Why can direction make a physiological vector clinically useful? Use the cardiac electrical axis as an example.

### The concept A vector provides information that a magnitude-only measurement cannot: it shows the direction of the measured effect. In cardiovascular physiology, the mean QRS vector is described as the heart's electrical axis. The material notes that examining this axis can help diagnose ventricular hypertrophy, which may shift the axis toward the enlarged side, and myocardial infarctions. ### What a good answer covers - State that a vector includes magnitude and direction. - Identify the mean QRS vector as the cardiac electrical axis. - Explain that axis direction can shift with ventricular hypertrophy. - Mention its clinical use in assessing cardiac pathology, including myocardial infarction. ### Terms - **Mean QRS vector**: the average direction of the heart's QRS electrical activity. - **Electrical axis**: the overall direction of cardiac electrical activity. - **Ventricular hypertrophy**: enlargement of a heart ventricle. - **Myocardial infarction**: injury to heart muscle associated with interruption of its blood supply. ### Remember A cardiac vector is clinically informative because its direction can change with disease.