13.2 Anesthetic Plan Development and Pre-anesthetic Assessment and Care
Key Takeaways
- The ASA Physical Status Classification (I normal healthy through V moribund, with E for emergency) standardizes patient risk communication and drives anesthetic plan intensity
- Fasting 4–6 hours in dogs and 6–8 hours in cats reduces regurgitation and aspiration risk while preventing the hypoglycemia that occurs with prolonged fasting in small or young patients
- A pre-anesthetic IV catheter is mandatory before induction so emergency drugs and fluids can be administered without delay
- Minimum pre-anesthetic bloodwork (PCV/TS, BUN/creatinine, ALT/ALP, glucose) screens for occult disease that would change drug choice or cancel anesthesia
- Balanced, multimodal anesthesia combines lower doses of drugs with different mechanisms to achieve analgesia, sedation, and unconsciousness while reducing each drug's side effects
A safe anesthetic event starts long before the vaporizer is turned on. The plan is built from the patient up: assess the patient, decide which drugs and equipment fit, prepare the airway and catheter, and premedicate so induction and recovery are smooth.
ASA Physical Status Classification
The American Society of Anesthesiologists (ASA) Physical Status Classification is adapted for veterinary patients and provides a common language for risk. Every patient is assigned a status at the pre-anesthetic exam:
| ASA PS | Definition | Example |
|---|---|---|
| I | Normal, healthy patient | Healthy dog for elective spay |
| II | Mild systemic disease, no functional limitation | Early mitral valve disease with no clinical signs; skin mass removal |
| III | Severe systemic disease, functional limitation | Pneumonia; compensated heart failure; anemia with mild lethargy |
| IV | Severe systemic disease that is a constant threat to life | Uncompensated heart failure; severe trauma; septic peritonitis |
| V | Moribund, not expected to survive without the procedure | Massive hemorrhage, catastrophic trauma |
| E suffix | Emergency status added to any of I–V (e.g., IIIE) | Gastric dilatation-volvulus; dystocia |
ASA status drives plan intensity. An ASA I dog for a routine castration may need only a light premedication and brief anesthesia. An ASA IV cat in septic peritonitis needs aggressive stabilization, minimal cardiovascular-depressant drugs, IV fluid support, invasive monitoring, and round-the-clock nursing.
Fasting, IV catheter, and the pre-anesthetic exam
Fasting reduces the volume of gastric contents and the chance of regurgitation and aspiration during induction and recovery. Traditional intervals — 4–6 hours in dogs and 6–8 hours in cats — balance aspiration risk against hypoglycemia, which is a real danger in small, neonatal, diabetic, or debilitated patients. Rabbits, guinea pigs, and other hindgut fermenters should generally not be fasted because cessation of eating rapidly causes GI stasis.
Every anesthetic patient must have a functional IV catheter placed before induction. The cephalic vein is standard in dogs and cats; the saphenous or jugular may be used in small or exotic patients. The catheter is your lifeline: emergency drugs (atropine, ephedrine, reversal agents), crystalloid fluids, and induction agents all run through it. Trying to place a catheter during a coding patient is a recipe for disaster.
The pre-anesthetic physical examination is the foundation of the plan. Record temperature, pulse, respiration (TPR), mucous membrane color, capillary refill time, hydration, auscultation of heart and lungs, and pain score. Any abnormality changes the plan — a murmur prompts cardiac workup; crackles prompt thoracic radiographs; dehydration prompts preoperative fluids.
Minimum bloodwork database
Pre-anesthetic bloodwork screens for occult disease that the physical exam may miss. The minimum database for a healthy adult patient includes:
- Packed cell volume / total solids (PCV/TS) — anemia, dehydration, hypoproteinemia
- BUN and creatinine — renal function
- ALT and ALP — hepatic function and cholestasis
- Glucose — hypoglycemia (small or young patients), diabetes mellitus
For older, debilitated, or ASA III+ patients, a complete CBC and full chemistry panel, plus electrolytes, total protein, albumin, and sometimes coagulation testing and urinalysis, are indicated. Cats over 7 years should have T4 checked for occult hyperthyroidism, which increases anesthetic risk (tachycardia, hypertension, arrhythmias).
Body weight is the dosing denominator for nearly every anesthetic drug. Use a calibrated scale, not a visual estimate — a 5 kg error on a 10 kg dog is a 50% dose error. Drugs are dosed in mg/kg (sometimes mg/m² for chemotherapy), so accuracy is non-negotiable.
Premedication: what it does and why it matters
Premedication is given 15–30 minutes before induction (longer for acepromazine or transmucosal buprenorphine in cats). A well-chosen premedication:
- Reduces stress and provides anxiolysis and mild analgesia
- Smooths the transition through Stage II on induction and recovery
- Reduces the dose of induction agent and inhalant needed (and therefore their side effects)
- Produces a calmer, safer recovery
- Allows lower-stress IV catheter placement
Typical combinations pair a sedative/tranquilizer (acepromazine, dexmedetomidine, or a benzodiazepine) with an opioid (morphine, hydromorphone, methadone, buprenorphine, fentanyl). The opioid supplies analgesia; the sedative supplies anxiolysis and muscle relaxation. Specific drugs are detailed in section 13.3.
Building the plan: balanced and multimodal anesthesia
Modern veterinary anesthesia relies on two related principles:
- Balanced anesthesia uses several drugs together, each at a lower dose than would be required alone, so the side effects of any single agent are minimized while the desired effects add up.
- Multimodal anesthesia targets different points in the pain pathway — for example, an opioid (opioid receptor), an NSAID (cyclo-oxygenase inhibition), and a local anesthetic block (sodium channel blockade) together provide superior analgesia at lower doses than any single class.
Agent selection depends on the procedure (a 15-minute cat castration differs from a 3-hour orthopedic), the species (cats are sensitive to alpha-2 bradycardia and propofol Heinz bodies), and the patient's health status (etomidate for cardiac disease, avoid acepromazine in boxers, avoid alpha-2 in cardiovascular collapse). The written anesthetic plan should specify premedication, induction agent, maintenance agent, fluid rate, monitoring interval, analgesic plan, and emergency drug doses calculated in advance. A plan that is not written down is a plan you cannot troubleshoot.
A 12-year-old cat presents for dental extraction. On auscultation, a grade III/VI systolic murmur is noted with a heart rate of 200 bpm, and the owner reports recent weight loss and increased activity. Which ASA physical status assignment is most appropriate, and what additional pre-anesthetic testing is most important?
Which of the following best illustrates the principle of multimodal anesthesia?
A 4-kg Yorkshire Terrier puppy (10 weeks old) is presented for elective ovariohysterectomy. The owner reports the puppy ate 2 hours ago. What is the most appropriate pre-anesthetic fasting decision?