Hepatic pressureflow relationships

Liver perfusion must be maintained in a wide variety of clinical conditions characterized by either low downstream right atrial pressures (e.g. during hypovolemic shock) or high downstream pressures (e.g. pulmonary hypertension). The existence of hepatic vascular waterfalls then represents a protective mechanism for the liver against high increases in central venous and hepatic venous pressures. By using an extracorporeal bypass from the right atrium to the pulmonary artery, flow can be controlled while right atrial pressure Pra is varied through a Starling resistor. Inflating the Starling resistor increases Pra and decreases flow, but Ppv begins to increase only after Pra reaches a certain level, demonstrating the existence of a vascular waterfall phenomenon downstream from the portal vein.

By using an in situ isolated liver bypass preparation with independent control of Qpv and Qha, during which hepatic venous pressure Phv could be maintained constant using a Starling resistor, vascular waterfalls have been demonstrated in both the portal venous and hepatic arterial circulations ( B§J.o.yc!Le.t,.§l; 1.995). In both circuits, the analysis of pressure-flow relationships was performed at incremental levels of Phv. In the portal venous circulation, the critical closing pressure was about 4 mmHg, with a classical waterfall physiology observed as Phv was raised. In the hepatic arterial circuit, the critical closing pressure (about 8 mmHg) showed a constant positive pressure difference of 5 mmHg over Phv as the latter was increased. Analysis of the slopes of the pressure-flow relationships showed that portal venous resistance decreased when Phv was greater than the portal venous critical closing pressure, consistent with recruitment/distention phenomena in this circuit. In contrast, hepatic arterial resistance was unchanged as Phv was increased (Bli.®DZ§...,§£,§( 1995).

During mechanical ventilation, particularly when positive end-expiratory pressure is applied, liver blood flow can be reduced by a decrease in venous return/cardiac output, an increase in transhepatic vascular resistance due to diaphragmatic descent, a transmission to the sinusoids of the increase in Pra/Phv, or an increase in abdominal pressure acting on intrahepatic closing pressures. Analysis of the pressure-flow relationships in the portal vein during mechanical ventilation demonstrated the existence of a passive distensibility with classic critical closing pressure, whereas the hepatic arterial critical closing pressure always remained higher than the outflow pressure (Brienzaefa/ 1995).

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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