Adjustment of cardiac output to step exercise in heart transplant recipients.

The maximal rate of O2 uptake (max VO2) during constant-load exercise is markedly reduced (about 30-50%) in heart transplant recipients (HTX) compared to peak aerobic performance of healthy sedentary controls (CTL). In order to evaluate the role of central and peripheral factors for O2 transport and utilization, breath-by-breath measurements of oxygen consumption (VO2) carbon dioxide output (VCO2), and beat-by-beat determination of cardiac output (Q) by transthoracic impedance cardiography were performed during upright submaximal square-wave cycloergometric exercise (50 W, 5 min) in a group of 21 HTX (age: 44 +/- 8 years; 23 +/- 29 months after transplantation, range 1.3-137 months) and 10 CTL (age: 37 +/- 10 years). The steady-state values (+/- SD) at 50 W for HTX and CTL, respectively, were: (VO2) 1.01 L/min +/- 0.10 and 0.98 L/min +/- 0.05 (NS); (VCO2) 1.07 L/min +/- 0.14 and 0.84 L/min +/- 0.06 (P < 0.01); (Q) 15.6 L/min +/- 5.3 and 12.8 L/min +/- 3.6 (NS). Heart rate (HR) in HTX, after an initial 60-90 s delay, increased linearly with time of exercise up to 122/min +/- 11. The half-time of the readjustment of Q at the onset of exercise was, in HTX and CTL respectively: 52 s +/- 16 and 40 s +/- 17 (NS). In spite of the time lag in the readjustment of HR, the Q on-response in HTX is only moderately delayed, indicating that the increase of stroke volume by augmented preload and the Frank-Starling mechanism at the onset of exercise compensates for the lack of autonomic control of the denervated allograft.(ABSTRACT TRUNCATED AT 250 WORDS)