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[1]. In young patients a history of recurrent syncope is associated with an increased risk of sudden death[2-5]. 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 ECG or patient-activated monitoring, exercise testing and invasive electrophysiological studies[1;6]. Empiric 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[8;9]. 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[8;10], by a mechanism similar to that described in aortic stenosis[11]. However, in some patients an inadequate cardiac output response to exercise may be responsible[12]. 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.