Final answer:
An electrocardiogram (ECG) graphically displays the heart's electrical activity during depolarization and repolarization. The direction of mean depolarization normally goes from the right shoulder to the left leg, corresponding to the heart's natural position. Changes in this direction are significant and can signal heart abnormalities.
Step-by-step explanation:
An electrocardiogram (ECG or EKG) is a graphical representation of the electrical activity of the heart as it undergoes depolarization and repolarization. Standard ECGs feature multiple leads, but for the basis of understanding, we focus on the limb leads, specifically lead II. Electrodes are placed on the right arm (RA), left arm (LA), and left leg (LL), measuring the voltage differences as the heart muscle depolarizes and repolarizes.
The axis formed by the limb leads views of the heart is depicted as a hexaxial reference system, where Lead I is the horizontal axis, and Lead II and III form angles of 30-degree segments with it. The direction of mean depolarization typically travels from the right shoulder (negative electrode) towards the left leg (positive electrode) in a downward and to the left direction because of the natural orientation of the heart within the chest.
Mean depolarization usually changes direction due to changes in heart morphology, conduction abnormalities, or pathologies such as myocardial infarction. The mean electrical axis is an indicative marker used to diagnose these conditions. Analysis of the lead II potential is particularly crucial as it correlates with the arterial blood pressure and provides insight into the heart's pumping function.