Cellular mechanisms of atrial contractile dysfunction caused by sustained atrial tachycardia.

Background: Transient atrial contractile dysfunction ("atrial stunning") follows conversion of atrial fibrillation (AF) to sinus rhythm and has significant clinical implications; however, the underlying mechanisms are poorly understood. We investigated the hypothesis that rapid atrial activation (as during AF) impairs cellular contractility and affects cellular Ca2+ handling.
Methods and Results: Edge detection and indo 1 fluorescence techniques were used to measure unloaded cell shortening and intracellular Ca2+ transients in atrial myocytes from control (Ctl) dogs and dogs subjected to atrial pacing at 400 bpm for 7 (P7) or 42 (P42) days. Atrial tachycardia reduced fractional cell shortening (0.1 Hz) from 7.3+/-0.4% (Ctl) to 4.3+/-0.3% and 2.0+/-0.3% in P7 and P42 dogs, respectively (P<0.01 for each). Resting [Ca2+]i was not altered in paced dogs, but the systolic Ca2+ transient was significantly reduced. Furthermore, cells from paced dogs showed slowed relaxation and use-dependent decreases of Ca2+ transients and cell shortening compared with cells from Ctl dogs. To determine whether changes in Ca2+ transients account fully for alterations in contractility, we varied [Ca2+]o to evaluate the relation between Ca2+ transients and cell shortening. Reductions in Ca2+ transients in Ctl cells reduced shortening to the level of paced cells; however, when Ca2+ transients in P42 cells were elevated to the range of Ctl cells, a significant reduction in cell shortening remained. Similar results were obtained in dogs that maintained 1:1 capture throughout the monitoring period and dogs that developed sustained AF over the course of the study.
Conclusions: Sustained atrial tachycardia causes important reductions in cellular contractility, in part by impairing cellular Ca2+ handling and decreasing systolic Ca2+ transients. These results provide direct evidence for the concept that AF induces atrial contractile dysfunction by causing a tachycardia-induced atrial cardiomyopathy.