What is the S wave in ECG?

As a cardiac electrophysiologist with extensive experience in interpreting electrocardiograms (ECGs), I can provide a detailed explanation of the S wave in an ECG. The S wave in an ECG is the first downward deflection that follows the R wave within the QRS complex. The QRS complex represents the electrical activity of the ventricles as they depolarize during a heartbeat. The S wave is particularly significant in leads that are looking at the heart from a direction where the depolarization wave is moving away from the electrode. It's important to note that an S wave may not be present in all ECG leads for a given patient. The size and presence of the S wave can vary depending on the lead and the patient's specific anatomy and physiology. In a normal ECG, the S wave is typically most prominent in the lead V1, and its amplitude tends to decrease as you move across to lead V6, where it may be almost nonexistent. The S wave is not only a normal finding but can also be indicative of certain conditions when abnormally large or deep, such as left ventricular hypertrophy (LVH) or other pathologies affecting the heart's conduction system.

The S wave is the first downward deflection of the QRS complex that occurs after the R wave. However, a S wave may not be present in all ECG leads in a given patient. In the normal ECG, there is a large S wave in V1 that progressively becomes smaller, to the point that almost no S wave is present in V6.