Class I or class III agents for atrial fibrillation: are asking the right question?

Role of potassium currents in cardiac arrhythmias

Abnormal excitability of myocardial cells may give rise to ectopic beats and initiate re-entry around an anatomical or functional obstacle. As K(+) currents control the repolarization process of the cardiac action potential (AP), the K(+) channel function determines membrane potential and refractoriness of the myocardium. Both gain and loss of the K(+) channel function can lead to arrhythmia. The former because abbreviation of the active potential duration (APD) shortens refractoriness and wave length, and thereby facilitates re-entry and the latter because excessive prolongation of APD may lead to torsades de pointes (TdP) arrhythmia and sudden cardiac death. The pro-arrhythmic consequences of malfunctioning K(+) channels in ventricular and atrial tissue are discussed in the light of three pathophysiologically relevant aspects: genetic background, drug action, and disease-induced remodelling. In the ventricles, loss-of-function mutations in the genes encoding for K(+) channels and many drugs (mainly hERG channel blockers) are related to hereditary and acquired long-QT syndrome, respectively, that put individuals at high risk for developing TdP arrhythmias and life-threatening ventricular fibrillation. Similarly, down-regulation of K(+) channels in heart failure also increases the risk for sudden cardiac death. Mutations and polymorphisms in genes encoding for atrial K(+) channels can be associated with gain-of-function and shortened, or with loss-of-function and prolonged APs. The block of atrial K(+) channels becomes a particular therapeutic challenge when trying to ameliorate atrial fibrillation (AF). This arrhythmia has a strong tendency to cause electrical remodelling, which affects many K(+) channels. Atrial-selective drugs for the treatment of AF without affecting the ventricles could target structures such as I(Kur) or constitutively active I(K,ACh) channels.

Targeted modification of atrial electrophysiology by homogeneous transmural atrial gene transfer

BACKGROUND

Safe and effective myocardial gene transfer remains elusive. Heterogeneous ventricular gene delivery has been achieved in small mammals but generally with methods not readily transferable to the clinic. Atrium-specific gene transfer has not yet been reported. We hypothesized that homogeneous atrial gene transfer could be achieved by direct application of adenoviral vectors to the epicardial surface, use of poloxamer gel to increase virus contact time, and mild trypsinization to increase virus penetration.

METHODS AND RESULTS

We "painted" recombinant adenovirus encoding the reporter gene Escherichia coli beta-galactosidase directly onto porcine atria. Investigational variables included poloxamer use, trypsin concentration, and safety. Using the painting method, we modified the atrial phenotype with an adenovirus expressing HERG-G628S, a long-QT-syndrome mutant. Our results showed that application of virus with poloxamer alone resulted in diffuse epicardial gene transfer with negligible penetration into the myocardium. Dilute trypsin concentrations allowed complete transmural gene transfer. After trypsin exposure, echocardiographic left atrial diameter did not change. Left atrial function decreased on postoperative day 3 but returned to baseline by day 7. Tissue tensile strength was affected only in the 1% trypsin group. HERG-G628S gene transfer prolonged atrial action potential duration and refractory period without affecting ventricular electrophysiology.

CONCLUSIONS

We show complete transmural atrial gene transfer by this novel painting method. Adaptation of the method could allow application to other tissue targets. Use with functional proteins in the atria could cure or even prevent diseases such as atrial fibrillation or sinus node dysfunction.

Ablation for atrial fibrillation: are cures really achieved?

During the past 10 years numerous studies on the treatment of paroxysmal atrial fibrillation (AF) by right and left atrial ablation procedures have been published. The results of studies based on follow-up periods of a few months have been repeatedly interpreted as providing evidence for curative therapy. However, insufficient focus on the variability of the natural history of paroxysmal AF, the inadequate detection of silent arrhythmic events, the eclectic post-interventional use of antiarrhythmic drugs, and the lack of appropriate control groups make the reports unconvincing. Randomized controlled trials are needed to confirm postulated long-term cure rates for AF.