Superior vena cava Phase 0 SA node Atrium AV node Overshoot 0 Phase 0 mV Purkinje -100 Tricuspid Resting potentia valve Mitral valve Action potential phases Ventricle 0:Upstroke 1:Early-fast repolarization 2:Plateau 3:Repolarization 4:Diastole ECG Q S PR- QT 200ms Figure 1.Schematic representation of the heart and normal cardiac electrical activity (intracellular recordings from areas indicated and ECG).Sinoatrial node, atrioventricular node,and Purkinje cells display pacemaker activity (phase 4 depolarization).The ECG is the body surface manifestation of the depolarization and repolarization waves of the heart.The P wave is generated by atrial depolarization, the QRS by ventricular muscle depolarization,and the T wave by ventricular repolarization.Thus,the PR interval is a measure of conduction time from atrium to ventricle,and the QRS duration indicates the time required for all of the ventricular cells to be activated (ie,the intraventricular conduction time).The QT interval reflects the duration of the ventricular action potential. Ionic Basis of Membrane Electrical Activity The transmembrane potential of cardiac cells is determined by the concentrations of several ions chiefly sodium (Na),potassium (K),calcium (Ca2),and chloride (Cl)on either side of the membrane and the permeability of the membrane to each ion.These water-soluble ions are unable to freely diffuse across the lipid cell membrane in response to their electrical and concentration gradients;they require aqueous channels(specific pore-forming proteins)for such diffusion.Thus,ions move across cell membranes in response to their gradients only at specific times during the 33 Figure 1. Schematic representation of the heart and normal cardiac electrical activity (intracellular recordings from areas indicated and ECG). Sinoatrial node, atrioventricular node, and Purkinje cells display pacemaker activity (phase 4 depolarization). The ECG is the body surface manifestation of the depolarization and repolarization waves of the heart. The P wave is generated by atrial depolarization, the QRS by ventricular muscle depolarization, and the T wave by ventricular repolarization. Thus, the PR interval is a measure of conduction time from atrium to ventricle, and the QRS duration indicates the time required for all of the ventricular cells to be activated (ie, the intraventricular conduction time). The QT interval reflects the duration of the ventricular action potential. Ionic Basis of Membrane Electrical Activity The transmembrane potential of cardiac cells is determined by the concentrations of several ions chiefly sodium (Na+ ), potassium (K+ ), calcium (Ca2+), and chloride (Cl- ) on either side of the membrane and the permeability of the membrane to each ion. These water-soluble ions are unable to freely diffuse across the lipid cell membrane in response to their electrical and concentration gradients; they require aqueous channels (specific pore-forming proteins) for such diffusion. Thus, ions move across cell membranes in response to their gradients only at specific times during the