An electrocardiogram (ECG or EKG) is a non-invasive test that provides a visual tracing of the heart’s electrical activity over time. Electrodes placed on the skin detect tiny electrical changes that govern the heart’s rhythmic, coordinated pumping action. The heart is essentially an electrical organ, where impulses generated by specialized cells spread outward to trigger muscle contraction. The ECG measures the magnitude and timing of these electrical signals, offering a detailed map of how the heart functions during each beat.
Translating Heart Activity into Waves
The electrical events within the heart are translated into characteristic deflections, or waves, on the ECG tracing. This activity is driven by two primary processes: depolarization and repolarization. Depolarization is the rapid electrical activation that moves through the muscle, causing the cells to shift their charge and subsequently contract. Repolarization immediately follows, representing the electrical recovery phase where cells return to their resting state, allowing the muscle to relax and refill with blood.
The complete cardiac cycle on the ECG is composed of three distinct segments: the P wave, the QRS complex, and the T wave. The P wave is the first small deflection, representing the depolarization of the upper chambers (atria). This atrial depolarization causes the atria to contract, pushing blood into the lower chambers.
Following a brief flat segment, the electrical signal reaches the ventricles, triggering a much larger and sharper deflection. This complex, known as the QRS complex, signifies the depolarization of the ventricles. The T wave marks the electrical recovery (repolarization) of the ventricular muscle, preparing the chambers for the next beat.
The QRS Complex: Ventricular Depolarization
The specific part of the ECG tracing that displays ventricular depolarization is the QRS complex. This complex is the most prominent feature because the ventricles contain a significantly greater mass of muscle tissue than the atria. The resulting stronger electrical current causes a large vertical deflection on the graph.
Ventricular depolarization is a rapid event, typically lasting 80 to 100 milliseconds. This speed is achieved because the electrical signal is distributed through a highly efficient network of specialized fibers. This rapid electrical spread ensures the entire ventricular mass contracts almost simultaneously, accounting for the QRS complex’s characteristic sharp, narrow appearance.
The QRS complex consists of up to three individual waves, though not all three are present in every recording lead. The rapid sequence of these waves collectively illustrates the swift electrical signal that precedes the mechanical contraction (systole) of the heart’s main pumping chambers.
Components of the QRS Complex
The Q wave is the initial downward deflection after the P wave, representing the depolarization of the interventricular septum.
The R wave is the first upward deflection and often the tallest point on the entire ECG, reflecting the depolarization of the main ventricular walls.
The S wave is the final component, appearing as a downward deflection immediately following the R wave, representing the depolarization of the base of the ventricles.
Completing the Cycle: Ventricular Repolarization
The electrical cycle for the ventricles is completed by the T wave, which represents the phase of ventricular repolarization. This is the necessary recovery period during which the ventricular muscle cells restore their negative electrical charge, allowing them to relax and prepare for the next depolarization signal. The T wave follows the QRS complex and the relatively flat ST segment, which marks the brief period when the ventricles are fully depolarized.
Unlike the fast, synchronized spread of depolarization, the process of repolarization occurs more slowly and does not rely on the heart’s specialized conduction system. This slower recovery process results in the T wave being wider and having a lower amplitude compared to the sharp, tall QRS complex. The T wave is typically a broad, upward deflection in most leads, even though the recovery current moves in the opposite direction of the activation current.
This final wave ensures the heart is primed for the next beat by completing the full cycle of ventricular activation and recovery. Analyzing the shape, amplitude, and timing of the T wave is informative, as abnormalities in repolarization can signify various issues affecting the health of the ventricular muscle.