Age-related heart rate response to exercise in heart transplant recipients. Functional significance

. The heart rate (HR) and O 2 uptake ( V^· O_2 ) responses to cycle ergometer exercise and the role of O 2 transport in limiting submaximal and maximal aerobic performance were assessed in 33 heart transplant recipients (HTR) [14 children (P-HTR), 11 young adults (YA-HTR) and 8 middle-age adults (A-HTR)] and in 28 age-matched control subjects (CTL). In 7 P-HTR ("responders") the HR response to the onset of exercise (on-response) was as fast as that of CTL, whereas in all other patients ("non-responders") the HR on-response was typical of the denervated heart. Compared with non-responder P-HTR, responder P-HTR were also characterized by a normal peak HR (177±16 vs. 151±25 beats/min), an equally slow time constant for the V^· O_2 on-response (τ: 54±11 vs. 62±13 s) and a similar low ( 60