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Hypertrophic cardiomyopathy (HCM), the most common inherited cardiac disorder, is defined by unexplained left ventricular hypertrophy. HCM is associated with a range of clinical expressions, including severe limitation, premature sudden death and asymptomatic survival to advanced age.1 A subset of people have left ventricular (LV) outflow tract obstruction (LVOTO), where systolic anterior motion of the mitral valve results in mitral regurgitation and mitral-septal contact that impedes LV ejection. Approximately 25% have obstruction evident at rest and many others have obstruction provoked by exercise. Although not all patients with obstruction are symptomatic, many are limited by chest pain, breathlessness, dizziness and syncope; LVOTO is an important therapeutic target.
Initial treatment with negative inotropes such as β blockers, verapamil and disopyramide often fails to control symptoms or is associated with intolerable side effects.1 Surgical left ventricular myectomy (LVM) was adopted as the first effective treatment for symptom relief2; with short atrioventricular (AV) delay pacing3 and alcohol septal ablation (ASA)4 subsequently developed as less invasive options.
Following observations of a potentially beneficial haemodynamic effect of pacing, placebo-controlled studies in the late 1990s divided expert opinion and pacing failed to gain widespread acceptance.5–7 Re-examination of the role of pacing in HCM is desirable for several reasons. First, the efficacy of ASA and LVM remain untested by randomised trial. Second, the randomised trials of pacing therapy have important limitations. Third, new device technology introduces novel treatment opportunities. Fourth, some patients are unsuitable for either ASA or LVM. Finally, recent data have demonstrated long-term symptomatic benefit from pacing in as many as 80% of patients.8–10
Rationale for dual chamber pacing
In obstructive HCM, right ventricular (RV) pacing reduces LVOTO acutely and chronically. The history of the development of DDD pacing for this indication and its likely mechanisms of action have been expertly summarised …
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