Cardiac resynchronisation therapy

Left ventricular dysfunction is often associated with myocardial conduction slowing which is usually seen clinically as left bundle branch block on the surface ECG. Left bundle branch block causes asynchronous contraction of the left ventricle with the ventricular septum contracting early and the lateral left ventricular wall contracting late. This leads to a reduction in cardiac output and myocardial contraction efficiency and systolic mitral regurgitation worsens. Patients with this combination of findings may benefit from the implantation of a pacing system that aims to normalise conduction and "resynchronise" the ventricles. This mode of cardiac stimulation is referred to as "biventricular" pacing and relies on the implantation of an additional pacing lead on the epicardial surface of the left ventricle. This is achieved by selectively cannulating the coronary sinus and passing a pacing lead via a posterolateral coronary sinus tributary to an appropriate location. This lead, as well as the two conventional right atrial and right ventricular leads, is then attached to a specialised pacemaker. The procedure may be challenging and usually takes 1-2h depending on the operator's experience. The QRS complex shortens as the lateral wall of the left ventricle becomes "pre-excited" and contracts in concert with the ventricular septum. Improved haemodynamics result immediately and favourable reverse left ventricular remodelling occurs over subsequent weeks. Recently, these physiologic advantages have been translated into real clinical gains for patients with biventricular pacemakers where improvements in exercise tolerance and quality of life and reduced hospitalisations for recurrent heart failure have been conclusively demonstrated.