PHAR 200 Study Guide - Midterm Guide: Qt Interval, Hypotension, Torsades De Pointes
Document Summary
Antiarrhythmic drugs: procainamide, metoprolol, amiodarone, verapamil and adenosine. Blockade of the sodium channel decreases automaticity by reducing the phase 4 slope in the sa node and increases threshold in myocardium by decreasing the phase 0 upstroke. It is important that most sodium channel blockers are state dependent. The sodium channel exists in an open state (ion flux) followed by an inactive state (refractory; closed but not repolarized). Finally, the channel attains a closed (i. e. ready to open) state when the membrane repolarizes. Most channel blockers bind to the open and/or inactivated states, and they dissociate from resting channels. So, the blockers bind better when the firing rate is high (i. e. more open and inactivated channels). They also dissociate more slowly in ischemic tissue where depolarization lasts longer (less resting channels). Finally, these drugs are not highly selective for sodium channels, and like procainamide, also affect other channels.
