COMPLICATION: PORTAL HYPERTENSION

Portal hypertension is the build-up of pressure in the portal vein (the vein connecting the intestines and the liver). Normally, the pressure is low compared with the arterial pressure, but slightly above the pressure in the other veins in our body system. The most common cause of portal hypertension and its clinical consequences is liver disease.

Portal hypertension is a manifestation of cirrhosis, however numerous other causes of portal hypertension can produce many of these same clinical manifestations, such as ascites (fluid within the abdominal cavity) and variceal bleeding (bleeding from veins in the neck, stomach, and elsewhere). Usually, portal hypertension is secondary to liver disease.

It is important to place portal hypertension within the proper context for the sake of this report. It is primarily related to progressive hepatic fibrosis. However, the cause of chronic liver disease is also important because it may produce specific symptoms early in the course of disease, such as itching with mechanical biliary obstruction or severe fatigue with autoimmune chronic active hepatitis (CAH). Severity of chronic liver disease is related to deposits of collagen, leading to fibrosis (scar tissue) in the liver. Therefore, we can define serious chronic liver disease as any hepatic process stimulating the progressive deposition of fibrous tissue. The deposition of collagen (fibrous tissue) causes intrahepatic shunting, disruption in the hepatic architecture, resulting in portal hypertension, and, eventually, formation of a fibrous encasement around the liver, which limits regeneration. To summarize at this point, portal hypertension is a complication of cirrhosis and fibrosis.

The liver has a dual blood supply from the hepatic artery and portal vein. Blood flow to the intestines, spleen and pancreas eventually drains into the portal vein and, from there, to the liver. All substances that enter the body orally are filtered by the liver after absorption by the intestines. Less than 3% of the liver by weight is normally made up of collagenous (fibrous) material. As fibrosis increases, a point is reached at which blood flow in hindered by distortion of the usual flow of blood and the portal pressure must increase to preserve flow. The hepatic flow of blood comes from both the hepatic artery (20%) and the portal vein (80%). As hepatic resistance increases from progressive hepatic fibrosis, portal hepatic flow must decrease and/or portal pressure must increase. Inevitably, both occur.

Intrahepatic portal hypertension contributes to fluid accumulation in the abdominal cavity (ascites) by forcing more fluid across the liver capsule into the abdominal cavity than the body is capable of draining. Acute portal hypertension may cause mild to moderate enlargement of the spleen, but marked enlargement requires sustained portal hypertension over a long period of time. Sustained elevations in pressure occur when cirrhosis is present. The blood volume is expanded in patients with chronic liver disease. Visceral congestion causes a larger percent of blood to be pooled in the portal system. It is analogous to water build-up behind a dammed-up stream. This occurs slowly as liver disease develops, allowing the body to compensate by formation of more blood. Thus, the total amount of blood in the body (intravascular blood volume) increases by 10% to 30%. When a stream is dammed up, water accumulates until the dam overflows or streams develop around the dam. The rate at which this occurs depends on the speed with which the water builds up. The portal pressure builds up in response to fibrosis. As the portal pressure increases, more blood flows into the liver, but some tries to find another route to the lower-pressure vena cava system. Portal hypertension allows some forward portal flow into the liver, but at the expense of a pressure gradient that produces collaterals (side branches/secondary blood vessels).

Collaterals form in areas where capillaries and venous systems are contiguous, and often, had been joined during the embryonic stage. They also form by reopening fetal connections between vascular systems that closed at the time of birth. Connections from the portal system to the inferior vena cava (IVC) system induced by portal hypertension include the hemorrhoidal veins, splenorenal collaterals, and some abdominal wall and retroperitoneal collaterals. Connections to the superior vena cava (SVC) system include collaterals via esophageal and gastric varices, the reopened umbilical vein through abdominal wall collaterals to the internal mammary system, and retroperitoneal collaterals.

The primary clinical importance of collaterals is their tendency to bleed and the diversion of blood away from the liver, thus avoiding the usual first-pass filtering of toxins and bacteria entering the portal system from the intestines. Esophageal varices, gastric varices, and hemorrhoids account for more than 85% of bleeding in patients with cirrhosis. The majority of life- threatening episodes of bleeding are from esophageal varices.

 

Metabolic Effects of Portal Hypertension

The liver is a major filter for the removal of substances in the blood. Substances that enter the body from the intestines must pass through the liver prior to entering the systemic circulation. This includes orally ingested foods and drugs, and products secreted into the intestine by the liver that are reabsorbed by the intestines, such as bile acids. The impact of liver disease on the blood levels of these substances can be profound. A decreased first-pass filtering & removal of hormones, toxins, and bacteria increases the amount of these substances reaching the systemic circulation.

Liver disease also decreases hepatic clearance of substances from the systemic circulation. Another effect of liver disease is to increase or decrease the portal input of portal substances such as ammonia with gastro-intestinal (GI) bleeding or bile acids by decreasing hepatic synthesis. Substances that are highly extracted by the liver may enter systemic circulation without the benefit of the first-pass filtering due to progressive liver disease. If the substance is a drug or toxin that bypasses the liver, serious side effects may ensue.

 

Diagnosis

The presence of significant portal hypertension is implied by ascites, collaterals, or encephalopathy that occurs in the presence of known chronic liver disease or peripheral clinical evidence of chronic liver disease (firm liver, enlarged spleen, spider angiomas, clubbing of fingers, palmar erythema).

An ultrasound test can assess the presence of collaterals (gastric varices or umbilical vein dilatation), the width of the portal vein, and portal blood flow with deep Doppler. Improvement in certain ultrasound techniques, including the FM ultrasound, may allow assessment of the degree of hepatic fibrosis. The CT scan can be used to assess the presence but not the degree of portal hypertension.

The demonstration of esophageal varices by endoscopy or barium swallow implies portal hypertension, as do significant intraabdominal collaterals at any site.