Effect of Acute Volume Loading on Heart Rate in the Conscious Dog

The effect on heart rate of rapid elevation of ventricular filling pressure by intravenous infusion of isotonic solution was studied in 31 conscious dogs with autonomic innervation intact and during vagal, beta-receptor, or combined vagal and beta-receptor blockade. When parasympathetic innervation was intact, heart rate always rose during infusion and there was a consistent relationship between mean left atrial pressure and heart rate described by f = fm − (fm − fl) e−kp where f is the heart rate at any left atrial pressure, p, fm the maximum heart rate, fl the initial heart rate, and k a rate constant. Sympathetic blockade reduced the magnitude of the response and parasympathetic blockade prevented the response. It is concluded that a steady rise in filling pressure during intravenous infusion in a normally innervated, conscious dog invariably results in a tachycardia, which is due to either reflex inhibition of the parasympathetic nervous system or direct mechanical stimulation of the sinoatrial node.