Cats can be difficult to safely anesthetize because of their small body size, fractious nature and altered metabolism of anesthetic drugs. Furthermore, cats are frequently undertreated for pain. This occurs for a variety of reasons, including difficulty in recognizing pain in cats and lack of knowledge of analgesic therapy for cats. Cats are now the most popular pet in the United States and it is imperative that we arm ourselves with safe, effective means of providing anesthesia and analgesia in cats.
Anesthesia / Analgesia Concepts
Regardless of the species or health status of the patient, anesthesia should be thought of as 4 distinct and equally important periods: 1) preparation/premedication; 2) induction; 3) maintenance and 4) recovery. We tend to diminish the importance of the phases of preparation/premedication and recovery and yet these phases contribute as much to successful anesthesia as the phases of induction and maintenance.
Anesthetic / Analgesic Drugs
a) No matter what anesthetic protocol is chosen, safe and successful anesthesia will be enhanced by the use of pre-anesthetic tranquilizers. This is evidenced by two facts: 1) Stress in the perioperative period is extremely dangerous physiologically. (Think about capture deaths that occur in wild animals.) and 2) Tranquilizers allow reduction in the dose of both induction and maintenance agents, thus increasing the distance between 'effective dose' and 'dangerous' or 'toxic dose' of drugs.
b) Also, no matter what anesthetic protocol is chosen, analgesia is imperative. Perioperative analgesia has two monumental advantages: 1) analgesia increases anesthetic safety by decreasing the necessary dosages of anesthetic drugs and 2) analgesia improves our medical success rate because adequate analgesia improves healing and allows a decreased incidence of postoperative stress-related complications. Regardless of which analgesic drugs are chosen, 3 basic tenets of pain management should always be followed: 1) analgesic drugs should be administered preemptively; 2) multimodal analgesia should be used whenever possible; and 3) analgesia should continue as long as pain is present.
c) Along with multi-modal or balanced analgesia, we should be providing multimodal or balanced anesthesia. Again, using a variety of drugs allows us to capitalize on the synergism between the drugs while decreasing the dose of each drug. With our current knowledge of pharmacology and the availability of safe, effective anesthetic and analgesic agents, anesthetizing a patient with a single agent (eg, no premedicant, inhalant induction, inhalant maintenance) is no longer appropriate. Nor is it safe.
A. Premedicants: Sedatives / tranquilizers / analgesics
Morphine, hydromorphone, methadone, oxymorphone, fentanyl; Butorphanol, buprenorphine
• Advantages: provide good analgesia, are reversible
• Disadvantages: May cause excitement in cats; generally don't provide enough sedation when used alone in young, healthy or excited patients
a) Full agonists (morphine, hydromorphone, oxymorphone, fentanyl)
Most potent class of analgesic drugs
Should be considered any time that pain is moderate to severe
Time to onset - < 5mins (< 1 min when administered IV)
Duration of action 4-6 hours (oxymorphone closest to 6 hrs; approx 4 hours for morphine and hydromorphone; 20 mins for fentanyl)
• Uses: IV (morphine not generally bolused IV), IM, SQ, transdermal patch (fentanyl), CRI, epidural space, intra-articular space, etc... Fentanyl CRIs are excellent in cats.
o May cause excitement in cats – use with sedatives.
o Respiratory depression is completely over-rated in animals.
o Minor to nonexistent cardiovascular depression.
o May cause hyperthermia in cats.
Recommendation: In cats, use a full agonist opioid for moderate to severe pain.
b) Partial agonists (buprenorphine), agonist-antagonists (butorphanol)
Not as potent as the full agonist class
o Don't confuse binding potency with analgesic potency
Should be considered when pain is mild to moderate
• May not be potent enough nor have a long enough duration (butorphanol) to be used alone as a premedicant for many of our surgical procedures.
Time to onset - < 5 mins (butorphanol); 10-20 mins (buprenorphine)
Duration of action - 45-90 mins (butorphanol); 6-12 hours (buprenorphine)
• Uses: IV, IM, SQ, transmucosal (buprenorphine), CRI? (not as effective as full agonist CRIs)
Similar to the effects of full agonists but not as pronounced
o Butorphanol provides moderate sedation, buprenorphine provides mild to no sedation.
Recommendation: Use buprenorphine transmucosally in cats, especially for at-home therapy; use butorphanol as a sedative but not as an analgesic (unless the duration of pain is expected to be VERY short).