5.4 Acute Hepatic Failure, Hepatic Encephalopathy & Biliary Emergencies

Acute Hepatic Failure, Hepatic Encephalopathy & Biliary Emergencies

The liver serves as the metabolic engine of the body, orchestrating carbohydrate homeostasis, albumin synthesis, coagulation factor production, and nitrogenous waste detoxification. When acute hepatocyte necrosis occurs—or when portal blood bypasses hepatic parenchyma via portosystemic shunts (PSS)—the loss of these vital functions precipitates life-threatening systemic crises.

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1. Critical Hepatic Functions & Acute Liver Failure (ALF)

Because the liver possesses immense functional reserve, clinical liver failure typically signifies the acute loss of >70% to 80% of functional hepatocyte mass.

Key Hepatic Functions in the ICU

• Carbohydrate Metabolism: Hepatic gluconeogenesis and glycogenolysis maintain baseline blood glucose. Acute failure results in profound, refractory hypoglycemia.
• Protein & Colloid Oncotic Synthesis: Hepatocytes synthesize almost all circulating plasma proteins, notably albumin (which accounts for 80% of plasma Colloid Oncotic Pressure [COP]). Hypoalbuminemia results in cavitary effusions (ascites) and peripheral edema.
• Coagulation Factor Synthesis: The liver produces all coagulation factors except Factor VIII and von Willebrand factor (vWF). It uniquely synthesizes the Vitamin K-dependent factors: II, VII, IX, and X, as well as endogenous anticoagulants (Protein C, Protein S, Antithrombin III).
• Nitrogen Detoxification (Urea Cycle): Ammonia ($NH_3$), generated by enteric bacterial degradation of dietary proteins and amino acid deamination, is cleared from portal blood and converted to non-toxic, water-soluble urea.

Common Toxic & Infectious Etiologies of ALF

2. Hepatic Encephalopathy (HE): Pathophysiology & Clinical Stages

Hepatic Encephalopathy (HE) is a complex neuropsychiatric syndrome resulting from the accumulation of gut-derived neurotoxins that enter the systemic circulation due to severe hepatic parenchymal dysfunction or congenital/acquired portosystemic shunts (PSS).

The Ammonia–Glutamine Astrocyte Hypothesis

1. Hyperammonemia: Elevated systemic ammonia ($NH_3$) readily crosses the blood-brain barrier into cerebral astrocytes.
2. Astrocyte Swelling: In the brain, astrocytes metabolize ammonia by combining it with glutamate via glutamine synthetase to form glutamine. Glutamine acts as a potent intracellular osmolyte, drawing free water into astrocytes and generating cytotoxic cerebral edema.
3. GABA/Benzodiazepine Up-Regulation: HE induces profound up-regulation of inhibitory $\gamma$-aminobutyric acid (GABA) and central benzodiazepine receptors, while increasing endogenous neurosteroids and false neurotransmitters (e.g., octopamine), depressing conscious central nervous system activity.

Clinical Staging of Hepatic Encephalopathy

• Stage I (Mild): Subtle behavioral alterations, mild lethargy, sleep-wake cycle disturbances, restlessness, mild unresponsiveness to commands.
• Stage II (Moderate): Pacing, mild ataxia, aimless wandering, head pressing, amaurosis (central cortical blindness with intact pupillary light reflexes), personality changes, and ptyalism/hypersalivation (especially in cats).
• Stage III (Severe): Marked stupor, severe ataxia, pronounced vocalization, muscle tremors, hyperreflexia, responsive only to noxious stimuli.
• Stage IV (Coma): Coma, unresponsiveness to deep pain, decerebrate posturing, generalized tonic-clonic seizures, and impending brainstem herniation.

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3. Medical Management of Hepatic Encephalopathy

Emergency medical management aims to decrease the production and systemic absorption of enteric nitrogenous neurotoxins:

1. Lactulose Therapy

• Mechanism of Action: Lactulose is a non-absorbable synthetic disaccharide (galactose-fructose). Colonic bacteria ferment lactulose into short-chain organic acids (lactic, acetic, and formic acids), dropping colonic pH below 5.0.
• Ammonia Trapping: In an acidic lumen, free diffusible ammonia ($NH_3$) is protonated into charged, non-diffusible ammonium ($NH_4^+$). Ammonium cannot cross colonic mucosal membranes and is trapped and evacuated in feces ("ammonia trapping"). Additionally, lactulose exerts an osmotic laxative effect, accelerating transit.
• Dosing Goal: Administered PO or via retention enema (diluted 1:3 with warm water) titrated to achieve 2 to 3 soft, formed stools per day.

2. Enteric Antimicrobial Suppression

• Targeting Urease-Producing Microbes: Administer antibiotics to suppress colonic bacteria that produce urease (the enzyme converting urea into ammonia):
  • Metronidazole: 7.5 to 10 mg/kg PO or IV q12h (use reduced dosing to prevent drug accumulation and neurotoxicity in patients with hepatic failure).
  • Neomycin: Non-absorbable aminoglycoside administered PO or via enema.
  • Rifaximin: Non-absorbable rifamycin derivative.

3. Critical Pharmacological Rules: Seizure Management

• First-Line Anticonvulsant: Levetiracetam (Keppra) (20 to 30 mg/kg IV) is the anticonvulsant of choice because it undergoes minimal hepatic metabolism (eliminated primarily via renal excretion) and does not bind GABA/benzodiazepine receptors.
• Contraindication of Benzodiazepines: Diazepam and midazolam are strictly contraindicated in hepatic encephalopathy! Because endogenous benzodiazepine receptor tone is already pathologically elevated in HE, exogenous benzodiazepines will precipitate profound, prolonged coma. (Flumazenil can be administered to reverse benzodiazepine-induced HE exacerbations).

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4. Coagulopathy of Acute Hepatic Failure

Coagulopathy is a hallmark of severe acute hepatic disease, developing through impaired factor synthesis, failure to clear activated clotting factors, and vitamin K malabsorption.

• The Short Half-Life of Factor VII: Among the vitamin K-dependent factors (II, VII, IX, X), Factor VII possesses the shortest circulating half-life (only 4 to 6 hours).
• Diagnostic Discrepancy: Because Factor VII belongs exclusively to the Extrinsic Pathway, Prothrombin Time (PT) prolongs first, well before the Activated Partial Thromboplastin Time (aPTT / Intrinsic Pathway) becomes abnormal.
• Therapeutic Interventions:
  • Vitamin K1 (Phytonadione): Administer 1.0 to 2.0 mg/kg SC or PO q12–24h using small-gauge needles to prevent hematoma formation (never give IV due to lethal anaphylaxis risk).
  • Fresh Frozen Plasma (FFP): Transfuse 10 to 20 mL/kg IV for patients with active cavitary hemorrhage or prior to invasive procedures (liver biopsy, abdominocentesis).

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5. Critical Biliary Emergencies

Gallbladder Mucocele

• Pathophysiology: Cystic mucosal hyperplasia of the gallbladder wall accompanied by excessive secretion of thick, gelatinous, inspissated bile and mucus. Common in older Shetland Sheepdogs, Miniature Schnauzers, and Cocker Spaniels, often associated with hyperlipidemia, Cushing's disease, or hypothyroidism.
• Ultrasound Hallmark: Pathognomonic "kiwi fruit" or stellate pattern (central organized hyperechoic mucus with radiating striations and an immotile, non-gravity-dependent lumen).
• Complications: Ischemic gallbladder wall necrosis, rupture, and secondary bile peritonitis. Curative therapy requires emergency cholecystectomy.

Extrahepatic Biliary Obstruction (EHBO) & Bile Peritonitis

• EHBO: Obstruction of the common bile duct by choleliths, pancreatitis, or mass lesions. Manifests with progressive hyperbilirubinemia, marked icterus, and acholic (clay-colored/white) feces due to absent stercobilin in the intestine.
• Bile Peritonitis Diagnostic Criteria:
  • Chemical and secondary bacterial peritonitis resulting from bile leakage into the peritoneal cavity.
  • Definitive Diagnosis: Peritoneal fluid total bilirubin concentration is GREATER THAN 2 TIMES (>2x) the concurrent peripheral serum total bilirubin concentration.
  • Cytology: Green-brown/golden amorphous or crystal-like bile pigment engulfed by degenerate neutrophils.