Wednesday, June 6, 2012

1016. Just-enough dosage of injectable anaesthesia for cat spay

What is the most effective optimal dosage of injectable anaesthesia for a cat spay without the use of isoflurane anaesthesia?  This answer is obtained from observations and the review of record keeping if the vet is interested. If not, the vet just has to give the cat isoflurane gas by mask to continue the surgery if the cat moves.

Obviously, an employee or associate vet has no need to account for the bottom-line of the practice and is not interested in reducing costs. So there is no motivation to conserve resources and use just-sufficient anaesthestics as they don't bear the expenses and responsibility for the economic health of the practice. If the practice closes down, just go to another practice to work!

Just give the cat isoflurane gas by mask to continue the spay if the cat is not sufficiently anaesthesized by xylazine and ketamine. So, more isoflurane and oxygen needs to be ordered if the vet has no interest in practical research in wanting to know what is best for the cat and for the practice economics.

Yesterday, June 6, 2012, a shy 3kg black and white cat from Bedok came in. I used this case to share my experience and demonstrate to Dr Daniel how I would use an effective optimal dosage of injectable anaesthesia WITHOUT the need of isoflurane gas top up. Toa Payoh Vets has the isoflurane gas facility and so, there is no need to catherise the cat or dog to top up, if the dosage of injectable anaesthesia is insufficient. However, catherisation means a waste of resource, more time spent in catherisation of the cephalic vein and in topping up to get surgical anaesthesia. This can make a spay surgery twice as long.

I prefer surgery to be simple and fast by giving the effective optimal dosage of xylazine and ketamine for a cat spay. I can get the whole spay done from first incision to stitching in 10-15 minutes if there is no need to top up.

As a guideline, a cat at 3 kg needs 0.15 ml xylazine and 0.6 ml ketamine IM to be sufficiently anaesthesized without the need of isoflurane gas or top up. Dr Daniel did not think this was possible and so the isoflurane gas was switched on as standby.

However, this amount lasted more than 30 minutes and he could see that the formula was sufficient. After the IM injection of 0.15 ml xylazine and 0.6 ml ketamine, there was a wait of 10 minutes. During this time, the cat's belly was shaved.

Obviously, if the vet takes a bit longer, isoflurane gas will be needed.


1. Don't snip off the SC fat if possible. Just undermine and see the linea alba.

2. The start of the incision is around 1.5 cm from the umbilical scar.

3. This cat had enlarged congested ovaries with follicles and uterus of over 8 mm in diameter although she was never "mated". So, it was difficult to hook out the womb. A longer incision was needed and more surgery time had to be spent. I could not believe when I saw the uterine horns being as large as 8 m min diameter and over 12 cm long. The ovarian blood vessels were enlarged and engorged. As if the cat was pregnant.

Could this be a case of cystic ovaries, pyometra or early pregnancy? However, the lady owner said: "My cat would never leave the HDB flat. Whenever she sees another cat, she would run away. It is not possible that she was mated."

"Is there a male cat in the apartment?" I asked.

"Yes, he was neutered some time ago at another vet practice."

"Did he try to mate with this caterwauling female 2 weeks ago? " I asked.

"Yes, the male cat was humping her."

So, this could be a case of false pregnancy with the signs of pregnancy in the uterus being developed. There was no foetal lumps. I did not cut into the uterine bodies to check for foetuses. The uterus was just swollen and thicker by 100x normal for a non-pregnant young cat of around one year old. It could be a case of early pyometra.


I put the cat carrier on the consultation table. A wire crate was ready. As soon as I opened the carrier door and tilted the carrier, the cat shot out like a rocket and jumped onto the floor to a corner of the back table. I expected the cat. Dr Daniel bent on his knees and talked to the cat. He got her back into the crate. Now, what to do?

"Put some telephone books inside the plastic crate to corner him and I will inject the anaesthestic," I said. But there were no phone books. So, he put a large dog carrier to corner the cat. "Ready for injection," he asked me. The cat had crawled to the top of the plastic crate and was moving.

"Can you inject him now?" Dr Daniel asked.

"Based on my experience, the cat will move when injected as she is not tightly cornered. Only half dose may be given."

"That should be OK," Dr Daniel replied.

"Sometimes the syringe needle may bend when the injection is given as the cat springs away," I said. In practice, it is so much different from in theory from the lectures.

"In any case, a full dose IM must be given to get the best optimal anaesthesia," I said.

As I tried to get the cat cornered, she climbed up the plastic crate top towards Dr Daniel and away from me.

"There is a gap in the side of wire crate which could not be closed properly after he had put in the plastic crate."

It was only a gap of 8 mm but the small cat's head went out. In an instant, she sprang out and jumped onto the floor back to the corner of the floor as before.

I was quite angry at this "waste of time". The side of the wire crate was not secured properly first after the plastic crate was put in. The cat was dropped into the wire crate from the top as there was a door. So, the cat escaped and I cursed. I hate wasting time which is much more precious to me at my age.

So, I left the consultation room for Dr Daniel to coax the cat back and put her into the plastic crate. Why not the wire crate? I was surprised but since now there was a new situation, I had to decide what to do, being the senior.

"If your hand is strong enough to grip the scruff of the cat and you do not mind being scratched," hold the cat up with your hand and I will inject the back muscle" I said.

Dr Daniel did that. He has a strong grip. The cat did not move at all when I inject. That is the norm as a strong grip on the scruff makes a cat not feeling any pain in injection, in my experience. A weak grip will result in the cat feeling the IM injection and clawing.

Then Dr Daniel put the cat inside the wire crate and from then on, he could see that a full dose IM according to my formula was really effective and no isoflurane gas was ever needed to spay this cat. The duration of analgesia was at least 30 minutes and that was more than enough time to spay a cat. 

As  mentor, many optimal and safe anaesthetic lessons are taught to the younger vets by "seeing is believing"

as each young vet has his or her own ideas of anaesthesia, basically from what the vet professors have had taught them and used in the Vet School. When they come out to work in the industry, they have to adapt to different anaesthetic and formulas which their professors had never used. This is because there is more than one combination to safely anaesthesized the cat and the vet practice has its own experienced method usually different from the Vet School which could teach and advise more injections (pre-med with ACP, sedate with another drug, atropine etc) as according to the theory.


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