The heart functions as an electrical pump, relying on precisely timed signals to coordinate muscular contractions. Medical professionals use an Electrocardiogram (ECG or EKG) to assess this process and diagnose potential issues. This non-invasive tool records the heart’s electrical activity, translating impulses into a traceable waveform. Analyzing the shape and timing of these deflections provides insight into the heart’s rhythm and overall function.
The Electrical Basis of the Heartbeat
The cardiac cycle begins with an electrical impulse generated in the sinoatrial (SA) node, the heart’s natural pacemaker, located in the upper right atrium. This impulse triggers depolarization, an electrical activation that causes muscle cells to contract. Following contraction, the cells reset themselves electrically through repolarization, which is the recovery phase.
The impulse first spreads across the atria, causing them to depolarize and pump blood into the lower chambers. The signal then travels to the atrioventricular (AV) node, where it pauses briefly. It is then rapidly conducted through the bundle of His and the Purkinje fibers. This specialized conduction network quickly distributes the signal throughout the ventricles, preparing them for contraction.
Mapping Electrical Activity to the ECG Trace
The ECG captures the heart’s electrical currents and displays them as distinct waves and complexes. Each deflection corresponds to a specific electrical event within the heart’s chambers. The P wave is the first small, rounded upward deflection, representing the depolarization of the atria.
The most prominent feature is the QRS complex, which represents ventricular depolarization. This complex marks the electrical activation of the large, muscular ventricles, immediately preceding their contraction. Due to the significantly larger ventricular muscle mass, the electrical activity generated is much stronger. This results in the QRS complex being the tallest and most noticeable part of the tracing.
The final major deflection is the T wave, which signifies ventricular repolarization, the electrical recovery of the ventricles. Atrial repolarization occurs but is a small electrical event hidden within the much larger QRS complex, making it generally invisible.
Understanding the QRS Complex
The QRS complex is composed of three sequential waves: the Q wave, the R wave, and the S wave. The Q wave is the initial downward deflection, often small or absent, representing the depolarization of the interventricular septum. The R wave is the first upward deflection, representing the electrical spread through the main mass of the ventricles and is typically the largest component.
The S wave is the final downward deflection, representing the electrical activation of the base of the ventricles. The entire QRS complex normally takes a very short time, typically lasting between 0.06 and 0.10 seconds in an adult, reflecting the rapid and coordinated electrical conduction through the Purkinje system. A wider QRS complex, one lasting 0.12 seconds or more, suggests a delay or abnormality in the ventricular conduction system, such as a bundle branch block.
Variations in the amplitude (height) of the QRS complex provide diagnostic clues. A significantly larger complex may suggest ventricular hypertrophy, an enlargement of the heart muscle mass that generates a stronger electrical signal. Conversely, changes in Q or R wave size can be a lasting sign of damage from a prior myocardial infarction, where the electrical signal cannot travel through scarred tissue.