Anesthesia for ophthalmic surgery (Proceedings)
Patients that present for ophthalmic surgery are often geriatric or pediatric, painful and anxious, and may present as emergency cases. Not uncommonly, the reason for their ocular complaint is a direct result of systemic disease, for example diabetic cataracts. Anesthetic challenges include safe choices of drugs in a patient who may be very young or very old, or in a patient with underlying disease, assuring a calm and pain-free recovery during which the patient does not traumatize their eye, minimizing increases in intra-ocular pressure, ensuring no sudden patient movement during intricate surgical steps, and, for intra-ocular procedures, a central and dilated pupil. Further anesthetic challenges (as with any patient) include maintaining good blood pressure and heart rate, good ventilation, maintaining adequate anesthetic depth and ensuring a smooth recovery without lengthening the hospital stay unnecessarily. Some of these goals may appear conflicting; for example, it will be necessary to provide excellent analgesia for the recovery phase, yet systemic analgesics such as opioids may prolong recovery and/or lengthen the hospital stay. Likewise, assuring that there is no risk of sudden patient movement during surgery may cause the anesthetist to make the animal excessively deep, which will in turn depress cardiovascular and respiratory function.
This lecture will take a case-based approach to anesthetic management of patients presenting for various ophthalmic procedures.
Entropion repair – Sharpei puppyThis is an otherwise healthy 6 month old female puppy. Anesthetic concerns would include: achieving good sedation for IV catheter placement, providing good analgesia for recovery without depressing ventilation excessively, maintaining normothermia, and maintaining good blood pressure, heart rate and ventilation during surgery. At 6 months of age, this puppy likely has relatively mature hepatic and renal function, but hypoglycemia and clearance of drugs may be a concern. Also, at this age blood pressure tends to be physiologically lower than in adults and blood pressure is more heart rate dependent. Thus, we want to prevent excessive bradycardia and we may want to choose anesthetic drugs that are either reversible or have extra-hepatic clearance.
Pre-anesthetic preparation would include a thorough physical exam and screening bloodwork, e.g. PCV, TP, glucose and BUN via azostick. Premedication choices should include a sedative and an agonist opioid for analgesia. For sedation acepromazine should only be used at very low doses because of its long duration and extensive hepatic metabolism. Other choices for sedation could include midazolam: desirable in that it is reversible and short-acting, but undesirable in that it provides little to no sedation when used alone in healthy young animals. A 3rd choice for sedation could be dexmedetomidine, which would certainly provide excellent sedation and is reversible if its effect is prolonged, but this sedative will cause bradycardia and depress ventilation. My personal choice would be a very low dose of acepromazine (0.01-0.02 mg/kg IM) combined with an effective agonist opioid. The inclusion of the opioid provides analgesia and synergistic sedation. Reasonable opioid choices could be hydromorphone (may cause panting), oxymorphone (expensive), or morphine (may cause nausea but this is offset by acepromazine to some extent). So let's say we decide to premedicate this patient with acepromazine and mid-dose morphine (0.5 mg/kg IM). Now 15-30 minutes later we have a moderately sedate animal that is amenable to IV catheterization (always a challenge in the Sharpei!). For this case the choice of induction drug is wide open, but propofol or ketamine with diazepam would be the two most likely choices. Ketamine may increase intra-ocular pressure, but this would not be a concern in an entropion repair. Induction would be followed by intubation and maintenance with inhalants (isoflurane or sevoflurane) in oxygen. Anesthetic management would include monitoring of temperature, blood pressure, heart rate, oxygenation (pulse oximetry), and ventilation (capnography or visual assessment), with occasional glucose checks to make sure this puppy is not becoming hypoglycemic. Fluid therapy would include any isotonic crystalloid at 10 ml/kg/hour. Once good blood pressure monitoring was in place, an NSAID such as carprofen could be administered for intra- and post-operative analgesia. Furthermore, lidocaine could be infiltrated into the eyelids for analgesia (Guiliano EA, Regional anesthesia as an adjunct for eyelid surgery in dogs. Topics Companion Animal Medicine, 2008 Feb; 23(1):51-6.).
For recovery of this patient, I would want to make sure that she does not paw at or traumatize her repair, and I would want to keep her temperature normal. While not truly a brachycephalic breed, we would also want to ensure that she could "protect" her airway after extubation and that she is awake enough to breath normally. For analgesia, if I have used morphine in my premedication and she has received an NSAID during surgery, she may not need any additional opioid at recovery, since morphine is relatively long acting (4-6 hours). If I used a shorter-acting opioid, however, then I would have a low dose of hydromorphone or oxymorphone on hand at extubation so that I could give a small amount quickly IV. I would not choose morphine in this instance as it can cause histamine release when given IV as a bolus.