THE HEPATIC CIRCULATION 



'395 



lar filling by contrast substance or injection mass is 

 most uncertain, and the assumptions required in esti- 

 mating volume from X-ray shadows are of dubious 

 validity. Nevertheless, further exploration in this 

 direction may prove fruitful. 



Changes in the volume of spleen and liver may be 

 more accurately measured plethysmographically in 

 animals, but the fixation of the organ and the sur- 

 gical handling required seriously impair the validity 

 of the values obtained (132). These devices permit a 

 rough estimation of engorgement or disgorgement of 

 the liver during vascular adjustments, but they 

 provide no information on the absolute volume of 

 blood in the liver. The same difficulties are encoun- 

 tered in studies of the volume or weight of the iso- 

 lated liver (10, 25, 61, 203) or spleen (163, 287). 

 Measurement of the volume of blood retained in or 

 expelled from the liver or splanchnic bed as a whole 

 may also be made on the basis of the difference in 

 blood inflow and outflow during a period of shifting 

 volume. 



It has proved extremely difficult, also, to deter- 

 mine the absolute volume of blood in the liver or 

 spleen and their tributaries by the direct approach. 

 With excision, blood runs off into the systemic veins 

 and is lost. Surgery in living animals, with care to 

 block inflow and outflow tracts simultaneously and 

 to avoid trauma that might induce physiologic re- 

 distribution of blood, is required to obtain reliable 

 values. The quantity of blood may then be evaluated 

 by extraction of hemoglobin and calculation of blood 

 volume from the hematocrit of arterial blood. A 

 serious difficulty arises at this point because the hema- 

 tocrit in the capillaries and sinusoids may differ 

 greatly from that in large vessels. Radioisotope label- 

 ing of plasma (I 131 -labeled human serum albumin, 

 Cr 51 tagging of plasma proteins, T-1824 bound to 

 plasma proteins) and of red cells (P 3 ' 2 , Cr 51 ) has proved 

 helpful in surmounting this obstacle. Recovery of 

 the isotope is relatively easy and blood volume can 

 be calculated on the basis of the radioactivity per 

 unit volume of arterial plasma and red cells. Allow- 

 ance must be made for the possible uptake of radio- 

 isotope by the liver cells or lymph. Though these 

 jmethods (106, 175, 181) provide approximate values 

 for hepatic blood volume in steady states, changes in 

 the same animals cannot be obtained. 



HEPATIC AND SPLANCHNIC BLOOD PRESSURES. Blood pres- 

 sure has been measured directly in the intra-abdominal 

 veins after laparotomy in animals and man (85). 

 Opening the abdomen may bring about changes in 



pressure gradients independently of the effects of 

 anesthesia and surgery, but on the whole these 

 measurements are acceptable and revealing, par- 

 ticularly when analyzed in terms of simultaneous 

 measurements of arterial pressure and blood flow. 

 Pressure measurements may be made in unanes- 

 thetized subjects by percutaneous splenic or hepatic 

 puncture (15, 17, 32). Atkinson & Sherlock (17) 

 found a statistically linear correlation between intra- 

 splenic pressure and portal venous pressure over a 

 wide range in 24 patients. With transhepatic punc- 

 ture of the portal vein (32) reliable records may be 

 obtained that possess an advantage over intrasplenic 

 pressure tracings in showing phasic or respiratory 

 fluctuations that are damped out in the splenic 

 pulp spaces. Portal venous pressure has also been 

 estimated on the basis of pressure in large readily 

 accessible collateral veins in the abdominal wall of 

 patients with portal venous obstruction. Though 

 this approach may yield valid figures for the subject 

 under study, it is not feasible in the normal and 

 does not yield values of general application. Care 

 must be taken to refer all values to the same reference 

 plane, preferably at the level of the right atrium 

 determined radiologically. Many workers use a level 

 5 cm posterior to the angle of Louis as the reference 

 plane in human subjects; and in general this is quite 

 satisfactory, though it appears to be a less dependable 

 guide to the level of the right atrium than the plane 

 10 cm anterior to the back (246). Changes in pressure 

 are usually of particular interest and the importance 

 of the zero reference plane is not often stressed, since 

 the accuracy of pressure differences is not affected 

 by it. When absolute values obtained by different 

 groups of workers are compared, however, apparent 

 discrepancies are encountered that may be due to 

 inexact definition of the reference point. 



The development of venous catheterization tech- 

 niques by Cournand, Richards, and their associates 

 (98, 246) has opened up a new approach to the study 

 of intravascular pressures. The insertion of a long 

 radiopaque ureteral catheter deep into the venous 

 system under fluoroscopic control is atraumatic, rela- 

 tively simple, and safe. The catheter is introduced 

 under local anesthesia into a vein in the antecubital 

 fossa (preferably lying at the medial aspect) in human 

 subjects and into a jugular vein in dogs. It is then 

 threaded into the right atrium and inferior vena cava. 

 A curved tip makes manipulation and control of 

 direction possible but at the same time interferes 

 with passage, since it may cause the catheter to move 

 in unintended directions or to catch at valvelike 



