Kinetics of the transplanted heart. Implications for the choice of field-test exercise protocol

The transplanted heart shows a slow increase (on transient) of both heart rate (HR) and oxygen consumption (VO2) at the beginning of exercise. The hypothesis used in this study was that this would lead to unacceptably large systematic errors when field predictions of maximal oxygen intake (VO2max) were derived from cycle ergometer tests with a steep ramp function.

The subjects (27 patients who had received orthotopic heart transplants 6 months previously, and 45 age-matched control subjects) each performed a rapid progressive cycle ergometer test (increments of 16.7 Watts [W] per minute to subjective exhaustion) and a "steady-state" test (two 6-minute stages at one third and two thirds of peak power output). Time constants for HR and VO2 were determined by fitting single exponent equations to the "steady-state" data and noting the time for the difference from the plateau value to reach 36.8% of its initial value. Heart rate and VO2 also were compared between rapid progressive and "steady-state" tests at one third and two thirds of peak power output.

At one third of peak power output (46 W in patients, 73 W in control subjects), the respective time constants (mean +/- SE [standard error]) were 60.4 +/- 6.4 and 40.5 +/- 3.0 seconds for VO2 (P < .01), and 130.0 +/- 14.3 and 67.9 +/- 10.0 seconds for HR (P < .001). At two thirds of peak power, the corresponding values were 49.2 +/- 5.5 and 34.7 +/- 2.7 seconds for VO2 (P < .05), and 147.9 +/- 13.0 and 122.2 +/- 4.8 seconds for HR (P < .10). Comparing rapid progressive and "steady-state" readings, the control subjects showed identical values for HR and VO2 at one third of peak power, but at two thirds, the rapid progressive test HR lagged behind the "steady-state" value by 8 +/- 3 beats per minute. The cardiac transplant patients showed a larger HR lag in the rapid progressive tests (109 +/- 12 vs 117 +/- 10 beats per minute, P < .05; 125 +/- 14 vs 141 +/- 14 beats per minute, P < .005). Oxygen consumption also tended to lag slightly at two thirds of peak power (118 +/- 76 mL/min, not significant).

The rapid progressive test protocol yields acceptable field estimates of aerobic power in normal individuals, but the slow acceleration of HR after cardiac transplantation leads to unacceptably large errors if the HR from a rapid progressive test protocol is used to predict maximal oxygen intake (VO2max) in such patients.