Objectives: 

Chronic physical exercise produces modifications on sinus automatism, atrioventricular conduction and intrinsic myocardial refractoriness in isolated heart. However, there is little information about the modifications of the intrinsic ventricular conduction velocity (CV). Refractoriness together with CV, determines the wavelength of the myocardial activation process, which is well-known to be closely related to the occurrence of reentrant arrhythmias. We hypothesize that chronic exercise increases intrinsic myocardial CV.

Materials: 

Nine male White New Zealand rabbits were submitted to a training program and ten were just housed. After training or housing periods, rabbits were anaesthetized (ketamine 10 mg/kg i.v.), euthanized, the hearts excised, isolated and perfused in a Langendorff system. A pacing electrode and a plaque with 256 recording electrodes were positioned on the left ventricle. Conduction velocity was determined using the extrastimulus test, and calculated as the quotient of the distance between two electrodes (positioned five interelectrode spaces in a direction perpendicular to the isochrones), and the difference between their activation times. Determinations of CV were performed both in the ventricular response to an extrastimulus delivered just before the effective refractory period, and in the one produced by the last stimulus of the fixed train. An unpaired Student t test was used to compare CV between control and trained group. Differences were considered significant when p<0,05.

Results: 

CV was higher in the trained group (103±14 cm/s, n=9 vs 85±8 cm/s, n=10) when measured at the extrastimulus response. No differences were found between groups when CV was determined using the response produced by the last stimulus of the train.

Conclusions: 

Chronic physical training seems to increase intrinsic CV, but only at high heart frequencies, as it occurs during acute exercise performance, but not at resting state.

Supported by MEC (DEP2006-56208-C03-01 and SAF2007-62412).