All parameters measured by both methods showed agreement by bias and precision (LOA) analysis, and mean percentage differences between measures were not clinically significant.
These results suggest that the USCOM device accurately measures cardiac function compared with the CardioWest flowmeter over a range of controlled outputs (5.2–9.3 L/min) in TAH HF patients. These measurements were possible at each examination and in all patients despite postcardiac surgical patients often being difficult to insonate and to acquire reliable echocardiographic signals.
The USCOM device has previously been validated against PAC in ICU postcardiac surgical patients7–10; however, the accuracy of PAC is uncertain so the use of a controlled circulation like the CardioWest to further test the accuracy of USCOM in the ICU is useful. Critchley et al.17 suggested a 30% LOA for acceptable clinical agreement between CO measurement methods in comparisons with PAC. However, Critchley et al. acknowledged that PAC has intrinsic variability in the order of 20 or more % suggesting that PAC may be a flawed reference standard for evaluating new CO measurement methods.
The ESCAPE trial, a 26 center randomized, controlled trial found no indication for routine use of PAC to adjust therapy during hospitalization for long-term HF and recommended that future trials should test noninvasive assessments with specific treatment strategies to better tailor therapy for both survival time and survival quality.1 However, the clinical ineffectiveness of PAC may be related either to the inaccuracy of the measurements or the ineffectiveness of the therapeutic protocols based on this information. This study confirmed the accuracy of USCOM measurements of circulation in TAH HF subjects, and further research should now be directed to develop and test therapeutic protocols to determine whether outcomes benefit can be proven using USCOM-guided management of circulation.
Importantly, the comparison of USCOM against the CardioWest circulatory model allows some assessment of the reliability of PAC as a reference standard for CO studies. Comparisons of precision between PAC and USCOM CO measurements in postcardiac surgical patients have been performed in four studies with a reported %LOA of 40%, 29%, 35% and 23%, and 15%, respectively, for a mean of 28%.7–10 This study of USCOM against CardioWest, a more stable and accurate measure of CO, reported a %LOA of 6.4%. Previous studies in animals of USCOM against transit time flowprobes, a true flow “gold standard” with accuracy of ±2%, also demonstrated high levels of precision with %LOAs of 13% and 5.5%.6,18 That USCOM shows higher precision when compared with more accurate methods of flow measurement, such as CardioWest and flowprobes than PAC, infers that USCOM may be a more accurate measure of hemodynamics than PAC, particularly where the flow CSA is already known. A more accurate method for measurement of CO may allow the development of improved circulatory management protocols with outcomes benefit in HF patients.
An accurate, noninvasive CO measurement device would not only significantly reduce the ICU infection risk but could also be used to extend the reach of ICU management to presurgical assessment, post-ICU monitoring, and postdischarge review, as well as to the outpatient setting for assessment and management of chronic cardiovascular disease, thus limiting acute ICU admissions.
In this study, averaged USCOM values of CO, SV, and HR were compared with the averaged CardioWest values; however, USCOM provides real-time, beat-to-beat measures of hemodynamic variables allowing calculation of SV variability (SVV). This beat-to-beat variability of SV is a diagnostic and prognostic indicator in HF and a guide to intravascular fluid volume optimization, thus adding value to the hemodynamic examination.19–21 Many invasive monitoring methods average SV measures from an extended time domain (up to 2 min for PAC) to generate a rolling CO readout which masks SVV. Unlike PAC, the USCOM device makes no measure of filling pressures. Although clinicians have traditionally used filling pressures to guide fluid administration, the measurement of filling pressure has neither been uniformly accurate4 nor resulted in management which has improved outcomes.2,21,22 Hofer et al.22 found that SVV was almost twice as predictive of fluid responsiveness as static measures of central venous pressure and pulmonary capillary wedge pressure. Michard and Teboul23 in a retrospective meta-analysis of fluid responsiveness studies found right arterial pressure accurately predicted fluid responsiveness in only 40% of studies and right ventricular end-diastolic volume in 33% of studies. In the same review, respiratory changes in pulse pressure and aortic blood velocity had positive and negative predictive values 94% and 91%, and 96% and 100%, respectively. Whereas Monnet and Teboul,24 in their review of the literature assessing predictability of fluid responsiveness, conclude that “static measures of cardiac preload are not appropriate to assess preload reserve.” The concept of preload-recruitable SV reserve is a more physiologic approach to fluid management and requires the use of an accurate beat-to-beat monitor of SV combined with endogenous fluid challenges, such as passive leg lifts, hepatic compression, or observations of respiratory changes in SV. In this study, USCOM accurately measured SV in TAH HF subjects and therefore has the potential to predict fluid responsiveness; however, further study will be required to develop this application.
Although signal acquisition was feasible in all subjects at each examination in this study, it is possible that there may be difficulty in signal acquisition in some subjects immediately after thoracotomy when both hemothorax and pneumothorax may limit acoustic transmission. We have noted that postural adjustment, usually rolling the patient to the left lateral decubitus position allows blood to settle to the dependent thorax and air to aggregate in the raised hemithorax, creating an acoustic access through the mediastinum.
A limitation of this study is that stable and controlled circulation was tested, and so results may not be duplicated in the independent circulation, which may be characterized by greater variability. Additionally, the CO, SV, and HR values measured in this study were those used therapeutically for circulatory optimization in TAH HF and, although similar to those found in high normal circulation, were greater than those of untreated HF. Importantly this validation at high normal outputs has implications for management of conditions associated with high CO, such as sepsis. Further study in subjects with low outputs will determine the usefulness of the device in the broader setting of HF; however, finding a reliable beat-to-beat gold standard reference measure may be difficult as PAC has established limitations for measurement of low CO.14,15,25–27
Another limitation is that this study was of artificially generated circulation and so the Doppler waveforms produced by the CardioWest may not be duplicated in other artificial or native hearts. However, the CardioWest driver has been engineered to generate optimized circulation simulating high normal native CO, thus creating flow profiles that are ostensibly similar to native signals. Additionally, CW Doppler has been well validated in industrial applications, circulatory models, and in clinical subjects across a range of COs and rhythms over 35 yr of clinical application suggesting that, although the profiles may differ in different clinical instances, the principle of Doppler measurement of hemodynamics allows extrapolation to other circulations.
This study found excellent agreement between the hemodynamic measurements made by the USCOM and the CardioWest across a range of COs from 5.2 to 9.3 L/min in TAH-treated HF subjects. USCOM is a feasible and accurate method for noninvasive measurement and monitoring of CO in TAH HF patients and may have additional diagnostic and therapeutic applications in circulatory management.
Uscom Ltd. did provide the USCOM device for the purpose of the study.
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