Anesthesia for our feline friends (Proceedings)

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.

     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.

  Anesthetic / Analgesic Drugs

A. Premedicants: Sedatives / tranquilizers / analgesics

   Opioids

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.

     • Effects

          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)

     • Effects

          o 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).

2. Alpha-2 adrenergic agonists (Domitor®, Dexdomitor®)

     • Advantages: provide analgesia, effects are reversible, can provide range of light to deep sedation.

     • Disadvantages: cause cardiovascular changes that are well-tolerated in patients with healthy hearts but are not appropriate for patients with cardiovascular disease.

     • Common protocols for the use of medetomidine and dexmedetomidine include:

          o Used alone, as described by the label of the drug (dexmedetomidine labeled for cats).

          o Combined with an opioid for improved sedation and analgesia

          • Generally 1/4-1/2 the label dose of medetomidine or dexmedetomidine (10-20 microg/kg) plus a standard dose of the opioid of choice.

          o Combined with an opioid and ketamine for brief periods of anesthesia.

          o As a premedicant for general anesthesia.

          • Alpha-2 agonists allow a decrease in the dosages of injectable and inhalant drugs necessary to induce/maintain anesthesia. This increases the safety margin of these drugs.

          o TIP: Use to treat patients recovering from anesthesia with excitement or pain.

     • My favorite use!

     • Patients having a rough recovery generally need both sedation AND analgesia.

     • ml IM (or 0.05 ml IV) for the average-sized cat.

     • Once the procedure is complete the alpha-2 agonist can be reversed with atipamezole.

     • The volume of atipamezole should be the same (or less) as the volume of medetomidine or dexmedetomidine that was administered.

     • The drug is generally used IM

     • Both sedation and analgesia will be reversed

     • Not all patients need to be reversed: May not want to reverse patients that have been excessively painful or patients that are easily excited.

3. Acepromazine

     • Advantages: Inexpensive, mild to moderate sedation, long-lasting

     • Disadvantages: No analgesia, may not be potent enough in some patients, long-lasting, may contribute to hypotension.

     • Recommendation: Do not use acepromazine in geriatric, neonatal or compromised patients. These patients may clear ace very slowly, contributing to prolonged recoveries and patients that are still sedate after discharge. The drug is acceptable for young, healthy patients or as a behavioral modifier (ie, for patients that are distraught or anxious – but not painful - for long periods of time).

     • Dosages: 0.01-0.1 mg/kg. Typical dosages are 0.03-0.05 for cats. Administered IM, SQ or IV.

4. Benzodiazepines - Diazepam (Valium®) and Midazolam (Versed®)

     • Advantages: minimal to no side effects, reversible

     • Disadvantages: generally don't provide adequate sedation when used alone in young, healthy or excited patients, no analgesia

     • Recommendation: Use midazolam when IM delivery is desired. Use diazepam when IV delivery is appropriate (diazepam is cheaper).

     • TIP: Use 0.1-0.2 mg/kg diazepam before propofol induction in critical patients. This will reduce the dose of propofol and decrease the occurrence of dose-related side effects.

     • NOTE: DO NOT use a benzodiazepine for sedation in aggressive or excited patients – they are HIGHLY LIKELY to become MORE aggressive or agitated.      • Dosages: 0.1-0.2 mg/kg IV (both) or IM (midazolam).

B. Induction drugs

    1. Propofol

     • Advantages: rapid induction and recovery, multiple routes of clearance from the body, good muscle relaxation.

     • Disadvantages: somewhat expensive, must be administered IV, causes mild to moderate respiratory and cardiovascular depression.

     • Dosage: 2-6 mg/kg IV

     • Tip: Administer 0.1-0.2 mg/kg diazepam IV just before inducing the patient with propofol. This will decrease the dose of propofol and thus decrease the propofol-mediated hypotension and hypoventilation.

     • Cat specific: Does not cause clinically significant amounts of Heinz bodies nor does it cause adverse effects in patients with hepatic lipidosis.

2. Ketamine

     • Advantages: inexpensive, can be administered IM, mild respiratory depression, no cardiovascular depression in heart-healthy patients.

     • Disadvantages:- very poor anesthesia and no muscle relaxation when used alone - must be combined with a sedative such as diazepam, midazolam, medetomidine or dexmedetomidine.

     • Dosage: 10-20 mg/kg IM (most useful in cats –volume too large for most dogs) or 5-10 mg/kg IV. Best combined with sedative/analgesic like dexmedetomidine or with a muscle relaxant like valium.

     • Tip: For combination with valium, draw up 1/2 ketamine and 1/2 diazepam BY VOLUME (rather than by concentration) in the same syringe and dose IV at 1 ml of the combo per 10-20 kg. Use the low end of the dose for very sedate or moribund patients.

     • Tip: Use in combination with dexdomitor and an opioid for anesthesia in cats.

3. Telazol

     • Advantages: potent, rapid induction, can be administered IM

     • Disadvantages: recoveries can be prolonged and rough (especially in dogs), fixed ratio of tiletamine:zolazepam

     • Dosage: 2-5 mg/kg IV or 5-10 mg/kg IM

     • Note: Physiological effects are similar to those described for ketamine/valium.

     • Tip: Reconstitute with butorphanol (2.5 mls of 10 mg/ml) and dexmedetomidine (2.5 mls) & use IM in cats.

4. Mask Induction or box induction Induction to anesthesia with inhalant anesthetic drugs alone ('masking' or 'boxing') should be avoided in all but the direst circumstances or for patients that need 'sedation' from inhalants rather than general anesthesia. Here are the reasons that induction to anesthesia with inhalant anesthetic drugs alone should not be your standard induction protocol for cats:

     a) Masking / boxing down is dangerous to the patient

     • Although inhalant anesthetic drugs are fairly safe, they do cause dose-dependent depression of the CNS, cardiovascular and respiratory systems. Thus, a LOW dose of inhalant gas is safe, but the HIGH dose required to induce a patient to anesthesia when no concurrent drugs are administered is technically an overdose and is NOT safe.

     • The excitement that the patient goes through with the resultant increase in the release of catecholamines and all of the physiological changes that occur secondary to catecholamine release (eg, tachycardia, hypertension, hyperventilation, etc...) is very dangerous and can result in complications in even healthy patients. These complications can be severe in compromised patients and mortality from cardiac or pulmonary arrest has occurred.

     • "Paradoxically, mask or box induction is frequently reserved for sick and compromised animals, the group that is least likely to tolerate the high concentrations of inhalant anesthetic required to induce anesthesia." Pete Hellyer DVM, MS, DACVA, Professor, College of Veterinary Medicine, Colorado State University

     • The release of catecholamines also delays induction to anesthesia and results in more drugs being administered to the patient before induction to anesthesia is achieved. By the time the patient is actually asleep, it may be so deeply anesthetized that it is extremely deep.

     • The maintenance period of anesthesia is also more dangerous if inhalant anesthetic agents are the only drugs utilized. The patient has no basal level of analgesia and / or sedation to help blunt responses to surgical stimulus and high concentrations of inhalant drugs are required to keep the patient asleep. Inducing and maintaining patients with inhalants alone adds to the risk of anesthetic fatalities (Brodbelt 2009). b) Masking / boxing is dangerous to the staff – no matter how tight the mask fits or how careful you are with the induction chamber, anesthetic gas will contaminate the environment and the staff will be exposed to inhalant gas. Although anesthetic gases are very safe, chronic exposure has been shown to lead to headaches and irritability and may lead to more serious health issues.

C. Maintenance Drugs

Inhalant anesthesia is generally the safest and most effective way to maintain anesthesia that will last 30 minutes or more. However, inhalant anesthetic agents should never be used as the sole anesthetic agent since this group of drugs causes significant DOSE-DEPENDENT hypotension, hypothermia, and hypoventilation. Our goal should always be to keep the vaporizer at the lowest possible setting.

     1. Isoflurane - moderate cardiovascular and respiratory depression, small % metabolized

     2. Sevoflurane - similar to isoflurane but faster induction, recovery and change of anesthetic depth

     3. Desflurane - similar to sevoflurane, very expensive vaporizer

     • Advantages of inhalants: easy to administer, relatively inexpensive, are eliminated with minimal metabolism (except for halothane), require oxygen for delivery, generally require intubation for delivery.

     • Disadvantages of inhalants: contribute significantly to hypoventilation, hypotension and hypothermia.

D. Analgesic Drugs Maintenance of anesthesia is much easier and safer if analgesia is provided prior to the painful stimulus. Most anesthetic agents, including the anesthetic gases, block the brain's response to pain but don't actually block pain. If the pain is severe enough, the brain can still respond and make the animal appear to be inadequately anesthetized. The result is that the vaporizer is turned up and the brain ceases to respond, but the patient is now too deeply anesthetized and can be at a very dangerous physiologic plane. A more appropriate response would be to block the pain and maintain anesthesia at a light, safe depth.

     1. Boluses of opioids or alpha-2 agonists

     2. Local anesthetic drugs

     • Advantages: Inexpensive, easy to administer, very effective, decrease response to painful stimulus AND decrease overall sensation of pain even after the block is no longer effective

     • Disadvantages: Relatively short duration of action compared to duration of pain

     3. Constant rate infusions

     • Advantages: Easy, cheap, a variety of drugs to choose from

     • Disadvantages: Patient must be on IV fluids

III. Monitoring and Support

Safe anesthesia is absolutely impossible without appropriate monitoring and support for the anesthetized patient. Anesthetic drugs cause functional changes in all organ systems but the changes in the CNS, cardiovascular and respiratory systems are the most immediately life-threatening. Thus, monitoring – and support - is focused on these 3 systems.

IV. Recovery drugs and support

Unfortunately, most unexpected anesthetic deaths occur in recovery. Patient monitoring and support must continue until the patient is conscious and pain-free (or at least comfortable). Young, healthy patients recover quickly, limiting the monitoring & support time that is required post-operatively. Aged and compromised patients, on the other hand, may recover very slowly and a longer monitoring & support period is required. Support should include IV fluids, active warming, analgesic drugs, oxygen, etc... Patients on the other end of the spectrum, i.e. those having a rough recovery, must also be addressed. Additional opioids or micro-doses of alpha-2 agonists are often needed to smooth recovery in these patients.

V: Sample Anesthesia Protocols

Healthy Cats (and Fractious Cats) Premed/Induction: 0.1-0.2 MLS/PER 4.5 KG BODY WT of each of the following: 0.1-0.2 mls medetomidine or dexmedetomidine per +0.1-0.2 mls buprenorphine or butorphanol (10 mg/ml) 0.1-0.2 mls ketamine Combine all 3 in same syringe and administer IM Cat will be anesthetized in 10 minutes

Maintenance: Inhalant anesthetic drugs may or may not be necessary depending on the patient and on the surgery. Dose inhalants 'to effect'. Any appropriate CRIs, local blocks, etc...

Post-Op: Opioids or dexmedetomidine, as needed. NSAIDs? CAN reverse alpha-2 agonist with 0.1-0.2 mls atipamezole but this will also reverse the analgesia. Transmucosal buprenorphine 0.01-0.02 mg/kg BID Geriatric or compromised cats Premed: 0.02-0.03 mg/kg buprenorphine (or 0.2 mg/kg butorphanol followed by 0.02 mg/kg buprenorphine) IM +/- 0.2 mg/kg midazolam IM Induction: 0.1-0.2 mg/kg diazepam IV +2-4 mg/kg propofol Maintenance: Sevoflurane or isoflurane, to effect Any appropriate CRIs, local blocks, etc... Post-op: Transmucosal buprenorphine 0.01-0.02 mg/kg BID