Signal transduction systems and atrial fibrillation

Angiotensin-converting enzyme inhibitors are not effective at inhibiting further fibrous changes in the atria in patients with chronic atrial fibrillation: speculation from analysis of the time course of fibrillary wave amplitudes

To examine whether angiotensin-converting enzyme (ACE) inhibitors are effective at inhibiting further fibrous changes in atria in patients with chronic atrial fibrillation, we retrospectively evaluated the time course of fibrillatory wave amplitudes and the effects of an ACE inhibitor on the changes. We reviewed medical records and electrocardiographic files. The patients were eligible for inclusion if they had chronic atrial fibrillation for more than 10 years. The fibrillatory wave with the greatest amplitude was measured in lead V1 of standard electrocardiograms. A total of 31 patients were enrolled and were divided into two groups according to treatment. Twelve patients were classified as the ACE inhibitor group and 19 as the non-ACE inhibitor group. There was no significant difference in the mean value of the fibrillatory wave amplitude at baseline between the 2 groups (ACE inhibitor group, 0.23 +/- 0.02 mV; non-ACE inhibitor group, 0.18 +/- 0.02 mV). The fibrillatory wave amplitude decreased significantly after 10 years in both groups (ACE inhibitor group, 0.10 +/- 0.02 mV; non-ACE inhibitor group, 0.11 +/- 0.01 mV) and the changes in the fibrillatory wave amplitude were similar between the 2 groups. These results suggest that ACE inhibitors are not effective at inhibiting further fibrous changes in atria in patients with chronic atrial fibrillation.

Proteomics in myocardial diseases

Recently, proteome analysis has been introduced to analyze differential protein expression and cellular protein composition in cardiovascular medicine. Proteins expressed by diseased hearts (myocardial proteomics) were first investigated over a decade ago using two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). However, while 2D-PAGE is very successful for the abundant and moderately expressed proteins, it struggles to identify proteins expressed at low levels. However, the sensitivity of mass spectrometry has increased considerably during recent years, and technical progress widens the detection limits of mass-spectrometric analysis. Proteomics now allows us to examine global alterations in protein expression in the diseased hearts, and will provide new insights into the cellular mechanisms involved in cardiac dysfunction. This review will summarize the present knowledge about the use of proteome analysis in myocardial diseases.

Atrial electrophysiology and mechanisms of atrial fibrillation

Atrial fibrillation is the most common cardiac arrhythmia in clinical practice, and its management remains challenging. A solid understanding of the scientific basis for atrial fibrillation therapy requires insight into the mechanisms underlying the arrhythmia, about which an enormous amount has been learned over the past 10 years. The basic information presently available about atrial fibrillation mechanisms is reviewed. The particular properties of normal atrial electrophysiology are discussed, including salient ionic determinants of the atrial action potential and key anatomic features. Reviewed are three crucial arrhythmia mechanisms long held to be involved in atrial fibrillation: 1) rapid ectopic activity, 2) single-circuit reentry with fibrillatory conduction, and 3) multiple-circuit reentry. The determinants of each and the evidence for their involvement in clinical and/or experimental atrial fibrillation are noted. The physiological consequences, various contributing mechanisms, and clinical implications of the role of atrial-tachycardia remodeling are analyzed. Atrial-tachycardia remodeling links the potential mechanisms of atrial fibrillation, since atrial fibrillation beginning by any mechanism is likely to cause tachycardia-remodeling and thus promote the maintenance of atrial fibrillation by multiple-circuit reentry. Atrial structural remodeling is discussed as a paradigm of atrial fibrillation in which the classic features required for reentry (reduced refractory period and reentrant wavelength) may be lacking. Finally, the importance of recent insights into potential genetic determinants of atrial fibrillation is reviewed. The classic understanding of atrial fibrillation pathophysiology saw the different possible mechanisms as being alternative and opposing hypotheses. We now consider the multiple potential mechanisms as contributing to the pathophysiology of the arrhythmia to a different extent in different clinical settings and interacting with each other in a dynamic way at various stages of the natural history in many patients. It is hoped that this improved mechanistic understanding will lead to the development of improved therapeutic options.

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.