Implantable demand pacemaker.

Adams-Stokes seizures can occur in patients with intermittent complete A-V block. The history of a single Adams-Stokes syncopal attack is an indication for intracardiac pacing. Fixed rate pacemakers employed in the presence of intermittent varying degrees of A-V block and sinus rhythm result in recurrent co-acting rhythms of artificial and natural beats (Linenthal and Zoll, 1962). This arrhythmia is usually considered to be innocuous. However, instances of repetitive firing have been reported when pacemaker stimuli fell in the vulnerable phase of the previous beat (Dittmar, Friese, and Holder, 1962; Elmqvist et al., 1963; Tavel and Fisch, 1964; Castellanos et al., 1964; Lemberg, Castellanos, and Berkovits, 1965; Bonnabeau et al., 1963; Robinson et al., 1965; Dressler, Jonas, and Rubin, 1965; Katz, 1965; Langendorfand Pick, 1965; Castellanos, Lemberg, and Gosselin, 1965a; Castellanos et al., 1966b). The potential hazards of an increase in cardiac rates due to summation of both rhythms are also to be considered (Lemberg et al., 1965; Nuniiez-Dey, Zalter, and Eisenberg, 1962). Occurrence of these complications justifies the use of modified pacemakers which can prevent these untoward reactions. A pacemaker system that meets these requirements is the demand pacemaker which was developed in 1964 for temporary transvenous use (Castellanos et al., 1964). Stimulation occurs when a preset interval following cardiac contraction has been exceeded. It will shut itself off when the natural rate exceeds that ofthe pacemaker. Sensing is immediate. This portable pacemaker has been useful in the treatment of various types of intermittent A-V block. The successful application of the demand pacemaker as a bedside unit stimulated the development of a synchronized implantable model.