Strength-interval relation for ventricular functional refractoriness

Determinants of slowed conduction in premature ventricular beats induced during programmed stimulations in perfused guinea-pig heart

NEW FINDINGS

What is the central question of this study? Is the slowed conduction upon premature ventricular activations during clinical electrophysiological testing attributable to the prolonged activation latency, or increased impulse propagation time, or both? What is the main finding and its importance? Prolonged activation latency at the stimulation site is the critical determinant of conduction slowing and associated changes in the ventricular response intervals in premature beats initiated during phase 3 repolarization in perfused guinea-pig heart. These relations are likely to have an effect on arrhythmia induction and termination independently of the presence of ventricular conduction defects or the proximity of the stimulation site to the re-entrant circuit.

ABSTRACT

During cardiac electrophysiological testing, slowed conduction upon premature ventricular activation can limit the delivery of the closely coupled impulses from the stimulation site to the region of tachycardia origin. In order to examine the contributing factors, in this study, cardiac conduction intervals and refractory periods were determined from left ventricular (LV) and the right ventricular (RV) monophasic action potential recordings obtained in perfused guinea-pig hearts. A premature activation induced immediately after the termination of the refractory period was associated with conduction slowing. The latter was primarily accounted for by the markedly increased (+54%) activation latency at the LV stimulation site, with only negligible changes (+12%) noted in the LV-to-RV delay. The prolonged activation latency was acting to limit the shortest interval at which two successive action potentials can be induced in the LV and RV chambers. The prolongation of the activation latency in premature beats was accentuated upon an increase in the stimulating current intensity, or during hypokalaemia. This change was related to the reduced ratio of the refractory period to the action potential duration, which allowed extrastimulus capture to occur earlier during phase 3 repolarization. Flecainide, a Na⁺ channel blocker, prolonged both the activation latency and the LV-to-RV delay, without changing their relative contributions to conduction slowing. In summary, these findings suggest that the activation latency is the critical determinant of conduction slowing and associated changes in the ventricular response intervals upon extrastimulus application during phase 3 of the action potential.

Programmed electrical stimulation protocols: variations on a theme

A series of prospective protocols were designed to determine the yield ratio (true positives vs. false positives = nonclinical) in various patient groups using a variety of programmed electrical stimulation (PES) variables. First, a PES protocol was used in 772 patients. Single, double, and triple extrastimuli were delivered in sequence (leaving each successive extrastimulus just beyond its refractory period before moving to the next extrastimulus) during sinus rhythm and two ventricular paced rates at the RV apex, before moving to the outflow tract and repeating the sequence and then moving on to isoproterenol infusion with the PES sequence repeated at the apex. This protocol met NASPE standards for induction of VT in patients with coronary artery disease and a history of VT, while failing to induce monomorphic VT in any control patient. The best yield ratios combined with the greatest likelihood of inducing clinical tachycardia were achieved with sinus rhythm and three extrastimuli, and pacing at the lower rate and three extrastimuli. Pacing at the faster rate and triple extrastimuli was highly inductive of clinical arrhythmias, but had a low yield ratio due to induction of more nonclinical arrhythmias than other steps. The next protocol was performed in 61 patients with inducible ventricular tachycardia. In each case, the protocol described above was completed at the RV apex, even if tachycardia was also induced at an earlier point in the protocol. This allowed for more accurate yield ratios to be established for each step in the protocol, since each patient was exposed to each of these steps.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of high-current stimulation in patients with sustained ventricular tachycardia rendered noninducible by antiarrhythmic drugs

Successful antiarrhythmic drug therapy for sustained ventricular tachycardia (VT) is presumed to be related to effects on myocardium within the re-entrant circuit. To test the hypothesis that prevention of VT induction may be related to effects on myocardium other than that directly involved in the tachycardia circuit, high-current stimulation was used to achieve shorter coupling intervals in 22 patients with sustained uniform VT that was rendered noninducible by antiarrhythmic agents during stimulation at twice threshold. Sustained uniform VT was induced in 10 patients in response to high-current stimulation (group 1), including 4 tachycardias with the same morphology observed in the baseline study. There were no inducible arrhythmias in 12 patients (group 2). Patients were receiving several different antiarrhythmic regimens, but there was no particular drug associated with the induction of VT using high-current stimulation. There was no statistically significant difference between groups 1 and 2 in baseline VT cycle length (247 +/- 41 vs 253 +/- 44 ms), drug-induced increase in effective refractory period (20 +/- 15 vs 16 +/- 7%), QRS duration (25 +/- 10 vs 20 +/- 17%) or maximal current strength delivered (10.9 +/- 5.3 vs 9.3 +/- 4.0 mA). There was no significant difference in local activation with high-current stimulation between groups 1 and 2. In conclusion, sustained uniform VT was induced in 45% (10 of 22) of patients whose arrhythmias were rendered noninducible by antiarrhythmic agents during programmed stimulation at twice threshold.(ABSTRACT TRUNCATED AT 250 WORDS)

Programmed electrical stimulation of the ventricle: an efficient, sensitive, and specific protocol

A relatively simple and efficient ventricular programmed electrical stimulation (PES) protocol was developed, capable of achieving high degrees of sensitivity and specificity. In a series of 481 subjects, 1, 2, and 3 extrastimuli (ES) were used successively during sinus rhythm and ventricular pacing at two drive cycle lengths, at one or more ventricular sites, together with rapid ventricular pacing, and other maneuvers such as isoproterenol infusion. Three ES were used immediately after two ES at each drive rate, rather than returning after completion of the protocol with two ES. Using the protocol, appropriate arrhythmias could be induced in 88% of all patients with ventricular fibrillation, 84% of all patients with sustained ventricular tachycardia (91% with underlying coronary disease), and 58% of patients with severe nonsustained ventricular tachycardia. There were significant differences in inducibility between patients whose ventricular arrhythmias were due to coronary artery disease and other causes. In contrast, sustained ventricular arrhythmias (all ventricular fibrillation) could be induced in only 5% of a control group of control patients, for a specificity of 95%. The protocol described is simpler and more efficient than those that use exhaustive testing of two ES before going to three ES. Three ES during sinus rhythm proved to be the most productive step, with a higher yield ratio (true: false-positives) than two ES or three ES during pacing, especially at faster rates. Greater efficiency is also achieved by leaving the timing of an extrastimulus just beyond its effective refractory period when an additional extrastimulus is to be added, compared to protocols in which the extrastimulus is moved later in the cycle and then decremented in tandem with the additional extrastimulus. Coupling intervals less than 200 msec produced some false-positives, but fewer overall than intervals greater than or equal to 200 msec, and with yield ratios comparable to other protocol steps. The protocol described meets NASPE standards for ventricular programmed stimulation protocols, and with its demonstrated specificity and relative simplicity and efficiency may be useful as a model for groups not yet committed to an alternative protocol.

Description of the strength-interval relation with external noninvasive pacing

The strength-interval relationship obtained by external pacing (EXP) was compared to curves obtained by endocardial pacing from a right ventricular site. There were 17 patients, age 45 +/- 17 years. Effective and functional ventricular refractory periods (ERP, FRP) were determined during a ventricular drive train of 8 stimuli at a cycle length 500 msec. Endocardial pacing threshold current averaged 0.5 +/- 0.3 mA (range 0.2-1.0 mA) and EXP threshold current averaged 64 +/- 14 mA (range 40-80 mA). At twice threshold, endocardial ventricular ERP was 235 +/- 32 msec and FRP was 262 +/- 29 msec. At 10 mA above threshold, EXP ventricular ERP was 276 +/- 29 msec and was significantly longer than endocardial ERP at twice threshold (P less than 0.001). EXP ventricular FRP shortened to 237 +/- 39 msec at twice threshold and was similar to endocardial ERP (P = 0.55). EXP ventricular FRP was significantly longer than endocardial ERP (280 +/- 29 vs 262 +/- 29 msec, P less than 0.001) at twice threshold. EXP strength-interval curves were similar to endocardial curves in 14 of 17 (82%) patients. We conclude, that at twice threshold, similar coupling intervals can be obtained with both endocardial and EXP. Therefore, EXP can provide critically coupled extrastimuli for programmed ventricular stimulation.

Prolongation of the conduction time of early premature beats: a marker of ventricular action potential duration

The conduction time of premature ventricular complexes, which are generally quite constant throughout late diastole, abruptly prolongs during repolarization as the refractory period of the ventricle is approached (conduction time breakpoint). The relationship between the conduction time breakpoint and the recovery properties of the ventricle was examined by relating the conduction time of premature ventricular complexes to the effective refractory period and the monophasic action potential derived from the ventricular suction electrode signal. In five anesthetized dogs during ventricular drive (cathode; 2.5 Hz; three times diastolic threshold), effective refractory period and conduction times of premature ventricular complexes were derived from computerized strength-interval curves. The recovery properties of the ventricle were altered by infusion of norepinephrine, calcium, and quinidine and by cooling (29 degrees to 34 degrees C). In these circumstances, the moment of conduction time breakpoint changes correlated well (r = 0.93) with the end of the negative slope of the ventricular suction electrode signal, which suggests that the conduction time breakpoint may be an indirect marker of the ventricular action potential duration. In seven additional dogs the interval between the effective refractory period and the conduction time breakpoint, which defines the relative refractory period, was increased by 5 to 8 minutes of ischemia (coincident with height of ventricular vulnerability) and was reversed by procainamide. Procainamide is known to diminish spontaneous and induced ventricular arrhythmias by reducing the disparity between the effective refractory period and the action potential duration.(ABSTRACT TRUNCATED AT 250 WORDS)