The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [577]
The rate of administration of a continuous or intermittent infusion may be calculated from the following equation (Pickstone 1999):
In this equation, 60 is a factor for the conversion of the number of hours to the number of minutes. Factors influencing flow rates are as follows.
Type of fluid
The composition, viscosity and concentration of the fluid affect flow (Pickstone 1999, Quinn 2000, Springhouse 2005, Weinstein and Plumer 2007). Irritating solutions may result in venospasm and impede the flow rate, which may be resolved by the use of a warm pack over the cannula site and the limb (Springhouse 2005, Weinstein and Plumer 2007).
Height of the infusion container
Intravenous fluids run by gravity and so any changes in the height of the container will alter the flow rate. The container can be hung up to 1.5 m above the infusion site which will provide a hydrostatic pressure of 110 mmHg (MHRA 2010b, Springhouse 2005). One metre above the infusion site would create 70 mmHg of pressure, which is adequate to overcome venous pressure (normal range in an adult is 25–80 mmHg) (Pickstone 1999). If it is hung too high then it can create too greater pressure within the vein, leading to infiltration of the medication (MHRA 2006a). Therefore any alterations in the patient’s position may alter the flow rate and necessitate a change in the speed of the infusion to maintain the appropriate rate of flow (Hadaway 2010, Weinstein and Plumer 2007). Positioning of the patient will affect flow and patients should be instructed to keep the arm lower than the infusion, if the infusion is reliant on gravity (Quinn 2008).
Administration set
The flow rate of the infusion may be affected in several ways.
Roller clamps (Figure 13.28) or screw clamps, used to adjust and maintain rates of flow on gravity infusions, vary considerably in their efficiency and accuracy which are often dependent on a number of variables such as patient movement and height of infusion container (Hadaway 2010). The roller clamp should be used as the primary means of occluding the tubing even if there is an anti-free flow device (MHRA 2010b).
The inner diameter of the lumen and the length of tubing will also affect flow. Microbore sets have a narrow lumen, so flow is restricted to some degree. However, these sets may be used as a safeguard against ‘runaway’ or bolus infusions by either an integrated antisyphon valve or anti-free flow device (Hadaway 2010, Quinn 2000, Weinstein and Plumer 2007).
Inclusion of other in-line devices, for example filters, may also affect the flow rate (Hadaway 2010, MHRA 2010b).
Figure 13.28 Roller clamp.
Vascular access device
The flow rate may be affected by any of the following.
The condition and size of the vein; for example, phlebitis can reduce the lumen size and decrease flow (Quinn 2008, Weinstein and Plumer 2007).
The gauge of the cannula/catheter (MHRA 2010b, Springhouse 2005, Weinstein and Plumer 2007).
The position of the device within the vein; that is, whether it is up against the vein wall (Quinn 2008).
The site of the vascular access device, for example, the flow may be affected by the change in position of a limb, such as a decrease in flow when a patient bends their arm if a cannula is sited over the elbow joint (Springhouse 2005).
Kinking, pinching or compression of the cannula/catheter or tubing of the administration set may cause variation in the set rate (MHRA 2010b, Springhouse 2005).
Restricted venous circulation; for example, a blood pressure cuff or the patient lying on the limb increases the risk of occlusion of the device and may result in clot