The efficacy and safety of autodecremental pacing (ADP) to interrupt ventricular tachycardia (VT) and atrial flutter was examined. Once tachycardia was recognized, ADP was initiated using a short train of stimuli with gradual shortening (3%) of the interstimulus interval. ADP was applied to 13 consecutive patients during 75 episodes of VT (mostly following induction by ventricular stimulation). Successful interruption of VT occurred in 88% of the episodes. In 6 episodes (8%), ADP resulted in ventricular fibrillation and in 3 episodes VT was unaffected by ADP. The only significant discriminator between the failure or success of ADP was the rate of VT. ADP was also applied to 17 consecutive patients with an atrial flutter that was resistant to conventional antiarrhythmic agents. Successful conversion of atrial flutter to sinus was seen in only 8 patients (47%). A temporary acceleration to atrial fibrillation appeared in 3 patients (18%), and in 6 patients atrial flutter was unaffected by ADP. ADP was successful in 70% (710) of patients with type 1 (<300 beats/min) atrial flutter. The authors conclude that ADP is beneficial in the interruption of VT and atrial flutter in a selected group of patients, especially with a slower rate of tachyarrhythmia (atrial rate during atrial flutter <300 beats/min and ventricular tachycardia <180 beats/min).

Reentry within the atrioventricular node or over accessory pathways are common causes of symptomatic tachycardia. These arrhythmias are frequently initiated by a spontaneous atrial or ventricular premature beat. Appropriately timed atrial or ventricular extrastimuli can, in some patients, render one limb of the reentrant circuit refractory and prevent induction of tachycardia. Currently available implantable devices are not suitable for this application. The authors have implemented a new implantable pacing algorithm capable of extremely short atrial and ventricular refractory periods, rapid triggered ventricular pacing rates, protection against encroachment on the ventricular vulnerable period, protection against tachycardia induction by ventricular premature beats, automated antitachycardia pacing in either atrium and ventricle, as well as an extensive event storage capability. This application has been made possible by a new, RAM-based microprocessor controlled dual chamber pacemaker (Medtronic Prometheus Model 6100). Its use in intraatrial tachycardia with atrioventricular block as well as Wolff-Parkinson-White syndrome is demonstrated. This device is capable of prevention of tachycardia induction in some patients. Where prevention is not feasible or fails, it is backed up by automated or manually activated antitachycardia pacing. The great flexibility of a completely reprogrammable software based pacemaker has enabled the implementation of a very complex experimental pacemaker that will permit evaluation in the implanted setting of this new pacing strategy.

The entrainment characteristics of orthodromic circus movement tachycardias occurring during autodecremental atrial and ventricular stimulation were studied in 9 patients with manifest Wolff-Parkinson-White syndrome. The phenomenon occurred in 34 of 38 episodes of tachycardia during autodecremental atrial stimulation. It was not seen in 4 episodes because the first impulse penetrating the circuit terminated the arrhythmia. Invariably, the HH and VV intervals were not equal to, but longer than, the stimulus-stimulus intervals, thus not fulfilling the definition of "classic" (constant cycle length) entrainment postulated by Okumura et al. Furthermore, the first 2 of the 3 diagnostic criteria were not demonstrated and the third only could be demonstrated in 7 episodes. Tachycardia termination was achieved in all 38 episodes. Entrainment occurred during autodecremental ventricular stimulation in 79 of 80 episodes, with the AA and H-H- intervals (when visible) being equal to the corresponding paced cycle lengths. Moreover, the intervals between the last paced ventricular beat and the first ventricular beat of the resumed tachycardia were invariably longer than the last stimulus-stimulus intervals. These characteristics were those which Okumura et al attributed to "concealed" entrainment. Tachycardia termination was achieved in 77 of 80 episodes. In summary: (1) autodecremental atrial pacing produced a specific form of entrainment that did not fulfill the "classic" definition of Okumura et al; (2) autodecremental ventricular pacing consistently produced "concealed" entrainment; and (3) autodecremental stimulation was very effective in terminating 115 of 118 (98%) of episodes of circus movement tachycardias.

We studied the effects of various pacing modes on cardiac hemodynamics and pulmonary gas alterations in chronic heart blocked dogs. Changing the pacing mode from an atrioventricular interval of 100 ms (AV100) to a ventriculo‐atrial interval of 100 ms (VA100) caused a significant fall in left ventricular pressure (117.64 ± 11.91 to 95.60 ± 16.58 mmHg) and cardiac output from 2.18 ± 0.24 to 1.46 ± 0.20 L/min. Following the change in pacing mode from AV100 to VA 100, there was an increase in the alveolar‐arterial O 2 gradient from 23.28 ± 6.97 to 28.74 ± 8.43 mmHg and a decrease in the arterial CO 2 tension from 32.42 ± 3.22 to 29.42 ± 3.22 mmHg. There was also a decrease in arterial CO 2 tension when the AV100 pacing mode was compared to asynchronous ventricular pacing (32.42 ± 3.22 versus 30.56 ± 2.82 mmHg). The minute volume of O 2 also decreased when the pacing mode was changed from AV100 to asynchronous ventricular pacing (0.134 ± 0.01 versus 0.126 ± 0.01 L/min) and decreased further at VA100 to 0.114 ± 0.01 L/min. Other significant changes were also observed: the percent of expired CO 2 decreased when the pacing mode was changed from AV100 to VA100 (3.68 ± 0.13 versus 3.37 ± 0.26%) or to asynchronous ventricular pacing (3.40 ± 0.31%). The end‐expiratory O 2 increased and CO 2 decreased when the pacing mode was changed from AV100 to VA100. The breath‐by‐breath correlation of end‐expiratory O 2 and CO 2 with left ventricular systolic pressure showed an almost immediate increase in O 2 and reduction in CO 2 concentration associated with decreasing systolic pressure. The decrease in pulmonary gas exchange appeared in part related to alterations in cardiac hemodynamics and particularly to the fall in cardiac output. It is speculated but not proven by these studies that alterations could be further explained by a fall in O 2 consumption or reflex shunting of blood in vascular beds due to the fall in cardiac output. However, the additional deleterious effects of atrial contraction against a closed AV valve on pulmonary gas exchange and hemodynamics were also apparent. Notably, these studies could provide a physiologic basis for some symptoms associated with the pacemaker syndrome produced by the absence of AV synchrony.

The efficacy and safety of a new antitachycardia pacing technique, self-adapting decremental overdrive pacing, was assessed in patients with clinical ventricular tachyarrhythmias who underwent programmed ventricular stimulation and serial drug testing. The three phases of this study involved a learning/experience phase, followed by intrapatient comparison of decremental overdrive pacing with conventional antitachycardia pacing modalities of overdrive burst ventricular pacing, and diastolic scanning with single (S2) and double (S2S3) ventricular extrastimuli. The final phase involved an intrapatient comparison of automated decremental overdrive pacing with overdrive burst ventricular pacing in patients with ventricular tachycardia (VT) cycle lengths of 280 msec or greater. Decremental overdrive pacing was superior to overdrive burst pacing and diastolic scanning (S2S3 and S2) (83% vs 38%, 50%, 9%) in patients with VT cycle lengths of 280 msec or greater. Automated decremental overdrive pacing as applied in the final phase was the most efficacious modality, terminating 92% of VT episodes compared with 56% for overdrive burst pacing in the same patients.

Thirteen patients who all had previously inserted temporary or permanent pacemakers (6, VVI; 7, A‐V sequential) were studied by two‐dimensional echocardiography and radionuclide gated blood pool ventriculography (RVG) for non‐invasive evaluation of cardiac performance. Patients were paced in both the VVI mode and during sinus/atrial or A‐V sequential pacing. Although there was no objective change of the ejection fraction during V‐pacing and atrial/A‐V sequential pacing or sinus rhythm, as has been previously reported, A‐V sequential pacing did result in significant improvement in overall cardiac function and output as judged by radionuclide ventriculography and blood pressure response in most of our patients. An appropriately timed atrial contribution to ventricular systole resulted in improved ventricular function in those individuals with pre‐existing systolic or diastolic myocardiol dysfunction and/or sick sinus syndrome in whom pacemaker therapy was indicated. Radionuclide ventriculography appears to be a reliable, accurate, non‐invasive method that can be used to evaluate patients before implantation of a permanent ventricular or atrioventricular pacemaker in order to decide which pacing mode is best for that particular individual.