Do plasma levels of digitoxin and digoxin correlate with cardiac response following intravenous administration in volunteers?
Plasma levels of digitoxin and digoxin correlate with their cardiac effects, but levels drawn prior to plasma-tissue equilibration will be higher than those seen during maintenance administration.
The value of plasma digitalis determinations will depend upon the accuracy with which they mirror the myocardial effects of digitalis. Ten volunteers received 0.8 mg of digitoxin intravenously, and after 2 hours the mean plasma digitoxin level was 45.6 ± 6 mµg/ml but declined over 4 to 6 hours during plasma-tissue equilibration. The cardiac response, as indicated by decreases in the left ventricular ejection time index (LVETI) and Q-S 2 intervals ( P < 0.01), was present at 2 hours and reached maximum at 4 to 6 hours. Subsequently, plasma level, LVETI, and Q-S 2 changed in the same direction during slow excretion. Six subjects received 1.0 mg of digoxin intravenously. Mean plasma digoxin at 30 min was 6.8 ± 0.3 mµg/ml; it fell after 3 to 4 hours to levels usually seen during maintenance digoxin administration (<3 mµg/ml) and then declined more slowly. Correlation of individual digoxin levels with ΔLVETI values during the first 4 hours was significant ( P < 0.01). The half-life (T ½ ) for the dominant slope of the plasma curve was 30.5 hours by plasma determinations and 29 hours by LVETI determination. Plasma levels of digitoxin and digoxin were related to their cardiac effects under the conditions studied. Prior to plasma-tissue equilibration, plasma determinations of digoxin and digitoxin will be higher than levels seen during maintenance administration of these drugs.
Shapiro et al. (1970) studied this question.