The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [578]
The patient
Patients occasionally adjust the control clamp or other parts of the delivery system, for example, change the height of the container, thereby making flow unreliable. Some pumps have tamper-proof features to minimize the risk of accidental manipulation of the infusion device (Hadaway 2010) or unauthorized changing of infusion device controls (Amoore and Adamson 2003).
Advantages and disadvantages of gravity infusion devices
A gravity flow system is simple to set up. It is low cost and the infusion of air is less likely than with electronic devices (Pickstone 1999). However, the system does require frequent observation and adjustment due to:
the tubing changing shape over time
creep or distortion of tubing made of polyvinyl chloride (PVC)
fluctuations of venous pressure which can affect the flow of the solution
the roller clamp can be unreliable, leading to inconsistent flow rates.
There can also be variability of drop size and if the roller clamp is inadvertently left open, free flow will occur. Infusion rates with viscous fluids can be reduced (particularly if administered via small cannulas) and there is a limitation on the type of infusion as it is not suitable for arterial infusions: this is because viscosity and arterial flow offer a high resistance to flow which cannot be overcome by gravity (Pickstone 1999, Quinn 2008). If more than one infusion is infusing and one is slower than the other or there is no flow in the second set then there is a risk of back tracking which leads to underinfusion or bolus delivery of medicines. The MHRA recommends that in these systems, the sets should include antireflux valves (MHRA 2007).
Gravity drip rate controllers
A controller is a mechanical device that operates by gravity. These devices use standard solution sets and although they look much like a pump, they have no pumping mechanism. The desired flow rate is set in drops per minute and controlled by battery- or mains-powered occlusion valves (MHRA 2010b).
Advantages and disadvantages of gravity drip rate controllers
Although they can maintain a drip rate within 1%, volumetric accuracy is not guaranteed and many of the disadvantages associated with gravity flow still remain. The main advantages are that they are relatively inexpensive and can usually use standard gravity sets. They also incorporate some audible and visual alarm systems (MHRA 2010b).
Infusion pumps
These devices use pressure to overcome resistance from many causes along the fluid pathway, for example length and bore of tubing or particulate matter in the tubing (Hadaway 2010). There are a number of general features required in infusion pumps.
Accuracy of delivery
In order to meet requirements for high-risk and neonatal infusions, pumps must be accurate to within ±5% of the set rate when measured over a 60-minute period although some may be as accurate as ±2% (Hadaway 2010, MHRA 2010b). They also have to satisfy short-term, minute-to-minute accuracy requirements, which demand smoothness and consistency of output (MHRA 2010b).
Occlusion response and pressure
Flow will occur if the pressure at the tip of an intravascular device is just fractionally above the pressure in the vein; the pressure does not need to be excessive. In an adult peripheral vein, pressure is approximately 25 mmHg, while in a neonate it measures 5 mmHg (Quinn 2000). Most pumps have a variable pressure setting which allows the user to use their own judgement about the pressure needed to deliver therapy safely. The normal pumping pressure is only slightly lower than the occlusion pressure (Hadaway 2010). Flow is dependent upon pressure divided by resistance. If long extension sets of small internal bore are used, the resistance to flow will increase (Pickstone 1999, Quinn 2000).
If an administration set occludes, the resistance increases and the infusion will not flow into the vein. The longer the occlusion occurs, the greater the pressure and the pump will continue to pump until an occlusion alarm is activated. There are two types of occlusions: