An examination of the measurement of mean circulatory filling pressure in the anaesthetized rat

Abstract

The measurement of mean circulatory filling pressure (MCFP) is widely used in research, yet the basic assumption of equal pressures throughout the body during MCFP measurement has not been adequately tested. Therefore, this study examined whether there is pressure equilibrium between tributary veins and the central vena cava during the MCFP manoeuvre. The time course needed for pressure equilibrium was also examined. Pressures in the iliac artery, the hepatic portal vein (HPVP), the renal vein (RVP) and the inferior vena cava (IVCP) were determined at 4-second intervals over a 20-second period of circulatory arrest, produced by inflating a right atrial balloon, under the conditions of normal blood volume (NBV), 10% blood volume depletion (-10% BV) and 10% blood volume expansion (+10% BV) in urethane-anaesthetized rats. HPVP was found to be higher than IVCP during the full period of arrest at -10% BV and during the first 16 seconds of arrest at +10% BV. At NBV, HPVP was significantly higher than IVCP only at the 4th second of the arrest. RVP was virtually equal to IVCP in the three volume states. At the 8th second of the arrest in -10% BV , HPVP was 6.2 ± 0.8 mmHg and IVCP was 3.4 ± 0.2 mmHg; in +10% BV, HPVP was 7.7 ± 0.5 mmHg and IVCP was 6.2 mmHg ± 0.4 mmHg, differences between HPVP and IVCP being significant (p < 0.01 for each of the comparisons). To test if this pressure disequilibrium resulted from reflex vasoconstriction, the MCFP manoeuvre was carried out following ganglionic blockade with hexamethonium and atropine. HPVP was found still to be significantly higher than IVCP for 12 seconds in both -10% and +10% BV conditions. It is concluded that during conditions of rapid blood volume alterations, HPVP is significantly higher than IVCP during the MCFP manoeuvre; physical transhepatic resistance at zero inflow appears to play a role in the constitution of this pressure disequilibrium. -- Keywords: mean circulatory filling pressure (MCFP) -- hepatic portal venous pressure -- blood volume alteration -- ganglionic blockade -- anaesthetized rats.