Plasma and myocardial digoxin concentrations in patients on oral therapy.

Methods are now available which are sensitive enough to detect digoxin concentration to levels of 5 mug./ml. Hitherto the degree of sensitivity of specific chemical methods was not acceptable for clinical application. Digoxin-specific antibodies have recently been prepared (Butler and Chen, 1967), and a test based on the radioimmunoassay principle has now been used to measure plasma digoxin levels (Oliver et al., 1968). The double isotope dilution technique is a specific and very sensitive method for measuring steroids in biological extracts, and it has been applied successfully to determining plasma digitoxin levels in man (Lukas and Peterson, 1966). However, there are no reports on its use for measuring plasma digoxin. The methods using the isotope dilution or radioimmunoassay techniques are technically difficult and laborious, but Lowenstein has used a biological assay technique based on the inhibition of the uptake of 86rubidium by human red cells in the presence of small quantities of glycosides (Lowenstein, 1965; Lowenstein and Corrill, 1966; Morgan and Binnion, 1967). Multiple plasma samples containing no acetyl strophanthidin gave a result above 2 m,ug./ml. in 1 in every 7 samples (P. F. Binnion, S. A. Hawkins, and L. M. Morgan, 1969, Acetyl strophanthidin tolerance test. The relationship between plasma levels of acetyl strophanthidin and potassium and induced dysrhythmias. Unpublished observations), but the method is relatively easy to perform and is as sensitive as more elaborate methods. The results of plasma digoxin concentrations in patients on oral therapy and also the myocardial concentrations of the glycoside determined in other patients undergoing mitral valvotomy are presented. In this way it is possible to see whether plasma levels reflect myocardial concentrations, for it must be the concentration of the digitalis glycoside in the