PHARMACOKINETICS IN PATIENTS WITH CARDIAC-FAILURE

Cardiac failure is often associated with disturbances in cardiac output, autonomic nervous system activity, central and systemic venous pressures and Na and H2O metabolism. These disturbances influence the extent and pattern of tissue perfusion, may lead to tissue hypoxia and visceral congestion and may alter gastrointestinal motility. By these mechanisms, cardiac failure potentially affects absorption and disposition characteristics of drugs, which may necessitate adjustment in dosage regimen for optimum therapy. Lignocaine is the drug most extensively studied in cardiac failure. Volumes of distribution and clearance are decreased. As a drug whose metabolism is largely limited by liver blood flow, decreased blood flow to the liver accounts for some of the change in clearance, but impaired hepatic metabolism appears also to play a role in some patients. Accumulation of active metabolites of lignocaine and procainamide in patients with cardiac failure can influence therapeutic and toxic effects. Theophylline metabolism, which is largely independent of blood flow, appears to be reduced significantly in patients with severe cardiac failure and necessitates reduction of dosage. In the presence of severe cardiac faulure, digoxin clearance may be less than anticipated on the basis of estimates of renal function. Quinidine plasma levels may be higher after single doses due to reduced volume of distribution. Quinidine metabolites are believed not to be pharmacologically active but may create confusion with nonspecific assays. Specific assays are recommended in cardiac failure, especially complicated by renal insufficiency. Data are lacking relating pharmacokinetic alterations to hemodynamic measurements in patients with cardiac failure. Whereas the direction of change in pharmacokinetic parameters may be predicted, variability in the magnitude of change is so great that determination of drug concentration in blood remains an essential adjunct to therapy.