Dosing for general anesthesia or even for standing sedation is tricky. There are species variations, diseases that impact drugs and drugs that impact diseases, change with age, etc. Not to mention our resources all come with ranges. So how do you know where to start? The following are my approaches to different situations. You will develop your own over time!
In general, if I can find the same dose in two separate references, I feel pretty comfortable. I will search Veterinary Clinics of North America or do a google search with edu added to the requirements (see extreme googling). I may also search by someone’s name (eg Hubbell for anesthesia stuff).
Generally I use the higher end of the range for oral antibiotics, figuring some will end up on me or on the floor, not in the animal. For IV, IM or SQ antibiotics, I go higher with the very safe drugs, lower with the unsafe ones. Neonates usually need more drug due to the higher volume of body water. Sick animals may need less frequent administration due to less effective metabolism of the drug. Most drugs are metabolized by the liver or kidneys and excreted by the kidney. If those organs are impaired, the drug will stay around longer. Drugs metabolized by the liver may also impact other drugs through the cytochrome P450 system. Those cases need to be researched more thoroughly.
I tend to go light on sedatives until I have more information on how the animal will respond. Clients may be able to tell you if the horse is a ‘lightweight” or “sucks down drugs”. Assume ruminants are lightweights. Overdosing can make your patient very sleepy. If you are trying for a standing sedation, you really don’t want them super sleepy. It is frustrating to spend all your time trying to get them to stop swaying or to perform your standing procedure with the animal on the ground.
There is nothing worse than an animal that will not hit the right plane of anesthesia but keeps waking up. This is more likely if you underdose the sedative and induction agents. For these, I go in the upper half of the range. If the animal is healthy, I go higher in the range than if they are sick. Intact males get the high end; testosterone really seems to interfere with the effect of the drugs (no neurons?). It is much easier to maintain a surgical plane of anesthesia if you get there on the first try. If there is a large range (ketamine), it is usually a safe drug. Remember many animals are sensitive to xylazine.
Standard drug concentrations
|Midazolam (or Diazepam)||5mg/ml||10ml|
|Penicillin-G (Procaine)||300,000 IU/ml||250ml|
|Sterile Diluent (for Telazol)|
|Xylazine (Equine version)||100 mg/ml||100ml|
Midazolam drip recipe: 5ml (25 mg) midazolam + 20 ml (2 g) ketamine +5 ml (100 mg) 20mg/ml xylazine in 1 liter bag fluids. About 1 drop/3 sec for 100kg BW
Xylazine/ketamine drip recipe: 50 mg xylazine and 1.0 to 1.5 g ketamine added to 1 L electrolyte solution. 1 drop/3 secs per 100kg BW
Double drip -Ketamine (1mg/ml or 1g/L) is added to 5% Guaifenesin
Muscle relaxation should be satisfactory if an adequate dose of xylazine is used at induction. If needed, relaxation can be improved by administering 25 to 50 mg of midazolam or diazepam intravenously as a bolus or by increasing the infusion rate of xylazine and ketamine. Approximately 2-4 ml/100# BW /hour.
See the following chapters for species specific doses and protocols.