Monitoring blood levels of cardioactive drugs.

The availability of specific, sensitive, and convenient analytic methods has simplified determinations of plasma concentrations of several cardioactive drugs. The available data indicate that, although in general the plasma concentrations of cardioactive drugs bear a more consistent relationship to their pharmacologic effects than the doses administered, they cannot substitute for careful clinical observation and judgment. Plasma concentrations of these drugs, following a given dose, are influenced by several factors which include patient's size, renal, hepatic, cardiac, and gastrointestinal function, bioavailability of the drug, and interplay with other concurrently administered drugs. These factors cannot be predicted with any degree of accuracy before starting therapy, but measuring plasma drug concentrations under appropriate conditions can be helpful in determining individual differences in drug disposition. The majority of patients can be treated without the use of plasma concentrations of digitalis and antiarrhythmic drugs. However, the availability of measurements can be helpful in a variety of specific clinical situations discussed in the text. In practice, plasma concentrations have been most valuable in the recognition of digitalis toxicity and as guides to therapy of resistant tachyarrhythmias requiring high dosages of single or multiple antiarrhythmic drugs.