STEP-BY-STEP SURGICAL MATH CALCULATIONS
IV FLUID CALCUATIONS
EXAMPLE: a 37lb dog
Step 1. Obtain the weight of the patient in kilograms. Usually, formulas for
fluid rates and drug calculations require a patient weight in kilograms (kg).
If you are starting with pounds, divide that number by 2.2.
EX: 37÷2.2=16.82kg
Step 2. Decide whether you will need a macro drip set or a micro drip set. If
the patient is less than 10kg use a micro drip set. A micro drip set is one that
delivers 60 drops of fluid to equal 1 mL. If the patient is larger than 10kg use
a macro drip set which is one that delivers 15 drops of fluid to equal 1 mL.
Macro drip sets also come in a 10 drop per mL variety. At VTI, we have the
15 drop per mL variety.
EX: This patient (37lb) will need a MACRO drip set, 15gtt/mL
Step 3. Do the calculation to determine your patient’s surgical fluid rate per
hour. *Remember the surgical fluid rate is 10mL/kg/hr*Round to 2 decimal
places* Multiply the patient’s weight (in kg) by 10
EX: 16.82 x 10 = 168.18 mL/hr
Step 4. Multiply the number in step 3 by the chosen drip set (10, 15 or 60).
*Also round this number two decimal places. In this example we should use
15gtt/ml.
EX: 168.18 x 15 = 2522.7
Step 5. Divide the number in Step 4 by 3600. 3600 is the number of seconds
in an hour. This answer is the number of drops that your patient should
receive each second. *Round to 2 decimal places.
EX: 2522.7÷3600 = 0.70
Your patient should receive 0.7gtt of fluid each second. HOW can you
possibly count 0.7gtt every second?! You can’t, so an easier thing to do is to
multiply by 10. This will give you the number of drops that your patient
should receive within 10 seconds.
EX: 0.70 x 10 = 7.0gtt/10seconds!!! MUCH EASIER TO COUNT!
NOW, LET’S LOOK AT THIS AS AN EQUATION…
THE EQUATION BEGINS WITH THE AMOUNT OF FLUID THAT THE
PATIENT SHOULD RECEIVE EACH HOUR UNDER ANESTHESIA.
168.18 mL x 15gtt x 1 hr
= 0.7gtt/sec x 10sec = 7 gtt/10 sec!!!
1hr
1 mL 3600 sec
If you don’t like fractions with the units attached to them, then plug in the
numbers to this equation:
(Patient’s fluid rate each hour) x (drip set) = _________gtt/sec x 10 = ______ gtt/10sec
3600
Other important fluid rates to remember:
Shock dose of fluids for dogs: 50-90mL/kg/hr
Shock dose of fluids for cats: 40-60ml/kg/hr
How to calculate drug dosages
Drug doses are usually given in the units mg/kg. Sometimes doses may be
given in mg/lb or mLs/lb or mLs/kg. Always read very carefully before
doing your calculations!
Example: 37 lb patient; Acepromazine 0.04mg/kg, conc:10mg/mL
Step 1: The dose is given in mg/kg, so convert the patient’s weight to kgs by
dividing by 2.2.
37÷2.2= 16.82kg
Step 2: Multiply the weight in kgs by the dose which is 0.04mg/kg.
16.82 x 0.04 = 0.67mg; Notice the units at this point…mg! By doing this
multiplication, we cancel out the unit “kg” and we are left with mg. So, we
know how many milligrams of Acepromazine the dog needs, but we need to
know how many mLs of Acepromazine to administer.
Step 3: Divide the mg to be given to the patient by the concentration of the
drug which is given in mg/mL.
0.67mg÷10mg/mL= 0.067 or 0.07mL
NOW, TO PUT IT ALL TOGETHER THE EQUATION LOOKS LIKE THIS:
WEIGHT X DOSE = _______mLs of drug (or the
CONCENTRATION
portion of
Tablet)
How to calculate O2 Flow rates
The oxygen flow rate for the patient will depend on two factors, the weight
and the type of breathing system used. For patients that are less than 7kg, a
non-rebreathing system is used and a higher O2 flow rate will be necessary.
For patients larger than 7kg, a rebreathing system will be used and a lower
flow rate will be necessary.
Non-rebreathing system: 250mL/kg/min
Rebreathing system: 40mL/kg/min
Example: 37 lb patient
Step 1: Determine what type of breathing system will be used. Since this
patient is greater than 7kg (37÷2.2=16.82kg) we should use a rebreathing
system.
Step 2: Multiply the patient’s weight in kg by the O2 flow rate for the
rebreathing system.
16.82kg x 40mL/kg/min = 672.8mL/min
Step 3: Remember, the O2 flowmeter on the machine is calibrated in L/min.
So, we need to convert our previous answer into liters per minute.
672.8mL÷1000=0.67L/min
**We will always round up to 0.8L/min if the calculation results in a flow
rate less than 0.8L/min**
How to calculate how long the oxygen tank will last
When the oxygen tank has been turned on with the “key” or wrench, the dial
on the tank pressure gauge should give a psi reading.
Example: 950 psi
Step 1: Multiply the psi reading on the tank by 0.3. This number is standard
for “E” tanks only. The answer will tell us how many liters of oxygen are in
the tank.
950psi x 0.3=285L of O2
Step 2: We really want to know how many minutes 285L will last. So we
divide this number by the patient’s oxygen flow rate. Above, we calculated
our patient’s O2 flow rate to be 0.67L/min, but we are going to round this up
to 0.8L/min.
285L÷0.8L/min=356.25 minutes or ~6hours
HOW TO CALCULATE RE-BREATHING BAG SIZE
Step 1: Obtain the patient’s weight in kgs
Step 2: Calculate the tidal volume, which is 10-15ml/kg
Step 3: Multiply the tidal volume by 5. (This is about 60ml/kg)
Step 4: Convert to liters, since the bags are measured in these units. To do
this, divide the answer from step 3 by 1000.
Step 5: Always round up
OK, HERE’S SOME PRACTICE PROBLEMS. REMEMBER TO
ALWAYS SHOW YOUR WORK!!!
1. A 20lb dog
a. calculate surgical fluid rate using 5ml/kg/hr
b. calculate Acepromazine, Atropine, and oral Rimadyl dosage
i. Ace: 0.04mg/kg; conc:10mg/mL
ii. Atropine: 0.02mg/kg; conc:0.54mg/mL
iii. Rimadyl: 1mg/lb; conc: 25mg tablets
c. calculate the O2 flow rate
d. the tank pressure gauge says 450 psi. How many minutes do we
have before we run out of oxygen?
2. A 7lb cat
a. calculate the surgical fluid rate using 5ml/kg/hr
b. calculate the low end and high end of shock dose fluids.
Indicate the number of mLs/hr AND the number of mLs every
15min.
c. calculate Buprenorphine dosage
i. Bupi: 0.015mg/kg; conc: 0.3mg/mL
d. calculate the O2 flow rate
e. the tank pressure gauge says 450psi. How many minutes do we
have before we run out of oxygen?