Anesthetic death is an unfortunate and unwelcome aspect of veterinary practice. Most, if not all, veterinarians have experienced the loss of a patient perioperatively. Especially unsettling is the death of a bird during the postoperative period when the patient is expected to make a complete recovery.
Photo 1: Indirect blood pressure measurement technique on a cockatoo.
There are numerous reasons why a bird might die including, but not limited to, underlying metabolic disturbances or other pathology, severe hypothermia, hypotension, human error or an idiosyncratic response to anesthesia. To lessen the likelihood that death occurs, the veterinary team should recognize the perioperative period as a critical time in the management of their patients and familiarize themselves with appropriate physiology, anesthetic protocols, patient monitoring and methods of supportive care for their patients.
The history and physical examination are extremely important when evaluating potential surgical patients. The history must thoroughly evaluate all aspects of husbandry, including the source of pet(s), length of ownership, diet, environment and previous or current illness(es) and therapy. Patients should be observed in their cage or transport carrier before the hands-on portion of the physical examination. This will allow veterinarians to identify signs of illness that may be difficult to detect once the patient is restrained. Thorough physical examination will help to assess physical condition, cardiopulmonary status, severity of illness if present and any other conditions that are unknown to the owner. The veterinary team can use this information to identify potential problems and to address them accordingly.
- Diagnostic testing: All patients should be as physiologically stable as possible prior to anesthesia, and veterinarians should use their judgment when performing pre-anesthetic screening for their avian patients. Ideally, the minimum database for procedures that require significant anesthesia time should include a complete blood count (CBC) and biochemical profile. In instances when the size of the patient limits the amount of blood that can be safely taken, a packed cell volume, total protein and blood glucose are acceptable; however the addition of uric acid, aspartate aminotransferase and white blood-cell count is preferable. If your hospital or clinic is equipped with biochemical analyzers that have avian/reptilian profile rotors, a complete biochemical panel can be obtained from 100 microliters of blood.
Hydration status should be noted and corrected with appropriate fluid therapy, if necessary. Subcutaneous fluids are suitable for birds that are mildly dehydrated. Intraosseous and intravenous fluids are required for patients that are moderately or severely dehydrated. In general, daily maintenance fluid requirement is 50 mg/kg/day. Fluid deficit is calculated by multiplying the body weight in grams by the percent dehydration (% dehydration x bodyweight[g] = fluid deficit in milliliters). To correct dehydration, the daily maintenance plus one-half of the fluid deficit is given during the first 12 to 24 hours, and then repeated during the second 24-hour period. Additional fluids are also given for ongoing losses resulting from continued regurgitation, vomiting or diarrhea. In emergency situations, a bolus of 10 ml/kg IV over a five-minute period is well-tolerated by most birds. Crystalloids or other balance fluid solutions with or without added dextrose (2.5% to 5%) are commonly used. During surgery, fluid should be given at a rate of 10 ml/kg/hour for the first hour, then 5 ml/kg/hour for the second hour and thereafter. Colloids such as hetastarch 6% (10-15 ml/kg IV or IO slowly) are administered if the patient is hypovolemic/hypoproteinemic and volume expansion is necessary to stabilize the patient.1,2 When using hetastarch with crystalloids, reduce the volume of crystalloids given by the volume of hetastarch used to avoid fluid overload.1
Fasting allows the upper gastrointestinal tract to empty, thereby reducing the likelihood that the patient will regurgitate or vomit and aspirate ingesta. Fasting also reduces proventricular and ventricular distension, thereby reducing potential interference with normal respiratory airflow or organ perforation during laparoscopic procedures.3 A prolonged fast greater than six hours is not recommended for most birds due to their small size and rapid metabolic rates. Some authors recommend fasting larger birds (>500 grams) for at least 12 hours while smaller birds (e.g. budgerigars and canaries) are fasted for six to 12 hours. In most instances, a three-hour fast is sufficient for most small birds.3 Raptors and especially waterfowl should be fasted for 12 to 24 hours if necessary.
Anesthetic protocols: There are numerous sources of information regarding anesthetic protocols for avian species. To become familiar with these protocols, I recommend purchasing an appropriate source (e.g. formulary or textbook) for more specific information.
When choosing an anesthetic protocol, select one that will allow you to complete the desired procedure with minimal to no physiologic changes to the patient. Most often isoflurane is the anesthetic of choice; however, sevoflurane is an acceptable alternative.
Emergency drugs (atropine and epinephrine) should be readily available or prepared in syringes prior to initiating anesthesia. This is a time-saving move that allows the veterinary team to address anesthetic emergencies (cardiopulmonary arrest) as soon as they occur. Patients under anesthesia for longer than 10 to 15 minutes should be intubated. Be careful not to manipulate the head and neck of intubated birds too much, since excessive movement of the endotracheal tube can result in hemorrhage or moderate inflammation within the tracheal lumen.
Anesthetic complications: Certainly, anesthetists are trained to monitor patients closely while they are under anesthesia and to maintain adequate ventilation. This is extremely important for all avian patients; however, special consideration should be given to African grey parrots (Psittacus erithacus sp) since some African grey parrots may suffer from hypocalcemia. Although the exact etiology of this condition is uncertain, dysfunctional parathyroid glands or parathyroid hormone (PTH) are suspected as a possible etiology.4 Additionally, isoflurane has been shown to decrease ionized calcium, myocardial contractility, cardiac index, mean arterial pressure and alter calcium homeostasis in cardiac and skeletal muscle in several mammalian species.5-7
Additionally, Krapf, Jaeger and Hulter (1992) revealed that chronic respiratory alkalosis due to hyperventilation may result in impaired renal response to PTH in humans8 and loss of calcium through the kidneys.
To manage anesthesia in African grey parrots, Edling et al (2001) suggest that intermittent positive pressure ventilation combined with capnography will help the anesthetist to maintain end-tidal CO2 within a range of 30 to 45 mmHg, which is adequate for and will improve anesthetic monitoring of greys.9 Supplemental calcium gluconate given five to 10 minutes prior to anesthesia may help to alleviate the problems associated with hypocalcemia.