Short-termrapid right atrial pacing induces left atrial mechanical and anatomical remodeling in a canine model

Objective The underlying mechanisms responsible for both electrical and mechanical remodeling of the atrium after cardioversion of atrial tachyarrhythmias may be similar, but they are still incompletely understood, and whether the changes in atrial myocardium structure after short-term rapid atrial activation is the basis for this remodeling is unknown. We aimed to investigate atrial mechanical function and atrial ultrastructural change before and after short-term rapid atrial activation, and the possible relation between lest atrial contractile and anatomical remodeling. Methods Seventeen anesthetized mongrel dogs were divided into experimental (n=12) and control (n=5) groups. The experimental group underwent insertion of a transvenous lead at the right atrial appendage and 5-hour of atrial pacing at 450bpm. Effective refractory period (ERP) and P-wave duration were measured before and after 5-hour pacing. Acoustic quantification (AQ) of left atrial waveforms was recorded before and after 5-hour pacing. All measurements were made in sinus rhythm. Dogs were killed and the myocardium in left atrial trabeculae and appendages was examined by light and electron microscopy. Control dogs did not undergo pacing, but the examinations were performed at the times corresponding to that for the experimental group. Results Despite the absence of changes in heart rate and left ventricular pressure after 5-hour rapid atrial pacing in the experimental group, left atrial reservoir and conduit function did not change, whereas atrial size increased and atrial booster function decreased. Marked changes were seen in cellular substructures, such as loss of myofibrils, accumulation of glycogen, and changes in mitochondrial shape and size. No changes were found in the control group. Conclusions Short-term rapid atrial activation can cause electrophysiological remodeling and left atrial contractile and anatomical remodeling, and anatomical remodeling may contribute to the development of atrial electrical and contractile remodeling.