Biological pacemakers: Concepts and techniques

The sinoatrial (SA) node is the dominant pacemaker of the heart which initiates the process of impulse generation in the cardiac tissue, thereby defining the rate and rhythm of cardiac contraction. The automaticity of the conduction cells in the SA node is due to ion channels which are inter-linked by molecular, histological and electrophysiological mechanisms causing spontaneous diastolic depolarization and generation of an impulse. The SA nodal action potentials are then transmitted to the ventricles by electrical coupling of the myocytes in different areas of the heart. Regulatory pathways overseeing cardiac impulse generation and conduction provide effective and safe pacing, and help maintain the rate according to the physiological demands of the individual's body. Failure of physiological pacing due to any pathology in the SA or atrioventricular node necessitates implantation of a permanent pacemaker. Implantable pacemakers, despite technological advances, are not without practical limitations including a defined battery life leading to lead and/or generator replacement at periodic intervals, vascular complications, occasional component failure, electronic interference from external/internal sources, e.g. myopotentials, electromechanical interference, etc., inadequate or incomplete physiological rate response to autonomic influences (devices have certain algorithms to address these issues) and most importantly the risk of infection. A biological pacemaker is therefore emerging as a promising technique to counter these challenges.