Anesthetic risk in cats is graded before the cat is clipped, not on the table. The scale runs ASA 1 to 5, and the grade sets the tests, the fast, and whether the cat goes home the same day. A healthy 3-year-old at 4.3 kg is ASA 1 and needs 4 hours without food. A 12-year-old with creatinine at twice the top of the range is ASA 3, needs blood pressure and a chest film, and may fast 6 hours. Setting the grade costs about 15 minutes of exam time and 20 minutes of lab time.
ASA 1 to 5 in plain terms
ASA 1 is a normal cat: 4.3 kg, temperature 38.4 C, heart rate 180, no murmur, nothing worse than mild tartar. ASA 2 is mild disease that does not limit activity: controlled hyperthyroidism on 2.5 mg twice daily, a grade 2/6 murmur, or 15 percent over ideal weight. ASA 3 is moderate disease that limits activity but is not an immediate threat: chronic kidney disease (CKD) IRIS stage 2 to 3, diabetes on 2 units of insulin twice a day, or hypertrophic cardiomyopathy with a 6 mm wall on echo. ASA 4 is severe disease that is a constant threat: congestive heart failure on 3 drugs, 15 percent weight loss over 8 weeks, PCV 16 percent. ASA 5 is moribund, meaning the cat will not survive 24 hours without the procedure.
Three findings push a cat up one grade on their own: PCV under 20 percent, systolic pressure over 160 mmHg, or a morning temperature under 37.5 C. Any one of them turns a 15-minute spay into a 45-minute procedure with a nurse on that cat alone.
Five checks on every cat
Five items are recorded before drugs are drawn: weight to 0.1 kg, rectal temperature, 60 seconds of heart and lung auscultation, skin tent under 2 seconds, and a blood panel. The panel is 6 values — PCV, total protein, BUN, creatinine, glucose, ALT — run on 0.5 mL of blood in 10 to 20 minutes.
Each number changes something. A 0.5 kg error in weight moves the dose by 10 to 12 percent. A temperature of 36.8 C means another 0.5 to 1.0 C will go across 45 minutes of anesthesia, so a forced-air warmer goes on before induction. A BUN of 40 mg/dL or a creatinine of 2.0 mg/dL moves fluid from 3 to 5 mL/kg/h and adds a 12-hour pre-op drip. Glucose under 60 mg/dL means 2.5 percent dextrose in the bag before the first drug, not in recovery.
Past age 7: add blood pressure and a chest film
From 7 years, 2 items join the list. Systolic pressure is taken 3 times, 30 seconds apart, with a cuff about 40 percent of limb circumference, and the lowest reading is kept. Two readings over 160 mmHg means hypertension: surgery is usually pushed back 7 to 14 days while amlodipine at 0.625 mg once daily is started and rechecked. The second is a 2-view chest film. Heart size is scored as a vertebral heart score, and anything over 9.3 gets an echo first.
The film also catches what a stethoscope misses, because mild bronchial patterns and small masses can be silent over 60 seconds of listening. A 7-year-old cat is near 44 in human years and a 12-year-old near 64, which is why the same 30-minute dental shifts from routine to a graded plan between those two ages. The arithmetic behind those conversions sits in the cat age calculator.
Fasting: 4 hours of food, 30 minutes of water
Cats are not small dogs. A healthy cat empties its stomach in 90 to 120 minutes on a normal meal, so 4 hours without food gives an empty stomach at induction, and water pulled at 30 minutes before induction limits what can come back up. The 12-hour fast still taught in some clinics adds 3 problems: 3 to 5 percent dehydration, a 10 to 15 percent drop in circulating volume, and a cat stressed enough to arrive with a heart rate 20 beats above its own baseline.
Three exceptions. Kittens under 16 weeks fast 2 hours, because glucose can fall inside 6 to 8 hours. A diabetic cat gets half the morning insulin dose and food 4 hours before induction, with glucose checked at 0, 2 and 4 hours. A cat with reflux fasts 6 hours and gets maropitant at 1 mg/kg an hour before premedication.
Six numbers written every 5 minutes
| Variable | Target | Act at | Fix within (min) | First fix |
|---|---|---|---|---|
| Heart rate | 140-220 /min | under 120 or over 240 | 2 | Check depth, then atropine 0.02-0.04 mg/kg |
| Respiratory rate | 10-30 /min | under 8 or over 40 | 1 | Ventilate at 10 mL/kg, 8-12 breaths |
| SpO2 | 95-100 % | under 92 % | 1 | Reposition head, flush circuit, FiO2 to 100 % |
| End-tidal CO2 | 35-45 mmHg | over 50 or under 25 | 2 | Lower minute volume, check tube depth |
| Temperature | 37.5-39.0 C | under 36.5 C | 5 | Forced-air warmer, fluids at 38 C |
| Systolic pressure | 90-140 mmHg | under 80 mmHg | 3 | Fluid bolus 5 mL/kg, reduce gas 20 % |
Fluids run at 3 to 5 mL/kg/h through a 24-gauge catheter; a cat under 2 kg goes to 10 mL/kg/h. Each of these 6 lines has a printed range, because one missing value in a 45-minute procedure is how a cat ends up with a 2-day stay.
Recovery is the risky 90 minutes
Most anesthetic deaths cluster in the first 90 minutes after the gas goes off, not in the 30 minutes of surgery. The tube comes out 2 to 4 minutes after the swallow reflex returns. Temperature is checked every 10 minutes until it passes 37.5 C, and below 36.5 C the cat goes into a forced-air warmer with fluids at 38 C. A cat that cannot lift its head by 60 minutes, or whose heart rate is under 120 at that point, stays in the clinic overnight.
Pain control belongs in this window too. A cat that walks at 40 minutes but hisses when the abdomen is touched at 90 needs its next analgesic now, not at the 6-hour mark; a cat that also scoots or licks the tail base may have a second problem worth checking (see anal sac issues in cats).
When to call the same night
Call the clinic if the cat refuses food for 12 hours, breathes over 40 times a minute at rest, runs a temperature over 39.5 C, vomits more than 2 times in 6 hours, or has an incision line open by 1 cm or more. These 5 signs are not normal recovery after a routine 30-minute procedure. Each has a 4 to 12 hour window where treatment is simple, and a 24-hour window where it is not.
Before the appointment
- Book the panel 3 to 7 days ahead, so a creatinine of 2.4 mg/dL is handled before surgery day rather than on it.
- Stop food at the 4-hour mark and water at 30 minutes, and write both times on the carrier.
- Ask for the ASA grade in writing; a grade 3 cat should have 1 nurse for that cat alone.
- Ask who watches the monitor: 1 person per cat, never 1 person for 3 tables.
- Keep the cat indoors 24 hours after discharge and look at the incision 2 times a day for 5 days.
Terms mentioned in this article
Frequently asked
Does a higher ASA grade mean my cat cannot be anesthetized?
No. The grade sets the plan, not a ban. An ASA 3 cat can be anesthetized safely with a pre-op drip, blood pressure support, and 1 nurse assigned to that cat, and most clinics will still proceed for a dental or a mass removal. What changes is the workup, the monitoring, and the odds of an overnight stay. Ask for the specific numbers behind the grade, such as creatinine 2.4 mg/dL or systolic pressure 170 mmHg, and what the clinic will do differently because of them.
Why only 4 hours of fasting when my dog fasted 12 hours?
A cat stomach empties in about 90 to 120 minutes, so 4 hours without food leaves it empty at induction. Longer fasts add dehydration of 3 to 5 percent and a drop in circulating volume of 10 to 15 percent without lowering vomit risk further. Kittens under 16 weeks fast only 2 hours to protect blood glucose, and diabetic cats get half the morning insulin dose with food 4 hours before the procedure.
What should I watch for the night after anesthesia?
Watch 4 things for 12 hours: breathing rate at rest, temperature if you have a pet thermometer, food intake, and the incision. Call the clinic for a resting rate over 40 per minute, a temperature over 39.5 C or under 37.5 C, no food for 12 hours, more than 2 vomits in 6 hours, or an incision open by 1 cm. A cat that is quiet but warm, breathing 24 times a minute, and eating by 8 hours is on the normal track.