An electrocardiogram (ECG or EKG) is a non-invasive test that provides a graphical tracing of the heart’s electrical activity over time. Electrodes placed on the body detect the tiny electrical currents generated by the heart muscle as it contracts and relaxes. The resulting waveform allows healthcare providers to assess the heart’s rhythm, rate, and the condition of its conduction system. The pattern of peaks, valleys, and flat lines is systematically broken down into waves, intervals, and segments, each representing a specific physiological event in the cardiac cycle.
The Electrical Action (Waves)
The waves on the tracing are the visible deflections, representing the active movement of electrical current through the heart muscle. Each wave is a direct reflection of depolarization (electrical activation) or repolarization (electrical recovery) in a specific chamber. The first deflection seen is the P wave, which signifies the electrical activation of the atria, the heart’s upper chambers. This depolarization begins in the sinoatrial node and spreads across the atrial tissue, preparing it for contraction.
Following the P wave is the QRS complex, a sharp, larger set of deflections representing the electrical activation of the ventricles, the heart’s main pumping chambers. Because the ventricular muscle mass is much larger than the atria, the voltage generated is significantly greater, resulting in the tallest deflections on the entire tracing. The Q wave is the first negative (downward) deflection, the R wave is the first positive (upward) deflection, and the S wave is the negative deflection that follows the R wave.
The QRS complex is a rapid event, reflecting the swift spread of electricity through the specialized Purkinje fibers to ensure near-simultaneous contraction of the ventricles. While the ventricles are depolarizing, atrial repolarization occurs, but it is masked by the much stronger QRS complex and is not usually observed. The T wave is the final major deflection, representing the electrical recovery (repolarization) of the ventricles.
Ventricular repolarization is a slower process than depolarization, which explains why the T wave is broader and more rounded than the sharp QRS complex. The coordinated timing of these electrical events drives the mechanical pumping action of the heart. Occasionally, a small U wave may be seen immediately following the T wave, though its origin is not fully understood.
Measuring Timing (Intervals)
Intervals are measurements of time on the ECG tracing that include at least one wave and the connecting segment, providing insight into the speed and coordination of the heart’s electrical system. The PR interval measures the time from the beginning of the P wave to the beginning of the QRS complex. This duration captures the entire journey of the electrical impulse from the atria, through the atrioventricular (AV) node, and into the ventricular conduction system.
The PR interval is an important measurement for assessing the function of the AV node, which acts as the gatekeeper, intentionally slowing the impulse to allow the atria to fully empty blood into the ventricles before the ventricles contract. A normal PR interval ranges from 0.12 to 0.20 seconds; deviations outside this range can indicate conduction blocks or abnormal pathways. Another significant time measurement is the QT interval, which spans from the start of the QRS complex to the end of the T wave.
The QT interval represents the total time required for the ventricles to depolarize and completely repolarize. Because its duration is dependent on heart rate, it is often corrected (QTc) for clinical assessment. Abnormally long QT intervals indicate a heightened risk for dangerous ventricular arrhythmias. The RR interval, measured between two consecutive R waves, determines the regularity of the heart’s rhythm and is the primary tool used to calculate the heart rate.
Periods of Rest (Segments and Baseline)
Segments are the flat, horizontal lines connecting the waves, representing periods where the electrical potential across the heart muscle is essentially zero, or isoelectric. The PR segment is the flat line that begins at the end of the P wave and stops just before the QRS complex begins. This segment corresponds to the brief electrical pause as the impulse travels slowly through the AV node and the bundle of His.
This moment of electrical quiet allows the atria to finish contracting before the ventricles are signaled to fire. The ST segment is the flat line between the end of the QRS complex and the beginning of the T wave. It represents the plateau phase of the ventricular action potential, a period when the ventricular muscle is fully depolarized and contracting.
Under normal circumstances, the ST segment should align with the baseline. Its elevation or depression is a hallmark sign of myocardial ischemia, meaning the heart muscle is not receiving enough blood flow.
The isoelectric line, or baseline, is the zero-voltage reference point from which all deflections are measured. It is the straight line present when there is no significant electrical current flowing.
The segment between the end of the T wave and the beginning of the next P wave, known as the TP segment, is the most accurate representation of the true baseline. Abnormalities in the segments, such as an elevated ST segment, are interpreted relative to this isoelectric line, providing evidence of acute injury or inflammation in the heart.