Atrial fibrillation may impair coronary blood flow by tachycardia and reflex vasoconstriction. It has not been documented, however, whether in the presence of coronary stenosis atrial fibrillation exceeds the effects of rhythmic atrial tachycardia. 2. The effects of experimentally induced atrial fibrillation compared with atrial tachycardia, therefore, were tested in 22 anaesthetized dogs. Stenosis of the left anterior descending coronary artery was induced to reduce coronary blood flow by about 40%. 3. In the presence of coronary stenosis, atrial fibrillation (ventricular rate: 234 +/- 21 beats/min) reduced coronary blood flow from 58 +/- 7 to 44 +/- 8 ml min-1 100 g-1 (P less than 0.001, mean +/- SEM) and subendocardial segment shortening (ultrasonic crystals) from 12 +/- 2 to 4 +/- 2% (P less than 0.0025), and resulted in a lactate production of 30 +/- 11% (P less than 0.005 vs sinus rhythm). 4. Atrial tachycardia (heart rate: 216 +/- 21 beats/min, NS vs atrial fibrillation) did not significantly change coronary blood flow and reduced segment shortening to 7 +/- 3% (P less than 0.05 vs atrial fibrillation). Significant lactate production did not occur. 5. Since mean arterial pressure fell from 100 +/- 4 mmHg at sinus rhythm to 89 +/- 3 mmHg (P less than 0.01) during atrial fibrillation but not during atrial tachycardia, it was held constant in 13 dogs by a pressurized blood reservoir. Coronary blood flow, however, fell from 43 +/- 6 to 36 +/- 5 ml min-1 100 g-1 (P less than 0.0025). 6. Thus atrial fibrillation may reduce coronary blood flow and induce myocardial ischaemia in the presence of coronary stenosis in excess of atrial tachycardia.
