The EKG Waveform

Wiggers Diagram
The Wiggers diagrams are good for showing that the electrical activity (depolarization) recorded on an EKG actually well before actual mechanical contraction of the atria and ventricles.
The EKG at the bottom of the Wigger's diagram (below), illustrates normal sinus rhythm. Atrial and ventricular contraction are shown as pressure changes in the atria, ventricles and aorta. The dotted lines have been included to help you note the time mechanical activity occurs relative to EKG activity.
Note that:
i. Atrial contraction 'a' begins the middle of the P-wave and continues throughout the PR interval. Note the atrial pressure increasing as the atria contract. As blood is pumped into the ventricles the ventricular pressure also rises. The PR interval corresponds to the delay necessary for the ventricles to fill after atrial contraction. Note that the atrial repolarization wave (electrical impulse) is usually hidden by the QRS complex and atrial muscle relaxation occurs after the QRS complex and is accompanied by a decrease in atrial pressure.
iii. Ventricular contraction begins at point 'b' which corresponds to the peak of the QRS complex, and continues during the ST segment and T-wave. At point 'b' the mitral (and bicuspid) valves close due to increase in ventricular pressure (as the ventricles contract). The closing mitral and biscupid valves produce the first heart sound.
Between point 'b' and 'c', ventricular pressure increases sharply since the semilunar valves are still closed and there is no blood flow (ventrular pressure is still below aortic pressure) . The semilunar valves open at point 'c' when the ventricular pressure equals the aortic pressure. The ventricular contraction forces blood into the aorta and an increase in both aortic and ventricular pressure is noted at 'd'. As blood is pumped from the ventricles and carried away in the aorta, ventricular pressure drops. When the pressure drops below aortic pressure, the semilunar valves slam shut 'e'.
Ventricular muscle repolarization begins at the end of the T-wave and causes further decrease in ventricular pressure. At 'f' the ventricular pressure falls below atrial pressure and the mitral and bicuspid valves open.

Last Updated: 01/03/00.
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