Effect of sudden rate acceleration on the human His-Purkinje system: adaptation of refractoriness in a dampened oscillatory pattern

Microvolt QRS Alternans Without Microvolt T-Wave Alternans in Human Cardiomyopathy: A Novel Risk Marker of Late Ventricular Arrhythmias

Background

Action potential alternans can induce ventricular tachyarrhythmias and manifest on the surface ECG as T‐wave alternans (TWA) and QRS alternans (QRSA). We sought to evaluate microvolt QRSA in cardiomyopathy patients in relation to TWA and ventricular tachyarrhythmia outcomes.

Methods and Results

Prospectively enrolled cardiomyopathy patients (n=100) with prophylactic defibrillators had 12‐lead ECGs recorded during ventricular pacing from 100 to 120 beats/min. QRSA and TWA were quantified in moving 128‐beat segments using the spectral method. Segments were categorized as QRSA positive (QRSA+) and/or TWA positive (TWA+) based on ≥2 precordial leads having alternans magnitude >0 and signal:noise >3. Patients were similarly categorized based on having ≥3 consecutive segments with alternans. TWA+ and QRSA+ occurred together in 31% of patients and alone in 18% and 14% of patients, respectively. Although TWA magnitude (1.4±0.4 versus 4.7±1.0 µV, P<0.01) and proportion of TWA+ studies (16% versus 46%, P<0.01) increased with rate, QRSA did not change. QRS duration was longer in QRSA+ than QRSA‐negative patients (138±23 versus 113±26 ms, P<0.01). At 3.5 years follow‐up, appropriate defibrillator therapy or sustained ventricular tachyarrhythmia was greater in QRSA+ than QRSA‐negative patients (30% versus 8%, P=0.02) but similar in TWA+ and TWA‐negative patients. Among QRSA+ patients, the event rate was greater in those without TWA (62% versus 21%, P=0.02). Multivariable Cox analysis revealed QRSA+ (hazard ratio [HR], 4.6; 95% CI, 1.5–14; P=0.009) and QRS duration >120 ms (HR, 4.1; 95% CI, 1.3–12; P=0.014) to predict events.

Conclusions

Microvolt QRSA is novel phenomenon in cardiomyopathy patients that can exist without TWA and is associated with QRS prolongation. QRSA increases the risk of ventricular tachyarrhythmia 4‐fold, which merits further study as a risk stratifier.

Human His-Purkinje System: Normal Electrophysiologic Behavior

The His-Purkinje system (HPS) plays a significant role in human pathophysiology, but knowledge is scattered. This article highlights some of the relevant concepts, phenomena, and mechanisms; clarifies, expands, confirms, or modifies commonly encountered clinical events; and adds new information, which is often available but obscure. Also included are the essentials of HPS anatomy and physiology. It is important to abandon inaccurate concepts that are still taught and occasionally appear in text books.

Wide complex irregularly regular tachycardia with QRS alternation

A 44-year-old white man was referred to our service because of a history of symptomatic rapid palpitations that had begun on January 2007. He had no previous history of syncope or family history of sudden cardiac death. The patient was otherwise well and had not been taking any medications. The electrocardiogram (ECG) recorded during sinus rhythm was within normal limits. A 12-lead ECG was performed during the last episode on July 2007 while he was working at his office. An irregular wide complex tachycardia with varying QRS morphology (left bundle-branch block with an alternating pattern) was documented. The differential diagnoses based on the 12-lead electrocardiogram (ECG) were supraventricular tachycardia with aberrant conduction and QRS alternans, atrial tachycardia with anterograde conduction over an accessory pathway, or ventricular tachycardia. An electrophysiology study was performed and the results are discussed.

Atrioventricular nodal re-entrant tachycardia with QRS voltage and cycle length alternation and aberrant conduction due to two distinct antegrade slow pathways

QRS voltage and cycle length alternation can be seen during supraventricular re-entrant tachycardias, especially in atrioventricular (AV) re-entrant tachycardia. We present a case of a 20-year-old man, in which AV nodal re-entrant tachycardia (AVNRT) shows alternation of QRS voltage and cycle length, as well as right bundle branch block aberration due to a re-entrant circuit using two distinct, beat-to-beat alternating slow AV nodal pathways antegradely and a single fast pathway retrogradely. Although more than one antegrade slow pathway exists, creation of a single lesion at the right posterior atrial septum using the conventional right-sided approach successfully eliminated AVNRT.

Electrocardiographic differentiation of typical atrioventricular node reentrant tachycardia from atrioventricular reciprocating tachycardia mediated by concealed accessory pathway in children

The value of the electrocardiogram (ECG) in children with supraventricular tachycardia (SVT) is unclear. The noninvasive differentiation of typical atrioventricular node reentrant tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT) mediated by concealed accessory pathway conduction is clinically important, as it helps in counseling and potentially facilitates ablation procedures. One hundred forty-eight ECGs showing narrow QRS complex SVT were obtained from children before successful radiofrequency catheter ablation. An initial 102 ECGs were analyzed by 3 blinded observers to assess the utility of various electrocardiographic findings. No electrocardiographic criteria were found to discriminate between SVT mechanisms on 1- to 3-channel Holter/event recorder tracings (n = 32); their interpretation mainly (55%) resulted in an incorrect SVT diagnosis. On 12-lead ECGs (n = 70), the 2 arrhythmias were accurately diagnosed in 76% of patients; 5 findings were found to be discriminators of tachycardia mechanism. Predictors of AVRT were visible P waves in 74% of cases (sensitivity 92%; specificity 64%), RP intervals of > or =100 ms in 91% (sensitivity 84%; specificity 91%), and ST-segment depression of > or =2 mm in 73% of cases (sensitivity 52%; specificity 82%). Pseudo r' waves in lead V(1) and pseudo S waves in the inferior leads during tachycardia predicted AVNRT in 100% of cases (sensitivity 55% and 20%, respectively; specificity 100% for both). Based on these results, we developed a new diagnostic 12-lead electrocardiographic algorithm for pseudo r'/S waves, RP duration, and ST-segment depression during tachycardia. Two observers tested the algorithm in 46 (21 AVNRT; 25 AVRT) additional cases; they correctly diagnosed the SVT mechanism in 91% and 87%, respectively. Thus, the stepwise use of diagnostically relevant 12-lead electrocardiographic parameters helps to more accurately differentiate mechanisms of reentrant SVT.

Conversion of atrial flutter by ibutilide is associated with increased atrial cycle length variability

OBJECTIVES

This study was designed to test the hypothesis that conversion of atrial flutter in humans by ibutilide, a new class III antiarrhythmic agent, is characterized by an increase in atrial cycle length variability.

BACKGROUND

Conversion of tachyarrhythmias has been associated with increased oscillations of cycle length.

METHODS

Electrograms and monophasic action potentials from the right atrium in 35 patients with spontaneous, sustained atrial flutter were recorded before, during and after intravenous ibutilide (0.005 to 0.025 mg/kg body weight, n = 25) or placebo (n = 10). Atrial cycle length, cycle length variability (coefficient of variation), diastolic interval and diastolic interval variability were measured from 10 consecutive cycles at baseline and 3 min before, 1 min before, 30 s before and immediately before conversion. Similar measurements were made in patients who received ibutilide or placebo but did not convert.

RESULTS

Ibutilide converted atrial flutter in 14 of 25 patients 25 +/- 16 min (mean +/- SD) after initiation of the infusion, whereas placebo converted no patients. Atrial cycle length was prolonged to the same extent in ibutilide converters and nonconverters (36 +/- 19 vs. 38 +/- 21 ms, p = NS) and was not affected by placebo. Beat-to-beat variability in atrial cycle length (baseline 1.2 +/- 0.7 vs. preconversion 7.3 +/- 4.9, p < 0.01) and diastolic interval (baseline 11 +/- 8 vs. preconversion 33 +/- 23, p < 0.05) increased significantly just before atrial flutter conversion and remained unchanged in ibutilide nonconverters and placebo group patients.

CONCLUSIONS

Ibutilide prolongs atrial cycle length, but conversion of atrial flutter by ibutilide is characterized by increased variability in atrial cycle length and diastolic interval.

Beat-to-beat behavior of QT interval during conducted supraventricular rhythm in the normal heart

To assess beat-to-beat behavior of QT interval under different conditions, high resolution recordings and computerized beat-to-beat analysis of the electrocardiogram were performed at rest, during recovery after short exercise, and during atrial pacing. Beat-to-beat variations of QT interval during sinus rhythm at rest and after short exercise were measured in ten healthy men. In an additional three patients with supraventricular tachycardia, beat-to-beat QT changes were studied after abrupt sustained acceleration and deceleration of heart rate by atrial pacing. Beat-to-beat changes in RR interval at rest are followed by minimal changes of the QT interval. The measured proportional change of the QT interval compared with the change in RR interval (delta QT/delta RR) was 0.02. This value represents 10% of the value expected for QT changes from Bazett's formula. Following short exercise QT interval did not change for 15 seconds and reached a maximal value 80 seconds later as compared to the RR interval (192 vs 115 secs, P < 0.001). The steady state of the QT interval during sustained atrial pacing was achieved after 132, 135, and 133 seconds for pacing intervals of 600, 500, and 600 msec, respectively. Our data indicate a relatively slow adaptation of the QT interval to changes in heart rate.

Changes of the RR interval and the QRS morphology in AV nodal tachycardia: further evidence for extranodal involvement

A young patient with AV nodal tachycardia was referred for ablation. During electrophysiological testing, a stable succession of up to four different RR intervals with concomitantly changing QRS morphologies were recorded. This observation might reflect the conduction of the reentry circuit through different extranodal "pathways" in the low right atrium. Radiofrequency current was applied near the ostium of the coronary sinus; this abolished conduction through the slow pathway, as dual AV conduction was no longer present. She remains free of recurrences for a follow-up period of 8 months.

Differentiation of paroxysmal narrow QRS complex tachycardias using the 12-lead electrocardiogram

OBJECTIVES

The purpose of this study was to evaluate the utility of the 12-lead electrocardiogram (ECG) for differentiating paroxysmal narrow QRS complex tachycardias.

BACKGROUND

Previous studies evaluating the utility of the 12-lead ECG for differentiating paroxysmal supraventricular tachycardia types have shown conflicting results on the usefulness of some ECG criteria, and some criteria that are considered to be useful have never been formally evaluated.

METHODS

Two hundred forty-two ECGs demonstrating paroxysmal narrow QRS complex (< 0.11 ms) tachycardia (rate > or = 120 beats/min) were analyzed. All ECGs were analyzed by an observer who had no knowledge of the mechanism of the tachycardia.

RESULTS

There were 137 atrioventricular (AV) reciprocating tachycardias, 93 AV node reentrant tachycardias and 12 atrial tachycardias. Six criteria were found to be significantly different between tachycardia types by univariate analysis. A P wave separate from the QRS complex was observed more frequently in AV reciprocating tachycardia (68%) and atrial tachycardias (75%). A pseudo r' deflection in lead V1 and a pseudo S wave in the inferior leads were more common in AV node reentrant tachycardia (58% and 14%, respectively); QRS alternans was present more often during AV reciprocating tachycardia (27%). When a P wave was present, an RP/PR interval ratio > or = 1 was more common in atrial tachycardias (89%). During sinus rhythm, manifest pre-excitation was observed more often in patients with AV reciprocating tachycardia (45%). By multivariate analysis, the presence of a P wave separate from the QRS complex, pseudo r' deflection in lead V1, QRS alternans during tachycardia and the presence of pre-excitation during sinus rhythm were independent predictors of tachycardia type. These criteria correctly identified 86% of AV node reentrant tachycardias, 81% of AV reciprocating tachycardias and incorrectly assigned the tachycardia type in 19% of cases.

CONCLUSIONS

Several features on the ECG are useful for differentiating supraventricular tachycardia type. However, approximately 20% of tachycardias may be incorrectly classified on the basis of analysis of the ECG; therefore, the ECG should not serve as the sole means for determining tachycardia mechanism.

Cardiac alternans: diverse mechanisms and clinical manifestations

OBJECTIVES

The purpose of this review is to assemble the widely dispersed information about cardiac alternans and to categorize the types and mechanisms of alternans, their clinical manifestations and possible therapeutic implications.

BACKGROUND

The phenomena of mechanical and electrical alternans have been of continuing interest to both physiologists and clinicians. Recent studies have enhanced this interest because of the reported association of alternans with experimental myocardial ischemia and cardiac arrhythmias.

METHODS

The review formulates concepts based on extensive review of published studies and personal observations.

RESULTS

Cardiac alternans has been subdivided into the following four categories: 1) mechanical, 2) electrical, 3) in association with myocardial ischemia, and 4) in association with cardiac motion. Mechanical alternans can be explained by hemodynamic or inotropic alterations, or both. Mechanical alternans in the ventricular muscle is accompanied by alternans of action potential shape. In the Purkinje fibers, action potential duration alternates without change in shape and is determined by the duration of the preceding diastolic interval. However, in ventricular muscle fiber, alternans can occur in the presence of constant diastolic intervals. T wave alternans reflects changes in action potential duration and is frequently associated with a long QT interval. Electrocardiographic manifestations of conduction alternans occur at many different sites within the conducting system and myocardium. During myocardial ischemia, additional mechanisms of repolarization alternans have been proposed. Alternans occurring in the presence of a large pericardial effusion is attributed to swinging motion of the heart maintaining two-beat periodicity.

CONCLUSIONS

Since its origin as "pulsus alternans" described by Traube in 1872, the definition of alternans has evolved into a term encompassing multiple physiologic and pathologic phenomena that, although united by the term cardiac alternans, diverge widely with respect to etiology, mechanism and clinical significance.

Electromechanical concordance in alternans during sustained tachycardias

Electrical alternans and mechanical alternans are intriguing phenomena that have been reported to occur during tachycardias. Their precise pathophysiologic mechanism and in particular their interrelation have not been well defined. This report documents an unusual electromechanical concordance in alternans during sustained supraventricular tachycardia and ventricular tachycardia in two patients, raising interesting possibilities on the underlying mechanism of the concordant phenomena. The findings highlight the principle of excitation--contraction coupling of the heart.

Repolarization inhomogeneities in ventricular myocardium change dynamically with abrupt cycle length shortening

BACKGROUND

In single heart cells, abrupt changes in stimulation rate elicit complex alterations in repolarization. The effects of rate change on dispersion of repolarization, however, have not been well characterized.

METHODS AND RESULTS

To determine the effects of abrupt cycle length (CL) shortening on spatial inhomogeneity of repolarization in a syncytium of ventricular cells, 124 action potentials were simultaneously recorded from Langendorff-perfused guinea pig hearts using high-resolution optical mapping with voltage-sensitive dye. The distribution of ventricular action potential durations (APDs) mapped during each cardiac cycle was used to calculate mean APD and repolarization dispersion index (DI), defined as the variance of the distribution. After abruptly shortening CL from 500 to 300 msec, mean APD declined exponentially in normoxic controls (by 23 +/- 3 msec, p less than 0.0001). This response was characterized by beat-to-beat oscillations of APD that were synchronized at all ventricular sites. After 30 minutes of hypoxia, mean APD decreased from 175.0 +/- 13.3 to 76 +/- 25.7 msec. However, during hypoxia, abrupt CL shortening lowered mean APD by only an additional 6 +/- 6 msec, and APD oscillations were no longer synchronized throughout the ventricle. In controls, beat-to-beat DI decreased significantly (-51.0 +/- 6.8%, p less than 0.01) by the sixth post-CL shortening beat and then recovered (by 15-20 beats). In contrast, DI failed to decrease during hypoxia (+7.1 +/- 23%). Two mechanisms for the transient decline of DI in controls were identified: synchronous APD oscillations and transient diminution of the apical-to-basal ventricular APD gradient.

CONCLUSIONS

These data demonstrate that inhomogeneity of ventricular repolarization, as measured by DI, changes dynamically with CL shortening. Furthermore, the hypoxic ventricle does not attenuate DI after abrupt CL shortening and thereby lacks a physiological response expected to diminish vulnerability to arrhythmias.

Monophasic action potential duration during programmed electrical stimulation

To examine changes in monophasic action potential duration (APD) with a pacing protocol similar to that used during electrophysiological testing, action potentials were recorded in vivo from the left ventricular apical endocardium of 12 normal mongrel dogs. The atrioventricular node was ablated and the dogs paced from the anterior right ventricle at a baseline cycle length of 1000 ms between interventions. Mean steady-state APD (APDss) was 266 +/- 7 ms at a pacing cycle length (PCL) of 1000 ms. Two pacing protocols were used. The first consisted of a sudden acceleration in pacing from a cycle length of 1000 ms to one between 300 and 600 ms. The second consisted of an 8-beat train at a cycle length of 400 ms followed by a premature beat at a coupling interval of 280 ms followed by a pause. The inter-train pause varied between 1 second and 32 seconds. With a sudden acceleration in pacing rate, steady-state values for APD at the faster PCLs were significantly smaller than APDss at 1000 ms with a change to cycle lengths of 600 ms (247 +/- 29 ms), 500 ms (229 +/- 21 ms), 400 ms (220 +/- 17 ms), and 300 ms (203 +/- 31 ms; P less than 0.01 for all comparisons). The time constant of the change in APD was shorter at a PCL of 300 ms (14.9 +/- 0.8 s) than 600 ms (20.3 +/- 4.7 s; P less than 0.05). With drive train pacing and incorporating an inter-train pause, the percent drop in steady-state APD compared to APD for the first train ranged from 10.1% with a 1-second inter-train pause to 2.1% with a 32-second pause. The difference in APD between the first drive train and drive trains after at least 3 minutes of pacing when APD had stabilized was not significant for an inter-train pause exceeding 8 seconds.

IN CONCLUSION

(1) with a sudden acceleration in pacing rate, endocardial APD in vivo decreases exponentially. The faster the new rate, the shorter the new steady-state APD and the shorter the time constant. (2) When pacing using an 8-beat drive train and an inter-train pause, there is a decremental shortening in APD for pause lengths shorter than 16 seconds. Thus, while performing programmed stimulation using a pause, a conditioning period of at least 2 minutes should be used prior to diastole scanning to allow APD to achieve a steady state.

Electrophysiological significance of QRS alternans in narrow QRS tachycardia

To investigate the electrophysiological significance of QRS alternans during narrow QRS tachycardia, transesophageal atrial pacing and recording was performed in 24 patients with a history of paroxysmal supraventricular tachycardia. Standard electrocardiograms showed ventricular preexcitation in 15 patients and normal QRS pattern in nine patients. The ventriculoatrial interval during tachycardia, as defined by means of transesophageal electrogram, allowed tentative diagnosis of the tachycardia mechanism. A 12-lead ECG was recorded either during spontaneous or induced tachycardia, as well as during transesophageal atrial pacing at increasing rates. Electrical alternans occurred spontaneously in eight patients (33%, group A): five with accessory pathway reentry (mean VA: 136 +/- 43 msec), and three with AV nodal reentry (mean VA: 48.3 +/- 12 msec). Tachycardia rate ranged between 170 and 230 beats/min (mean 200.7 +/- 16). In two patients, alternation of the QRS occurred only in the presence of a heart rate exceeding 180 and 190 beats/min, respectively. The amplitude of QRS remained stable during tachycardia in 16 patients (67%, group B): 14 had accessory pathway reentry (mean VA: 137.5 +/- 32 msec), and two had AV nodal reentry (mean VA: 45 +/- 7 msec). In this group, the tachycardia rate ranged from 150 to 210 beats/min (mean 175 +/- 12). Incremental transesophageal atrial pacing up to rates equal to that of tachycardia was performed in five patients from group A and in five patients from group B. Electrical alternans could not be induced in both groups with pacing at progressively increasing rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Cumulative effects of cycle length on ventricular refractoriness in man

This study examined the time course of changes of ventricular effective refractory period (VERP) following an abrupt change of cycle length (CL) in man. Stimulation at the right ventricular apex consisted of 19 cycles of an initial CL, followed by a variable number of cycles (0 to 50 cycles) of a new CL, and an extrastimulus to test for VERP. Fifteen patients were enrolled in each part of the study. In part A, initial CLs were long (mean +/- standard error, 650 +/- 20 msec) and new CLs were short (325 +/- 10 msec). VERPs were 259 +/- 6 msec after the long cycles, 238 +/- 6 msec after one short cycle (p less than 0.05), 224 +/- 5 msec after 10 short cycles, and 210 +/- 6 msec after 50 short cycles (p less than 0.05 versus 1 or 10 short cycles). Thus 43% of total shortening of VERP occurred in the first short cycle and 57% occurred in subsequent short cycles. In part B, initial CLs were short and new CLs were long. VERPs were 212 +/- 7 msec after the short cycles, 237 +/- 7 msec after one long cycle (p less than 0.05), 239 +/- 7 msec after 10 long cycles, and 247 +/- 7 msec after 50 long cycles (p less than 0.05 versus 1 or 10 long cycles). Thus 71% of total lengthening of VERP occurred in the first long cycle and 29% occurred in subsequent long cycles. In conclusion, following an abrupt change of CL in man, VERP changes markedly in the first new cycle (immediate effect) and then undergoes further, more gradual change over a large number of subsequent cycles (cumulative effects). Cumulative effects appear to be greater following shortening of CL than following lengthening of CL.

Oscillations of diastolic interval and refractory period following premature and postmature stimuli in canine cardiac Purkinje fibers

The effective refractory period (ERP) of His Purkinje tissue is known to oscillate following sudden changes in stimulus frequency. It has been proposed that the oscillation of ERP is caused by oscillation of the diastolic interval. This hypothesis was tested in vitro in canine cardiac Purkinje fibers superfused with normal and hyperkalemic (10 mM KCl) Tyrode's solution. The ERP of responses to premature stimuli, responses to postextrastimuli, and responses to postmature stimuli were measured. ERP of premature responses and postextrasystolic responses oscillated in normal Tyrode's solution. Oscillations of ERP of premature responses and postextrasystolic responses were damped in hyperkalemic Tyrode's solution. Determinations of the restitution of ERP as a function of diastolic interval revealed that oscillation of ERP of premature and postextrasystolic responses was for the most part predictable from the diastolic interval during superfusion with normal Tyrode's solution. In contrast, both hyperkalemic Tyrode's solution and postmature stimuli shifted the restitution curves, resulting in ERPs that were markedly longer than those predicted from the diastolic interval alone. Shifts in the restitution curves completely accounted for damping of the oscillation of ERP in hyperkalemic Tyrode's solution and with postmature stimuli. These data may relate to alterations of refractoriness during sudden changes in ventricular rate, such as occur during atrial fibrillation or ventricular tachycardia.

Oscillations of conduction, action potential duration, and refractoriness. A mechanism for spontaneous termination of reentrant tachycardias

The mechanism of cycle length oscillation and its role in spontaneous termination of reentry was studied in an in vitro preparation of canine atrial tissue surrounding the tricuspid orifice. Reentry occurred around a fixed path with incomplete recovery of excitability. Among 18 experiments, there was complete concordance between the occurrence of spontaneous cycle length oscillation and spontaneous terminations; both were observed in 10 experiments and neither in the other eight (p less than 0.001). Local changes in conduction during oscillations resulted from the dependence of both conduction velocity and action potential duration on the preceding local diastolic interval. Interval-dependent changes in action potential duration contributed to the oscillation by altering the next diastolic interval. Because of changes in action potential duration, changes in cycle length were poorly correlated with changes in diastolic interval and, therefore, with local conduction velocity. Complex oscillations resulted from variations in conduction time at multiple sites in the circuit. Oscillations caused most spontaneous terminations. The critical event was an exceptionally long diastolic interval preceding the next-to-last cycle that accelerated local conduction (which tended to shorten the last cycle) and prolonged action potential duration and refractoriness at the site of block. Ninety-two of 99 recordings of spontaneous termination showed evidence of oscillation of conduction and refractoriness causing block.

Cycle length dependence of human action potential duration in vivo. Effects of single extrastimuli, sudden sustained rate acceleration and deceleration, and different steady-state frequencies

Using a new method for long-term recording of monophasic action potentials from the human heart, we studied in 17 patients the effects on ventricular action potential duration (APD) of three clinically pertinent cycle length perturbations: (1) single extrastimuli, (2) abrupt sustained rate acceleration and deceleration, and (3) different steady-state cycle lengths. Results were: (a) APD after single extrastimuli at progressively longer cycle lengths were related to the extrastimulus cycle length with a biphasic electrical restitution curve which after an initial steep rise and a subsequent transient descent rose again more gradually to a plateau at cycle lengths above 800-1,000 ms. (b) After a sustained step decrease in cycle length, the first APD shortened abruptly while final steady-state adaptation required up to several minutes. The transition between the rapid and slow phase of APD change was characterized by a variable alternans of APD which correlated inversely with the preceding diastolic interval. (c) In the steady state, APD correlated linearly with cycle length, increasing an average of 23 ms per 100 ms cycle length increase (r = 0.995). The divergence between steady-state and non-steady-state APD, and the slowness of steady-state adaptation, are important factors to be considered in clinical electrophysiologic studies and in rate correction algorithms of APD or QT intervals, respectively.

Alternans of action potential duration after abrupt shortening of cycle length: differences between dog Purkinje and ventricular muscle fibers

The purpose of this study was to determine whether the alternans of action potential duration (APD) occurring in Purkinje and ventricular muscle fibers after an abrupt shortening of cycle length can be explained by the two factors controlling the cycle length-dependent APD changes (i.e., restitution and memory effect). Action potentials were recorded simultaneously from dog Purkinje fibers and ventricular muscle fibers using conventional microelectrode techniques. APD change during alternans was dependent on the preceding diastolic interval in the same manner as during restitution in Purkinje fibers but not in ventricular muscle fibers. The course of memory change was not affected by the presence of alternans in either fiber type. In Purkinje fibers, APD alternans was attenuated by a Ca2+ channel blocker, nisoldipine (2 X 10(-6) M), and augmented by a Ca2+ channel agonist, Bay K 8644 (3 X 10(-8) M). These effects were attributed to the changes in the kinetics and the amplitude of restitution. In ventricular muscle fibers, APD alternans was always preceded and accompanied by alternans of action potential shape. Alternans of both action potential shape and APD was suppressed by nisoldipine (2 X 10(-6) M) and attenuated by Bay K 8644 (3 X 10(-8) M). These results show that in Purkinje fibers, APD during alternans can be explained by restitution and memory effect. However, in ventricular muscle fibers, the mechanism of APD alternans is linked to factors controlling action potential shape. These findings are compatible with the hypothesis that APD alternans in Purkinje fibers depends on the differences in the recovery of membrane currents generated by the preceding action potential and in ventricular muscle fibers on the differences in the concentration and/or handling of intracellular calcium.

Effects of upright posture on anterograde and retrograde atrioventricular conduction in patients with coronary artery disease, mitral valve prolapse or no structural heart disease

To assess the effects of posture on anterograde and retrograde atrioventricular conduction, electrophysiologic testing was performed in 25 patients in both the supine and 45 degrees upright positions on a tilt table. Retrograde conduction was present during ventricular pacing in 17 patients in the supine position; all 17 continued to manifest retrograde conduction in the upright position. In all patients with absent retrograde conduction while supine, retrograde conduction could not be demonstrated while upright. Upright posture significantly (p less than 0.05) shortened the sinus cycle length (from 808 +/- 34 to 678 +/- 26 ms, mean +/- standard error of the mean), AH interval during sinus rhythm (78 +/- 6 to 69 +/- 6 ms), and AH interval during atrial pacing at cycle length 500 ms (123 +/- 13 to 91 +/- 9 ms). Total atrioventricular conduction time during atrial pacing shortened significantly (from 169 +/- 13 to 136 +/- 10 ms), as did ventriculoatrial conduction time during ventricular pacing (from 192 +/- 9 to 178 +/- 7 ms). Upright posture also significantly shortened both anterograde block cycle length (390 +/- 20 to 328 +/- 17 ms) and retrograde block cycle length (466 +/- 27 to 354 +/- 18 ms). However, the effect of upright posture on retrograde block cycle length was significantly greater than on anterograde block cycle length: a 21% decrease retrograde vs a 14% decrease anterograde (p less than 0.05). These effects may produce clinically important changes in characteristics of arrhythmias that depend on the properties of anterograde and retrograde conduction.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of QRS alternans during narrow QRS tachycardia

This study was designed to prospectively determine the incidence of QRS alternans during various types of narrow QRS tachycardia and to clarify the determinants of QRS alternans. An electrophysiologic study was performed in 28 consecutive patients with a narrow QRS tachycardia. Persistent QRS alternans was observed in 6 (43%) of 14 patients during orthodromic reciprocating tachycardia, 5 (71%) of 7 patients during atrial tachycardia and 3 (43%) of 7 patients during atrioventricular (AV) node reentrant tachycardia. Incremental atrial pacing during sinus rhythm resulted in QRS alternans in patients who had QRS alternans during tachycardia, unless the shortest pacing cycle length associated with 1:1 AV conduction exceeded the tachycardia cycle length. In patients without QRS alternans during narrow QRS tachycardia, incremental atrial pacing during sinus rhythm resulted in persistent QRS alternans in five patients in whom the shortest pacing cycle length associated with 1:1 AV conduction was 60 to 180 ms less than the tachycardia cycle length. In an additional 20 patients without a narrow QRS tachycardia, persistent QRS alternans was observed during incremental atrial pacing in 11 (55%) of the patients. In six of six patients who had QRS alternans during abrupt rapid atrial pacing, QRS alternans was not observed when the same pacing rates were achieved gradually. Among the patients with narrow QRS tachycardia, the mean tachycardia cycle length in those who had QRS alternans (mean +/- SD 288 +/- 44 ms) was significantly shorter than in those who did not (369 +/- 52 ms, p less than 0.001). The presence of QRS alternans was not related to the tachycardia mechanism, relative or functional refractory period of the His-Purkinje system (at a drive cycle length of 500 ms), age, presence of structural heart disease, direction of input into the AV node or concealed retrograde conduction in the His-Purkinje system. In conclusion, QRS alternans during narrow QRS tachycardias is a rate-related phenomenon that depends on an abrupt increase to a critical rate and is independent of the tachycardia mechanism.

Characterization of oscillations in ventricular refractoriness in man after an abrupt increment in heart rate

Oscillations in ventricular myocardial refractoriness after a change in rate have not been described in man. During 25 baseline paced cycle lengths (SB-SB) of 400 to 800 msec in 14 patients, a shorter cycle length (S'-S') was introduced that was 10 to 20 msec (mean 15 +/- 5) greater in duration than the ventricular effective refractory period (VERP) determined after 12 beats of the respective baseline cycle length (group 1 trials). In addition, during 14 of the 25 baseline cycle lengths, a second shorter cycle length was introduced that was 50 to 80 msec (mean 65 +/- 10) greater than the VERP of the respective baseline cycle length (group 2 trials). In all 39 group 1 and group 2 trials the VERP was determined after each of at least 3 beats (S'1, S'2, S'3) of the shorter cycle length and in six of the 25 group I trials the VERP was determined after each of at least 12 beats of the shorter cycle length. In all group 1 trials oscillations of the VERP were observed, with a mean S'1 VERP of 202 +/- 28 msec, an S'2 VERP of 228 +/- 25 msec (p less than .001 vs S'1 VERP), and an S'3 VERP of 210 +/- 26 msec (p less than .001 vs S'2 VERP). Oscillations dampened within 4 beats, but persisted at a lower amplitude in four of the six group I trials during which the shorter cycle length was maintained for at least 12 beats.(ABSTRACT TRUNCATED AT 250 WORDS)