Equine Colic Surgery: Anesthetic Management and Postoperative Analgesia

Key concerns: (1) The horse is cardiovascularly compromised (HR 60, CRT 3 s, lactate 4.5) indicating hypovolemia and possibly endotoxemia. Xylazine at 0.5 mg/kg IV can cause significant hypotension and reduce cardiac output; a lower dose (0.2–0.3 mg/kg IV) or detomidine (0.01–0.02 mg/kg IV) may be safer. Butorphanol provides minimal visceral analgesia; better choices include methadone (0.1–0.2 mg/kg IV) or morphine (0.05–0.1 mg/kg IV) but these can cause excitement; use with caution. (2) Induction with ketamine and diazepam is acceptable, but in a hypovolemic horse, ketamine can cause hypotension; ensure adequate fluid resuscitation before induction. (3) Maintenance with isoflurane causes dose-dependent hypotension; consider adding a lidocaine CRI (loading dose 1.5 mg/kg IV over 5 min, then 50 µg/kg/min) to reduce volatile requirement and provide analgesia. (4) Fluid rate of 5 mL/kg/h is likely insufficient for a hypovolemic horse; calculate deficit (e.g., 5–10% dehydration) and administer boluses (10–20 mL/kg) to target MAP ≥70 mm Hg. (5) Lack of invasive blood pressure monitoring is suboptimal; place an arterial catheter for continuous monitoring. (6) Positive pressure ventilation is appropriate but may reduce venous return; monitor and adjust. (7) Postoperative analgesia: phenylbutazone q24h is inadequate for severe pain; use q12h (4.4 mg/kg IV q12h) or switch to flunixin meglumine 1.1 mg/kg IV q12h. Butorphanol q6h provides poor visceral analgesia; consider a lidocaine CRI (50 µg/kg/min) or a fentanyl patch (but absorption variable) or a ketamine CRI (0.5–1 mg/kg/h) for somatic pain. Multimodal approach: add an NSAID (flunixin or phenylbutazone) plus an opioid (morphine 0.05–0.1 mg/kg IM q6h) or a lidocaine CRI. (8) Monitor for endotoxemia: consider polymyxin B or hyperimmune plasma. (9) Postoperative monitoring: pain score (e.g., equine pain scale), HR, PCV/TP, lactate, and gastrointestinal motility.

Intraoperative pain management: (1) Ensure adequate depth of anesthesia with isoflurane and add a lidocaine CRI (1.5 mg/kg IV loading dose over 5 min, then 50 µg/kg/min) for visceral analgesia and to reduce volatile requirement. (2) Consider an opioid infusion: morphine (0.05–0.1 mg/kg IV) or methadone (0.1–0.2 mg/kg IV) can be given as boluses, but they may cause ileus and excitement; use judiciously. (3) A ketamine CRI (0.5–1 mg/kg/h) can provide somatic analgesia and antihyperalgesia, but may cause ileus; use with caution. (4) Local anesthetic techniques: consider an incisional line block with bupivacaine (1–1.5 mg/kg) or a transverse abdominis plane (TAP) block if skilled. Postoperative pain management: (1) Use a validated equine pain scale (e.g., composite pain scale) every 4–6 hours. (2) Multimodal analgesia: NSAIDs (flunixin meglumine 1.1 mg/kg IV q12h or phenylbutazone 4.4 mg/kg IV q12h) for somatic pain; lidocaine CRI (50 µg/kg/min) for visceral pain and to promote motility; opioids (morphine 0.05–0.1 mg/kg IM q6h or butorphanol 0.05 mg/kg IV q6h) for rescue. (3) Avoid excessive opioids if ileus is present; monitor for decreased borborygmi and reflux. (4) Consider a fentanyl patch (50–100 µg/h) for sustained analgesia, but absorption may be variable in horses. (5) Monitor for endotoxemia: lactate, PCV, TP, and cardiovascular parameters; treat with fluids, NSAIDs, and possibly polymyxin B. (6) Promote gastrointestinal motility with lidocaine CRI, walking, and avoiding opioids that inhibit motility. (7) Provide a calm environment and monitor for signs of pain (rolling, pawing, flank watching). Adjust analgesia based on pain score and clinical status.