Demand pacing, its rationale, its advantages, and the evolution and testing of safe, effective instruments have been presented in previous communications.'" After laboratory evaluation of the American Optical (AO) unit for both utility and safety in the face of all hazards that we could contrive, we implanted a prototype of such a unit in August, 1966. The demand pacemaker imitates the natural escape mechanism of an automatic myocardial fiber. It initiates only nonparasystolic escape beats when the interval after the last ventricular contraction permits pacemaker escape. It has no refractory period and emits a stimulus only when required for pacing. This presentation will be limited to: ( a ) clinical indications for demand pacing, both temporary and permanent; (b) proper monitoring and definition of the endocardial signal; and (c) a review of our clinical experience to date. Indications for Demand PacingTemporary demand pacing formerly was not satisfactory because the demand function depended on the monitoring of the surface electrocardiogram. Extrinsic noise, patient motion, poor lead application, and improper monitor adjustment resulted in unreliable sensing. With proper sensing of the depolarization wave through the endocardial electrode, demand pacing has become feasible. Patients who may require temporary demand pacing include those with myocardial infarctions complicated by heart block, patients with Adams-Stokes syncope due to heart block, and those with bradycardia due to drug intoxication. Similarly, patients who undergo open-heart surgery can be protected against bradycardia from heart block or slow sinus activity through temporary myocardial wires implanted at the time of surgery. Temporary demand pacing may be used during replacement of nonfunctioning implanted units in symptomatic patients.Early indications for permanent implantation of demand pacemakers included demonstrated or predictable competition in patients with a predominant normal sinus rhythm who needed protection against atrial bradycardia, arrest, or atrioventricular block. The unit has been used in conjunction with drugs such as digitalis and propranolol to control supraventricular tachybradyarrhythmias where use of the drug alone was prohibited because of bradycardia. Permanent demand pacing was used in one patient with sinus bradycardia and refractory angina. His angina was successfully controlled with propranolol, and the hazard of further slowing was obviated by insertion of a demand pacer.With early reliability, the use of demand pacing was extended to all indications for permanent pacing. Support for demand pacing for presumed permanent block

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