PROBLEMS ASSOCIATED WITH THE MEASUREMENT OF MEAN CIRCULATORY FILLING PRESSURE BY THE ATRIAL BALLOON TECHNIQUE IN ANESTHETIZED RATS

To examine the existence of pressure equilibrium between tributary vei ns and the central vena cava during the mean circulatory filling press ure manoeuvre, pressures in the hepatic portal vein, renal vein, and i nferior vena cava were determined at 4-s intervals over a 20-s period of circulatory arrest induced by inflating a right atrial balloon in n ormal blood volume, 10% volume depletion, and 10% volume expansion sta tes in urethane-anaesthetized rats. Portal vein pressure determined 8 s after arrest during volume depletion and expansion was significantly higher than vena caval pressure (6.2 +/- 0.8 vs. 3.4 +/- 0.2 and 7.7 +/- 0.5 vs. 6.2 +/- 0.4 mmHg (1 mmHg = 133.32 Pa), respectively; p < 0 .01); this pressure disequilibrium continued for 16 s during volume ex pansion and for the entire 20 s during volume depletion. Renal vein pr essure was equal to vena caval pressure during this manoeuvre. Portal vein pressure at normal blood volume was significantly different from vena caval pressure following circulatory arrest (4.6 +/- 0.3 vs. 3.8 +/- 0.4 mmHg, respectively). Following ganglionic blockade, portal vei n pressure was still significantly higher than vena caval pressure for 12 s during volume alterations. At the 8th s of the arrest the portal pressure determined in volume depletion was 3.6 +/- 0.3 mmHg and the inferior vena caval pressure was 2.6 +/- 0.4 mmHg (p < 0.05). Under th e volume expansion condition, the respective values were 6.5 +/- 0.3 a nd 5.3 +/- 0.4 mmHg (p < 0.05). We conclude that, under conditions of blood volume alterations, there is no pressure equilibrium between the portal vein and the inferior vena cava when mean circulatory filling pressure is measured by this technique; a transhepatic barrier indepen dent of reflex control during the measurement of mean circulatory fill ing pressure appears to play a role in obstructing the establishment o f pressure equilibrium within the venous system.