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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



shelves, such as the Eustachian "valve" in the right 

 atrium at the point of entry of the inferior vena cava. 

 With experience it is usually possible to maneuver 

 the catheter tip past these obstacles and into the de- 

 sired vessel. Human subjects can assist in this opera- 

 tion by making voluntary movements of the arms, 

 shoulders, neck, and trunk or by deep breath holding, 

 thus shortening, lengthening, or straightening the 

 venous channels in accommodating the passage of 

 the catheters. Untrained dogs must be anesthetized 

 and external movement of the body employed as an 

 aid. Rappaport (77) has described a device for 

 "guided catheterization" which can be used to bend 

 the tip of the catheter after it is placed close to the 

 orifices of the hepatic veins. The angulation obtain- 

 able permits catheterization of an hepatic vein from 

 below, an approach which is impossible with the 

 Cournand catheter owing to the relatively fixed 

 obtuse angle of its tip. In at least 1000 catheterizations 

 of the hepatic veins in man in several laboratories 

 not a single fatality has occurred despite the fact 

 that many subjects were seriously ill. This good 

 record is undoubtedly attributable to the fact that 

 the right heart is not entered and thus a dangerous 

 source of arrhythmia is avoided. The use of a rela- 

 tively soft catheter is advisable even though it makes 

 manipulation more difficult. Excessive buckling or 

 coiling should be guarded against, since knots can 

 be tied in the catheter. Indeed, a knot in a catheter 

 which included the chorda tendinae of the tricuspid 

 valve has been observed in the dog. The procedure is 

 therefore not without hazard and should always be 

 used cautiously with the catheter under direct ob- 

 servation throughout. In man, a vein draining the 

 right lobe is easiest to enter; in the dog, a vein drain- 

 ing the left lobe. Since these lobes are the largest 

 hepatic lobes in man and dog, the catheter can be 

 inserted to a depth that permits reliable measure- 

 ments of intrahepatic venous pressure. 



Taylor & Myers (286) have shown that thrusting 

 the catheter deep into the liver and obstructing the 

 hepatic vein provides a means of measuring portal 

 venous pressure. Occlusion is assured by introducing 

 the catheter until it buckles slightly within the he- 

 patic vein. The '"occluded hepatic venous pressure" 

 theoretically reflects the pressure transmitted from 

 the portal vein or venules through a stationary column 

 of blood extending from the tip of the catheter. The 

 outflow tract obstructed is probably quite large, 

 presumably consisting of a "wedge" of convergent 

 hepatic venules, sublobular veins, and sinusoids into 

 which both hepatic arterioles and portal venules 



empty. The hepatic venous "end pressures" therefore 

 may mirror the mean pressure attained when flows 

 into and out of the obstructed area have reached 

 equilibrium and may more closely approximate 

 sinusoidal pressures than portal venous pressure. 

 The small gradient of pressure between the portal 

 vein and the sinusoids probably accounts for the good 

 agreement with the portal venous and intrasplenic 

 pressures reported by a number of workers (76, 135, 

 244)- 



Indirect Methods 



Hepatic venous catheterization has also proved of 

 major importance in the development of indirect 

 methods for the appraisal of the hepatic circulation. 

 Accurate measurement of changes in the blood as it 

 passes through the liver makes it possible to apply 

 the Fick principle in the estimation of flow, to study 

 hepatic clearances, and to follow the dilution of 

 isotopes within the splanchnic blood volume. Since 

 hepatic blood flow and hepatic arteriovenous dif- 

 ferences can be determined simultaneously, hepatic 

 removal of various substances from the blood can 

 be subjected to analysis. Under appropriate condi- 

 tions maximal hepatocellular activity can be em- 

 ployed as a means of approximating the mass of 

 tissue perfused by blood in order to permit more 

 precise definition of ischemia, hyperemia, and re- 

 distribution. On the basis of such analyses, more 

 sophisticated clearance techniques have been de- 

 veloped that may circumvent venous catheterization. 

 The blood volume in the splanchnic bed and the 

 distribution of flow and volume have been adduced 

 from studies of the time required for the movement of 

 tracers such as I 131 human serum albumin across the 

 splanchnic bed, in relation to flow. These approaches 

 have been opened up in the past 15 years and are 

 already yielding a rich harvest of new information 

 regarding hepatic hemodynamics. 



hepatic blood flow. The hepatic blood flow can be 

 estimated indirectly by three somewhat different 

 methods. In one the total quantity of some substance 

 removed from or added to the blood each minute 

 by the liver is determined and divided by the hepatic 

 arteriovenous concentration difference, i.e., the 

 amount removed from or added to each milliliter 

 of blood perfusing the liver. In a second procedure, 

 the percentile disappearance of some substance 

 more or less completely cleared from the blood per- 

 fusing the liver is measured and hepatic blood flow 



