The Heart's Electrical Sequence

The synchronized electrical sequence of the heart is initiated by the SA node, the heart's natural pacemaker. The firing of the SA node sends out an electrical impulse via its neurons to the right atrium, left atrium, and AV node simultaneously. Since the right atrium is closer to the SA node, it depolarizes first, resulting in pumping action by the right atrium before the left atrium. At the AV node, the impulse is delayed to allow for the ventricles to fill up with blood. After the delay, the AV node sends the impulse to the Bundle of His and the Purkinje fibers. This triggers the contraction of the ventricles to send blood either to the lungs or out to the body.

Electrocardiograms

The electocardiogram or ECG is a major diagnostic tool for the assessment of the health of the heart. It is a measurement taken at the surface of the skin which reflects the electrical phenomena in the heart when the SA node triggers the electrical sequence that controls heart action.

Vector Electrocardiograms

The electocardiograph or ECG is a major diagnostic tool for the assessment of the health of the heart. It detects the electric field produced by the electrical polarization of the heart muscle as it progresses through its pumping sequence. The voltage between each pair of electrodes measures the scalar magnitude of that electric field, but the electric field itself is a vector quantity. But with a set of at least three electrodes, one can measure two components and obtain a value for the magnitude and direction of the field at the surface.

Many different combinations of electrodes have been used clinically, up to twelve leads. From these combinations the electric field vector representing the heart's electrical activity can be determined as a function of time.

With an appropriately positioned set of electrodes, a representation of the electric field of the heart muscle's electrical activity as a function of time can be obtained.