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

The heart rate (HR) and O2 uptake (V̇O2) responses to cycle ergometer exercise and the role of O2 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 V̇O2 on-response (τ: 54±11 vs. 62±13 s) and a similar low (∼60% of that of CTL) peak V̇O2 (28±7 vs. 26±10 ml/kg per min). On the other hand non-responder YA-HTR and A-HTR were characterized by a relatively low peak HR (151±21 and 144±29 beats/min, respectively), a slow τ for the V̇O2 on-response (63±12 and 70±11 s) and a low peak V̇O2 (28±7 and 19±6 ml/kg per min). In conclusion, a sizeable number of paediatric patients (responder P-HTR) may reacquire the normal HR response to exercise, both in terms of kinetics and maximal level. Despite the almost complete recovery of cardiovascular function, and, probably, oxygen delivery, both the kinetics of the V̇O2 on-response and the maximal aerobic power of the responder P-HTR were similar to those of non-responder P-HTR. The latter finding is probably attributable to peripheral limitations, due to inborn and/or pharmacological muscle deterioration.