An Electrocardiogram (ECG) is a non-invasive test that records the electrical activity of the heart over time, translating the heart’s cycle of depolarization and repolarization into a visible waveform tracing. This tracing is composed of repeating patterns, known as complexes, which represent a single heartbeat. The repeating pattern is systematically labeled using the sequence of letters P, Q, R, S, and T to standardize interpretation. Understanding how these components—waves, intervals, and segments—are defined and measured is foundational to reading the heart’s electrical signature.
The Deflections: Waves P, QRS, and T
The letters P, QRS, and T label the electrical deflections, or waves, that correspond to the heart muscle’s depolarization and repolarization. The P wave is the first small, typically upward deflection, representing atrial depolarization. This electrical event signals the activation of the atria, causing them to contract.
Immediately following the P wave is the QRS complex, which represents the electrical impulse spreading through the ventricles, causing ventricular depolarization. This complex is the largest feature on the tracing because the ventricular muscle mass is significantly larger than the atria. The QRS is termed a complex because it is usually comprised of three distinct waves: Q, R, and S.
The Q wave is the first downward deflection of the complex, while the R wave is the first upward deflection. The S wave is the first downward deflection that follows the R wave. Not every QRS complex contains all three waves; for instance, a complex with only an upward deflection is still referred to as an R wave. Atrial repolarization also occurs during this time, but it is typically hidden within the larger QRS complex and is therefore not seen as a separate wave.
The final major deflection is the T wave, which follows the QRS complex. It represents ventricular repolarization, the electrical recovery phase where the ventricular muscle cells reset before the next heartbeat. In a normal heart, the T wave is generally upright and slightly asymmetrical.
Measuring Duration: Intervals
An interval is an ECG measurement defining a duration of time, incorporating at least one wave and the connecting baseline segment. Intervals are measured in seconds or milliseconds and reflect the time required for the electrical impulse to travel between specific points in the heart’s conduction system.
The PR interval is measured from the very beginning of the P wave to the start of the QRS complex. This duration reflects the time it takes for the electrical impulse to travel from the atria, through the atrioventricular (AV) node, and into the ventricles. A normal PR interval typically falls between 0.12 and 0.20 seconds (120 to 200 milliseconds).
The QRS duration is the measurement of the entire QRS complex, from its first deflection (Q or R) to its last deflection (S). This interval represents the total time required for ventricular depolarization. In a healthy adult, the QRS duration is relatively short, usually ranging from 0.06 to 0.10 seconds (60 to 100 milliseconds).
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. This measurement encompasses the entire electrical activity of the ventricles, representing both depolarization and subsequent repolarization. Because the length of the QT interval changes with heart rate, a corrected value (QTc) is often calculated to standardize the measurement for interpretation.
Electrical Baselines: Segments
A segment on the ECG refers to the flat, isoelectric line that connects two waves. Segments represent brief periods when the entire mass of the relevant heart chamber is either uniformly depolarized or fully repolarized, meaning no net electrical current is flowing.
The PR segment is the baseline found between the end of the P wave and the start of the QRS complex. During this period, the electrical impulse moves slowly through the AV node. This delay produces an electrical signal too weak to be recorded on the surface ECG, resulting in a flat line. This segment is often used as the baseline reference point for measuring the amplitude of other waves.
The ST segment is the isoelectric line that connects the end of the S wave to the beginning of the T wave. This segment represents the period when the ventricles are fully depolarized, but repolarization has not yet fully begun. The point where the QRS complex ends and the ST segment begins is specifically labeled the J point. The morphology of the ST segment is particularly important in clinical assessment as it can indicate conditions such as acute cardiac ischemia.