Intravenous infusions

Infusion rate calculations for drugs and fluids often seem challenging to the newcomer, yet they are really very simple. When using a solution set for giving fluids follow these three steps:

1. Check the drip set factor, which is the number of drops per ml your set delivers. Usually 20 for a standard set and 60 for a paediatric set.

2. Calculate the fluid infusion rate in millilitres per hour for your patient. Fluid infusion rates are commonly specified in millilitres per kilogram of bodyweight per day (ml/kg/day), maintenance for a dog is often assumed to be 50 ml/kg/day. It's a simple matter to calculate the total daily requirement for a given patient. Dividing this by 24 gives the hourly rate. If you plan to use an infusion pump or syringe driver this figure is then used to set the infusion rate.

3. Calculate the drip interval. Divide by 60 to get the fluid rate in ml per minute (ml/min), then multiply by the drip set factor (20 for a standard set such as 0274.21) to calculate drops per minute, 60 divided by the result gives the drip interval. For example, 1 ml per minute is 20 drops per minute, which is 60/20 = 3 seconds drip interval.

For drug infusions, using an infusion set or syringe driver:

• Calculate total dose per hour. An infusion dose rate will be specified for the drug. Usually it is in milligrams (mg)/kg/min. A rate per square metre of body surface area may be used where accuracy is important. Micrograms (mcg) may be used as the unit of weight for potent drugs. Total dose per hour in mg or mcg equals infusion dose rate multiplied by patient weight (or surface area) multiplied by 60.

• Decide what concentration of the drug is required in the fluid to be administered by considering the total dose per hour. A practical infusion rate that the syringe driver, infusion pump or giving set can deliver is required, using an appropriate total volume of solution. Advice on final drug dilution may be given in the manufacturer's drug data sheet.

• Calculate the infusion rate. This is hourly dose divided by drug concentration administered. For example, a 20kg dog is to be infused short-term with an analgesic during anaesthesia at 1 mcg/kg/min. Hourly dose = 1x20x60 = 1200 mcg/hr. The surgical procedure to be performed takes over an hour and we wish to use a 20ml syringe driver, so we choose a drug concentration of 100 mcg/ml and dilute the manufacturer's preparation in saline accordingly. The infusion rate is 1200/100 = 12 ml/hr. Our filled syringe will last 20/12 hrs = 1hr 40min. The same method of calculation can be used for higher volume infusions using either infusion pumps or just a giving set alone for low precision applications.

Infusion products from Vygon Vet:
Infusion devices
CVP measurement
Stopcocks
Connectors
Bungs and stoppers

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