Mechanical effects of right atrial pressure on heart rate in the conscious rabbit

We attempted to detect and quantify the mechanical effects of change in right atrial pressure on the chronotropic properties of the cardiac pacemaker, when blood volume was altered in an isohaemic fashion by +/- 27% in 8 conscious rabbits. Under control conditions there was a strong negative association between heart rate and right atrial pressure, attributable to the baroreceptor-heart rate reflex. After deletion of the main afferent sources of reflex effects on heart rate, by denervating the arterial baroreceptors and chemoreceptors, there was an insignificant association between heart rate and right atrial pressure or arterial pressure. Then, in addition, the main neurohumoral effects on the cardiac pacemaker were eliminated by three different pharmacologic treatments: (1) the cardiac nerves were blocked by instilling 2% procaine into the pericardial sac, (2) cardiac beta-adrenoceptors and cholinergic receptors were blocked with intravenous propranolol and hyoscine methyl bromide, (3) the above treatments were combined with autonomic ganglion-blockade by intravenous pentolinium. The combination of cardiac afferent and efferent blockade (1 and 3) revealed a small but consistent positive association between heart rate and right atrial pressure of 1.3 and 0.7 beats/min/mmHg, respectively. With efferent blockade alone (2), there was a steeper positive association of 2.3 beats/min/mmHg. We conclude that in the conscious rabbit there may be a mechanical effect of right atrial pressure on the cardiac pacemaker, but that it is so small as to be of little importance under physiological conditions.