Right atrial volume during hemorrhage in the dog

Atrial B receptors elicit reflex changes after hemorrhage and respond to changes in atrial volume, but the latter have not been measured after hemorrhage. An impedance catheter that was originally designed to measure ventricular volume was advanced into the atrium from the femoral vein in each of 10 splenectomized dogs that were anesthetized with alpha-chloralose. The rate of hemorrhage and reinfusion was 2% of the estimated blood volume per min. Cardiac output (CO) was measured with dye dilution. The components of the atrial volume signal that were most sensitive to hemorrhage were the filling that occurred during late ventricular systole (Vs) and the absolute volume at the end of ventricular systole, (Vabs). Both variables were significantly less than base line (P less than 0.05) after 10% hemorrhage. The rate of change of Vs decreased significantly after 20% hemorrhage. However, the volume ejected with each atrial beat (Ve) decreased significantly (P less than 0.05) only after 30% hemorrhage. Atrial output (AO) as computed by multiplying Ve by heart rate (HR) did not change after hemorrhage. The ratio of AO to CO increased in direct proportion to HR (P less than 0.01) so that after 30% hemorrhage, for which HR was high, the decreases in CO from base line in individual dogs correlated significantly with the changes in AO (P less than 0.01). Since the B receptors are known to fire at the end of the ventricular systole at a rate that is dependent on blood volume, their firing may be a function of the concomitant atrial signals, Vs and Vabs, that are most sensitive to blood volume, and these atrial signals may serve as indexes of B fiber activity.(ABSTRACT TRUNCATED AT 250 WORDS)