Fluke Biomedical IDA-4 Plus - Manual (Page 63)

Infusion Pump Analyzer
Infusion Devices Delivery Methods
A
A-3
Control against over-infusion is effective, but increased resistance to flow may result in
under-infusion. The better controllers incorporate a flow status system in which users are
provided with a visual indication of increased resistance to flow, which could mean that
there is a ‘positional IV’ problem (catheter impinging on the vein wall) or even infiltration
(extra-vascular infusion in which the catheter has punctured the vein and the infusion
solution is infiltrating surrounding tissue). An alarm operates when there is a significant
departure from the set drip-rate. No alarm should ever be regarded as being a nuisance.
Controllers are recommended for use in many lower risk applications, involving fluid
replacement therapy where their low delivery pressure, early warning of problems at the
infusion site, inability to pump air and the reduced risk of extra-vascular infusion (due to
the low delivery pressure) are considerable advantages.
Drip-Rate Controllers
use standard solutions sets and although they look much like a pump, they have no
pumping mechanism. They are sometimes marked CONTROLLER. The flow rate is
selected in drops per minute and controlled by battery or mains powered line occlusion
valves. Controls tend to be few and simple to operate. More advanced models incorporate
a flow status system, which gives a visual indication of resistance to flow. A drop sensor
is always included.
Drip-rate controllers are suitable for the majority of lower risk infusions in which
volumetric accuracy is of lesser importance, see below.
Accuracy of Drip Controlled Devices
Because of the high accuracy of drop counting mechanisms used by gravity controllers, it
is natural to assume that volume delivery will be correct according to the conversion used -
it may not be.
The normal drops/ml for a set, or the value taken from a conversion chart, are approximate
and the actual drops/ml may differ considerably. The volume of fluid in a drop depends on
a number of variables: the composition of the fluid, its temperature and surface tension; the
drip rate set; the size, shape and condition of the drop forming orifice. Most simple
aqueous solutions of electrolytes, lactates or dilute sugars, give drop volumes fairly close
to the expected nominal drops/ml, except at very low drip rates. However, total parenteral
nutrition (fat-soluble vitamins, solutions containing alcohol and some amino-acid
solutions) produce much smaller drops. The flow rate will therefore be lower than expected
and the infusion time will be increased. With all fluids, the drops/ml value falls (drop
volume rises) with increasing delivery rate.
For the majority of infusions, these variations are acceptable, but where volumetric
accuracy is considered to be critical, the use of a volumetric pump (or a syringe pump if
the volume to be infused is small) is essential.
Volumetric Controllers
are calibrated in milliliters per hour, have a drop sensor, may have more controls and
alarms than a drip-rate controller and may need a dedicated solution set or a disposable
‘rate clip’.
Volumetric controllers include compensation for drop size. The main problem is knowing
what compensation to apply, as drop size varies due to several factors. Volumetric
accuracy is dependent upon the compensation being correct under all circumstances. With
present technology, a number of compromises have to be accepted and a fluid code system
is usually adopted which groups like infusates together. It is generally considered that the
accuracy of delivery of these devices cannot be much better than
± 10% over a one-hour
period.
The only advantage of using a volumetric controller, rather than a drip-rate controller, is
that no calculations have to be made for the drip rate of the administration set and so this
potential source of error is eliminated.