An electrocardiogram (ECG or EKG) is a quick, non-invasive test that provides a graphic representation of the heart’s continuous electrical activity. Electrodes placed on the body’s surface detect minute changes in electrical potential as the heart muscle contracts and relaxes. The ECG measures the rate and rhythm of the heartbeat, offering insight into the function of the heart’s electrical conduction system and the health of the cardiac muscle.
The Underlying Electrical Rhythm
The entire tracing is a visual record of two fundamental electrical events: depolarization and repolarization. Depolarization is the electrical discharge that travels through the heart cells, triggering the muscle to contract and pump blood. This movement of positive charge is registered by the ECG machine as a wave deflection from the baseline.
Immediately following depolarization, heart muscle cells reset their electrical state in a process known as repolarization. This electrical recovery phase is also recorded on the ECG as a wave or segment. The orderly movement of these electrical charges throughout the cardiac chambers translates into the characteristic pattern of waves and flat segments. This cycle of electrical activation and recovery is necessary for the mechanical pumping action of the heart.
Atrial Activation: The P Wave
The first small, rounded deflection is the P wave, which represents the electrical activation of the heart’s upper chambers, the atria. This upward deflection corresponds to atrial depolarization, the impulse that causes the atria to contract and push blood into the ventricles. The P wave is relatively small because the atria possess a significantly smaller muscle mass compared to the ventricles.
Immediately following the P wave is the PR segment, a flat line that illustrates a brief, deliberate pause in electrical conduction. The signal has reached the atrioventricular (AV) node, where it is intentionally delayed before moving to the ventricles. This delay allows the ventricles sufficient time to fill completely with blood from the contracting atria.
The PR interval is the entire duration from the beginning of the P wave to the start of the next large complex. It reflects the total time required for the electrical signal to travel from the atria to the ventricles.
Ventricular Contraction: The QRS Complex
The QRS complex is the most visually prominent feature of the ECG tracing, representing the powerful electrical event preceding the heart’s main pumping action. This complex marks ventricular depolarization, the rapid electrical spread through the lower chambers (ventricles), causing them to contract forcefully. The complex is composed of three potential deflections: the Q wave (downward), the R wave (large upward), and the S wave (final downward).
The electrical activity of the QRS complex is notably taller and sharper than the P wave due to the ventricles’ much larger muscle mass. Since the ventricles pump blood to the lungs and the rest of the body, they require a greater electrical signal. The rapid, widespread electrical transmission through the specialized Purkinje fibers results in the QRS complex’s characteristic narrow duration, typically lasting between 0.06 and 0.10 seconds.
While the ventricles are depolarizing, the atria simultaneously undergo repolarization. However, the electrical signal generated by atrial repolarization is so small that the massive electrical forces of the QRS complex completely obscure it. Consequently, this atrial recovery phase is not visible on the standard ECG tracing.
Ventricular Recovery: The T Wave
Following the electrical event of the QRS complex, the heart begins its recovery phase, represented by the T wave. This final wave indicates ventricular repolarization, the process where the ventricular muscle cells electrically reset themselves for the next heartbeat. Repolarization is a slower, more gradual process than depolarization, which gives the T wave a broader, more rounded appearance.
The flat line connecting the end of the QRS complex to the beginning of the T wave is called the ST segment. This segment represents the plateau phase of the ventricular action potential, a brief moment when the ventricles are fully contracted before recovery begins. The ST segment should normally be level with the baseline; any deviation can indicate myocardial injury or ischemia.
The T wave completes the basic electrical cycle, illustrating the full electrical recovery of the ventricles before the next P wave starts the sequence anew. The shape and direction of the T wave, along with the appearance of the ST segment, offer information regarding the health of the ventricular muscle. The entire pattern of the P wave, QRS complex, and T wave represents a single, complete heartbeat.