Heart failure or asymptomatic left ventricular (LV) systolic dysfunction (LVSD) is seen in up to 40% of patients with permanent pacemakers,1 related to the percentage of right ventricular (RV) pacing and the medical background of the patient. It is likely that RV pacing has a detrimental effect on LV function, although many patients receiving RV pacemakers already have or will subsequently develop other risk factors for LV dysfunction.1 Whether patients at risk of heart failure benefit from receiving a cardiac resynchronisation therapy (CRT) device at the time of the initial implant rather than a standard pacemaker is unknown. The recent publication of the Biventricular versus Right Ventricular Pacing in Heart Failure Patients with Atrioventricular Block (BLOCK-HF) study in the New England Journal of Medicine sets out to answer this question.2

BLOCK-HF started recruitment in 2003. Enrolment criteria included first-degree block with a PR interval of >300 ms (when paced at 100 bts/min), second-degree block or third-degree block, in the presence of LVSD with an ejection fraction (EF)≤50%. All patients were implanted with CRT devices (either pacemakers or defibrillators), and subsequently randomised in a 1:1 allocation to have the LV coronary sinus lead active or not in a double-blind fashion. Out of 918 patients assessed, 809 were listed for an implant, of whom 757 (94%) were successfully implanted (484 CRT-pacemakers and 207 CRT-defibrillators) with 691 randomised (349 to biventricular …