While heart transplant recipients have significantly improved tolerance for exercise post-transplant, their aerobic exercise capabilities remain 40 - 60 per cent below normal. Blood vessel dysfunction, skeletal muscle wasting, and surgical severing of the nerves to the heart have all been implicated as factors contributing to reduced aerobic capabilities in individuals following heart transplantation. Heart transplant recipients also have dramatic pressure increases in the blood vessels within their lungs during exercise. These abnormally high pressures may result in heart dysfunction and breathlessness, causing impaired exercise tolerance. The drug Sildenafil has been shown to reduce pressure in the blood vessels of the lung. Previous research has shown that Sildenafil improved heart function during exercise among heart failure patients. Ben Esch is investigating whether Sildenafil is also beneficial to heart transplant recipients during exercise. He is testing aerobic exercise capacity among 20 heart transplant recipients both with and without Sildenafil — assessing their heart function (using cardiac ultrasound) and oxygen uptake. The results from this investigation may have important implications for cardiac rehabilitation in heart transplant recipients. If Sildenafil is shown to have a positive effect on cardiac function and exercise tolerance, its use could help heart transplant recipients train at higher intensities for longer duration during their exercise rehabilitation.