A. J. Camm

The Ventricular Paced QT Interval—The Effects of Rate and Exercise

Changes in the QT and QTc intervals in 19 patients were studied at a ventricular paced rate difference of 50 beats/min. In all patients the measured QT interval shortened as the pacing rate was increased, from a mean value of 441 ms to 380 ms (p < 0.001), but when correct ed for heart rate the QTc‐ lengthened from a mean value of 518 ms to 575 ms. In 11 patients the QT in terval was measured at rest and immediately following exercise sufficient to increase the atrial rate by approximately 50 beats/min at identical ventricular paced rates. In all patients exercise‐induced QT interval shortening from a mean value of 433 ms to 399 ms (p < 0.001). These results show first that Bazetts formula is unsuitable for correction of QT interval changes induced by ventricular pacing, and second that heart rate and changes in sympathetic tone independently influence the duration of the QT interval. It is suggested that these resuits are relevant to the design of physiological pacemakers in which the duration of the QT interval influences the discharge frequency of the pacemaker and to the consideration of ventricular pacing for the treatment of abnormal repolarization syndromes. (PACE, Vol. 5, May‐June, 1982)

The response of regular re-entrant supraventricular tachycardia to right heart stimulation.

The study was designed to assess the effect of various forms of right atrial or ventricular stimulation on the termination of re‐entrant “supravenlricular” tachycardias. LStandard eleclrophysio‐logical techniques were used in 81 patients to study 86 stable tachycardias. All tachycardias were initiated by single or double atrial or ventricular premature stimuli or incremental atrial pacing. Eight groups of tachycardia circuit were defined in terms of the anterograde and retrograde pathways. Termination of each tachycardia was studied by atrial underdrive, ventricular underdrive. rapid atrial stimulation and single or double atrial and ventricular premature extrastimuli. fntranodal re‐entrant tachycardias formed 33% of the total and WPW tachycardias as a whole formed 55% of the total number of arrhythmias. The remainder were comprised of utrial tachycardia (5%). tachycardias in association with a partial AV nodal bypass (3%) and pre‐excited tachycardias (5%). A single atrial extrastimulus was most effective where the circuit involved the right atrium. Atrial underdrive was consistently Jess successful than a single atrial exlrastimulus in all groups. Rapid atrial pacing was effective in all groups, but caused transient atrial flutter or fibrillation in a proportion of each group except one. Ventricular underdrive stimulation was most effective in those groups where the right ventricle was involved in the circuit, but tended to be less effective than programmed single or double ventricular extrastimuli. Pacemakers designed to deliver appropriately timed single or double extrastimuli may offer an important alternative to other pacing modalities.

The long QT syndrome: Effects of drugs and left stellate ganglion block

Four patients, two with congenital QT prolongation (Romano-Ward syndrome) and two with acquired idiopathic QT prolongation not related to bradycardia, drug toxicity, electrolyte imbalance, or neurological disorder were investigated for the onset of recurrent palpitations and/or syncope. The effects on the measured QT interval of intravenously administered propranolol (QTp), an infusion of isoproterenol (QTi) and left stellate ganglion block (QTs) were assessed at identical atrial paced rates and during sinus rhythm, corrected for rate change (QTc). Propranolol shortened the QTc in all patients. The QTp shortened only in those with congenital QT prolongation. Isoproterenol lengthened the QTc in the three patients studied. However, the QTi lengthened in the congenital syndrome whereas it shortened in the acquired syndrome. The QTs was uninfluenced by left stellate ganglion block in all patients. It is suggested that the congenital and acquired forms can be differentiated by pharmacological interventions and that the efficacy of propranolol in the former may result from its ability not only to increase the threshold for ventricular fibrillation, but also its ability to shorten the QT interval.

The use of active fixation electrodes for permanent endocardial pacing via a persistent left superior vena cava.

Two patients underwent permanent endocardial pacing for complete atrioventricular block. In each case a persistent left superior vena cava was either suspected or known to be present. An active fixation electrode was passed down the left superior vena cava and the tip positioned in the apex of the right ventricle. Stable ventricular pacing was achieved for the follow‐up period of approximately six months. With the availability of such active fixation electrodes the presence of a persistent left superior vena cava no longer mandates insertion of an endocardial electrode via the right superior vena cava. when present, or implantation of an epicardial pacing system. (PACE, Vol. 5, March‐April, 1982)

The proarrhythmic effects of antiarrhythmic drugs.

Although antiarrhythmic drugs are effective for controlling cardiac arrhythmias, they may also induce or exacerbate them. Case reports have appeared implicating all classes of antiarrhythmic drugs. It is difficult to assess the size of the problem in practice, as it varies with different subgroups of patients, but rates of up to 13% have been found where proarrhythmic effects were actively sought. Their occurrence is affected both by the electrophysiologic characteristics of the drugs and by the arrhythmia substrate. Mechanisms of proarrhythmic effects may be classified according to the electrophysiologic and hemodynamic effects of the drugs. Detection of drug-induced arrhythmias depends on appreciation of the problem by physicians and, although there are few clear predictors, some form of monitoring of antiarrhythmic drug treatment is recommended. Management of such arrhythmias when they occur involves withdrawal of the offending agent, correcting contributory factors, and reassessing the initial arrhythmia and the strategy for its management.

Recognition of Mutiple Tachyarrhythmias by Rate-Independent Means Using a Small Microcomputer

New implantable devices are now available that can offer different therapies for different arrhythmias but they need a method of discriminating between these rhythms. Heart rate analysis is predominantly used to discern between sinus rhythm (SR) and pathological tachycardias but this may be of limited value when the rates of the rhythms are similar. An enhanced form of Gradient Pattern Detection (GPD) has been developed using an 8‐bit microcomputer that can distinguish between Sfl and up to three other arrhythmias in real time. This is a method based on electrogram morphology where each rhythm s specific electrogram is classified by a sequence of gradient zones. The microprocessor of the computer is of similar processing power to ones used in current pacemakers. Five patients with multiple arrhythmias were studied. Four had ventricular tachycardia (VT) and one had three conduction patterns during supraventricular tachycardia (SVT). Bipolar endocardial right ventricular electrograms were recorded during SR and tachycardia in all patients. The computer would first learn about each different rhythm by a semi‐automatic means. Once all the rhythms were learned the program would enter the GPD analysis phase. The computer would output a series of real‐time rhythm specific marker codes onto a chart recorder as it recognized each rhythm. Sixteen different arrhythmias (13 VT, 3 SVT) were examined for this study. All rhythms (including SR) were distinguished from each other except in the case of one patient with six VTs where two VTs had identical shapes and therefore could not be detected apart. The method would be a useful addition to heart rate analysis for future generations of microprocessor assisted pacemakers.

Rapid autonomic tone regulation of atrioventricular nodal conduction in man

The changes in P-P intervals and atrioventricular nodal (AVN) conduction during the Valsalva maneuver were studied in 17 patients. In spite of a significant decrease in the sinus P-P interval during phase II of the maneuver (733 +/- 143 to 520 +/- 86 msec, p less than 0.005) and prolongation during phase IV (884 +/- 171 msec, p less than 0.01), there was no change in the AH interval (control: 78 +/- 15: phase II: 76 +/- 15: phase IV: 72 +/- 14 msec, N.S.). In six patients consecutive P-P intervals during phase II were recorded in solid-state memory and were used to trigger pacing of the high right atrium at rest. This showed a significant increase in the AH interval (75 +/- 10 to 123 +/- 45 msec, p less than 0.05). Valsalva maneuver during constant rate atrial pacing resulted in a significant decrease in the AH interval during phase II (115 +/- 36 to 80 +/- 15 msec, p less than 0.001). During phase IV there was prolongation of the AH interval (156 +/- 58 msec) but in 11 patients (61%) a variable degree of Wenckebach periodicity appeared. Thus autonomic tone modulates the changes in AVN conduction induced during physiologic heart rate variation, resulting in maintenance of adequate 1:1 AVN conduction.

A microcomputer-based system to evaluate cardiac pacing for the treatment of tachycardias

A microcomputer-based pacemaker system for the evaluation of pacemaker treatment of tachycardia is described. The system may be used to study tachycardia initiation, tachycardia termination or a combination of the two. The software incorporates a visual display unit screen handling package which provides the user-system interface. System-patient interfacing is performed by a separate pacing and sensing unit which communicates with the computer via standard digital input/output lines. Several pacing options are available, selectable from a screen-displayed menu. Each selection also has an associated set of programmable parameters which may be adjusted, within allowed limits, to suit particular studies. Examples of the use of the system for tachycardia termination are given. The main programming language for the controlling software was Fortran IV. Some routines were necessarily written in assembly language. The system is useful for evaluation purposes and forms the basis of a cardiac pacemaker development tool.