This patient should undergo coronary angiography. He underwent heart transplantation 10 years ago and presents with exertional dyspnea. The two most common causes of dyspnea in post–cardiac transplant patients are rejection and cardiac allograft vasculopathy. The prevalence of cardiac allograft vasculopathy is approximately 50% by year 5 post-transplant and is the most common cause of mortality in patients after the first year post-transplant. Because the transplanted heart is denervated at the time of transplant, vasculopathy and subsequent ischemia may occur without the classic symptoms of angina. Therefore, this diagnosis must be suspected in long-term transplant patients presenting with symptoms compatible with ischemia without chest pain. In this patient with exertional dyspnea 10 years after transplantation, the most likely cause is cardiac allograft vasculopathy, and therefore proceeding to coronary angiography to confirm the diagnosis is the appropriate next step. Dobutamine stress echocardiography would be a reasonable option in lower-risk patients (such as those with a relatively recent coronary angiography study).

If the patient had undergone heart transplantation within the past year, the suspicion for rejection would be high. However, the incidence of rejection after the first year is low unless patients are not compliant with their immunosuppressive medications. Therefore, endomyocardial biopsy to evaluate for rejection is not the most appropriate step.

Because of the patient's significant history of tobacco use, pulmonary function testing might be a reasonable consideration for evaluation of possible underlying lung disease. However, the rapid onset of respiratory symptoms in a previously asymptomatic patient who is currently a nonsmoker would make this diagnosis less likely, and testing would not be appropriate before excluding a cardiac cause.

The transplanted heart is denervated, and without the normal vagal tone, a normal heart rate for transplant patients is between 90/min and 110/min. Because sinus tachycardia may also be present in patients with pulmonary embolism, it may be more difficult to assess tachycardia as a possible presenting sign in patients who are post-transplant. However, this patient is not at increased risk for pulmonary embolism, and his heart rate of 102/min is not unusual and should not increase suspicion for this diagnosis. Therefore, a ventilation-perfusion lung scan to test for this possibility would not be an appropriate next step.