Total vascular pressure-volume relationship in the conscious rat

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Abstract
Mean circulatory filling pressure (MCFP), a useful index of total body venous capacity, was measured in conscious unrestrained rats following inflation of an indwelling right atrial balloon that briefly arrested the circulation. During circulatory arrest, venous pressure increased and arterial pressure decreased to plateaus within 4-5 s; MCFP was calculated from these pressures and the arterial-to-venous compliance ratio. MCFP can be determined within 4-5 s after balloon inflation. Reflex venoconstriction began 10-13 s after balloon inflation in lightly anesthetized rats. The arterial-to-venous compliance ratio was 1:60. Correlation of the calculated MCFP with MCFP that was measured by transferring blood from the arterial to the venous side of the circulation was excellent and the deflated right atrial balloon-tipped catheter had no effect on cardiac output. Reproducibility of MCFP measurements was good. Norepinephrine and hexamethonium, which affect arteriolar and venular tone increased and decreased MCFP, respectively, while causing opposite or insignificant effects on blood volume (BV). Hydralazine, predominantly an arteriolar dilator with little effect on veins, had no effect on MCFP or BV. MCFP can be used to assess total body venous capacity in conscious rats.

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