An electrocardiogram (ECG or EKG) is a non-invasive procedure recording the heart’s electrical activity. Electrodes detect small electrical changes as the heart muscle depolarizes and repolarizes. The tracing provides a graphical representation of the heart’s rate, rhythm, and electrical impulse timing. Analyzing this pattern helps identify irregularities, signs of a previous heart attack, or poor blood flow. This guide explains the foundational principles and key visual components of the ECG tracing.
The Electrical Foundation of the Heart
The heart’s ability to pump blood depends upon a coordinated electrical system. This system begins with the sinoatrial (SA) node, the heart’s natural pacemaker located in the upper right chamber. The SA node spontaneously generates the electrical impulse, setting the pace for the entire heart (sinus rhythm).
From the SA node, the electrical signal rapidly spreads across the atria, causing the upper chambers to contract. The impulse then reaches the atrioventricular (AV) node, which serves as a gatekeeper between the atria and the ventricles. The AV node briefly delays the signal, allowing the atria to fully empty their blood before the lower chambers begin to contract.
After this delay, the signal travels down the bundle of His and into the Purkinje fibers, rapidly distributing the impulse throughout the ventricular walls. This ensures the ventricles contract simultaneously, generating the push needed to circulate blood. The process involves depolarization (activation leading to contraction) and repolarization (recovery necessary for relaxation).
The surface electrodes detect the cumulative electrical currents generated by these cycles. Movement toward or away from the electrodes produces the characteristic upward and downward deflections. The ECG only displays the stronger activity of the contracting atrial and ventricular muscle, as SA and AV node potentials are too small to be recorded.
Decoding the Key Components: Waves and Intervals
The ECG tracing is composed of recognizable waves, complexes, and segments, each corresponding to a specific physiological event. Interpreting the tracing begins with the P wave, a small upward deflection reflecting the initial depolarization of the atria. The P wave should not exceed 0.12 seconds, representing the time the impulse takes to spread across the atrial muscle.
Following the P wave is the PR interval, measured from the beginning of the P wave to the start of the next deflection. This interval represents the time required for the impulse to travel from the SA node, through the atria, and pass through the AV node to the ventricles. A normal PR interval is 0.12 to 0.20 seconds; deviation outside this range can indicate a problem with AV node conduction speed.
The most prominent feature is the QRS complex, a sharp, three-part deflection signifying ventricular depolarization. It is composed of the Q wave (first downward deflection), the R wave (first upward deflection), and the S wave (subsequent downward deflection). The QRS complex is large because the substantial ventricular muscle mass generates a strong electrical signal.
The duration of the QRS complex is an important measurement, normally lasting between 0.06 and 0.10 seconds. This reflects the speed of the electrical signal through the specialized conduction system within the ventricles. A longer duration suggests the electrical impulse is traveling slowly, possibly due to a block or delay in the bundle branches.
Immediately after the QRS complex is the ST segment, a flat line connecting the S wave to the T wave. It aligns with the baseline, representing the moment the ventricular muscle is fully depolarized. Changes in the height of this segment often signal reduced blood flow or injury to the heart muscle.
The final component is the T wave, an upward deflection representing the repolarization of the ventricular muscle. Atrial recovery is obscured because it happens simultaneously with the larger QRS complex. The T wave is longer and broader than the QRS complex, reflecting the slower process of muscle cells returning to their resting state.
Determining Heart Rate and Rhythm
The ECG tracing is printed on grid paper, allowing for precise measurement of time and voltage. Small squares measure 0.04 seconds horizontally, and large squares (five small squares) represent 0.20 seconds.
To quickly calculate the heart rate for a regular rhythm, the “300 method” is frequently used, which relies on the distance between consecutive R waves in the QRS complex. The calculation involves dividing 300 by the number of large squares separating one R wave from the next. For instance, if the R waves are separated by three large squares, the heart rate is 100 beats per minute (bpm).
For rhythms that are irregular or too slow to use the 300 method, the “6-second strip method” provides a reliable estimate. This technique requires identifying a 6-second segment of the tracing, which corresponds to 30 large squares on the paper. The next step is to count the number of QRS complexes, or R waves, that appear within that 6-second strip and multiply that count by 10 to obtain the rate in beats per minute.
The concept of “rhythm” refers to the regularity and source of the electrical impulses. A normal sinus rhythm is characterized by a regular rate between 60 and 100 bpm. Crucially, every QRS complex must be preceded by a P wave, and the PR interval must be constant, indicating the SA node is correctly initiating the beat and the signal is conducted consistently.
What Abnormal Readings Indicate
Deviations from the normal ECG pattern indicate conditions affecting the heart. Rate abnormalities are classified as tachycardia (rate exceeding 100 bpm) or bradycardia (rate below 60 bpm). While these can be normal responses to exercise or rest, inappropriate occurrence suggests the heart’s pacing mechanism is malfunctioning.
Rhythm issues, or arrhythmias, involve disorganized electrical activity. Atrial fibrillation, for example, appears as a chaotic, irregularly irregular rhythm with no clear P waves, indicating disorganized electrical firing in the atria. Premature beats show up as an early, often wide QRS complex that interrupts the regular pattern.
The shape and position of the ST segment and T wave indicate myocardial ischemia (reduced blood flow). Ischemia may cause the ST segment to appear depressed (lower than the baseline), while a serious acute injury or heart attack can cause ST segment elevation. An inverted or abnormally peaked T wave also suggests the heart muscle is not receiving sufficient oxygen.
Changes in QRS complex width indicate the origin of the electrical problem. A wide QRS complex (longer than 0.12 seconds) often suggests the impulse is traveling slowly through the ventricles, potentially due to a bundle branch block. Narrow complexes indicate the impulse uses the heart’s normal, fast conduction pathway but may originate from an abnormal location above the ventricles.