Type of publication: popular article
Author: Macklin D.


Physical principles of intravenous administration


During the numerous courses on venous access devices, one realizes that the physical principles of the flow of a fluid in venous lines are often not known, despite the fact that this knowledge would allow to prevent and solve the malfunction of the venous line. It is known that elements such as the viscosity of the drug or the distance between the bag and the exit point of the catheter, or the caliber of the catheter itself, influence the flow. However, it is not always known why this occurs.


Troubleshooting an intravenous therapy delivery system can be very frustrating, especially in the home care setting where the practitioner is without the collaboration available to nurses in the institutional setting. However, because many infusion problems relates to physics, a basic understanding of some of the physical principles that impact flow rates will enable the practitioner to more clearly understand the issues that may impact a less-than-optimal flow rate. Here are some of the factors that impact flow, and a few pointers to improve your troubleshooting methods: Understanding Flow Flow occurs as a result of the relationship between resistance and pressure. Resistance is simply a property of the path of flow. There are three resistors: viscosity, distance (length of the tubing, catheter), and internal diameter of the tubing and the catheter. Resistance is proportional to length times viscosity divided by the internal diameter to the fourth power. Resistance increases directly with the length of the path the fluid must take. Adding long lengths of IV tubing to an infusion setup increases the total length the fluid takes. Also, if your are infusing by gravity into an adult patient through a PICC line, the flow will be slower than the same fluid flowing through a one-inch peripheral catheter of similar gauge size. If you double the length of the catheter, the flow rate will be halved if no other changes are made in the IV system. Remember that PICCs are as much as 20 times longer than peripheral catheters. A 20-gauge PICC will thus achieve a slower flow rate than a 20-gauge, one-inch peripheral catheter. Adding tubing for increased patient mobility also increases the resistance and will affect flow rate. Length of the line is not the only factor that affects resistance. Resistance also increases directly with viscosity. If you double the viscosity of the fluid, the flow rate will be halved. Infusion nurses should be aware that the temperature of the fluid being infused impacts viscosity. For example, cold blood flows slowly initially because it is very viscous. As the blood warms, the viscosity is lower and the blood flows faster if the infusion system is not recalculated. Need to slow the infusion rate of an elastomeric pump to promote hemodilution of the drug? Infuse it directly from the refrigerator, but be sure to apply a warm towel to the client’s arm to minimize the potential for venous spasm. Also affecting resistance is the internal diameter of the catheter. If you double the diameter of a catheter, the flow rate is increased to the fourth power. Gauge size is standardized and reflects the outer circumference of the catheter. The wall thickness impacts the size of the internal diameter of the catheter. Venous access devices with similar gauge size have different internal diameters depending upon catheter design and catheter material. Silicone catheters of equal gauge size will achieve slower flow rates than polyurethane catheters because their internal diameter is smaller due to the thicker walls of the silicone catheter. With some small gauge silicone PICCs, a pump must be used to ensure a flow rate. Internal diameter differs between Teflon and polyurethane peripheral catheters. For example, a 24-gauge Insyte (polyurethane) catheter has the same approximate flow rate as a 22-gauge Angiocath (Teflon). It is important to know the manufacturer’s published flow rates for each catheter. Think how much easier it might be to place a thin wall 24-gauge peripheral catheter in a patient with extremely poor access. Troubleshooting using pressure and force Pressure is equal to the force applied divided by the area over which it is applied. Pressure can be static, such as is caused by the patient’s blood pressure or the elevation of the bag. This type of pressure changes when you increase the height of the bag or the patient’s blood pressure increases. Static pressure does not vary as a result of the resistance the system is generating by the fluid flowing through the IV system. However, dynamic pressure varies with the resistance generated by the fluid flowing through the IV system and is thus a symptom of resistance. As resistance rises, so does the dynamic pressure. Fluid flowing at a fast rate through a large tube produces little resistance and therefore little pressure is generated. The same rate flowing through a very small tube causes a large amount of resistance and also a rise in dynamic pressure. Pump is a constant source of force A pump is a constant source of force. The force a pump exerts is equal to the rate times the amount of friction caused by the path of flow. The pump has a sensor that alarms when a predetermined pressure limit is reached. This limit is usually around 9 pounds per square inch (PSI). When you hear the alarm, it means that the pressure in the system has surpassed the predetermined limit. Since dynamic pressure is a symptom of resistance, the warning alarm indicates that resistance has risen in the system. Investigate the intravenous system for causes of the increased resistance. Is the IV tubing kinked? Is there tape wrapped around the arm or placed in such a way as to partially occlude the vein or catheter? The peripheral catheter tip may be in a valve or positioned against the vein wall. If you flush the catheter you may not feel any resistance. This is because the pressure you are generating with your thumb pushing on the syringe plunger is greater than the pump pressure limit that sounds the alarm. The pump is more sensitive than your thumb. Disconnect the IV line from the catheter and see if the IV fluid flows freely. If it does not, the problem is within the line. If a filter is in place, check and see if it has an air lock. Air is 5 microns in size. If a .22-micron filter is filled with air, fluid will be unable to flow through it. Invert the filter and tap it several times to vent some air. When the air lock is relieved, fluid will again flow. If fluid flows when not attached to the patient, then the resistance is being caused within the catheter or the vein. With peripheral catheters, you can take off the dressing, reposition the catheter by pulling back and redress. You might try attaching the IV fluid to the repositioned catheter before completely securing the dressing and catheter to see if the fluid flow is correct and if the pump alarm does not sound. With midline catheters it is usually necessary to pull the catheter back since the resistance is typically caused by poor tip location. If you have a pump that monitors resistance, use the resistance readings to fully understand how your device is working. As resistance rises, you will know to investigate. Early detection can mean that you intervene before a complication such as infiltration occurs. Infiltration is not a complication a pump normally can detect. Fluid will have to continue to fill the tissue until the pressure alarm point is surpassed. With low flow rates for instance, resistance is low, therefore the pressure is low and an infiltration can go undetected for extended periods of time. When you know that the flow rate is not creating a much resistance, you must pay additional attention to assessing the IV site for infiltration. It is important to remember these physical principles allow flow to occur. When they are understood they can be used to troubleshoot the IV system. Many times the solution is simple if you remember the relationship of pressure and resistance.