DETERMINANTS OF HEART-RATE-VARIABILITY IN HEART TRANSPLANTED SUBJECTSDURING PHYSICAL EXERCISE

Respiratory sinus arrhythmia has been described in heart transplanted subjects. In order to investigate the mechanisms involved in the gener ation of this condition in the transplanted heart and its evolution af ter surgery, graded exercise was performed (0-75 W in 25 W steps) on a cycle ergometer by 41 subjects (mean age 44 years) who had undergone heart transplantation 28 months (range 3-60) earlier and by six age ma tched-control subjects. R-R interval, respiratory signal, O-2 consumpt ion (VO2) and CO2 production (VCO2) were measured. Respiratory sinus a rrhythmia was assessed by the autoregressive power spectrum of the R-R interval and respiration. All subjects reached the anaerobic threshol d (heart transplants: 60% at 50 W, 40% at 75 W Controls: 150 W). In co ntrol subjects, the respiratory sinus arrhythmia was higher than in he art transplanted subjects(5.80 +/- 0.30 vs 1.45 +/- 0.16 ln ms(2)) and it decreased significantly (4.66 +/- 0.30 ln ms(2), P < 0.05) during exercise, despite the increase in breathing rate and depth. When the g roup of heart transplanted subjects was considered as a whole, respira tory sinus arrhythmia was found to be present in all conditions. It si gnificantly increased at 25 W (from 1.45 +/- 0.16 to 2.00 +/- 0.17 ln ms(2), P < 0.01), then significantly fell below baseline during recove ry (to 0.97 +/- 0.23 ln ms(2), P < 0.01). Multiple regression analysis showed that a linear combination of heart rate (inverse correlation) and VO2 (direct correlation) together with months having passed since transplantation surgery, could explain the observed changes in heart r ate during exercise (multiple regression: r = 0.658, P < 0.0001). In f ive long-term transplanted subjects, non respiratory-related low frequ ency (0.1Hz) waves were present on the R-R spectrum, but respiratory s inus arrhythmia is also present in the recently transplanted heart and depends on the opposing effects of ventilation and heart rate. In a f ew cases, sympathetic modulation (re-innervation) could not be exclude d.