Mechano-electric feedback and atrial fibrillation

Cardiorespiratory interactions in patients with atrial flutter

Respiratory sinus arrhythmia (RSA) is generally known as the autonomically mediated modulation of the sinus node pacemaker frequency in synchrony with respiration. Cardiorespiratory interactions have been largely investigated during sinus rhythm, whereas little is known about interactions during reentrant arrhythmias. In this study, cardiorespiratory interactions at the atrial and ventricular level were investigated during atrial flutter (AFL), a supraventricular arrhythmia based on a reentry, by using cross-spectral analysis and computer modeling. The coherence and phase between respiration and atrial (γ[Formula: see text], φAA) and ventricular (γ[Formula: see text], φRR) interval series were estimated in 20 patients with typical AFL (68.0 ± 8.8 yr) and some degree of atrioventricular (AV) conduction block. In all patients, atrial intervals displayed oscillations strongly coupled and in phase with respiration (γ[Formula: see text]= 0.97 ± 0.05, φAA = 0.71 ± 0.31 rad), corresponding to a paradoxical lengthening of intervals during inspiration. The modulation pattern was frequency independent, with in-phase oscillations and short time delays (0.40 ± 0.15 s) for respiratory frequencies in the range 0.1–0.4 Hz. Ventricular patterns were affected by AV conduction type. In patients with fixed AV conduction, ventricular intervals displayed oscillations strongly coupled (γ[Formula: see text]= 0.97 ± 0.03) and in phase with respiration (φRR = 1.08 ± 0.80 rad). Differently, in patients with variable AV conduction, respiratory oscillations were secondary to Wencheback rhythmicity, resulting in a decreased level of coupling (γ[Formula: see text]= 0.50 ± 0.21). Simulations with a simplified model of AV conduction showed ventricular patterns to originate from the combination of a respiratory modulated atrial input with the functional properties of the AV node. The paradoxical frequency-independent modulation pattern of atrial interval, the short time delays, and the complexity of ventricular rhythm characterize respiratory arrhythmia during AFL and distinguish it from normal RSA. These peculiar features can be explained by assuming a direct mechanical action of respiration on AFL reentrant circuit.

Role of multidetector computed tomography in the anatomical definition of the left atrium-pulmonary vein complex in patients with atrial fibrillation. Personal experience and pictorial assay

PURPOSE

This study aimed to illustrate the typical anatomical pattern and anatomical variants of the left atrium-pulmonary vein (LA-PV) complex studied by 16-slice multidetector computed tomography (MDCT) in a population of patients with atrial fibrillation (AF) undergoing percutaneous transcatheter left atrial ablation. Accurate knowledge of this anatomical region is fundamental for increasing the efficiency, efficacy and accuracy of the procedure and for reducing the risk of complications.

MATERIALS AND METHODS

From January 2004 to March 2007, we studied 75 patients (57 men, 18 women) affected by paroxysmal and chronic AF by using MDCT. In 63 patients, the MDCT examination was performed using retrospective cardiac electrocardiographic (ECG) gating and dose modulation, with reconstructions performed at 75% of R-R interval. In the remaining 12 patients, ECG gating was not possible due to high-frequency AF.

RESULTS

We identified 286 PV: 157 right and 129 left. On the right side, eight PV were supernumerary and one was a common trunk, whereas on the left side, we found 22 common trunks and one supernumerary vein. In 61.3% of patients, the anatomical pattern was typical (two right and two left PV). In the remaining 38.7%, it was atypical [two right PV-left common trunk (26.6%); three right PV-two left PV (6.7%); three right PV-left common trunk (2.6%); three right PV-three left PV (1.3%); right common trunk-two left PV (1.3%)]. MDCT identified branching of the right inferior PV in 94.5%, of the right superior PV in 75.6%, of the left superior PV in 7.5% and of the left inferior PV in 7.5%; 3/8 of the right supernumerary veins presented branching. With respect to the left PV ostia, the position of the orifice of the 74 recognised appendages was high in 85.1%, intermediate in 12.1% and low in 2.8%. There was no association between PV anatomical variants and clinical presentation of AF (paroxysmal or chronic).

CONCLUSIONS

MDCT represents a fundamental diagnostic imaging tool in the anatomical definition of the LA-PV complex, which is characterised by considerable variability. Radiologists must be familiar with the anatomical variants and help the referring interventional electrophysiologist understand their importance.

Atrial fibrillation in end-stage renal disease

End-stage renal disease (ESRD) is associated with increased cardiovascular morbidity and mortality. Recent studies indicate that atrial fibrillation (AF) is prevalent among ESRD patients while it adversely affects the clinical outcome. Despite these considerations, AF management in this population is problematic. Notably, most ESRD patients with AF are deprived of the benefits of anticoagulation therapy because of the fear of hemorrhagic complications. This article provides a concise and critical overview of the complex pathophysiology, epidemiology, and discusses the clinical issues regarding the emerging association between ESRD and AF.

A model for mechano-electrical feedback effects on atrial flutter interval variability

Atrial flutter is a supraventricular arrhythmia, based on a reentrant mechanism mainly confined to the right atrium. Although atrial flutter is considered a regular rhythm, the atrial flutter interval (i.e., the time interval between consecutive atrial activation times) presents a spontaneous beat-to-beat variability, which has been suggested to be related to ventricular contraction and respiration by mechano-electrical feedback. This paper introduces a model to predict atrial activity during atrial flutter, based on the assumption that atrial flutter variability is related to the phase of the reentrant activity in the ventricular and respiratory cycles. Thus, atrial intervals are given as a superimposition of phase-dependent ventricular and respiratory modulations. The model includes a simplified atrioventricular (AV) branch with constant refractoriness and conduction times, which allows the prediction of ventricular activations in a closed-loop with atrial activations. Model predictions are quantitatively compared with real activation series recorded in 12 patients with atrial flutter. The model predicts the time course of both atrial and ventricular time series with a high beat-to-beat agreement, reproducing 96+/-8% and 86+/-21% of atrial and ventricular variability, respectively. The model also predicts the existence of phase-locking of atrial flutter intervals during periodic ventricular pacing and such results are observed in patients. These results constitute evidence in favor of mechano-electrical feedback as a major source of cycle length variability during atrial flutter.

Mechanoelectric feedback leads to conduction slowing and block in acutely dilated atria: a modeling study of cardiac electromechanics

Atrial fibrillation, a common cardiac arrhythmia, is promoted by atrial dilatation. Acute atrial dilatation may play a role in atrial arrhythmogenesis through mechanoelectric feedback. In experimental studies, conduction slowing and block have been observed in acutely dilated atria. In the present study, the influence of the stretch-activated current ( Isac) on impulse propagation is investigated by means of computer simulations. Homogeneous and inhomogeneous atrial tissues are modeled by cardiac fibers composed of segments that are electrically and mechanically coupled. Active force is related to free Ca2+ concentration and sarcomere length. Simulations of homogeneous and inhomogeneous cardiac fibers have been performed to quantify the relation between conduction velocity and Isac under stretch. In our model, conduction slowing and block are related to the amount of stretch and are enhanced by contraction of early-activated segments. Conduction block can be unidirectional in an inhomogeneous fiber and is promoted by a shorter stimulation interval. Slowing of conduction is explained by inactivation of Na+ channels and a lower maximum upstroke velocity due to a depolarized resting membrane potential. Conduction block at shorter stimulation intervals is explained by a longer effective refractory period under stretch. Our observations are in agreement with experimental results and explain the large differences in intra-atrial conduction, as well as the increased inducibility of atrial fibrillation in acutely dilated atria.

Modification of ventricular fibrillation activation patterns induced by local stretching

INTRODUCTION

We hypothesize that local modifications in electrophysiological properties, when confined to zones of limited extent, induce few changes in the global activation process during ventricular fibrillation (VF). To test this hypothesis, we produced local electrophysiological modifications by stretching a circumscribed zone of the left ventricular wall in an experimental model of VF.

METHODS AND RESULTS

In 23 Langendorff-perfused rabbit hearts frequency, time-frequency and time-domain techniques were used to analyze the VF recordings obtained with two epicardial multiple electrodes before, during, and after local stretching produced with a left intraventricular device. Acute local stretching accelerated VF in the stretched zone reversibly and to a variable degree, depending on the magnitude of stretch and the time elapsed from its application. In the half time (5 minutes) of the analyzed period, a longitudinal lengthening of 12.1 +/- 4.5% (vertical axis) and 11.8 +/- 6.2% (horizontal axis) in the stretched zone produced an increase in the dominant frequency (DFr) (15.2 +/- 1.9 versus 18.8 +/- 2.5 Hz, P < 0.0001), a decrease in mean VV interval (63 +/- 8 versus 53 +/- 6 msec, P < 0.001), and an increase in the complexity of the activation maps-with more areas of conduction block and more breakthrough patterns (23% versus 37%, P < 0.01), without significant changes in the percentages of complete reentry patterns (9% versus 9%, ns). Simultaneously, in the nonstretched zone, no variations were observed in the DFr (15.2 +/- 2.1 versus 15.3 +/- 2.5 Hz, ns), mean VV intervals (66 +/- 8 versus 65 +/- 8 msec, ns), or types and percentages of maps with breakthrough (25% versus 20%, ns) or reentry patterns (12% versus 8%, ns). No significant correlation was observed between the DFr in the two zones (R = 0.24, P = 0.40).

CONCLUSION

Local stretching increases the electrophysiological heterogeneity of myocardium and accelerates and increases the complexity of VF in the stretched area, without significantly modifying the occurrences of the types of VF activation patterns in the nonstretched zone.

Cardiac mechano-electric feedback: past, present, and prospect

Mechanical effects on heart rhythm have been known to the clinical community for well over a century, and documented cases include both arrhythmogenic and pro-rhythmic consequences of mechanical stimulation. The intracardiac pathway that leads from changes in the cardiac mechanical environment to altered electrical activity is referred to as mechano-electric feedback (MEF). Fundamental research into the mechanisms underlying cardiac MEF is 'engineering-intensive', and much of the current insight would have been impossible without the introduction of novel techniques for the study of isolated cardiac cells. Clinical and basic research into MEF have developed over different time scales, often uninformed of each other, and utilizing disparate concepts and terminology. Bridging the gap between the two domains is not straightforward, as physicians and scientists tend to publish in different journals and attend different meetings. There is, however, a growing interest in 're-uniting' the clinic and basic MEF research, as witnessed by an increasing number of dedicated journal issues and international meetings, including events hosted by major European and American professional organisations such as the ESC and NASPE. Last year alone saw an international workshop on Cardiac MEF & Arrhythmias at Oxford, as well as dedicated sessions at NASPE's 23rd annual meeting in San Diego, CardioStim 2002 in Nice, and the UK Physiological Society meeting in Leeds. This volume of Progress in Biophysics and Molecular Biology incorporates clinical and basic science results, and it is fitting that its publication coincides with a special session on cardiac MEF at the 2003 meeting of NASPE.

Mechano-electrical feedback underlying arrhythmias: the atrial fibrillation case

Mechanoelectrical feedback (MEF) has become firmly established as a mechanism in which mechanical forces experienced by myocardial tissue or cell membranes convey alterations in electrophysiologic characteristics of such tissue. Observations to date mainly concern mechanically induced changes in action potential duration, resting and active potential amplitude, enhanced pacemaker frequency, or afterdepolarizations. While some of these changes (i.e. after depolarizations) may give rise to premature beats, a role of MEF in explaining sustained ventricular tachyarrhythmias has so far been elusive. Here, we review recent findings showing that acute atrial dilatation facilitates atrial fibrillation (AF) and that two stretch-activated channel (SAC) blockers (gadolinium and GsMTx-4) are able to suppress stretch-facilitated AF. These findings strongly support a role of MEF and SACs in promoting sustained arrhythmias and point to a new class of antiarrhythmic drugs.

The role of atrial dilatation in the domestication of atrial fibrillation

Numerous clinical investigations as well as recent experimental studies have demonstrated that atrial fibrillation (AF) is a progressive arrhythmia. With time paroxysmal AF becomes persistent and the success rate of cardioversion of persistent AF declines. Electrical remodeling (shortening of atrial refractoriness) develops within the first days of AF and contributes to the increase in stability of the arrhythmia. However, 'domestication of AF' must also depend on other mechanisms since the persistence of AF continues to increase after electrical remodeling has been completed. During the first days of AF in the goat, electrical and contractile remodeling (loss of atrial contractility) followed exactly the same time course suggesting that they are due to the same underlying mechanism. Contractile remodeling not only enhances the risk of atrial thrombus formation, it also enhances atrial dilatation by increasing the compliance of the fibrillating atrium. In goats with chronic AV-block atrial dilatation increased the duration of artificially induced AF-episodes but did not change atrial refractoriness or the AF cycle length. When AF was maintained a couple of days in these animals, a shortening of the atrial refractory period did occur. However, the AF cycle length did not decrease. Long lasting episodes of AF with a long AF cycle length and a wide excitable gap suggest that in this model AF is mainly promoted by conduction disturbances. Chronic atrial stretch induces activation of numerous signaling pathways leading to cellular hypertrophy, fibroblast proliferation and tissue fibrosis. The resulting electroanatomical substrate in dilated atria is characterized by increased non-uniform anisotropy and macroscopic slowing of conduction, promoting reentrant circuits in the atria. Prevention of electroanatomical remodeling by blockade of pathways activated by chronic atrial stretch therefore provides a promising strategy for future treatment of AF.