What is the difference between a demand and an asynchronous pacemaker?
A demand pacemaker discharges in the absence of intrinsic electrical activity. For example, a demand ventric-ular pacemaker (VVI) will only pace when it does not sense R waves from the ventricular electrode. This is advanta-geous because it does not interfere with intrinsic cardiac rhythm when the ventricular rate is adequate and will not stimulate the ventricle during the vulnerable repolar-ization interval (and potentially induce a dysrhythmia). Extraneous electrical activity may interfere, however, by inhibiting the pacemaker when, in fact, no intrinsic cardiac activity is present. A classic example of this is electro-cautery inhibition of a ventricular pacemaker which is set to sense ventricular activity. In this situation, the pacemaker will remain inhibited despite bradycardia or asystole.
An asynchronous pacemaker discharges continuously regardless of the heart’s intrinsic rhythm. Almost all cardiac pacemakers will convert to the asynchronous mode (at least temporarily) during the placement of a magnet on the skin overlying the pacemaker generator. Asynchronous pacemaker modes are generally not used in patients outside the operating room. The operating room is unique, however, in that there are multiple poten-tial sources of electrical interference which may prevent normal functioning of demand pacemakers.