Thoracic Vein Arrhythmias

Critical Requirements for the Initiation of a Cardiac Arrhythmia in Rat Ventricle: How Many Myocytes?

Cardiovascular disease is the leading cause of death worldwide due in a large part to arrhythmia. In order to understand how calcium dynamics play a role in arrhythmogenesis, normal and dysfunctional Ca²⁺ signaling in a subcellular, cellular, and tissued level is examined using cardiac ventricular myocytes at a high temporal and spatial resolution using multiscale computational modeling. Ca²⁺ sparks underlie normal excitation-contraction coupling. However, under pathological conditions, Ca²⁺ sparks can combine to form Ca²⁺ waves. These propagating elevations of (Ca²⁺)_i can activate an inward Na⁺-Ca²⁺ exchanger current (I_NCX) that contributes to early after-depolarization (EADs) and delayed after-depolarizations (DADs). However, how cellular currents lead to full depolarization of the myocardium and how they initiate extra systoles is still not fully understood. This study explores how many myocytes must be entrained to initiate arrhythmogenic depolarizations in biophysically detailed computational models. The model presented here suggests that only a small number of myocytes must activate in order to trigger an arrhythmogenic propagating action potential. These conditions were examined in 1-D, 2-D, and 3-D considering heart geometry. The depolarization of only a few hundred ventricular myocytes is required to trigger an ectopic depolarization. The number decreases under disease conditions such as heart failure. Furthermore, in geometrically restricted parts of the heart such as the thin muscle strands found in the trabeculae and papillary muscle, the number of cells needed to trigger a propagating depolarization falls even further to less than ten myocytes.

Vagal effects on the occurrence of focal atrial fibrillation originating from the pulmonary veins

BACKGROUND

There is evidence that the autonomic nervous system may be involved in the mechanism of focal atrial fibrillation (AF), so the present study investigated the effects of the parasympathetic nervous system on the occurrence of focal AF originating from the pulmonary veins (PVs).

METHODS AND RESULTS

In 10 mongrel dogs, programmed stimulation and local burst stimulation (12.5 Hz, impulse duration 0.5 ms) were performed at each of the PVs. Pacing thresholds at different sites were determined and shown as a terraced distribution. The closer to the ostium of the PV, the lower was the pacing threshold (P<0.05-0.001). The local effective refractory period (ERP), AF induction and AF threshold were measured at baseline and during bilateral vagal nerve stimulation (VNS). VNS led to local ERP shortening at each of the PV sites (P<0.05-0.001), increased the inducibility of AF at all sites in the 4 PVs (P<0.05-0.001), and decreased the AF threshold at most sites, especially in the distal portions of the 4 PVs (P<0.05-0.01).

CONCLUSIONS

VNS changes the electrophysiological characteristics of the PVs and facilitates the induction of AF. Interaction between the autonomic nervous system and local cardiac autonomic nerve system may be a potential mechanism.