John/ Susanne
The Short-term accuracy /constancy index you refer to is highlighted in page 53, MDA BD2003(02).
http://nww.medical-devices.nhs.uk APPENDIX 1 Choosing infusion devices according to therapy category.
Information about the technical performance of infusion devices is provided by the MDA Infusion Pump Evaluation programme of the Device Evaluation Service.
http://nww.mda.nhs.uk/pumpevaluation Short-term accuracy
Minute-to-minute variability of flow becomes very important where drugs with short half-lives are being administered. The importance derives from the need to prevent undesirable fluctuation of effect-site concentration of the drug. There is documented evidence that minute-to-minute variability of flow can cause variation of physiological parameters and consequent difficulties in management, in both adults and neonates, where the half-life of the drug is short. Short-term accuracy is expressed by the concept of constancy index. This is the shortest period during the pump's steady-state operation over which measurement of output consistently falls within ±10% of the mean rate. The data are derived from the flow tests performed over 24 hours at 1ml/h. Flow is recorded at 30 second intervals over the final 18 hour period. And the average rate compared with flow over each short period. The principle is that the constancy index of the pump should be less than or equal to the half-life of the drug used. Syringe pumps have a shorter constancy index than volumetric pumps i.e. a low constancy index indicates good short-term flow rate accuracy. Battery operated ambulatory pumps frequently have a constancy index in excess of 31 minutes making these particular devices unsuitable for use with short half-life drugs. For the purposes of this bulletin the half-life of a drug is the time taken for the concentration of that drug to be reduced by 50 % either by metabolism or excretion.
Notes
1. The half-life of a drug cannot usually be specified precisely, and may vary from patient to patient. As a rough guide, drugs with half-lives of the order of five minutes or less might be regarded as "short' half-life drugs.
2. Diamorphine is a special case. The injected agent (diamorphine) has a short half-life, whilst the active agent (the metabolite) has a very long half-life. It is safe to use a device with performance specifications appropriate to the half-life of the metabolite.
Bill
