PURPOSE

Thoracic electrical bioimpedance (TEB) is an alternative to invasive monitoring of hemodynamic parameters including cardiac output, stroke volume, and cardiac index. The Centers for Medicare and Medicaid Services (CMS) requested a technology assessment by the Agency for Health Care Research and Quality (AHRQ) to evaluate data on the clinical effectiveness of thoracic electrical bioimpedance (TEB) for several cardiovascular applications. The Tufts-New England Medical Center was asked to conduct a technology assessment on the literature published since an earlier report published in 1992 by the Agency for Health Care Policy and Research (now AHRQ).

MATERIALS AND METHODS

We conducted a systematic review and meta-analysis of the TEB literature. We searched MEDLINE using synonyms for “impedance cardiography;” the search strategy was restricted to the English language, to human subjects, and was conducted for the period from 1966 through January 2002. This search yielded more than 8000 articles. An updated search was performed on July 22, 2002. Inclusion criteria for articles included date of publication 1991 and onward, reporting on the methodology of TEB as a diagnostic and/or monitoring tool or TEB in comparison to another diagnostic technique for the clinical indications of interest. Seventy-seven articles were included in the evidence tables of this report. We performed meta-analyses by constructing subgroups (i.e., inpatient, outpatient, emergency department, and years of publication – to account for most recent technology) for selected comparison techniques, equations used by the devices and hemodynamic parameters --- cardiac output, cardiac index, and stroke volume.

RESULTS

1. ACCURACY OF BIOIMPEDANCE DEVICES: The overwhelming majority of studies reported only the correlation coefficient of bioimpedance when compared to alternative techniques, such as thermodilution (TD). Correlation coefficients were in the range of −0.01 to 0.97. Correlation coefficients have serious limitations when used to summarize diagnostic test data, and there are no methodologic crosswalks which can allow correlation coefficients to reflect well-established parameters of accuracy, such as sensitivity and specificity. There was significant between-study heterogeneity due to factors other than the factors that we used to stratify studies. The majority of the studies were done on the NCCOM device, a device that is no longer commercially produced. There is wide variation in results across the instruments; the variation could be due to differences in instrument performance, but there is not enough data available on any one instrument to draw conclusions about this. We also reported the bias (systematic error) and limits of agreement (random variation) in studies of TEB. The test for heterogeneity across studies was statistically significant for bias and limits of agreement for cardiac output for TD, suggesting that there may be patient populations where TEB measurements can be much farther from the TD measurement than the combined limits of agreement indicate. Errors in placement of the leads, and clinical factors such as patient weight and presence of pulmonary edema, have been reported to affect results of measurements. Data on the effect of these factors have not been adequately reported in published literature with currently available commercial devices on the outpatient population of interest. 2. CLINICAL RESULTS: A. MONITORING IN PATIENTS WITH SUSPECTED OR KNOWN CARDIOVASCULAR DISEASE: No studies provided information on health outcomes, patient management, or on clinical endpoints to address the usefulness of TEB in monitoring or management. B. ACUTE DYSPNEA: No studies were found that evaluated the clinical impact on patient management and/or improved health outcomes from the use of TEB monitoring for the differentiation of cardiogenic from pulmonary causes of acute dyspnea. C. PACEMAKERS: There were no well-designed studies for this indication that provided information on the clinical impact on patient management or improved health outcomes. For example, since none of the studies reported health outcomes after adjustment of the atrioventricular delay (AV) setting, the evidence is insufficient to conclude whether TEB optimization of the AV delay improves health outcomes. D. INOTROPIC THERAPY: No studies were found that evaluated the clinical impact on patient management and/or improved health outcomes from the use of TEB monitoring of patients in need of inotropic therapy. E. POST-HEART TRANSPLANT EVALUATION: Only one study reported sensitivity and specificity of TEB as a diagnostic test. In this study, TEB had a sensitivity and specificity of 71% and 100%, respectively, for detecting rejection in heart transplant patients, suggesting that, if this finding were replicated, TEB might be a useful adjunct to the standard test, myocardial biopsy. F. CARDIAC PATIENTS WITH A NEED FOR FLUID MANAGEMENT: Several studies were identified which assessed congestive heart failure patients with a need for fluid management with whole body impedance, but no such studies involving TEB were found. G. HYPERTENSION: Only one study reported patient outcomes, and this was a randomized study of the use of TEB compared to specialist care in guiding management of patients with resistant hypertension. In this study, patients who were monitored with TEB had a small, but statistically significant, lower blood pressure at the end of the study, compared to patients treated using clinical judgment. Blood pressure is a well-accepted intermediate result for health outcomes of interest such as lower rates of stroke. Despite the randomized design, the TEB group had a lower average blood pressure at the beginning of the study. The difference in blood pressure between groups at the end of the study was not much larger than the difference at the beginning. Patients in both the control and TEB groups had large reductions in blood pressure compared to their starting pressures, suggesting that the majority of the benefit may have been due to intensive management by expert specialists. The results may not be generalizable to community practice.

CONCLUSION

Due to limitations in the studies, no meaningful conclusions can be drawn about the accuracy of TEB, compared to alternative measures of hemodynamic parameters. There is also little conclusive evidence regarding TEB’s usefulness in the specific clinical areas addressed. This was largely due to the lack of focus on clinical outcomes by researchers in this area. The clinical reports on the use of TEB for a variety of clinical indications in reports published from 1991 onwards suggested that this non-invasive method is of interest and may potentially support some of these indications, but there is little evidence that directly addressed how this monitoring technique can affect patient outcomes.