Intrinsic cardiac rate regulation in the anaesthetized rabbit

Intrinsic cardiac regulation, the direct effect of changes in right atrial pressure on heart rate, was examined in rabbits under chloralose/urethane anaesthesia. Autonomic influences on the cardiac pacemaker were eliminated by cervical vagotomy and intravenous propranolol. Right atrial transmural pressure was monitored as the difference between right atrial and pleural pressures. Blood volume expansion (5–15%) increased right atrial transmural pressure and heart rate and produced a sinus arrhythmia associated with respiration-linked fluctuations in right atrial transmural pressure. The gain of intrinsic cardiac rate regulation was calculated as 0.96 ± 0.24 beats min^-1 mmHg^-1 at a heart rate of 218 ± 6 beats min^-1 (values as the mean± SEM, n= 12). When heart rate was reduced by electrical stimulation of the peripheral end of the right vagus nerve, gain increased to 2.25± 0.57* and 4.61 ± 1.6* beats min^-1 mmHg^-1 at heart rates of 180 ± 4 and 130 ± 4 beats min^-1, respectively (n= 6 and n= 10; *P < 0.05 compared with pre-stimulation values). During vagally-induced bradycardia, rapid infusion of blood into the left superior vena cava produced a brief marked cardiac acceleration. We conclude that right atrial pressure has a small direct influence on heart rate, and this is enhanced by background cardiac parasympathetic stimulation.