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.
I. Anesthesia / Analgesia Concepts
a) 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.
b) 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:
• Stress in the perioperative period is extremely dangerous physiologically. (Think about capture deaths that occur in wild animals.)
• Tranquilizers allow reduction in the dose of both induction and maintenance drugs, thus increasing the distance between 'effective dose' and 'dangerous or toxic dose' of drugs.
c) 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 affects the quality of life of the patient.
d) 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 drugs, anesthetizing a patient with a single agent (eg, no premedicant, inhalant induction, inhalant maintenance) is no longer appropriate. Nor is it safe.
II. Anesthetic / Analgesic Drugs
A. Sedatives / tranquilizers
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 metabolize 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 0.01-0.03 for dogs and 0.03-0.05 for cats. Can be administered IM, SQ or IV.
2. Alpha-2 adrenergic agonists (Domitor®, Dexdomitor®)
• Advantages: provide analgesia, effects are reversible, can provide anything from 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 (only dexmedetomidine is labeled for cats).
o Combined with an opioid for improved sedation and analgesia: Generally ¼-½ the label dose of medetomidine or dexmedetomidine plus a standard dose of the opioid of choice.
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. The dose is very small – generally about 1/8 to 1/10 the label dose (Dexdomitor is labeled for preanesthetic use in dogs so use the label dose – or lower.)
o Combined with an opioid and ketamine for brief periods of anesthesia.This is especially useful in cats. Don't forget to add more analgesia – use buprenorphine or butorphanol as part of the combination.
• TIP: Use alpha-2 agonists to treat patients recovering from anesthesia with excitement or pain.
o My favorite use! Patients having a rough recovery generally need both sedation AND analgesia. The dose is the same as the premed dose or even a little lower.
• Once the procedure is complete, the alpha-2 agonist can be reversed with atipamezole
o The volume of atipamezole should be the same as the volume of medetomidine or dexmedetomidine that was administered.
o Both sedation and analgesia will be reversed and 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. 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.
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).
• Morphine, hydromorphone, methadone, oxymorphone, fentanyl, butorphanol, buprenorphine
• Advantages: provide moderate to profound analgesia, are reversible
• Disadvantages: may not provide enough sedation when used alone in young, healthy or excited patients
• Full agonists (morphine, hydromorphone, methadone, fentanyl)
o Most potent class of analgesic drugs
o Should be considered any time that pain is moderate to severe
o Excellent premedicants since they provide analgesia and sedation in dogs; combine with a sedative in cats
o Time to onset - < 5mins (< 1 min when administered IV)
o Duration of action 2-6 hours (oxymorphone closest to 6 hrs; approx 4 hours for morphine and hydromorphone)
o Uses: IV (morphine not generally bolused IV), IM, SQ, transdermal patch (fentanyl), constant rate infusion (CRI), epidural space, intra-articular space, etc...
• Sedation - good effect pre-op and post-op in dogs
• Vomiting - good effect when used pre-op, empties stomach
• Respiratory depression? Not a big issue like it is in human beings; almost directly related to degree of sedation.
• Minor to nonexistent cardiovascular depression.
o Recommendation: In dogs, use a full agonist opioid as your standard opioid, use other opioids in the rare instance when a full opioid isn't the best choice. Use full opioid agonists with sedatives in cats.
• Partial agonists (buprenorphine), agonist-antagonists (butorphanol)
o Not as potent as the full agonist class
o Don't confuse binding potency with analgesic potency
o Should be considered when pain is mild to moderate
o 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.
o Time to onset - < 5 mins (butorphanol); 10-20 mins (buprenorphine)
o Duration of action - 45-90 mins (butorphanol); 6-12 hours (buprenorphine)
o Uses: IV, IM, SQ, transmucosal (buprenorphine)
• Similar to the effects of full agonists but not as pronounced
• Butorphanol provides moderate sedation, buprenorphine provides mild to no sedation.
o 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).
B. Induction drugs
• 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 use alone - must be combined with a sedative such as valium, medetomidine or dexmedetomidine.
• Dosage: 5-20 mg/kg IM (most useful in cats –volume too large for most dogs) or 2-10 mg/kg IV. Best combined with sedative/analgesic like dexmedetomidine or with a muscle relaxant like valium.
• Tip: For combination with valium, draw up ½ ketamine and ½ 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.
• 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.
• 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: Dogs metabolize the zolazepam portion of telazol prior to metabolizing the tiletamine portion. Thus, dogs have a rough recovery if no additional sedatives are added to the protocol.
• Advantages: no cardiovascular changes
• Disadvantages: expensive, poor muscle relaxation, vocalization
• Dosage: 1 mg/kg IV
• Tip: Make sure patient has adequate sedation prior to etomidate injection – this will decrease the likelihood of muscle rigidity and vocalization. As with propofol, administer a benzodiazepine just prior to administering the etomidate.
5. 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. Here are the reasons that induction to anesthesia with inhalant anesthetic drugs alone is not recommended for most patients:
Masking / boxing down is dangerous to the patient
The dose of the drug is extremely high and side effects are dose-dependent; excitement causes increased need for drugs; excitement causes negative effects
Masking/boxing down is dangerous to the staff
Chronic exposure to anesthetic gases can cause health problems
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 drugs should never be used as the sole anesthetic 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. We often need to add extra analgesia to allow a decrease in the vaporizer setting.
• Advantages of inhalants: easy to administer, relatively inexpensive, are eliminated with minimal metabolism, require oxygen for delivery, generally require intubation for delivery.
• Disadvantages of inhalants: contribute significantly to hypoventilation, hypotension and hypothermia. MONITOR, MONITOR, MONITOR.
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
D. Other analgesic drugs
1. Local anesthetic drugs
Advantages: CONTROL INFLAMMATION! Easy to use, not controlled
Disadvantages: Not appropriate for patients with renal, hepatic or GI disease
2. Non-steroidal anti-inflammatory drugs
Advantages: Easy to use, effective, inexpensive; Disadvantages: Really none