Chapter 2 Practical Methods of Anesthesia
Anesthesia is an integral part of the practice of companion animal medicine. In addition to surgical applications, some form of anesthesia may be required for a wide variety of procedures, such as radiography, endoscopy, cerebrospinal fluid collection, and bone marrow aspiration.
GENERAL PRINCIPLES
Preoperative Assessment (Table 2-1)
• A history of vomiting or a recent meal is an indication for postponing surgery or using techniques that produce rapid induction, allowing rapid endotracheal intubation that minimizes the potential for aspiration of gastric contents.
• Perform a physical examination with emphasis on the cardiovascular and respiratory systems (Table 2-2 provides normal values).
• Evaluate traumatized patients more extensively because of the potential for blood loss, cardiac arrhythmias (ventricular tachycardia), and thoracic trauma (pneumothorax).
• Evaluate abnormalities discovered after auscultation, percussion, palpation, and examination of mucous membranes with ancillary tests, such as thoracic radiography and electrocardiography.
• Weigh the animal. Estimate the lean body weight if the animal is obese to more accurately calculate correct anesthetic drug doses.
• Determine packed cell volume and total plasma protein to establish a baseline for reference if hemorrhage occurs. Check renal function with blood urea nitrogen (BUN) or creatinine concentrations in older (>7 years) animals. Increases in BUN or creatinine values may dictate a more careful perianesthetic fluid management to prevent further renal dysfunction (see Table 2-2).
Table 2-1 PREANESTHETIC CHECKLIST
History and Physical Examination
Pulse rate, rhythm, and strength
Hydration, mucous membrane color, and capillary refill time
Laboratory Data
Blood urea nitrogen, creatinine, or serum dipstick analysis (Azostick) if less than 7 years
Endotracheal Intubation
A patent airway is essential to any anesthetic protocol.
• Clean, thoroughly rinse, and dry endotracheal tubes between uses. Sterilization of endotracheal tubes between uses is not necessary on a routine basis. Use chemical sterilization or discard the tube if a known pathogen is present. Glutaraldehyde is a safe disinfectant if the tubes are thoroughly rinsed following sterilization.
Table 2-3 APPROXIMATE ENDOTRACHEAL TUBE SIZES
| Weight (kg) | Cuffed Tube Diameter (mm) | |
|---|---|---|
| Dogs | 3–7 | 3.0–5.0 |
| 7–15 | 5.0–7.5 | |
| 15–30 | 7.5–9.5 | |
| >30 | 9.5–12.0 | |
| Cats | 2.5–4.0 | |
PRODUCING A TRACTABLE ANIMAL
The doses of drugs alone and in combination are listed in Table 2-4 (dogs) and Table 2-5 (cats).
Table 2-4 ANESTHETIC DRUGS AND DOSES IN DOGS
| Drug | Intravenous Dose (mg/kg) | Intramuscular or Subcutaneous Dose (mg/kg) |
|---|---|---|
| Anticholinergic | ||
| Atropine | 0.02–0.04 | 0.02–0.04 |
| Glycopyrrolate | 0.005–0.010 | 0.005–0.010 |
| Tranquilizer/Sedative | ||
| Acepromazine | 0.05–0.20 | 0.1–0.3 |
| Xylazine | 0.3–0.8 | 0.5–1.5 |
| Medetomidine | 0.007–0.020 | 0.01–0.04 |
| Diazepam | 0.10–0.25 | 0.10–0.25 |
| Midazolam | 0.05–0.20 | 0.1–0.2 |
| Analgesic | ||
| Morphine | NR* | 0.2–0.5 |
| Oxymorphone | 0.05–0.10 | 0.1–0.3 |
| Fentanyl | 0.002–0.005 | 0.004–0.008 |
| Meperidine | 0.4–2.0 | 1.0–4.0 |
| Butorphanol | 0.1–0.2 | 0.1–0.4 |
| Nalbuphine | 0.5–2.0 | 0.5–2.0 |
| Buprenorphine | 0.005 | 0.005 |
| Hydromorphone | 0.05–0.2 | 0.1–0.4 |
| Anesthetic | ||
| Tiletamine/zolazepam (Telazol) | 0.5–4.0 | 4–10 |
| Thiopental | 6–10 | NR |
| Etomidate | 1–4 | NR |
| Propofol | 2–6 | NR |
| Combination | ||
| Acepromazine/oxymorphone | 0.05–0.10/0.01–0.02 | 0.1–0.2/0.1–0.2 |
| Acepromazine/butorphanol | 0.05–0.10/0.1–0.2 | 0.1–0.2/0.1–0.2 |
| Ketamine/acepromazine | 2–5/0.05–0.10 | 5–10/0.1–0.2 |
| Ketamine/xylazine | 1–5/0.1–0.8 | 5–10/0.3–1.5 |
| Ketamine/diazepam (50:50) | 1 ml/10 kg | NR |
Table 2-5 ANESTHETIC DRUGS AND DOSES IN CATS
| Drug | Intravenous Dose (mg/kg) | Intramuscular or Subcutaneous Dose (mg/kg) |
|---|---|---|
| Anticholinergic | ||
| Atropine | 0.02–0.04 | 0.02–0.04 |
| Glycopyrrolate | 0.005–0.010 | 0.005–0.010 |
| Tranquilizer/Sedative | ||
| Acepromazine | 0.05–0.20 | 0.1–0.3 |
| Xylazine | 0.4–1.0 | 0.8–1.8 |
| Medetomidine | 0.01–0.03 | 0.03–0.08 |
| Diazepam | 0.10–0.25 | 0.10–0.25 |
| Midazolam | 0.05–0.20 | 0.1–0.2 |
| Analgesic* | ||
| Oxymorphone | 0.01–0.04 | 0.05–0.10 |
| Butorphanol | 0.05–0.20 | 0.1–0.3 |
| Nalbuphine | 0.5–1.5 | 0.5–1.5 |
| Buprenorphine | 0.005 | 0.005 |
| Hydromorphone | 0.05–0.1 | 0.1–0.3 |
| Anesthetics | ||
| Ketamine | 4–10 | 10–20 |
| Tiletamine/zolazepam (Telazol) | 0.5–4.0 | 4–12 |
| Thiopental | 6–10 | NR† |
| Etomidate | 1.0–4.0 | NR |
| Propofol | 2–6 | NR |
| Combination | ||
| Acepromazine/oxymorphone | 0.05–0.07/0.01–0.04 | 0.1–0.2/0.05–0.20 |
| Acepromazine/butorphanol | 0.05–0.07/0.07–0.15 | 0.1–0.2/0.1–0.2 |
| Ketamine/acepromazine | 4–8/0.05–0.10 | 7–15/0.1–0.2 |
| Ketamine/xylazine | 4–8/0.1–0.8 | 7–15/0.3–1.5 |
| Ketamine/diazepam (50:50) | 1 ml/10 kg | NR |
* Higher doses can be associated with nervousness and excitement.
Tranquilizers
Acepromazine
• A potent phenothiazine tranquilizer that produces sedation in the dog. It also has antiemetic and antiarrhythmic properties.
• Calms excitable dogs. Aggressive dogs or cats may not become tractable. Combine with opioids or cycloheximides to produce the desired effect (see Tables 2-4 and 2-5).
• Avoid in animals with epilepsy, shock, bleeding disorders (inhibition of platelet function), or liver disease.
Xylazine
• Produces profound cardiopulmonary depression, including bradycardia, first- and second-degree atrioventricular blockade, catecholamine sensitization, and decreased respiratory rate.
• Combine with an anticholinergic (atropine or glycopyrrolate), particularly if large doses are administered.
• Reverse effects with yohimbine, tolazoline, or atipamezole (see “Recovery and Reversal” for doses).
Medetomidine
• An α2-adrenergic agonist that produces sedation with muscle relaxation and analgesia (see Tables 2-4 and 2-5 for doses).
• Similar to but approximately 100 times more potent than xylazine with a longer duration of action.
• Produces dose-dependent cardiopulmonary depression including bradycardia, decreased arterial blood pressure, and slowing of respiratory rate.
• Combine with an anticholinergic (atropine or glycopyrrolate), particularly if large doses are administered.
• Reverse effects with yohimbine, tolazoline, or atipamezole (see “Recovery and Reversal” for doses).
Diazepam and Midazolam
• Benzodiazepine derivatives that produce mild sedation in dogs and cats. Neither is effective in calming an excited patient when used alone. Both are anticonvulsants.
• Use to enhance tractability in depressed patients or in combination with other agents (primarily opioids).
• Administer diazepam IM or slowly IV with caution. The drug is solubilized in a propylene glycol base that can produce bradycardia and hypotension.
• Use in patients with cardiorespiratory compromise or another metabolic disease. Both agents produce minimal cardiopulmonary side effects and provide muscle relaxation.
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