Drop size and time tapes
I had to get some nurses to help me with this, and give me their insider tips. After working out the standard volume and time/rates for your meds and fluids, you need to know how to count this. The trick is to figure out the drop size: drip sets can have slightly different drop sizes. Look for the drop factor on the packaging, listed in drops per ml, or gtt/ml. Or just a drop icon with a number next to it.
For a standard drip set, mostly, it’s between 10 and 20 ggt/ml, and usually 20. Now you know how many drops you need per milliliter, so you need to set an approximate rate and actually stand there and count. The nurse trick is to hold your watch next to the drip chamber to make this as painless as possible.
After it’s set, remember this is not a controlled infusion pump. There are no alarms, the infusion rates can change over time. You need to check it regulary and have some markers to go by. Set yourself bag markings by making a time tape. Place a pice of tape on the bag, next to the volume markers. Mark the time at your current volume, and mark the tape with the calculated times for when the infusion should reach those set volumes. Bam, easy to check and control.
This is used by ward nurses all the time for low risk infusions, but in an extreme situation and with some vigilance, it can be used for sedation, anaesthesia, paralytics and cardiovasoactive drugs. And often it can be controlled with the patient as a feed-back loop.
Physiological feed-back loop
For some infusions the time volume infused is critical, as for the most scary infusions that are hyperpotent cardovasoactive drugs. But the upside is that they’re quick acting and short lasting. So you have a feed-back loop through clinical response, pulse and BP. If you have a weak solution, you can adjust the drip to effect, without worrying too much about the absolute drip rate. A quick and dirty trick is to add 1 mg of adrenaline or noradrenaline to a bag of 1000 ml of crystalloid, giving you adrenaline 1 microg/ml, and start that as a drip infusion.
For a 70 kg patient, a slow infusion of adrenaline 0.03 mcg/kg/min through a 20 drops per minute drop set would equal 0.03 x 70 = 2.1 mcg = 2.1 ml. Each ml is 20 drops, so 0.03 x 70 x 20 = 42 drops per minute. Or about 1.5 seconds between drops. A slowish drip.
A high-ish infusion of 0.1 mcg/kg/min through a 20 drop per minute drop set for the same 70 kg patient would equal 0.1 x 70 x 20 = 140 drops per minute. A bit over 2 drops per second. A quick drip. Use that a starting point and then adjust to effect. If you ever need to do this and need high doses, consider doubling the solution strenght to avoid infusing large fluid volumes in a critical patient.
Also check out our post on Basic Ketamine Infusion Anaesthesia, applying the same principles.