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

The HR and O2 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 (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 (“responder” P-HTR) the HR on-response was as fast as that of CTL, whereas in all other patients HR on-response was the typical one for 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) ; b) equally slow time constant of the O2 on- response (t : 54±11 vs 62±13 s) ; c) similar low (~60% of that of CTL) peak O2 (28±7 vs 26±10 ml/kg.min). On the other hand “non responder” YA- HTR and A-HTR were characterized by relatively low peak HR (151±21 and 144±29 beats/min, respectively), slow t of the O2 on-response (63±12 and 70±11 s), and low peak O2 (28±7 and 19±6 ml/kg.min). In conclusion, a sizeable amount of children (“responder” P-HTR) may resume normal HR response to exercise both in terms of kinetics and maximal level. Despite the almost complete recovery of cardiovascular function, and, likely, of oxygen delivery, both the kinetics of O2 on-response and maximal aerobic power of “responder” P-HTR are similar to those of “non responder” P-HTR. The latter finding is likely attributable to peripheral limitations, due to inborn and/or pharmacological muscle deterioration.