An electrocardiogram (ECG or EKG) is a non-invasive medical test that records the electrical activity of the heart over a period of time. Electrodes placed on the skin detect the tiny electrical changes that occur as the heart muscle contracts and relaxes with each beat. The resulting tracing provides a graphical representation of the heart’s electrical cycle, which is composed of distinct waves, segments, and intervals. Understanding these waveforms allows clinicians to assess the heart’s rhythm and overall function. This article focuses on defining the S wave and explaining what its characteristics reveal about the heart’s electrical health.
The Basics of Cardiac Electrical Activity
The heart’s ability to pump blood relies on a precise electrical impulse that coordinates the contraction of its muscular walls. This electrical signal originates in specialized pacemaker cells and travels through the heart chambers in a specific sequence. The waves seen on an ECG are a direct visualization of two main electrical processes: depolarization and repolarization.
Depolarization is the electrical activation of heart muscle cells, which immediately precedes and triggers muscle contraction. This process involves a rapid influx of positively charged ions, causing the cell’s internal charge to switch from negative to positive. Repolarization is the electrical recovery phase, where the cells reset their charge back to the resting state, preparing for the next beat.
The electrical impulse begins in the upper chambers, the atria, and then moves rapidly down to the lower, larger chambers, the ventricles, which perform the main pumping action. The P wave on the ECG represents atrial depolarization, while the QRS complex and the T wave represent the depolarization and repolarization of the ventricles, respectively. The direction of the electrical current flow through the heart determines whether the resulting wave moves upward (positive deflection) or downward (negative deflection) relative to the recording electrode.
Placing the S Wave: Understanding the QRS Complex
The QRS complex is the most visually prominent and central feature on a standard ECG tracing, representing the rapid depolarization of the heart’s powerful ventricles. This depolarization event is responsible for triggering the ventricular contraction that pushes blood out to the lungs and the rest of the body. The entire QRS complex normally completes its cycle in a very short duration, typically lasting between 0.06 and 0.10 seconds in an adult.
The complex is named for its three potential components, the Q, R, and S waves, which occur in rapid succession. The Q wave is defined as the first downward deflection immediately following the P wave, often reflecting the initial activation of the interventricular septum. Following this is the R wave, which is the first upward or positive deflection and represents the depolarization of the main mass of the ventricles.
The Q, R, and S waves are grouped together because they collectively map the entire electrical activation sequence of the ventricles. Even if all three waves are not distinctly visible in every electrode view, the entire waveform is still referred to as the QRS complex.
What the S Wave Specifically Represents
The S wave represents the final electrical event in the ventricular depolarization sequence and is defined as the first negative deflection that immediately follows the R wave. Physiologically, the S wave signifies the completion of the electrical activation as the impulse spreads through the final, basal portions of the ventricles. This electrical activity includes the depolarization of the upper posterior wall of the left ventricle and the pulmonary conus.
The reason the S wave is typically a downward (negative) deflection relates to the direction of the electrical current relative to the recording electrodes. An electrical impulse moving toward a positive electrode creates an upward deflection, while an impulse moving away creates a downward deflection. The final vector of ventricular depolarization, which corresponds to the S wave, moves upward and backward toward the base of the heart, effectively moving away from the commonly placed chest electrodes like V3 and V4, thus causing the deep negative deflection often observed in those leads.
The depth and duration of the S wave can vary significantly across the twelve different views, or “leads,” recorded by the ECG machine. S waves are often deepest in the right-sided chest leads (V1 and V2) and progressively become smaller as the electrical view moves toward the left side of the chest (V5 and V6). This transition, known as R-wave progression, is a normal finding and reflects the changing perspective of the electrical axis as the electrodes move across the chest.
Clinical Significance of S Wave Variations
Variations in the normal appearance of the S wave provide important diagnostic clues for clinicians evaluating heart health. Any deviation from the typical depth, width, or overall shape of the S wave can suggest an underlying cardiac issue. The width or duration of the S wave is a specific measure of how long the final ventricular depolarization takes to complete.
A significantly widened S wave indicates a delay in the electrical conduction through the ventricles, a common sign of a condition called a bundle branch block. In this condition, the impulse travels through one ventricle normally but is delayed in the other, causing the QRS complex, including the S wave, to be prolonged beyond the normal 0.10-second limit.
Furthermore, an abnormally deep S wave, particularly when combined with a very tall R wave, can be a marker for ventricular hypertrophy, which is the thickening of the ventricular muscle mass. Since the electrical current is proportional to the muscle mass, an enlarged ventricle generates larger electrical potentials, leading to a deeper S wave in specific leads. Deep S waves are also associated with conditions like pulmonary hypertension or certain cardiomyopathies. Changes in the S wave amplitude, such as a decrease in depth, can also be observed during episodes of acute myocardial ischemia, indicating a lack of blood flow to the heart muscle.