1. In 50 subjects with atrial fibrillation we have attempted to demonstrate variation of ventricular rate with respiration, as evidence of cardioregulatory reflex activity. 2. The electrocardiogram was recorded for 3 min during spontaneous respiration. The presence of respiratory variation of R-R intervals was analysed by multiple regression against a cosine function (cosinor analysis), making it possible to determine the phase of respiration when the intervals were longest. 3. Variation in ventricular rate with respect to respiration was demonstrated ( P < 0.05) in seven (14%) cases. On average, R-R intervals were longest at the end of inspiration; this contrasts with sinus rhythm where P-P, P-R and R-R intervals are longest around the time of end-expiration. 4. These results suggest that in atrial fibrillation the beat-to-beat ventricular rate may be under the influence of cardioregulatory reflexes, but the effect of respiration is weak and paradoxical.

1. The electrocardiogram was recorded for 3 min during spontaneous respiration in 70 subjects aged 15–86 years who were in sinus rhythm. Using a signal-averaging approach, the presence of respiratory variation of P-P intervals was analysed by multiple regression against a cosine function (cosinor analysis). 2. By cosinor analysis the phase of respiration when the intervals were longest was determined, together with the amplitude of the variation of the intervals around their mean value. 3. Respiratory variation of P-P intervals (respiratory sinus arrhythmia) was demonstrated in 84% of subjects; its amplitude decreased with age and respiratory rate. On average, the duration of P-P intervals varied by 2.8% around the mean, and the maximum duration occurred around the time of end-expiration. 4. By cosinor analysis, and allowing for variation of heart rate, P-R intervals showed an independent respiratory variation in 39% of cases, and its average amplitude was 1.2% around the mean. 5. The respiratory variation of P-R and P-P intervals showed similar phase relationships to respiration, suggesting that during respiration there is parallel alteration of sinoatrial and atrioventricular node function.