Understanding veterinary anesthesia
Editor's Note: This month, DVM Newsmagazine spoke with veterinary anesthesia expert Victoria Lukasik, DVM, Dipl. ACVA, of the Southern Arizona Veterinary Specialists in Tucson, Ariz. Lukasik has published numerous professional articles and has contributed to textbooks on the field of anesthesia. She is a nationally and internationally recognized speaker and has been presented with several professional awards, including one for compassion.
Lukasik is a 1990 graduate of the Washington State University College of Veterinary Medicine. She completed her residency training in veterinary anesthesiology at Cornell University. She is also involved in cancer imaging research at the Arizona Health Sciences Center at the University of Arizona and a member of the Radiology and Anesthesiology faculties.
DVM: If you had one myth to bust about anesthesia, what would it be?Lukasik: No one special breed is overly sensitive to anesthetic drugs. All of the specific breeds of dogs or cats can be successfully anesthetized using appropriate drugs and appropriate doses. It may be an individual animal's response that is atypical in that it becomes overly sedate or can have a problem of some kind with ventilation. And there certainly are more appropriate doses and less appropriate doses depending on how old or sick or young our patients are.
DVM: What kinds of questions do you field frequently when it comes to analgesia or anesthesia?
Lukasik: One of the questions I get frequently is what protocol to use when you have an aggressive cat or one that would be dangerous to us in the clinic.
We try to use the lowest doses in combinations of drugs to create a calm animal that has become amenable to be manipulated. That combination of drugs might include a tranquilizer and analgesic drug, or sometimes we need heavier sedatives. Combinations might include midazolam, oxymorphone and the addition of dexmedetomidine at a very low dose. That combination will usually achieve nice sedation with minimal cardiovascular and respiratory compromise.
I usually recommend a dexmedetomidine dose of 1 to 3 µg/kg. And the dosing of midazolam and oxymorphone would be more the mid-range doses. My favorite protocol is midazolam at 0.2 or 0.3 mg/kg, oxymorphone at 0.05 mg/kg and dexmedetomidine at 0.001 to 0.003 mg/kg. The dexmedetomidine is such a small volume that we need to draw it up in an insulin syringe while the other two drugs (midazolam and oxymorphone) can be drawn up in a regular 1-ml syringe. Those are all combined in one syringe and administered intramuscularly. We sometimes only get one chance at an intramuscular injection. I take my one chance and give all three at the same time. Fractious animals are actually hard to sedate with single drug protocols; they respond so much more reliably to drug combinations like this.
DVM: You talk about the four phases of anesthesia, could you describe each?
Lukasik: During the physical examination, we begin to choose drugs and doses that are most appropriate for that patient and its needs. Then we premedicate it so we have a calm patient that has received preemptive analgesia and will allow manipulations such as placing an intravenous catheter, monitoring with equipment and preoxygenating with a face mask. The monitoring in this first phase to establish a baseline blood pressure and a electrocardiogram. It's important to note that the trend is often more important than the number itself. Where is the blood pressure going? Where is the heart rate going? Where is the respiratory rate going?
The second phase of anesthesia is induction, and that includes an understanding of the anesthesia continuum, which is from awake to dead. What we really want to do is keep the patient closer to awake. So we titrate the induction dose to get it just asleep enough to intubate it, but not too asleep. The second phase with the injectable induction is to go ahead and take the patient from the awake state into sleep. This phase also includes the transition onto an inhalant anesthetic. As the induction drug wears off, we need to have enough inhalant anesthetic concentration from the anesthetic machine to smoothly transition the patient from the injectable onto the inhalant.
The third phase of anesthesia is maintenance of the inhalant anesthetic. It is a active phase. We are looking at our patient every three to five minutes to assess how deep it is. Is it too light under anesthesia or too deep under anesthesia—or is it just right? We have to remember that to find "just right," we have to find "too light." Everyone is afraid to find this too light state, but it is achievable when monitoring really subtle signs—not just when the patient is moving. Some of the ways we measure for those signs include monitoring for heart rate and respiratory rate and other trends in physiology like blood pressure, eye position, changes in palpebral reflex and jaw tone. Generally, blood pressure, respiratory rate and heart rate decrease as the depth of anesthesia increases, and the eye will change position and the jaw muscles relax, signaling the patient is moving into the deeper planes of anesthesia.
Recovery is the fourth phase of anesthesia. We want to be sure that in recovery the patient can regain its ability to guard its airway and regain mentation as well as smooth coordination but also maintain appropriate analgesia for whatever its needs are postoperatively. Recovery can be short or long, depending on how quickly the patient is coming around. This recovery time depends on the patient's temperature; hypothermia will delay recovery. Hypotension can also delay recovery if the patient had really low blood pressure while it was asleep. Patients that did not breathe very well while under anesthesia could also have a delayed recovery. There are lots of other reasons, too.
Recovery is often one of the most dangerous times for a patient under anesthesia. One study told us that most of the patients that die in the perianesthetic period actually die in the first three hours after extubation.1 It underscores that recovery, which includes that initial time after extubation to the time the patient regains its coordination, mentation and muscle strength, is a really critical time. We have to keep caring for our patients long after extubation.
DVM: What are some of the most common mistakes made by general practitioners when it comes to anesthesia monitoring, induction and recovery?
Lukasik: The most common mistake is being too hurried in your approach to anesthesia and losing your attention to the patient. It takes time to look at where the patient is and, more important, where it is going in terms of recognizing a trend. We have to slow down and not rush into things too quickly. We all need to take the time to recognize problems in this process. Take a really good look at the patient before, during and after the procedure. Another area is to use better preemptive analgesic protocols or more appropriate analgesic drugs for the patient we have in front of us rather than just using the things we always have used.
1. Brodbelt DC, Blissitt KJ, Hammond RA, et al. The risk of death: the confidential enquiry into perioperative small animal fatalities. Vet Anaesth Analg 2008;35(5):365-373.