An electrocardiogram (ECG or EKG) is a non-invasive medical tool that provides a visual representation of the heart’s electrical activity. This activity drives the rhythmic contraction and relaxation of the heart muscle, allowing it to pump blood effectively throughout the body. By attaching electrodes to the skin, the ECG machine detects the electrical signals generated by the heart and translates them into a continuous wavy line. Analyzing the pattern, timing, and shape of this line allows healthcare providers to assess the heart’s function and define a normal, healthy tracing.
How the ECG Records Heart Activity
The visual output of the ECG is a graph showing voltage changes over time, tracking the process of electrical activation and recovery within the heart muscle. Heart function relies on two primary electrical events: depolarization and repolarization. Depolarization represents the electrical activation that precedes a muscle contraction, while repolarization signifies the electrical recovery, or reset, that allows the muscle to relax.
When an electrical wave moves toward a positive electrode, the ECG records an upward spike (positive deflection). Conversely, an electrical wave moving away results in a downward spike (negative deflection). The flat line that appears when there is no electrical activity is called the isoelectric line or baseline. This sequence creates the repeating pattern of a single heartbeat on the tracing.
Decoding the ECG Waveform Components
A normal heartbeat tracing is composed of three distinct electrical waves and one major complex, each representing a specific phase of the heart’s cycle. The P wave is a small, rounded, upward deflection. This wave signifies atrial depolarization, the electrical signal that initiates the contraction of the heart’s two upper chambers, the atria.
Following the P wave is the QRS complex, a sharp, tall sequence of deflections that dominates the tracing. This complex represents ventricular depolarization, the powerful electrical signal that causes the ventricles to contract. The Q wave is the first downward stroke, the R wave is the large upward spike, and the S wave is the final downward stroke of this complex. Atrial repolarization is generally too small to be seen and is masked within the larger QRS complex.
The final component is the T wave, a broader, rounded wave that follows the QRS complex and represents ventricular repolarization. This wave shows the ventricles electrically resetting themselves to prepare for the next impulse. A normal T wave is typically slightly asymmetrical. The entire sequence, from the P wave through the T wave, reflects the complete electrical cycle of one normal heartbeat.
Measuring Time and Distance Between Waves
A normal ECG requires precise timing, measured by analyzing the segments and intervals between the waves. The standard ECG paper moves at a fixed speed, which allows the horizontal distance between points to be converted into time.
The PR interval measures the time from the start of the P wave to the start of the QRS complex, and it normally ranges from 0.12 to 0.20 seconds. This duration represents the time it takes for the electrical impulse to travel from the atria, through the atrioventricular (AV) node, and into the ventricles. The QRS duration (the width of the QRS complex) should be narrow, typically lasting 0.06 to 0.10 seconds, reflecting the rapid spread of the signal through the ventricles.
The ST segment is the flat line between the end of the S wave and the beginning of the T wave, and in a normal tracing, it should lie flat on the isoelectric baseline. This segment represents the period when the ventricles are fully depolarized before they begin to repolarize. The QT interval measures the total time required for the ventricles to depolarize and fully repolarize. A normal corrected QT interval is 0.44 seconds or less, adjusted based on the heart rate.
Defining a Normal Heart Rhythm
A normal heart rhythm, known as Normal Sinus Rhythm, is defined by the combination of correct waveform shapes and precise timing measurements. The rhythm is called “sinus” because the electrical impulse originates from the heart’s natural pacemaker, the sinoatrial (SA) node. For a rhythm to be considered normal, the heart rate must fall between 60 and 100 beats per minute (bpm) for an adult at rest.
The rhythm must be regular, meaning the distance between consecutive beats should be consistent across the tracing. Every QRS complex must be preceded by a P wave, showing that the electrical signal always begins in the atria before traveling to the ventricles. The P wave must also have a consistent, upright morphology, confirming the impulse is following the standard pathway from the SA node. When all these criteria of rate, regularity, and waveform sequence are met, the ECG demonstrates healthy, coordinated electrical function.