Does thoracic bioimpedance accurately determine cardiac output in COPD patients during maximal or intermittent exercise?

Study objectives: The monitoring of cardiac output (CO) during exercise rehabilitation in patients with COPD, often including strenuous exercise, is advisable. Invasive methods (thermodilution, Fick method) are accurate, but for clinical routine use noninvasive CO estimation is required. We have shown that impedance cardiography (Physio Flow; Manatee Biomedical; Macheren, France) is reliable in COPD patients at rest and during a recumbent, light-intensity exercise. The aim of our study was to evaluate the validity of this noninvasive device in COPD patients during a maximal incremental exercise test (IET) and also during a strenuous intermittent work exercise test (IWET). Design: Prospective comparative study of the impedance cardiograph vs the direct Fick method applied to oxygen. Patients: Eight patients with moderate-to-severe COPD (59 +/- 6 years old; FEV1, 38 +/- 15% predicted; residual volume, 194 +/- 64% predicted) [mean +/- SD]. Measurements and main results: Forty-nine simultaneous measurements of CO by means of the direct Fick method (COFICK) and CO measured by the impedance cardiograph (COPF) were obtained during the IET, and 108 measurements were made during the IWET. The correlation coefficients between the two measurements were r = 0.85 and r = 0.71 for the IET and the IWET, respectively. COPF was higher than COFICK. The difference between the two methods was 3.2 +/- 2.9 L/min during the IET and 2.5 +/- 2.1 L/min during the IWET. Expressed as a percentage of the mean of the two measurements, this corresponded to 31 +/- 21% and 25 +/- 20%, respectively. Conclusions: The relatively high number of values differing by > 20% precludes the use of impedance cardiography in clinical routine in such a difficult setting (hyperinflated patients and intense exercise).