Pacemakers during open heart surgery
Since the initiation of the modern era of cardiac pacing by Bigelow and Callaghan in 1951, and the subsequent development of the implantable pacemaker by Chardack in 1960, great strides have been made in refining pacing technology.
Types of pacemakers
Various types of pacemakers are available and can be generally classified as asynchronous, synchronous, and sequential. The asynchronous type is also called a fixed-rate pacemaker and, as this implies, emits an impulse about 70 times a minute, regardless of the electrical activity of the heart. The synchronous pacemakers are ventricular inhibited, ventricular triggered, and atrial. The ventricular inhibited model is the one most commonly used. This device is suppressed by the occurrence of a QRS complex, and thus in the absence of this complex, will pace the ventricle. The ventricular triggered pacemaker senses the R wave and fires harmlessly into the absolute refractory period. When an R wave is not sensed, the unit is preset to fire automatically. The atrial pacemaker uses a double electrode system, senses the P wave and triggers ventricular contraction after a long refractory period corresponding to a prolonged PR interval, which can be set. The long refractory period prevents the ventricles from contracting more than 140/min. When no P wave is sensed, the unit converts to an asynchronous type.
Various types of atrioventricular sequential pacemakers are available. However, units that rely on sensing the P wave are rarely used as it is generally accepted that electrical potentials, particularly from abnormal atria, are not reliable to . . .