Dose Calculations and Administration of Drugs

Objective:

To provide a basic introduction on the proper calculaitons of drug dosages and how drugs are commonly administered.

 

Contents:

Dose Calculation: 

Dose calculation is a relatively simple exercise requiring minimal knowledge of mathematics. Nevertheless, having a calculator handy is often a very good idea!

USE THE METRIC SYSTEM! This is standard scientific notation and is the measurement system used in most countries.

Accurate dosing is critical to the proper utilization of all pharmaceuticals. To calculate the correct dose of drug you need to know the concentration of the drug, the weight of the animal, and the recommended dose rate of the drug in question for the specific animal you are administering the drug to.

 

Concentration of the drug:

READ THE DRUG LABEL!

mg/ml

Manufacturers usually provide concentrations of their product in milligrams (mg) of drug per (ml) of solvent.

%

Some concentrations might be expressed as percent. For example a 10% solution of xylazine is 100mg/ml and a 2% solution of xylazine is 20mg/ml.

IU/ml

International Units per ml of solvent is used for some preparations like penicillin and some of the fat soluble vitamins. This is actually a measurement of activity and doses use the same unit of measure to make calculations easier.

powders

You may receive drugs in a powdered form and be given the mg of active drug in the vial. For example, Telazol (tiletamine and zolazepam) comes in powdered form with 500mg per vial. The labeling instructs you to add 5ml of sterile water for injection to the vial thus providing 5ml of 100mg.ml drug. However, if you measure this you discover that you actually have 5.5ml of 90.91mg/ml. An obvious inconsistency that you need not follow. Telazol is a nice compound because we can reconstitute it at higher concentrations: we normally add 2ml of sterile water for injection to make 2.5ml of a 200mg/ml solution. Other concentrations are also possible but be careful about trying to make it too concentrated or you may not achieve a reasonable solution.

 

Recommended dose rate of the drug:

READ THE AVAILABLE LITERATURE! Always look up the drug dose for the species you are working with - it often varies.

 

Weight of the animal:

Always best to use a scale and get an accurate weight. However, if you cannot weigh the animal prior to injection (as in wildlife capture operations) you need to be experienced in estimating the weight.

 

Let's try it out:

You need to administer xylazine at a dose rate of 0.1mg/kg to a 300kg muskox. You are using 2% (20mg/ml) xylazine.

Therefore, the proper dose for a 300kg muskoxen is 30mg of xylazine and, the correct volume of 2% xylazine to give is 1.5ml

 

The proper dose to anesthetize arctic fox is 10mg/kg ketamine and 0.05mg/kg medetomidine. Your fox is estimated to be 5kg.

Ketamine:

Medetomidine:

ketamine comes in vials of 100mg/ml and medetomidine comes in vials of 1mg/ml

Ketamine:

Medetomidine:

Therefore you need to adminsiter 0.25ml of medetomidine and 0.2ml of ketamine

 

It works the other way too. If you immobilized an adult moose with 6mg carfentanil and 100mg xylazine and, after you captured it you weighed it to find out it weighs 476kg. What dose did you actually give?

of carfentanil

of xylazine

 

ADVISE FOR TINY ANIMALS

If you are administering injectables to mice or other extremely small animals, you will soon discover that the volumes to be administered are too tiny for practical purposes. Therefore you will need to prepare dilutions. When diluting pharmaceuticals you MUST use sterile water for injection (assuming you are diluting an aqueous solution). You cannot use water, distilled water, or even sterilized filtered water because, although sterile, pyrogens may still be present. Sterile water for injection is commercially available, inexpensive and safe for the animal.

For stock solution recipes for common drug combinations in tiny animals see Appendix 3 in:

 

 

Administration of Drugs:

Intravenous

IV

Absorption is circumvented thus giving potentially immediate effects.

Suitable for large volumes. Must inject slowly. Not suitable for oily solutions or insoluble substances.

Intramuscular

IM

Prompt absorption from an aqueous solution (i.e. most of our injectable anesthetics and immobilizing agents). Slow and sustained from repository preparations (i.e. long acting penicillin). Preferred route for most of our anesthetic agents.

Suitable for some moderate volumes, oily vehicles, and some mildly irritating substances.

Subcutaneous

SC

Similar to IM although much slower absorption. Absorption of some immobilizing agents from this site may be too slow to achieve desired blood levels.

Not suitable for large volumes. Suitable for some insoluble suspensions and for implantation of pellets or silastic implants.

Intraperitoneal

IP

Used most commonly for injecting anesthetics or other substances into rodents since muscle mass is so small and veins are difficult to find. Rapid absorption (almost as fast as IV) due to large peritoneal surface.

Some substances may cause mild-moderte irritation. Possible accidental injection into the urinary bladder or loop of bowel.

Oral

per os

Variable; depends upon many factors.

Most convenient, economical and usually more safe (depends on what you are administering).

Intrathoracic

 

Not routinely used. Usually only reserved for euthanasia with barbiturates. IP or IV is preferred.

Intrathoracic injection of barbiturates for euthanasia damages the tissues rendering them useless for study.

Intradermal

ID

Usually reserved only for injection of an antigen to test cellular immunity (i.e. tuberculin test).

 

Respiratory

 

Inhalant anesthetics and some other drugs are absorbed through the lungs. Absorption (as well as excretion) tends to be rapid since the alveolar lining has such a large surface area and intimate relationship with capillaries.