Calculation methods for the continuous intrathecal infusion of a solution containing two or three drugs

The continuous intrathecal infusion of drugs with implantable and programmable pumps is commonly used for the treatment of otherwise intractable pain and spasticity. There is now a consensus that it is possible to use mixtures of two or even three drugs in selected cases. The calculation of the parameters of the pump is easy for a single drug, but it becomes more complicated for a combination of two or three drugs. This paper offers some algebraic formulae that make easier and faster the calculation of the following values: the flow of the pump, the volume of each drug solution to be added to the final mixture, the concentration of each drug in the mixture and the duration of the infusion. These values are determined from the desired quantity of each drug to be infused in 24h, the concentration of each drug solution and the volume of the reservoir. It is essential that the drugs do not react with one another, that they will remain stable in the reservoir at body temperature and that they are safe for a long-term infusion.