Electrophysiologic Basis Of Digitalis Induced Arrhythmias
More the 200 years have elapsed since the ublication in 1785 of William Witherings classic iccount of Foxglove and its medical uses. With the inical usage of digitalis came its toxicity and a gamut of troublesome arrhythmias. In the 1960s and early 1970s the reported incidence of digitalis loxicity was 25% with a mortality as high as 41%’.
Atrial and Ventricular Muscle Fibres
Changes in the duration of action potential in ventricular fibres, resembles the HPS though of lesser magnitude leading to decrease in QT interv al. An increase in slope of phase 2 and a decrease in slope of phase 3 accounts for the ST-T changes. In higher, toxic concentrations there is an overall decrease in the resting membrane potentials amplit ude of the action potential and the rate of depolariz ation of phase o. This decrease conduction veloci ty and ultimately causes inexcitability. These dram atic effects are seldom seen in clinical settings. Digitalis may lead to delayed after potentials in atrial or ventricular fibres leading to increased autom aticity.