Exercise hemodynamics and oxygen delivery measurements using rebreathing techniques in heart transplant patients

The cardiorespiratory response to graded exercise in cardiac transplant patients differs from that in normal subjects partly because denervated hearts are much less responsive to changes in autonomic nervous system input. Previous studies with transplant patients have shown a delayed and often diminished heart rate response to exercise compared with normal subjects. 1,2 Although it has been assumed that increases in stroke volume, using the Frank-Starling mechanism, compensate for the delayed heart rate response during exercise, little is known about the matching of cardiac output (Q̇ T) to oxygen uptake (V̇O 2) in cardiac transplant patients. During exercise the metabolic requirements of skeletal muscle, a function of the work load and the size of the active muscle mass, are the major determinants of Q̇ T and systemic O 2 transport. 3 The relation between Q̇ T and V̇O 2 in normal subjects during exercise has been studied extensively and found to be relatively constant at 5 to 6 liters of blood flow, Q̇ T, per liter of change in V̇O 2/min. 4–6 This is approximately a 1:1 ratio between exercise-induced increases in O 2 transport relative to levels at rest. Because the normal heart relies primarily on an increase in heart rate to increase Q̇ T during exercise, with only minimal change in stroke volume, it is reasonable to expect that the transplant patient will have a suboptimal response to exercise. This study was conducted to determine the characteristics of the cardiovascular response to upright bicycle exercise in cardiac transplant patients. In particular, the Q̇ T to V̇O 2 relation during exercise in the transplant patients was compared with that of normal subjects to determine if Q̇ T was appropriately matched to V̇O 2 during exercise.