An electrocardiogram (ECG or EKG) is a non-invasive medical test that provides a visual representation of the heart’s electrical activity. The procedure involves placing small electrode patches on the skin to record the tiny electrical impulses generated by the heart muscle. The primary purpose of the ECG is to check the heart’s rate and rhythm. By recording these signals, healthcare professionals can assess heart function and look for signs of conditions like poor blood flow or an irregular heartbeat. The resulting graph of voltage versus time is a powerful diagnostic tool.
The Heart’s Electrical Pathway
The heart’s pumping action is governed by a specialized electrical conduction system. The process begins in the right upper chamber, or atrium, at the Sinoatrial (SA) Node, which acts as the heart’s pacemaker. The SA Node generates an electrical impulse, typically 60 to 100 times per minute in a resting adult, which spreads rapidly across the atria, causing them to contract.
The electrical signal then travels to the Atrioventricular (AV) Node, located near the center of the heart. The AV Node momentarily slows the impulse, ensuring the ventricles fill with blood. After this delay, the signal races down the Bundle of His and the Purkinje fibers, rapidly distributing the impulse throughout the ventricles and triggering their forceful contraction. This orderly sequence of electrical activation is what the ECG records.
Decoding the P, QRS, and T Waves
The characteristic pattern of a normal ECG tracing is composed of three main deflections: the P wave, the QRS complex, and the T wave, each representing a specific electrical event. The small, rounded P wave is the first deflection, signifying the electrical activation (depolarization) of the atria. This depolarization causes the atria to contract. A normal P wave is smooth, no more than 2.5 millimeters tall, and less than 0.11 seconds in duration.
Following the P wave is the QRS complex, a sharp and much larger set of deflections representing the rapid depolarization of the ventricles. The Q wave is the first negative deflection, the R wave is the first positive deflection, and the S wave follows the R wave. This complex’s large size reflects the greater muscle mass of the ventricles, indicating the electrical impulse spreading to cause the main pumping action. The entire QRS complex is normally very narrow, lasting between 0.06 and 0.10 seconds.
The final significant deflection is the T wave, which reflects the electrical recovery (repolarization) of the ventricles. Repolarization is when the heart muscle cells recharge electrically, preparing for the next beat. A normal T wave is usually slightly asymmetric and follows the same general direction as the preceding QRS complex. Atrial recovery also occurs but is hidden within the much larger QRS complex.
Standard Measurements for Intervals and Segments
The timing and duration of the waves are measured by specific intervals and segments, which are crucial for defining a normal ECG. The ECG paper itself is a grid where the horizontal axis measures time and the vertical axis measures voltage (amplitude). At a standard speed of 25 millimeters per second, each small square represents 0.04 seconds, and each large square (five small squares) represents 0.20 seconds.
The PR interval measures the time from the beginning of the P wave to the beginning of the QRS complex, reflecting the impulse travel time from the atria through the AV Node. A normal PR interval falls between 0.12 to 0.20 seconds, or three to five small squares on the grid. The QRS duration measures the width of the ventricular depolarization, and a normal duration is less than 0.12 seconds.
The ST segment is the flat line immediately following the QRS complex and leading up to the T wave. In a normal tracing, this segment is isoelectric, resting on the baseline, representing the brief period where the ventricles are fully depolarized. Finally, the QT interval measures the total time from the start of the QRS complex to the end of the T wave, encompassing the entire electrical cycle of ventricular activity. A corrected QT interval (QTc) is generally considered normal if it is 0.42 seconds or less.
Defining Normal Sinus Rhythm
The term “Normal Sinus Rhythm” (NSR) describes a healthy, coordinated heartbeat, indicating that the SA Node is the heart’s pacemaker. This diagnosis requires that all wave shapes and intervals fall within normal parameters. The defining features of NSR relate to the heart rate and the consistent relationship between the atria and the ventricles.
For an adult, the heart rate must be between 60 and 100 beats per minute. The rhythm must be regular, with each P wave distinctly followed by a QRS complex. This consistent one-to-one relationship confirms that the heart’s electrical system is working harmoniously to pump blood efficiently.