Exercise responses following heart transplantation: 5 year follow-up

Heart transplantation is an established treatment for end stage heart failure. In addition to increased life expectancy, heart transplant recipients report a remarkable improvement in symptoms and functional capacity. Exercise performance following heart transplantation, however, remains impaired even in the absence of exertional symptoms. We have assessed the response to exercise in 47 patients with cardiac failure prior to and then at yearly intervals to five years post transplantation. All patients performed incremental symptom limited exercise tests during which minute ventilation (V'_E), oxygen consumption (V'O₂) and carbon dioxide production (V'CO₂) and heart rate (HR) were measured. Ventilatory response (V'_E/V'CO₂), anaerobic threshold (V'O₂ AT %predicted) and heart rate response (HR/NO₂) were calculated. The dead space to tidal volume ratio (V_D/N_T) and alveolar-arterial oxygen gradient (A-aO₂) were computed from transcutaneous monitoring. Despite substantial improvement in subjective functional capacity, heart transplant recipients continue to have limited exercise performance [Maximal V'O₂% predicted pre-transplant 41.3 (2.2); 1 year 48.6 (1.7), p <0.001: V'O₂ AT% 31.5 (1.1); 1 year 35.6 (1.0); respectively p<0.05]. The maximal oxygen uptake continued to improve at two years post-transplant but, thereafter, there was no further significant change at up to 5 years post transplant [50.9 (1.5)]. At one year post-transplantation peak HR [65.2 (0.9) vs 79.1(1.4)] and the HR/VO₂ response [3.0(1.8) vs 79.6(4.2)] were significantly reduced compared to pre-transplant values. The heart rate response remained lower compared to predicted at 5 years post-transplant although there was a significant increase compared to one year post-transplant (32.9 vs 24.0mls/bt). There was a weak but significant relationship between maximal VO2 and peak HR (0.39, p<0.05) and HR/VO2 (r= 0.37, p<0.05) at one year post-transplant. Prior to transplantation the ventilatory response to exercise was elevated [V'_E/V'CO₂ 45.6 (2.5)] and decreased significantly following transplantation [1 yr 34.1 (1.3), respectively p<0.001]. In addition, despite significant improvement in V_D/V_T after transplantation, it remained higher than normal Pre V_D/V_T at maximum exercise 0.35 (0.02); 1 yr 0.31 (0.02); p<0.05]. There was a further fall in the V_E/VCO₂ and V_D/V_T at two years post-transplantation with no further change at up to 5 years post transplantation [V_E/VCO₂ 32.0 (1.0); V_D/V_T 0.29 (0.01)]. Although cardiac output is markedly improved after transplantion, due to chronotropic incompetence associated with denervation, its response remains subnormal and this may explain the residual abnormalities of ventilation and gas exchange responses to exercise following transplantation.