Types of Hepatic Shunts #
Hepatic shunts are broadly classified as extrahepatic or intrahepatic, with distinct characteristics that influence anesthetic planning.
Extrahepatic shunts occur predominantly in small breed dogs such as Yorkshire Terriers, Maltese, Miniature Schnauzers, and Shih Tzus, as well as in cats. These shunts exist outside the liver parenchyma, typically connecting the portal vein directly to the caudal vena cava or azygos vein. The most common extrahepatic shunts include portocaval, portoazygous, and portophrenic communications. While these shunts are generally more accessible for surgical intervention, the small size of affected patients often creates additional anesthetic challenges, particularly related to thermoregulation and vascular access.
Intrahepatic shunts occur more frequently in large breed dogs, particularly Labrador Retrievers, Irish Wolfhounds, Golden Retrievers, and Australian Cattle Dogs. These shunts form within the liver parenchyma and frequently represent a persistent ductus venosus that failed to close after birth. Intrahepatic shunts can be further classified based on their anatomical location:
- Left divisional shunts (most common): involve the left hepatic vein
- Central divisional shunts: connect to the central hepatic vein
- Right divisional shunts: involve the right hepatic vein
Surgical correction of intrahepatic shunts typically involves more complex interventions with higher risk, which significantly impacts anesthetic planning and management.
Common Perioperative Morbidities #
Patients with hepatic shunts are predisposed to several perioperative complications that directly influence anesthetic management:
Hypoalbuminemia resulting from decreased hepatic protein synthesis affects drug binding and distribution, often necessitating reduced dosages of highly protein-bound drugs. The reduction in oncotic pressure may also contribute to fluid shifts and tissue edema during prolonged procedures.
Coagulopathies arise from impaired synthesis of clotting factors, increasing the risk of hemorrhage during surgical manipulation. Preoperative evaluation of coagulation profiles and potential vitamin K supplementation may be necessary before proceeding with anesthesia.
Hypoglycemia occurs frequently due to compromised glycogenolysis and gluconeogenesis, particularly during periods of fasting. Regular monitoring and dextrose supplementation are essential components of anesthetic management.
Hepatic encephalopathy presents a significant risk, as accumulated neurotoxins, particularly ammonia, can lead to altered consciousness and seizures. The stress of anesthesia and surgery may exacerbate these neurological manifestations.
Reduced drug metabolism significantly impacts pharmacokinetics and pharmacodynamics, potentially prolonging drug effects and recovery times. Drugs primarily dependent on hepatic clearance should be used cautiously with appropriate dose reductions.
Intraoperative Complications #
Several specific complications may arise during surgery for hepatic shunts:
Portal hypertension represents perhaps the most serious intraoperative complication, particularly following acute complete shunt ligation. Gradual attenuation rather than complete ligation is generally preferred to prevent this potentially fatal complication. Monitoring for signs of portal hypertension, including intestinal cyanosis, pancreatic edema, and splenic congestion, is essential during surgical manipulation.
Hemorrhage may occur during dissection and manipulation of vascular structures, particularly with intrahepatic shunts. Readily available blood products and careful surgical technique are crucial preventive measures.
Hypotension frequently develops during vascular manipulation, particularly when temporary occlusion of the caudal vena cava is required. Careful fluid management and vasopressor support may be necessary during these critical periods.
Cardiac arrhythmias may develop during manipulation near the heart or from electrolyte disturbances. Continuous ECG monitoring and maintenance of electrolyte balance are essential.
Hypothermia develops rapidly in these often small, compromised patients and can further impair drug metabolism and coagulation.
Postoperative Complications #
The postoperative period presents continued challenges for patients with hepatic shunts:
Seizures and neurological deterioration may indicate worsening hepatic encephalopathy or development of portal hypertension. Immediate intervention including mannitol administration, anticonvulsant therapy, and potential surgical revision may be necessary.
Persistent portal hypertension can manifest as ascites, gastrointestinal congestion, or shock. Close monitoring of abdominal distention, pain, and cardiovascular parameters is essential.
Prolonged recovery is common due to altered drug metabolism. Extended monitoring in a controlled environment with supportive care is typically required.
Respiratory complications may develop, particularly in brachycephalic breeds with concurrent upper airway abnormalities. Careful extubation and oxygen supplementation can minimize these complications.
Hypoglycemia may persist into the postoperative period, necessitating continued dextrose supplementation and frequent monitoring of blood glucose.
Key Considerations for Anesthetic Planning #
Successful anesthetic management of patients with hepatic shunts requires careful consideration of several factors:
Preoperative stabilization should address hepatic encephalopathy, coagulopathies, and metabolic abnormalities. Lactulose, antibiotics to reduce intestinal bacterial load, vitamin K supplementation if indicated, and correction of electrolyte and acid-base disturbances should be implemented prior to anesthesia.
Drug selection should favor agents with minimal hepatic metabolism or those not significantly affected by reduced hepatic function:
For premedication, opioids should be selected carefully, with fentanyl (2-5 μg/kg IV) or low-dose methadone (0.1-0.2 mg/kg IV) typically well-tolerated.
Induction can be safely accomplished with propofol at reduced doses (2-4 mg/kg IV to effect), though propofol undergoes hepatic metabolism, its short duration of action (due to redistribution) makes it generally acceptable. Alfaxalone (1-2 mg/kg IV to effect) offers an excellent alternative with minimal cardiovascular effects and partially extrahepatic metabolism.
Maintenance is best achieved with inhalational anesthetics such as isoflurane or sevoflurane, which undergo minimal hepatic metabolism. Balanced anesthesia incorporating constant rate infusions of fentanyl (3-5 μg/kg/hr) or lidocaine (25-50 μg/kg/min) can reduce inhalant requirements while providing additional analgesia.
Fluid therapy should include crystalloids at maintenance rates (5-10 ml/kg/hr), with dextrose supplementation (2.5-5%) to prevent hypoglycemia. Colloids may be necessary for patients with severe hypoalbuminemia.
Monitoring must be comprehensive, including continuous ECG, capnography, pulse oximetry, and blood pressure measurement. Regular blood glucose checks are essential, with central venous pressure monitoring when feasible. Core body temperature requires close attention, with active warming measures implemented throughout the procedure.
Recovery should occur in a controlled environment with continued monitoring and support. Extended recovery times should be anticipated, with readily available interventions for potential complications.
Anesthetic management of patients with hepatic shunts demands a comprehensive understanding of the pathophysiology, careful drug selection and dosing, thorough monitoring, and vigilant supportive care. By recognizing the unique challenges these patients present and implementing appropriate protocols, veterinary anesthesiologists can significantly improve outcomes for this complex patient population.
References #
- Mayhew PD, Weisse C. Liver and biliary system. In: Tobias KM, Johnston SA, eds. Veterinary Surgery: Small Animal. 2nd ed. Elsevier; 2018:1744-1787.
- Greenhalgh SN, Reeve JA, Johnstone T, et al. Anesthesia and perioperative management of dogs and cats with portosystemic shunts. J Am Vet Med Assoc. 2014;245(7):718-733.
- Mathews K, Kronen PW, Lascelles D, et al. Guidelines for recognition, assessment and treatment of pain. J Small Anim Pract. 2014;55(6):E10-E68.
- Pasloske K, Sauer B, Perkins N, et al. Plasma pharmacokinetics of alfaxalone in both premedicated and unpremedicated Greyhound dogs after single intravenous administration of Alfaxan. J Vet Pharmacol Ther. 2009;32(5):510-513.
- Murphy LA, Russell JD, Clifford CA, et al. Physiological effects of selective venous occlusion during surgical attenuation of congenital extrahepatic portosystemic shunts in dogs. Am J Vet Res. 2017;78(3):305-312. T
