Right and left atrial diameters during incremental atrial pacing in pigs

The relationship between right and left atrial diameters and heart rate was examined in 14 open-chest barbiturate-anesthetized pigs by an ultrasonic technique. The maximal atrial diameter was read at the top of the v wave, which decreased in both atria when heart rate was increased from 124 (118-130) to 159 (156-160) (median and 95% confidence interval) beats/min. Right and left atrial maximal diameter fell significantly from 25.7 (23.6-32.6) to 24.9 (22.9-29.5) mm and from 30.8 (25.1-37.2) to 29.8 (23.4-36.3) mm, respectively, when heart rate was increased from 124 to 180 (177-182) beats/min. At higher pacing frequencies, both right and left atrial maximal diameter progressively increased, and at 220 (216-221) beats/min the maximal diameter regained the values obtained at 124 beats/min. The minimal atrial diameter, which was read at the end of the a wave, remained unchanged at heart rates below 180 beats/min but rose significantly at higher rates. Our findings indicate progressively reduced atrial filling with increasing heart rate and hampered atrial emptying and atrial distension at the highest heart rates.