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Physics and flushes: The science supporting why we do what we do

Davenport, Deborah E. MSN, RN, CNE; Utterback, Virginia Ann PhD, RN, CNE

doi: 10.1097/01.NURSE.0000399593.20910.76
Department: I.V. ROUNDS

Understanding the physics of flushes

Deborah E. Davenport is an assistant professor at Texas Tech University Health Sciences Center, Anita Thigpen Perry School of Nursing, in Lubbock, Tex., and Virginia Ann Utterback is an assistant professor at Texas A&M Health Science Center College of Nursing in Bryan, Tex.

UNDERSTANDING why helps nurses internalize new information to mindfully alter existing practice. Although many elements are required to safely use and maintain vascular access devices (VADs), you can benefit from understanding the rationale for using evidence-based flushing techniques.1 This article explains the importance of syringe size and positive-pressure techniques for flushing VADs, with the focus on negative fluid displacement device systems.

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Size matters

According to the Infusion Nursing Standards of Practice, the purpose of flushing a VAD is to maintain patency and prevent contact between incompatible medications or fluids to prevent precipitation.1,2 Even though the volume of solution to be flushed varies by type of VAD (such as tunneled, nontunneled, central venous catheters, peripherally inserted central catheters, or implanted ports), the 10-mL syringe size should remain constant.

When considering the syringe size to use for flushing, the most essential dimension is the diameter of the syringe barrel.1 Flush volume should be at least twice the volume capacity of the VAD and add-on devices.3 Recommended flush volumes depend on several considerations: the type of VAD, the manufacturer's guidelines, the location of the inserted or implanted device, and the patient's size.

Regardless of the volume required to successfully flush a VAD, you should carefully consider the syringe barrel diameter. The barrel diameter of a 10-mL syringe ranges from 14.5 to 15.9 mm depending on the manufacturer. This is the recommended minimum diameter to prevent complications associated with using smaller-diameter syringes.1,4,5 Using a smaller syringe can damage the catheter and increase the risk for thrombus formation.1,3 Using a syringe larger than 10 mL affords the same protective advantages while accommodating larger flush volumes.3 Flush volumes typically range from 1 to 20 mL, but smaller volumes require the larger-diameter barrel.6

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Considering the physics

The rationale for using a 10-mL syringe is supported by physics: The smaller-diameter syringe will generate more pressure than a larger-diameter syringe.1 The additional pressure exerted by using a smaller-diameter syringe increases the risk of damage to the VAD tip, consequently increasing the risk of thrombus formation.

Routine flushing requires applying sufficient force to the syringe plunger to move fluid down the syringe and into the VAD lumen. An increase in intraluminal pressure is expected when resistance is met while applying force to the plunger. But differences in individual hand strength make it difficult to measure or standardize the amount of force being applied to the plunger. Applying excessive force to the plunger to overcome catheter resistance can lead to catheter rupture, regardless of syringe size.1,3

To better understand this concept, you need to understand pounds per square inch (PSI), the primary unit of measurement for pressure. PSI is defined as the pressure a gas or liquid exerts on the walls of its container, whether it's an oxygen cylinder, scuba tank, or tire.4

Because it's difficult to standardize the force applied to the syringe plunger, calculations of pressure applied at 3.5 PSI to both 3- and 10-mL syringes will serve to illustrate this concept. (See Comparing intraluminal pressures.) Using the 10-mL syringe markedly reduces intraluminal pressures when flushing VADs and is the basis for manufacturers marketing prefilled 10-mL syringes for flushing.3,7

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Preventing backflow

Additional concerns about VAD flushing include preventing the backflow of blood into the lumen.1,2 Flushing fluid into the VAD lumen requires positive pressure applied by hand. When all the fluid in a syringe has been pushed into the catheter, the catheter tip may be compressed as the plunger reaches the end of the barrel. At this point, you'll release direct pressure on the plunger while moving to disconnect the syringe from the catheter hub. Releasing pressure on the plunger leads to a reflux of blood into the lumen from the reexpansion of the compressed tip. Either syringe compression of the plunger rod or syringe/tubing disconnection can result in blood reflux into the lumen.1

To combat the reflux problem, manufacturers have developed negative, positive, and neutral fluid displacement devices.8 These types of devices depend on specific flushing techniques. It's essential to understand the type of device in use before attempting to flush. Negative fluid displacement devices use positive pressure on the plunger rod while the clamp is being closed on the catheter or extension set. Positive fluid displacement devices are flushed by simply closing the clamp after flushing while neutral fluid displacement devices aren't dependent on a specific flushing technique.8

A negative fluid displacement device (for example, blunt cannula, standard Luer tip syringe, or extension tubing) allows blood to reflux into the VAD lumen when the tubing or syringe is disconnected.1 To offset reflux, a positive-pressure flushing technique is required. Positive pressure can be described as a constant and even force within a VAD lumen that prevents reflux of blood. This technique can be accomplished by clamping while injecting or by withdrawing from the catheter hub while injecting.1,7,8 Activating a clamp during positive-pressure flushing on a VAD for intermittent use prevents blood from refluxing and is the policy in some facilities.

Figure. C

Figure. C

For totally implantable ports, applying positive pressure during needle withdrawal reduces the incidence of reflux by nearly 80%.9 In the absence of positive pressure, reflux may increase significantly depending on the septum diameter and needle gauge.

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Rely on the science

It's important to understand flushing techniques for VADS and follow manufacturers' guidelines.10 An evidence-based approach is the key to understanding the safe use and maintenance of VADs. Being both informed and empowered promotes safe practice and helps articulate proper VAD flushing methods when teaching patients, families, or peers.

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1. Hadaway L. Technology of flushing vascular access devices. J Infus Nurs. 2006;29(3):129–145.
2. Infusion Nurses Society. Infusion nursing standards of practice. J Infus Nurs. 2011;34(1 suppl):S1–109.
3. Standards: Site care and maintenance. Flushing: 56. J Intraven Nurs. 23(6S):S53-S54 .
4. SensorsONE. Pressure measurement instrumentation. PSI: pounds per square inch pressure unit .
5. Syringe selection guide .
6. Hadaway L. Flushing vascular access catheters: risks for infection transmission. Infection Control Resource. 2009;4(2):1–7.
7. Becton, Dickinson and Company. Positively unique: BD PosiFlush pre-filled syringes. 2007 .
8. Hadaway LC. Targeting therapy with central venous access devices. Nursing. 2008;38(6):34–40.
9. Lapalu J, Losser MR, Albert O, et al. Totally implantable port management: impact of positive pressure during needle withdrawal on catheter tip occlusion (an experimental study). J Vasc Access. 2010;11(1):46–51.
10. RMS Medical Products. Freedom60 Syringe Infusion System :
© 2011 Lippincott Williams & Wilkins, Inc.