Using gravity

Last updated: Tuesday, July 12, 2022

Using gravity is the simplest method of infusing drugs and fluids. The infusion container (usually a bag or bottle) is hung above the patient at a specific height to create the desired flow pressure and flow rate. As mentioned on the previous page in an acute hospital setting in the UK medicines are usually administered by electromechanical pumps, but if these are not available then using gravity may be an option for some medicines in selected patients after a risk assessment.

A drip chamber (on the left) and roller clamp (on the right)
A clear administration set is attached to the infusion container. This consists of tubing which attaches to the patient's vascular access device (e.g. peripheral intravenous cannula), a 'roller clamp' and a 'drip chamber'. The drip chamber enables the number of drops falling per minute to be counted and the roller clamp is used to 'pinch' the tubing partially or fully closed, thereby controlling the flow of the infusion fluid into the patient.

Gravity infusion can be used to administer fluids without additives (e.g. sodium chloride 0.9%) and drugs where the rate is not critical and serious adverse effects are not anticipated (e.g. metronidazole). Care is required if the height between the infusion container and the patient changes (such as if the patient sits up in bed) as this may change the flow pressure, and in turn the rate at which the infusion is delivered.

Gravity calculations
The rate of administration is calculated by taking into account the infusion volume, the desired infusion time and the number of drops per millilitre of fluid. For crystalloid fluids (e.g. sodium chloride 0.9%) given through a standard administration set there are approximately 20 drops per millilitre. For more viscous fluids such as blood there are approximately 15 drops per millilitre.
    Drops per minute = Number of drops per mL x volume to be infused (in mLs) / 60 x desired infusion time (in hours) 
For example, a patient is prescribed metronidazole 500mg in 100mL intravenous infusion to be given over 1 hour. The drop rate is calculated as shown;
    Drops per minute = 20 drops per mL x 100mL / 60 x  1 hour  = 33 drops per minute
Therefore the person administering this metronidazole infusion will need to use the roller clamp to adjust the rate so that one drop is falling through the drip chamber approximately every 2 seconds. 

Now over to you. 
Your patient is prescribed clarithromycin 500mg in 250mL intravenous infusion over 60 minutes. Calculate the administration rate in drops per minute.  Click to reveal the answer.
Drops per minute  = 20 drops per mLx 250mL/ 60 x 1 hour = 83 drops per minute.