Pathophysiology of physiologic cardiac pacing: advantages of leaving well enough alone.

HE IMPLANTABLE CARDIAC PACEMAKER WAS FIRST introduced in 1960, with a primary goal of maintaining adequate heart rates in individuals with symptomatic bradycardia. In such patients, singleventricle stimulation was effective even though it was suboptimal from a physiologic standpoint. However, with sustained bradyarrhythmia requiring frequent ventricular activation, problems with this approach became apparent. Ventricular pacing disrupted the normal temporal sequence of atrial-ventricular systole, resulting in AVdissociation, mitral and tricuspid valvular regurgitation, and cardiac cycles with variable filling and thus systolic dysfunction. For patients in whom cardiac performance was compromised, loss of effective atrial contraction and elevated filling pressures worsened symptoms. The first solution to this problem came in the late 1970s with development of dual-chamber pacing, whereby native atrial activation was either sensed or provided if it did not occur at a preset rate, and subsequent ventricular activation was then timed to maintain more physiologic coordination between the chambers. Now ventricular stimulation would be used only in patients with more distal conduction delay, whereas pure sinus bradyarrhythmia could be dealt with by atrial stimulation alone. Dual-chamber pacing, with better maintenance of normal-range atrial rates and atrioventricular timing, has been shown to reduce clinical symptoms of cardiac failure, enhance quality of life, and help prevent arrhythmias such as atrial fibrillation. 1,2 These issues are important as the prevalence of bradyarrhythmias has increased with more patients living to advanced age and with more widespread use of -blockers and antiarrhythmic agents. One group of patients that has routinely received pacemakers even in the absence of bradycardia consists of those with severe ventricular arrhythmia requiring treatment with an implantable cardioverter defibrillator (ICD). These devices have long been coupled with pacemakers to deal with bradyarrhythmia that might occur immediately following any cardioversion and to provide minimum backup stimu