Fluid flow accuracy of syringe drivers
Gavin Hughes, Joanna Ford, Pete Phillips
Syringe drivers are used for controlled administration of fairly small quantities of drugs. They can be set at a particular flow rate or controlled by the patient/healthcare professional in the case of analgesia. This article focuses on the accuracy of flow from syringe drivers that are set at a particular flow rate. Manufacturers often make claims regarding the flow rate accuracy of their products but they do not always stand up to the scrutiny of laboratory tests.
Factors that could potentially affect the accuracy of flow rate in syringe drivers could include the temperature of the medication (increased temperatures decrease the viscosity of fluids) (Capes et al., 1997) or the height of the syringe driver compared with the patient (Donald et al., 2007).
This paper sets out to investigate flow patterns and accuracy using one model of syringe driver over the course of an administration period.
SMTL was requested by a client to assess the delivery accuracy of Luer Lock syringes from a standard syringe pump using the method in the International Standard ISO 60601-2-24. A range of different size syringes from 2 different manufacturers (known as A and B throughout this report) were used during this test. One calibrated syringe pump/driver was used for all the tests reported here. Throughout testing, the syringe driver software was locked for one model of syringe (syringe B). Syringe A is intended to be a drop-in replacement for Syringe B.
The test apparatus was assembled as shown in the diagram below:
The syringe was filled with de-ionised water and attached to an administration line with an 18 gauge cannula at the distal end. The tip of the cannula was submerged in a beaker containing a small amount of water and a thin layer of oil to prevent evaporation. The container was placed on an analytical electronic balance. The tip of the cannula and the pump chamber were positioned at the same height. The syringe driver was set to a particular flow rate as seen in the table below. Once the system was up and running, the mass of the infused solution was measured every 30 seconds by the analytical balance and recorded on a lap top coupled to the balance. The table below shows the syringe sizes used and the flow rates the syringe driver was set to in each case.
|Syringe size (ml)||
Set flow rate (ml per hour)
For each syringe size, 2 syringes (from each company) were tested over a 2 hour period.
The 2 'start-up' graphs below show the actual flow rate recorded every 30 seconds compared with the 'set' flow rate (shown as a dashed line). The results shown here are from 20 ml syringes set at 4 ml/h (results from supplier A are on the left and supplier B on the right).
Start up graph for syringe A (20 ml) set at 4 ml/h Start up graph for syringe B (20 ml) set at 4 ml/h
The 'trumpet' graphs below show the maximum positive and negative percentage deviation from the set flow rate in relation to the time interval over the first 31 minutes of administration. In general, the longer the observation period (the time interval over which the test is run), the more accurate the dose. This gives the graph its trumpet appearance. Again, the results are for 20 ml syringes set at 4 ml/h, (supplier A on the left and supplier B on the right).
Trumpet graph for syringe A (20 ml) set at 4 ml/h Trumpet graph for syringe B (20 ml) set at 4 ml/h
Results and Discussion
In general, results showed that flow rates were not smooth but fluctuated over time, above and below the set level. There were differences in accuracy between the two designs and also between syringes from the same supplier (data not shown).
The results clearly demonstrate that flow from syringe drivers is not steady and regularly fluctuates during the period of administration.
One of the potential reasons for this is 'stiction', where the friction between the surfaces of the syringe plunger and the barrel can cause irregularities in movement as the plunger progresses down the barrel. In addition, syringes are cheap, bulk produced items which are not necessarily identical, and this may lead to differences in flow rate accuracy between syringes from the same manufacturer (Ferrari and Beech '95).
When syringe driver manufacturers quote a percentage maximum and minimum deviation from the set flow rate, these figures may be misleading, especially if they are based on the average flow rate over the full time period compared with the set amount expected (Ferrari and Beech '95). Quoting the maximum and minimum percentage deviation from the set flow rate for fixed time points (as shown in the trumpet graphs) is a far more useful indication of flow accuracy, and enables users to determine if the driver/pump is fit for purpose.
Capes D, Martin K and Underwood R. Performance of a Restrictive Flow Device and an Electronic Syringe Driver
for Continuous Subcutaneous Infusion. Journal of Pain and Symptom Management 1997; 14(4):210-217
Donald A, Chinthamuneedi M and Spearritt D. Effect of changes in syringe driver height on flow:
a small quantitative study. Critical Care and Resuscitation. 2007; 9 (2) :143-147
Ferrari R. and Beech D. R. Infusion pumps:guidelines and pitfalls. Aust Prescr. 1995; 18:49-51