Episodic syncope in hypertrophic cardiomyopathy: evidence for inappropriate vasodilation.

Symptoms of impaired consciousness (syncope and pre-syncope) occur in 15-25% of patients with hypertrophic cardiomyopathy (HCM). In young patients a history of recurrent syncope is associated with an increased risk of sudden death. Syncope usually occurs without warning or symptoms suggestive of the cause. Detailed investigations identify a probable mechanism in a minority, usually paroxysmal atrial fibrillation or ventricular tachycardia. In the majority however no likely mechanism is found despite repeated 24-hour ambulatory echocardiography (ECG) or patient-activated monitoring, exercise testing and invasive electrophysiological studies. Empirical treatment with amiodarone, a pacemaker or an implantable cardioverter-defibrillator is commonly employed, but is often unsuccessful in relieving the symptoms. We have previously observed that approximately 30% of patients with HCM have abnormal blood pressure response during maximal upright exercise. This was due in the majority of patients to an exaggerated fall in systemic vascular resistance, possibly arising from abnormal activation of stretch-sensitive left ventricular mechanoreceptors, by a mechanism similar to that described in aortic stenosis. However, in some patients an inadequate cardiac output response to exercise may be responsible. We hypothesised that abnormal vasodepressor-mediated hypotension may also occur during daily life in patients with HCM, and that this may be an important mechanism of syncope when conventional investigations fail to reveal a cause. [ABSTRACT FROM AUTHOR]