Determination of free disopyramide in blood serum in situ using a disopyramide-sensitive membrane electrode

Free disopyramide levels in blood serum, the monitoring of which is especially effective for antiarrhythmic therapy, were determined in situ using a disopyramide-sensitive membrane electrode. Sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate was incorporated as the ion-exchanger and 2-fluoro-2′-nitrodiphenyl ether as the solvent mediator in a poly(vinyl chloride) membrane matrix to form the electrode. The lower limit of the detection for free disopyramide in serum was 0.8 μM. The free disopyramide concentrations measured by the ion-selective electrode agreed well with those determined by an established fluorescence polarization immunoassay that requires an additional ultrafiltration procedure to remove disopyramide bound to serum protein.