IV Administration Sets: priming volume vs residual volume

Q: A patient gets daily IV antibiotic infusion at home. The antibiotic is mixed in a 50mL 0.9% Sodium Chloride minibag by pharmacy. The home care nurse administers the infusion via gravity drip using a macrodrip primary tubing via the patient’s PIV lock. When the IVPB bag is completely empty, the nurse stops the infusion, disconnects the line and flushes the PIV lock. The IVPB bag and tubing is disposed properly. During a recent home visit, the patient had a guest who is a nurse and questioned the home care nurse about the medication left in the IV tubing. The guest was asking about the priming volume and claimed the patient did not get the full dose because of the residual volume. Is this true?

A: First, let’s begin by defining some terms.

Priming volume – the amount of fluid required to fill the entire length of the IV administration set eliminating the air in line. At the beginning of an infusion, all IV sets are primed to avoid delivering air through the line. After the IV bag is spiked with the IV administration set, the IV solution/fluid flows through the tubing to remove the air. The priming volume of IV administration sets vary by manufacturer and affected by the length of the tubing and any add-on devices. The priming volume information can be found on the packaging label of the IV administration set.

Drug volume – after priming the tubing, this is the volume left in the IVPB/medication bag.

Residual volume: the amount of fluid or medication left in the IV administration set after the IVPB bag is empty or drug volume is infused. Some call this “dead volume”.

Most primary IV administration sets have priming volumes ranging from 15mL – 27mL. The longer the tubing, more fluid is needed to prime the tubing so the priming volume is more. What’s unknown in your scenario is if you are using an extension set and/or an add on in line filte, which adds length and more volume. For this discussion, let’s say you are using a primary gravity IV administration set only, no filter or extension set and let’s use the priming volume of 25mL.

IVPB Bag (50mL) – Priming volume (25ml) = Drug Volume (25mL) 

When the IVPB bag is empty, you have delivered the drug volume(25mL) to the patient.

If you stopped the infusion at this point  (bag is empty)  but the drip chamber and the entire length of the tubing below is still filled with fluid, this is the residual volume, the remainder of the medication the patient did not receive since you disconnected at this point and discarded the bag and tubing. In your case, it’s 25mL or half of the bag.

Residual volumes are often overlooked and misunderstood by many nurses, not realizing the serious implications of the inaccurate and incomplete administration of prescribed dosages may lead to suboptimal treatment outcomes or other adverse effects for the patient. When the residual volume is not cleared, it has been reported that a 40% loss of the prescribed antibiotic dose occurs. In your case, it’s 50%

priming volume

So how can nurses manage the “residual volume”?

1. Know the numbers: We all need to check the priming volume of the IV administration set we are using. Always check the package for the priming volume before you throw it away. In this scenario, we are discussing the primary set, not the secondary set, which also has its own priming volume. Note that any add on devices to the primary IV administration set, ie filter or extension set, can increase the priming volume. Check the package of the add on devices for priming volume.

2. Empty the tubing: Since you are infusing by gravity using a primary IV set, after the bag is empty, take the time to allow for the fluid to drain as far as possible into the patient, You may need to help this process along by holding the tubing above the IV site to allow fluid to flow into the patient. Since you are disposing the IV tubing after the infusion, the issue of air in tubing for next dose is not a concern. This will be more challenging for those using IV pumps or intermittent secondary IV set. (topic for another day)

3. Flush bag: For patients who are not on fluid restrictions, you can use a second 50mL IV bag of plain saline to replace the empty medication bag to flush the line. This will allow for the residual volume to be completely infused keeping in mind the tubing’s primary volume.

4. IV bag overfill – many of us have been told that commercially available IV bags have overfill of up to 16% of the labeled volume the bag. However, this overfill doesn’t address the possible loss of drug in the residual volume. Depending on the volume of the overfill, the concentration of the medication in the IV bag after the medication is added may differ. Bottom line, we still need to be vigilant with managing residual volumes.

5. Alternative strategies: collaborate with pharmacy on clinical strategies to manage residual volumes such as but not limited to:

  • IV push method – investigate which antibiotics can be administered this way in the home.
  • Use of syringe pumps with microtubing
  • Use of higher doses of medication to compensate for the loss
  • Use of higher solution volumes – 250mL vs 50mL if no fluid restriction
  • Extended infusion times to compensate for residual volumes

There is a lack of standardized approach on dealing with residual volumes yet we know this is an issue. There is still a heavy reliance on the vigilance of the nurses and the organization’s guidelines or best practices. Keep in mind, less manipulation, less invasive is preferred to decrease risk of contamination.

Thank you for your question and I hope you find this post helpful.