An electrocardiogram (ECG) is a graphic representation of the electrical activity that drives the heartbeat. This non-invasive diagnostic tool records the electrical currents generated by the heart muscle as they spread throughout the body. Translating these impulses into a wave pattern, the ECG provides a snapshot of the heart’s rhythm and overall function. The fundamental unit of this tracing is the PQRST complex, which illustrates the full sequence of one cardiac cycle.
The Standard PQRST Sequence
The electrical events of a single heartbeat are organized into the predictable PQRST sequence. The P wave, the first deflection, reflects the electrical activation (depolarization) of the atria, the heart’s upper chambers. Following the P wave, the electrical signal passes through a brief pause, represented by the flat PR segment, before reaching the ventricles.
The QRS complex is the most prominent feature of the tracing, representing the rapid depolarization of the ventricles, the heart’s main pumping chambers. The S wave is the final component of this ventricular depolarization complex. It appears as the first negative (downward) deflection that immediately follows the R wave, the large, positive central peak.
The Electrical Meaning of the S Wave
The S wave’s electrical origin lies in the final moments of ventricular depolarization. While the Q and R waves represent the initial spread of activation through the septum and the main muscle mass, the S wave reflects the electrical impulse moving through the upper and basal portions of the ventricular walls. This terminal spread of activation often moves back toward the atrioventricular (AV) junction.
The characteristic downward deflection occurs because the net electrical vector of this final activity is traveling away from the positive exploring electrode of the ECG lead. A negative deflection is recorded whenever the electrical signal moves away from the positive electrode. Therefore, the S wave signifies the completion of the electrical signal that triggers the muscular contraction of the ventricles.
Assessing a Normal S Wave
The S wave in a healthy heart is a sharp, downward deflection whose appearance varies depending on the electrode placement. Its depth (amplitude) is measured vertically from the baseline, while its duration (width) is measured horizontally using the small squares on the ECG paper. On standard ECG paper, each small box represents 0.04 seconds horizontally and 0.1 millivolt vertically.
The duration of the entire QRS complex, which includes the S wave, normally falls between 0.06 and 0.10 seconds. In the precordial leads placed across the chest, the S wave is small in V5 and V6 (reflecting the left side of the heart), but deep and prominent in leads V1 and V2 (over the right side). This variation reflects the shift in the electrical axis as the signal moves across the heart.
What Abnormal S Waves Can Indicate
Deviations in the S wave’s depth or width are markers for underlying cardiac conditions. A significantly deep S wave in the right-sided leads (V1 or V2) may suggest right ventricular hypertrophy, an enlargement of the right ventricle muscle. This increase in muscle mass generates a larger electrical signal, causing the prominent S wave deflection. Conversely, a deep S wave combined with a tall R wave in the left-sided leads suggests left ventricular hypertrophy.
An S wave that is abnormally wide or prolonged, often exceeding 0.04 seconds, indicates a delay in the electrical conduction system. This widening can be the visual signature of a condition like a bundle branch block, where the electrical impulse is slowed as it spreads through the ventricles. For example, in a right bundle branch block, the delayed activation of the right ventricle causes a late, wide S wave to appear in the left-sided leads. Attenuated or absent S waves in expected leads can also suggest previous damage to the heart muscle, as electrically inactive scar tissue from an old myocardial infarction fails to generate the final electrical forces.