Zones of atrial vulnerability. Relationships to basic cycle length

Effects of acute autonomic modulation on atrial conduction delay and local electrograms duration in paroxysmal atrial fibrillation

Slowed atrial conduction may contribute to reentry circuits and vulnerability for atrial fibrillation (AF). The autonomic nervous system (ANS) has modulating effects on electrophysiological properties. However, complex interactions of the ANS with the arrhythmogenic substrate make it difficult to understand the mechanisms underlying induction and maintenance of AF.

AIM

To determine the effect of acute ANS modulation in atrial activation times in patients (P) with paroxysmal AF (PAF).

METHODS AND RESULTS

16P (9 men; 59±14years) with PAF, who underwent electrophysiological study before AF ablation, and 15P (7 men; 58±11years) with atrioventricular nodal reentry tachycardia, without documentation or induction of AF (control group). Each group included 7P with arterial hypertension but without underlying structural heart disease. The study was performed while off drugs. Multipolar catheters were placed at the high right atrium (HRA), right atrial appendage (RAA), coronary sinus (CS) and His bundle area (His). At baseline and with HRA pacing (600ms, shortest propagated S2) we measured: i) intra-atrial conduction time (IACT, between RAA and atrial deflection in the distal His), ii) inter-atrial conduction time (interACT, between RAA and distal CS), iii) left atrial activation time (LAAT, between atrial deflection in the distal His and distal CS), iv) bipolar electrogram duration at four atrial sites (RAA, His, proximal and distal CS). In the PAF group, measurements were also determined during handgrip and carotid sinus massage (CSM), and after pharmacological blockade of the ANS (ANSB). AF was induced by HRA programmed stimulation in 56% (self-limited - 6; sustained - 3), 68.8% (self-limited - 6; sustained - 5), and 50% (self-limited - 5; sustained - 3) of the P, in basal, during ANS maneuvers, and after ANSB, respectively (p=NS). IACT, interACT and LAAT significantly lengthened during HRA pacing in both groups (600ms, S2). P with PAF have longer IACT (p<0.05), a higher increase in both IACT, interACT (p<0.01) and electrograms duration (p<0.05) with S2, and more fragmented activity, compared with the control group. Atrial conduction times and electrograms duration were not significantly changed during ANS stimulation. Nevertheless, ANS maneuvers increased heterogeneity of the local electrograms duration. Also, P with sustained AF showed longer interACT and LAAT during CSM.

CONCLUSION

Atrial conduction times, electrograms duration and fractionated activity are increased in PAF, suggesting a role for conduction delays in the arrhythmogenic substrate. Acute vagal stimulation is associated with prolonged interACT and LAAT in P with inducible sustained AF and ANS modulation may influence the heterogeneity of atrial electrograms duration.

Inducibility of atrial fibrillation during electrophysiologic evaluation is associated with increased dispersion of atrial refractoriness

The impact of atrial dispersion of refractoriness (Disp_A) in the inducibility and maintenance of atrial fibrillation (AF) has not been fully resolved.

AIM

To study the Disp_A and the vulnerability (A_Vuln) for the induction of self-limited (<60 s) and sustained episodes of AF.

METHODS AND RESULTS

Forty-seven patients with paroxysmal AF (PAF): 29 patients without structural heart disease and 18 with hypertensive heart disease. Atrial effective refractory period (ERP) was assessed at five sites--right atrial appendage and low lateral right atrium, high interatrial septum, proximal and distal coronary sinus. We compared three groups: group A - AF not inducible (n=13); group B - AF inducible, self-limited (n=18); group C - AF inducible, sustained (n=16). Age, lone AF, hypertension, left atrial and left ventricular (LV) dimensions, LV systolic function, duration of AF history, atrial flutter/tachycardia, previous antiarrhythmics, and Disp_A were analysed with logistic regression to determine association with A_Vuln for AF inducibility. The ERP at different sites showed no differences among the groups. Group A had a lower Disp_A compared to group B (47+/-20 ms vs 82+/-65 ms; p=0.002), and when compared to group C (47+/-20 ms vs 80+/-55 ms; p=0.008). There was no significant difference in Disp_A between groups B and C. By means of multivariate regression analysis, the only predictor of A_Vuln was Disp_A (p=0.04).

CONCLUSION

In patients with PAF, increased Disp_A represents an electrophysiological marker of A_Vuln. Inducibility of both self-limited and sustained episodes of AF is associated with similar values of Disp_A. These findings suggest that the maintenance of AF is influenced by additional factors.

Moderate and chronic hemodynamic overload of sheep atria induces reversible cellular electrophysiologic abnormalities and atrial vulnerability

OBJECTIVES

The aim of this study was to evaluate the myocardial consequences of a chronic volume overload of the left atrium (LA).

BACKGROUND

Atrial dilation is a major risk factor for atrial fibrillation (AF), but the underlying mechanisms are poorly understood.

METHODS

A left-right aorto-pulmonary artery shunt (APS) was created in sheep. The cardiopathy was characterized by echocardiography, electrophysiologic testing, and histologic analysis. Cellular action potential (AP) and calcium current (I(Ca)) were recorded by means of microelectrode and patch clamp techniques.

RESULTS

Three to four months after surgery, all animals in the APS state had a dilated LA (146.2 +/- 35.4 cm(2)/m(2) vs. 91.7 +/- 10.4 cm(2)/m(2) in the control state; p = 0.0024) but remained in sinus rhythm. Repetitive atrial firing was triggered by a single extra beat in five of six animals in the APS state and in two of six animals in the control state. Moreover, in two animals in the APS state, a single extra beat triggered sustained AF. Myocytes were enlarged and 39.8% showed some degree of myolysis. In animals in the APS state, the AP had no plateau phase or small amplitude and numerous myocytes were unexcitable. The I(Ca) density was 45.2% lower in APS animals than in control animals. Beta-adrenergic stimulation normalized I(Ca) and restored the plateau phase of the AP. After shunt suppression, the electrophysiologic properties of the atria returned to normal.

CONCLUSIONS

The APS induced moderate, isolated LA dilation, which was sufficient to cause major changes in cellular electrophysiologic properties and to render the atria vulnerable to fibrillation. These effects were reversed by shunt suppression.

Syncopal episodes in a young amateur body builder

A 36 year old male weight training enthusiast suffered several syncopal episodes. An electrocardiogram confirmed atrial fibrillation with normal ventricular response. The patient admitted to taking anabolic steroids and bromocriptine. The atrial fibrillation was considered to be due to bromocriptine misuse.

Atrial fibrillation induction and determination of atrial vulnerable period using very low energy synchronized biatrial shock in normal subjects and in patients with atrial fibrillation

The atrial vulnerable periods (AVP) for shock induction of atrial fibrillation (AF) in humans have not been clearly defined. Furthermore, the safety and efficacy of using low energy biatrial shock delivered transvenously for AF induction are unknown. We tested the safety and efficacy of using very low energy biatrial shocks, delivered between the right atrium and the coronary sinus for AF induction and used this technique to characterize the AVP in nine controls and nine patients with AF. Thirty-volt and 60-V 3/3-ms biphasic shocks were delivered, starting from 50 ms before the atrial effective refractory period with 20-ms increments until the end of the QRS interval to determine the AVP front, AVP end, and the AVP duration. Successful AF induction could be achieved in eight (89%) of the nine controls and in nine (100%) of the nine patients with AF without any complication. In patients with AF, the AVP front started significantly earlier within the QRS complex, and the AVP duration and the AVP duration/QRS percent ratios were also significantly greater as compared to controls. Furthermore, a higher induction shock energy in patients with AF was associated with an increase in AF inducibility and significantly increased the AVP duration and AVP duration/QRS percent ratio as compared to the controls. This study demonstrated the safe and efficacy of delivering a very low energy biatrial shock during the AVP within the R wave for AF induction. The characteristics of AVP in patients with AF were significantly different from normal subjects.

Experimental study of the effects of multi-site sequential ventricular pacing on the prophylaxis of ventricular fibrillation

Previous studies report a significant prophylactic effect on the occurrence of atrial fibrillation by simultaneous multi-site atrial pacing. We investigated the effects of multi-site sequential ventricular pacing (MSVP), which may be preferable to simultaneous multi-site pacing in terms of the prophylaxis of the occurrence of ventricular fibrillation (VF). Needle electrodes were inserted at ten different epicardial sites on both ventricles for MSVP in 12 adult beagle dogs. Four premature ventricular extrastimuli (PVE) were introduced to provoke VF reproducibly from a separate electrode in the left ventricle. The 4 PVE were applied to try to provoke VF during MSVP in a comparable fashion to the activation sequence during sinus rhythm. We compared the prophylactic effects of MSVP on the inducibility of VF by changing the number of stimulation sites to either 1, 3, 5, or 10 epicardial sites. We performed a total of 363 trials of induction and suppression of VF. The occurrence rates of VF by the 4 PVE for the various number of epicardial stimulation sites of MSVP, i.e., at 1, 3, 5, and 10 sites, were 0.8263, 0.4286, 0.4450, and 0.2857, respectively (p < 0.05). There was a significant prophylactic effect of MSVP on the inducibility of VF, and this effect became stronger as the number of MSVP sites was increased from 3 to 10. The hemodynamic state was relatively stable during MSVP. MSVP seems to be a promising method with which to reduce the occurrence of VF, and a larger number of stimulation sites would be more effective in terms of the prophylaxis of VF.

The comparative effects of drive and test stimulus intensity on myocardial excitability and vulnerability

The number and intensity of stimuli that set basic cycle length in cardiac electrophysiological studies can influence the electrical properties assessed by extrastimuli. The relative contribution of drive (S1) and test (S2) stimulus intensity in defining myocardial excitability and vulnerability has not been reported. The purpose of this investigation was to assess this interaction and to determine whether atrial and ventricular findings differed. The effects of S1 and S2 intensity on atrial and ventricular stimulus-intensity-refractory-period curves were determined in open-chest dogs: comparisons were made between curves with S1 intensity varied between diastolic threshold (DT) and 10 mA and S2 intensity maintained at DT and those with S1 intensity maintained at DT and S2 intensity varied between DT and 10 mA. S1-S1 was held constant and S1-S2 varied. The effects of different stimulation sites, cycle length, number of stimulations, and neural blockade were assessed. S1 intensity amplification shifted atrial stimulus-intensity-refractory period curves in the direction of increased excitability and vulnerability; the changes were more pronounced than those obtained by modulating S2 intensity. The changes produced by increasing S1 intensity were evident at different cycle lengths and were enhanced by an increased number of stimulations, but were not evident when S1 and S2 were delivered at different atrial sites. Although beta-blockade attenuated the effects of increasing S1 intensity somewhat, the addition of cholinergic blockade virtually abolished it. Ventricular refractoriness was also changed by modulation of S1 intensity, but the changes were less striking. In the atrium, modulation of S1 intensity has greater effects of stimulus-intensity-refractory-period relations than modulation of S2 intensity; in the ventricule, the converse is true.

Electrophysiological properties of the left atrium evaluated by coronary sinus pacing in patients with atrial fibrillation

Repetitive atrial firing (RAF), marked fragmentation of atrial activity (FAA), and interatrial conduction delay (CD) have been shown to be electrophysiological features of the atrium in patients with atrial fibrillation (AF). Moreover, it has been observed that atrial extrastimuli are more likely to induce AF when delivered from the right atrial appendage (RAA) than from the distal coronary sinus (CSd). We examined the electrophysiological properties of the atrial muscle by CS and RAA stimulation in patients with paroxysmal AF. Patients were divided into two groups: group I, consisting of 18 patients with clinical paroxysmal AF; and group II, consisting of 22 patients with various cardiac arrhythmias in which the substrate does not exist in the atrium. In group I, the following values of electrophysiological parameters of the atrium indicated that AF was more likely to be induced during RAA pacing than CSd pacing: atrial effective refractory period (RAA vs CSd: 201 +/- 28 ms vs 240 +/- 35 ms, P < 0.001), RAF zone (16 +/- 25 ms vs 0 +/- 0 ms, P < 0.03), FAA zone (38 +/- 37 ms vs 5 +/- 19 ms, P < 0.01), maximum interatrial conduction time (144 +/- 19 ms vs 93 +/- 19 ms, P < 0.0001) and CD zone (53 +/- 21 ms vs 9 +/- 18 ms, P < 0.0001). The values of the electrophysiological parameters of the atrium evaluated by CSd pacing in group I patients were not significantly different from those in group II patients. In conclusion, when coronary sinus stimulation is performed, electrophysiological properties of the atrium in patients with AF show a significant decrease in atrial vulnerability compared to stimulation from RAA and also show similar values to those in patients without AF. It might be suggested that the left posterior or posterolateral atrium is electrophysiologically stable even in patients with paroxysmal AF.

The spatial dispersion of atrial refractoriness and atrial fibrillation vulnerability

The local dispersion of conduction and refractoriness has been considered essential for induction of atrial arrhythmias. This study sought to determine whether a difference of refractoriness and vulnerability for induction of atrial fibrillation between trabeculated and smooth as well as high and low right atrium may contribute to initiation of atrial fibrillation in dogs. In 14 healthy mongrel dogs weighing 22.4 +/- 1 kg, closed-chest endocardial programmed stimulation was performed from four distinct right atrial sites. Atrial refractory periods and vulnerability for induction of atrial fibrillation or premature atrial complexes were determined during a basic cycle length of 400 and 300 ms and an increasing pacing current strength. For a pacing cycle length of 300 ms, atrial refractory periods were longer on the smooth, as compared to the trabeculated right atrium (102 +/- 25 vs. 97 +/- 17 ms, p < 0.05), whereas for a pacing cycle length of 400 ms, there was no significant difference. The duration of the vulnerability zone for induction of atrial fibrillation was longer on the smooth right atrium, for a cycle length of both 400 ms (40 +/- 30 vs. 31 +/- 22 ms; p < 0.05) and 300 ms (33 +/- 25 vs. 23 +/- 21 ms; p < 0. 01). When comparing high and low right atrium, refractory periods were longer on the the low right atrium, for a cycle length of both 400 ms (111 +/- 23 vs. 94 +/- 24 ms; p < 0.01) and 300 ms (104 +/- 20 vs. 96 +/- 23 ms; p < 0.01). For a pacing cycle length of 300 ms, the duration of the atrial fibrillation vulnerability zone was longer for the high, as compared to the low right atrium (34 +/- 22 vs. 22 +/- 22, p < 0.01). Seven dogs with easily inducible episodes of atrial fibrillation demonstrated significantly shorter refractory periods as compared to 7 non-vulnerable dogs, regardless of pacing site and current strength. In conclusion, significant differences in refractoriness and vulnerability for induction of atrial fibrillation can be observed in the area of the crista terminalis in healthy dogs. Thus, local anatomic factors may play a role in the initiation of atrial fibrillation.

Dynamic effects of intravenous procainamide infusion on the electrophysiological properties during atrial fibrillation

Although the mechanism of atrial fibrillation (AF) is still controversial, multiple wandering reentry is considered the primary mechanism in most AF. It has been suggested that prolongation of the wavelength would make it impossible for the reentry to continue and would lead to the termination of the AF. In the present study a dynamic fluctuation in the electrophysiological properties was observed with procainamide infusion during AF. In 12 patients, both the local electrogram and monophasic action potentials (MAP) during AF were recorded from the right atrium before, during and after infusion of procainamide (10 mg/kg). The minimum AF cyclelength (CLmin), MAP duration at 90% repolarization (MAPD90) and widths of the intraatrial potentials (WAP) were measured with custom-made computer software. The conduction velocity index (CVI) was determined from the WAP. The wavelength index (WLI=CVIxCLmin) and postrepolarization refractoriness (PRR= CLmin-MAPD90) were calculated. In 6 patients, AF was terminated by procainamide infusion (group A), but not in the other 6 patients (group B). Group A patients showed a biphasic change in the parameters following procainamide infusion. In phase I, the CLmin, MAPD90 and PRR increased, while the CVI decreased, and the WLI remained unchanged. In phase II, the PRR, CVI and WLI increased and the AF was terminated. No restoration of the CVI nor increase in the WLI were observed in group B. The biphasic fluctuation in the CVI and the remarkable increase of the PRR and WLI were observed before termination of AF by procainamide infusion.

Susceptibility to atrial fibrillation: a study in an ovine model of pacing-induced early heart failure

INTRODUCTION

The development of susceptibility to atrial fibrillation (AF) is a common consequence of many forms of cardiovascular disease, especially heart failure. In this study we used a sheep model of pacing-induced stable early heart failure to describe, quantify, and relate the level of susceptibility to AF to changes in structural and electrophysiologic parameters.

METHODS AND RESULTS

Epicardial electrodes were implanted on the atria and right ventricles of nine sheep. The AF threshold, atrial vulnerability period, atrial effective refractory period (ERP), and interatrial conduction time were examined during control and over a 6-week period of ventricular pacing at 190 beats/min. Left atrial (LA) area and left ventricular (LV) fractional shortening were monitored using echocardiography. There were significant increases in LA susceptibility to AF (P < 0.0003), LA area (P < 0.0002), and LA ERP400 (P < 0.0002). Rate of increase in LA area was related positively to AF susceptibility (P = 0.02) and inversely to LA ERP400 (P = 0.002). LV fractional shortening decreased to approximately 50% of control value (P < 0.00001). No changes were observed in right atrial electrophysiology.

CONCLUSION

In this study, susceptibility (the ability of an extrastimulus to induce AF) was rigorously measured within a predetermined format. Significant relationships were found to exist between susceptibility, certain of the observed changes in atrial electrophysiology and structure.