1. Chronic sodium depletion in dogs is associated with a blunting of the pressor response to carotid occlusion. After section of the vagus nerves this pressor response reverts to normal although atropine is without effect, suggesting a possible role for increased activity of cardiopulmonary vagal afferents in suppressing sympathetic outflow. Since increases in central blood volume stimulate vagal afferents, cardiopulmonary haemodynamics were studied by radioisotope dilution before and after 3 and 4 weeks of dietary sodium restriction together with frusemide.
2. Sodium depletion was associated with significant decreases in cardiac output, ejection fraction and total blood volume; the cardiopulmonary blood volume increased but the change was not statistically significant (P<0.2). These changes were accompanied by a significant increase in the ratio cardiopulmonary blood volume/total blood volume and a decrease in the ratio cardiac output/cardiopulmonary blood volume.
3. Results indicate that sodium depletion may be accompanied by a paradoxical translocation of blood to the cardiopulmonary region in part due to depressed ventricular performance.