A physiological model for the distribution of injected agents, with special reference to pethidine.

The model is based on Mapleson's Model P for inhaled anaesthetics, but has compartments for lungs, peripheral shunt, kidneys, portal bed, liver, other viscera, muscle, other lean, fat, brain, i.m. injection site and for various blood "pools". Intracellular and extracellular fluids are represented separately in each compartment and in blood. In each fluid, four forms of the agent are distinguished: unionized dissolved in water ("standard" form), ionized dissolved in water, unionized dissolved in lipid, and bound to protein. Equations define the equilibrium between these four forms in any one fluid and between blood and tissue. After changing two of the more uncertain numbers in the quantification for pethidine, good agreement was obtained between computed and published venous concentrations after single i.v. and i.m. injections, continuous i.v. infusions and repeated i.m. injections. The model can be used to make a wide variety of "what if" predictions.