Electrocardiogram(ECG)

The electrical activity of the heart can be recorded to monitor cardiac changes or diagnose potential cardiac problems. Because body fluids contain electrolytes, they are good electrical conductors. Electrical impulses generated in the heart are conducted through body fluids to the skin, where they can be amplified, detected, and printed out by a machine called an electrocardiograph. This printout is called electrocardiogram, or ECG. A standard ECG is recorded from 10 electrodes placed in specific anatomical locations. These 10 electrodes correspond to 12 leads that represent different views of the heart. Three basic components of the ECG represent important aspects of cardiac function:

• The P wave;
• The QRS complex;
• The T wave.

The P wave represents atrial depolarization and occurs when the electrical impulse travels from the SA node through the atria to the AV node. The QRS complex represents ventricular depolarization and occurs as the impulse spreads from the AV bundle to the Purkinje fibers and through the ventricles. The T wave represents ventricular repolarization. Atrial repolarization cannot be seen, because it occurs during ventricular depolarization(QRS complex).

Electrocardiograms are often obtained during exercise as clinical diagnostic tests of cardiac function. As exercise intensity increases, the heart must beat faster and work harder to deliver more blood to active muscles. Indications of coronary artery disease, not evident at rest, may show up on the ECG as the heart increases its rate of work. Exercise ECGs are also invaluable tools for research in exercise physiology because they provide a convenient method for tracking heart rate and rhythm changes during acute exercise.