SOME OBSERVATIONS ON THE MECHANISM OF PRESSURE RELATED ATRIAL-FIBRILLATION

In order to investigate the effect of atrial pressure on the propensit y of the atria to fibrillate and the mechanism of this association, th e right atrial pressure was changed acutely by transfusion-bleeding in 12 anaesthetized open-chest dogs. Under various atrial pressures the conduction time was measured between two pairs of hook electrodes posi tioned on the two atrial appendages respectively. The effective refrac tory period was measured by continuous pacing of the right atrium at a 250 ms cycle length at double threshold intensity and interpolating a progressively earlier stimulus after each eighth paced beat. The prop ensity of fibrillation was studied by rapid (450 min(-1)) pacing of th e atria at double threshold intensity for 10 s at different atrial pre ssures. At a high (greater than or equal to 14 mmHg) atrial pressure t he conduction time (45.7 +/- 14.2 ms) was significantly (P<0.01) longe r, the effective refractory period (157.9 +/- 15.2 ms) significantly ( P<0.01) longer and the atrial fibrillation (11/19 or 57.9%) significan tly (chi(2) = 9.95, P<0.001) more common than at a low (less than or e qual to 10 mmHg) pressure (35.2 +/- 11.6, 146.2 +/- 12.4, 3/24 or 12.5 %, respectively). Analysis of variance showed that the probability of atrial fibrillation was significantly affected by the atrial pressure but not by either the conduction time or the effective refractory peri od. The findings suggest that an increase in right atrial pressure by acute volume overload prolongs the inter-atrial conduction time and ri ght atrial refractoriness and increases the propensity of the atria to fibrillate by rapid atrial stimulation. The effect of atrial pressure on fibrillation does not seem to be mediated by the prolonged atrial refractoriness or conduction time.