methodology. We found that the FloTrac TM CO device provided CI measurements that had limited correlation with those obtained using the PAC. Although there is no convenient gold standard for CO determination, the bolus injection thermodilution PAC has been well validated for CO measurements [8], as has the CCO PAC [9, 10, 11, 12]. Despite the poor precision of the FloTrac TM device relative to the PAC, no consistent bias was observed. These preliminary data are limited by the small and varying number of CO estimations obtained for each patient. However, even in this small group of patients, the findings clearly demonstrate the limitations of the device and produce proof of concept that further refinements in its software, frequency of sampling and time of averaging are probably required and that some calibration is needed. Measurements obtained using intermittent bolus PAC showed closer correlation with the FloTrac measurements than those obtained with the CCO PAC. It has been suggested that the CCO PAC is inaccurate when used soon after cardiopulmonary bypass [13]. In our study, however, the majority of CO measurements were performed beyond the first few hours post cardiopulmonary bypass. Another possible explanation for this difference is that the timing of CO readings may have adversely influenced the correlation due to different averaging times for the CCO PAC and FloTrac TM CO devices. The FloTrac TM device averages measurements over a 20-s time frame, whereas the CCO PAC measurement is averaged over minutes. The shorter averaging time for the FloTrac TM device will result in greater variability in the measured CO due to less smoothing. This could not be taken into account in this study. Heart rhythm was observed to have an influence on the correlation of the FloTrac TM and PAC measurements of CI. Better correlation was seen in measurements performed during paced than spontaneous rhythms, and was poorest during atrial fibrillation. The different methodologies in determining CO utilised by these devices could contribute to this finding. The FloTrac TM algorithm for CO estimation is highly dependent on heart rate and arterial waveform, whereas both bolus and CCO PAC methodology does not rely on either for measuring CO. Although the comparisons made in this study are limited by the small number of measurements and the different PAC CO methodologies employed, the practical application of the FloTrac TM device as it currently stands would provide results as seen in this pilot study. McGee studied the device in a similar cohort of patients and reported almost identical bias and precision [14]. One of the appealing features of this device is its simplicity, particularly the absence of requirement for calibration. Other devices using arterial waveform analysis for CO determination that have demonstrated acceptable correlation with existing CO determination technologies incorporate a calibration step such as transpulmonary thermodilution (PiCCO, Pulsion) or lithium dilution (LiDCO plus) [15, 16]. Inclusion of a calibration step for the FloTrac TM device may be one way of improving its accuracy. Currently this device requires further evaluation before it could be recommended for use in any clinical scenario. If, with further evaluation and modifications, the FloTrac TM device is found to be sufficiently precise, then, as well as being an alternative to the PAC and arterial pulse contour CO devices where they are currently used, this device may find broader application. This might include the assessment of shocked patients in the emergency department and the peri-operative monitoring of high-risk surgical patients. A further possible role is that of a screening tool to determine whether more invasive haemodynamic monitoring is warranted, although the current cost of the disposable transducer might limit such use. In conclusion, the FloTrac TM CO monitor has potential advantages due to its relative non-invasiveness and simplicity. This pilot study indicates, however, that CO values obtained using this device are imprecise compared with the PAC. Further evaluation and technical modifications are required before the FloTrac TM device can either be discarded or embraced as a welcome addition to assist clinicians in the optimisation of haemodynamic management. Acknowledgements. This study was funded by the Austin Hospital Anaesthesia and Intensive Care Trust Fund.

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