Fluids 7: Medical Math for Fluid Therapy: Lesson 50

Dose

amount of medication measured (mg, mL)

Dosage

amount of medication based on units per weight of the animal (50 mg/kg or 10 mL/kg for example)

Concentration

on the bottle if dealing with a drug in mg/mL(or mg/tablet)

Reconstitution of solutions

A percent solution means “per one hundred” 1% solution= 1/100 5% solution= 5/100

1% solution = 1 gram / 100 ml or 1% solution = 1000mg/100ml

This means that in 1ml there is 1000/100 = 10mg/ml

This means that for the 5% solution there is 5 gram of product in 100 ml fluid

We are trying to make a 5% solution

5% dextrose = 5 grams dextrose / 100ml = 50gram/liter

• = 5000mgs per 100ml = 50mg per ml

So if we want a 1 litre solution with 50mg/ml… how many mg of dextrose do we need to add to the solution?

1000 x 50 = 50,000 mg of dextrose need to be added to the 1 litre bag

How many mls of 50% dextrose are needed to put 50,000mg of dextrose into the 1 litre bag?

• Remember: a 50% solution is 500mg/ml

• So 50,000 / 500 = 100ml. We need to add 100ml

How about calculations with electrolytes?

Electrolytes are given in mEq = milliequivalent

patient needs 10 mEq of potassium chloride, needs to be added to a 1-liter bag of LRS. How many milliliters should you draw up to obtain 10 mEq?

On the bottle you can see that the concentration is 2 mEq/ml. If you need 10 mEq you would need 10 / 2 = 5 ml

You attend to a cow many miles away from your practice and need to administer fluids for dehydration but the only thing you brought in your truck is a gallon of distilled water. The farmer has salt in his house. How do you prepare a 0.9% saline solution with the available material?

0.9x(3785/100)=34g per gallon

medical math needs when considering the rate of fluid therapy.

Hourly rate (ml/hr) = daily volume (ml)/ 24

Converting ml/hr to drops/minute = drops/ml* x ml/hr 60 (minutes)

IV sets have different drop factors: macro drip set = 20 drops/ml, micro drip set = 60 drops/ml

Small animal vs Large animal fluid plans

Small animal fluid plans: often initially created for 12-24 hours (with reassessment in between)

Large animal fluid plans: often initially created for 24 hours (with reassessment in between)

Regular monitoring is always essential to be able to adjust and avoid potential side effects.

Infusion rate guide: This chart works for a macro drip set

Multiply by 3 if a micro drip set is used

Fluid rate example

A 5 lb puppy is 10% dehydrated meaning that the fluid deficit is 10% of its body weight

What is the fluid deficit for this puppy in ml?

2.3 kg x .1 =.23 L x 1000= 230 mL

Drip rate from fluid deficit

320 ml per day is 320 / 24 =13.3 ml/hr

13.3 / 60 = 0.22 ml / min, 0.22 / 60 = 0.0037 ml / sec

With a drip set that does 60 drops per ml per second it will do 60 x 0.0037 = 0.22 drops per second for 0.0037 ml

We cannot measure 0.22 drop per second, but it is possible to put it on a rate of 2 drops per 10 seconds