Augmentation of respiratory sinus arrhythmia in response to progressive hypercapnia in conscious dogs

Respiratory sinus arrhythmia (RSA) may serve to enhance pulmonary gas excha nge efficiency by matching pulmonary blood flow with lung volume within eac h respiratory cycle. We examined the hypothesis that RSA is augmented as an active physiological response to hypercapnia. We measured electrocardiogra ms and arterial blood pressure during progressive hypercapnia in conscious dogs that were prepared with a permanent tracheostomy and an implanted bloo d pressure telemetry unit. The intensity of RSA was assessed continuously a s the amplitude of respiratory fluctuation of heart rate using complex demo dulation. In a total of 39 runs of hypercapnia in 3 dogs, RSA increased by 38 and 43% of the control level when minute ventilation reached 10 and 15 l /min, respectively (P < 0.0001 for both), and heart rate and mean arterial pressure showed no significant change. The increases in RSA were significan t even after adjustment for the effects of increased tidal volume, respirat ory rate, and respiratory fluctuation of arterial blood pressure (P < 0.001 ). These observations indicate that increased RSA during hypercapnia is not the consequence of altered autonomic balance or respiratory patterns and s upport the hypothesis that RSA is augmented as an active physiological resp onse to hypercapnia.