September 21, 2012
Achieving low mortality and low morbidity is not good enough in 2012. Our goals should also include excellent analgesia, patient comfort and minimal stress.
The American Animal Hospital Association recently published another great white paper, this time on the art of anesthesia. The “guidelines are not meant to establish a universal anesthetic plan or legal standard of care,” explain the authors (R. Bednarski et al., “AAHA Anesthesia Guidelines for Dogs and Cats,” JAAHA 2011, Vol. 47, p. 377–385). But they are an excellent review of common practices.
Full disclosure: What follows is not a summary of the article, but the first of a two-part review of a few great points made by the authors.
Despite common concerns expressed by owners of certain breeds, “Few are documented,” the authors remind us. Two exceptions:
Brachycephalic dogs and cats clearly have a higher risk of upper airway obstruction.
Sight hounds may have a slower recovery when propofol or thiopental are used. Since thiopental is not available in the U.S., this is a good reminder to be cautious with propofol in those breeds.
We asked Richard Bednarski, a board-certified anesthesiologist at The Ohio State University and first author of the article, to clarify this important point about greyhounds.
“Studies indicate that the effects of propofol last twice as long as in other breeds. If you consider that it takes half an hour for other breeds to recover from a single propofol bolus, then a greyhound may take an hour.
“Therefore this may not be a significant clinical issue. Studies have identified a deficient cytochrome metabolic pathway in greyhounds. It may be the same one involved with delayed recovery from barbiturates.”
The patient’s temperament should be taken into consideration. An aggressive or fearful patient may require higher drug dosages, whereas lower dosages may suffice for a quiet or depressed patient.
Patients under general anesthesia, “Sedated patients require appropriate monitoring and supportive care.” These patients may require intubation and/or oxygen supplementation.
“There is no evidence to indicate the minimum time frame before anesthesia within which laboratory analysis should be performed.” So that time frame is up to the clinician. It should be reasonable in order to pick up changes that increase the anesthetic risk. Any pre-existing disease, the patient’s particulars and recent changes in the patient’s health are reasons to consider recent blood work.
Considering your patient’s American Society of Anesthesiologists score is a good way to prepare your team for potential risks. Studies show that, logically, patients with a higher ASA score have a higher risk of anesthetic complications.
“There are routine surgeries, but there is no routine anesthesia,” as the saying goes. Risks should be verbally explained to pet owners, and should be summarized in the consent form, which the owners sign before anesthesia. “The proposed anesthetic plan and any available medical or surgical alternatives” should be discussed with the client.
Pediatric patients (6 weeks to 4 months) should be allowed to eat up to four hours before surgery. Beyond 4 months, food should be withheld overnight. One exception exists: emergency situations. In such cases, pay extra attention to airway protection in case of vomiting or regurgitation and subsequent aspiration.
Benefits of premedication include “lowered patient and staff stress, ease of handling, and reduction of induction and inhalant anesthetic doses.”
However, it is important to anticipate the side effects of “premeds.” They include “dysphoria related to benzodiazepines, bradycardia related to alpha-2 agonists and opioids, and hypotension related to acepromazine.” Appropriate dosing (adapted to each patient and each situation) and balanced anesthesia can circumvent these side-effects.
We are very fortunate to have a variety of analgesic drugs and techniques at our disposal, which often work synergistically. They include opioids, NSAIDs, local blocks (SQ lidocaine), nerve blocks, CRIs (Constant Rate Infusions) and NMDA receptor antagonists (e.g. ketamine).
The authors are very clear: “No one anesthetic drug or drug combination is better for renal disease; most important is to maintain blood pressure and adequate renal perfusion.”
To achieve this important goal, azotemic patients could be perfused with crystalloids at 1.5–2 times maintenance to hopefully lower their azotemia before anesthesia is planned.
During anesthesia, crystalloid fluid rates as high as 20–30 mL/kg/hr have been recommended in kidney patients. Fluid therapy is then continued during the post-anesthetic phase.
Alternatively, low-dose mannitol can induce diuresis by increased renal perfusion pre-operatively.
One important risk with cardiac disease is fluid overload, which can lead to congestive heart failure. The IV fluid rate should therefore be tailored to each patient, taking into consideration blood pressure, central venous pressure, oxygenation and lung sounds.
In spite of the required preop fasting period, “Cardiac medications should be administered normally the day of surgery.” It is important to understand potential side-effects of the patient’s drug regimen. For example, ACE inhibitors and beta-blockers may lead to hypotension, so inotropes may be required intraop.
Proof that universal recipes are not ideal for all patients: Some medications may be less than ideal in cardiac patients. For example, ketamine may be contraindicated in patients with hypertrophic cardiomyopathy, because it increases heart rate at higher doses. Alpha-2 agonists are not ideal in patients with mitral valve disease. Balanced anesthesia should help mitigate such side-effects.
Read PART 2 of this series here.
For more practical tips, visit AAHA's website. To watch a video of AAHA's anesthesia guidelines in action at Newport Harbor Animal Hospital, click here.
Dr. Phil Zeltzman is a mobile, board-certified surgeon near Allentown, Pa. He is the co-author of “Walk a Hound, Lose a Pound: How You and Your Dog Can Lose Weight, Stay Fit, and Have Fun Together.
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