The overall accuracy of cardiac output measurements made by impedance cardiography during maximum exercise was studied in man. Initially, the systematic error of the technique was assessed over the range 3.5 to 18 litre . min-1 by comparing with simultaneous measurements of cardiac output made using the direct Fick method. No systemic error was demonstrated in 40 estimations made in 20 subjects. The random error was assessed in 4 subjects in a steady state at rest and during exercise at 80 and 130 W and found to be less than 5% in each subject. The reproducibility of maximum exercise response was assessed in six healthy male subjects (age 26.2 +/- 4.4 years, +/- SEM) who underwent maximum exercise tests twice, 1 week apart, on a bicycle ergometer. Simultaneous recordings of cardiac output and oxygen uptake (VO2) at rest and during each 3 min stage of exercise were made. Highly significant correlations were obtained in the stroke volume (r = 0.84, p less than 0.001), cardiac output (r = 0.98, p less than 0.001) and VO2 (r = 0.98, p less than 0.001) between the two tests. Average maximum cardiac output was 27.0 +/- 1.2 litre . min-1 (+/- SEM) and maximum VO2 was 4.4 +/- 0.2 litre . min-1 (+/- SEM). These results show that measurements of cardiac output were reproducible over one week. Impedance cardiography is non-invasive technique which is as accurate as invasive methods and can be used for maximal exercise testing.

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Invasive studies in patients with left ventricular dysfunction show that data at rest (e.g. ejection fraction-EF) are poor predictors of the changes in cardiac output (CO) which occur with exercise. This investigation was undertaken to determine whether impedance cardiography could be used in such patients to assess CO response to exercise. The method was compared with the direct Fick method. Over a range of COs between 4 and 18 min-1 there was no systematic error. Reproducibility for CO over one week was highly significant (r = 0.94; P less than 0.001). Impedance cardiography was incorporated into routine exercise testing on a bicycle ergometer for a group of 15 patients (mean age 53.2 +/- 3.0 yrs, SEM) who had sustained a major myocardial infarct 6 to 12 months previously, (EF 38.1 +/- 3.5%, SEM). CO was measured at the end of each 3-min stage. In eight patients (EF 40.0 +/- 3.4%, SEM) CO response was abnormal with either a decrease or a failure to increase with increasing workloads. Conventional end-points i.e. angina, attainment of 85% of predicted maximum heart rate, abnormal blood pressure response or excessive dyspnoea did not indicate consistently a need to terminate the test. It is suggested that impedance cardiography is a useful non-invasive method of evaluating patients with left ventricular dysfunction.