Comparison of electrical and mechanical events during one cardiac cycle, using a normal electrocardiogram tracing.

In Figure 16-8, point 1 shows early ventricular diastole, when the atria and ventricles are at rest. Blood from the large veins is passively filling both atria. As the atria fill, the pressure in the atria exceeds the pressure in the ventricles, and the AV valves open in response to the pressure gradient. The blood from the atria now passively fills the ventricles.

At point 2, the beginning of late ventricular diastole, both ventricles remain relaxed and are about three fourths full. The SA node fires spontaneously (due to automaticity), and both atria depolarize, generating a P wave. The atria contract, and blood is actively moved from the atria into the ventricles: this “atrial kick” supplies approximately 20% to 30% of the ventricular blood volume.

At point 3, late in the PR interval, the action potential begun in the SA node is being delayed and “collected” in the AV node and travels to the bundle of His. The atria and ventricles are at rest.

At point 4, the action potential moves to the septum, which depolarizes and leads to the Q wave. Septal depolarization is rapidly followed by action potential movement down the right and left bundles into the Purkinje fibers to all cardiac muscle cells. These electrical events are seen as the RS wave on the ECG and are followed rapidly by mechanical contraction of both ventricles. The AV valves close, and the aortic and pulmonic valves open.

At point 5, the heart returns to early ventricular diastole, and the ventricles repolarize. This repolarization shows as a large, wide T wave. The aortic and pulmonic valves close about midway through repolarization.