The 12-lead electrocardiogram (ECG or EKG) is a non-invasive tool that records the electrical activity of the heart. By placing electrodes on the limbs and chest, the machine captures 12 different views, or “leads,” of the heart’s electrical current. This mapping provides a snapshot of cardiac function, which is fundamental for diagnosing a wide range of heart conditions, from rhythm disturbances to damage of the heart muscle. Interpreting this complex tracing requires a methodical and structured approach to ensure no subtle abnormalities are overlooked. A systematic review of rate, rhythm, electrical axis, and waveform morphology allows for an accurate assessment of the heart’s electrical health.
Establishing the Basics: Rate and Rhythm
The initial step in ECG interpretation involves determining the heart rate and assessing the regularity of the cardiac rhythm. The heart rate is calculated by measuring the interval between successive R waves. For regular rhythms, two methods are commonly used to estimate the rate. The “300 method” divides 300 by the number of large squares between two consecutive R waves for a rapid estimation.
The “1500 method” provides a more precise measurement by dividing 1500 by the number of small squares separating two R waves. For irregular rhythms, the “6-second method” is used. This requires counting the number of QRS complexes within a six-second strip and multiplying that count by ten to derive the beats per minute.
Rhythm determination focuses on identifying the origin and regularity of the electrical impulse. Normal sinus rhythm originates in the sinoatrial (SA) node and is characterized by a rate between 60 and 100 beats per minute. To confirm sinus rhythm, a P wave must precede every QRS complex and be positive in lead II.
The regularity is checked by measuring the R-R interval. If these intervals vary by less than 0.12 seconds, the rhythm is considered regular. Irregular rhythms are categorized as either regularly irregular (a pattern exists) or irregularly irregular (no clear pattern).
Analyzing Electrical Orientation: Determining the Axis
The next step involves determining the heart’s electrical axis, which represents the average direction of electrical spread during ventricular depolarization. This vector is influenced by the mass of the ventricles; normally, the current flows downward and to the left, with the QRS axis ranging between -30 and +90 degrees.
Axis determination uses the “quadrant approach,” focusing on the net deflection of the QRS complex in leads I and aVF. Lead I represents the horizontal plane, and lead aVF represents the vertical plane. If the QRS complex is positive in both leads I and aVF, the axis is normal.
If the QRS is positive in lead I but negative in lead aVF, it indicates Left Axis Deviation (LAD). Conversely, a QRS that is negative in lead I but positive in lead aVF suggests Right Axis Deviation (RAD).
Axis deviation provides information about underlying cardiac issues. LAD can suggest left ventricular hypertrophy or a conduction delay. RAD may be associated with right ventricular strain, which can occur in conditions like pulmonary hypertension.
Detailed Waveform Analysis: Intervals and Morphology
The systematic interpretation continues with a detailed examination of the individual waveforms and intervals, reflecting the sequential steps of the cardiac cycle.
P Wave
The P wave, the first deflection, represents atrial depolarization, or the electrical activation of the upper chambers. A normal P wave should have a duration of less than 0.12 seconds (three small squares) and an amplitude of less than 2.5 millimeters.
PR Interval
The PR interval measures the time from the beginning of the P wave to the start of the QRS complex. This encompasses the time taken for the impulse to travel from the atria through the atrioventricular (AV) node. This measurement is normally between 0.12 and 0.20 seconds (three to five small squares). A measurement exceeding this upper limit suggests a delay in conduction, known as a first-degree AV block.
QRS Complex
The QRS complex represents ventricular depolarization. Its normal duration is narrow, measuring less than 0.12 seconds. A widened QRS complex indicates a delayed or abnormal electrical spread through the ventricles, typically seen in conditions like bundle branch blocks.
A pathological Q wave signifies myocardial necrosis or permanent damage to the heart muscle. A Q wave is considered pathological if its duration is 0.04 seconds or longer, or if its amplitude is 25% or more of the height of the subsequent R wave in the same lead.
ST Segment and T Wave
The ST segment represents the plateau phase between ventricular depolarization and repolarization and should normally be isoelectric (resting on the baseline). Deviations in the ST segment are significant, often indicating myocardial ischemia or injury. ST segment elevation is a hallmark of acute injury, while depression suggests subendocardial ischemia. The T wave represents ventricular repolarization and is generally in the same direction as the QRS complex.
QT Interval
The QT interval measures the total time for ventricular depolarization and repolarization. It must be corrected for heart rate (QTc) using formulas like Bazett’s to accurately determine if it is prolonged. A prolonged QTc interval is associated with an increased risk for life-threatening ventricular arrhythmias.
Identifying Common Clinical Findings
The final stage involves synthesizing all measurements and morphological observations to recognize specific disease patterns.
Ischemia and Infarction
The ECG is essential for identifying myocardial ischemia and infarction. Ischemia, a lack of blood flow, is often suggested by ST segment depression or T wave inversion. Acute injury involves ST segment elevation. Pathological Q waves mark transmural necrosis, indicating that the heart muscle tissue has died. The location of these changes across the 12 leads helps pinpoint the specific area of the heart affected and the likely blocked coronary artery. For example, ST elevation in leads II, III, and aVF suggests an inferior wall injury, typically associated with the right coronary artery.
Conduction Blocks
Conduction blocks are recognized by the duration of the intervals and the shape of the QRS complex. A prolonged PR interval signifies a first-degree AV block. A QRS duration of 0.12 seconds or greater indicates a bundle branch block, where one of the main electrical pathways within the ventricles is impaired. This is often seen with characteristic patterns in the chest leads (V1 and V6).
Ventricular Hypertrophy and Arrhythmias
The ECG can also provide evidence of ventricular hypertrophy, which is the enlargement of the heart muscle. Left Ventricular Hypertrophy (LVH) is suggested by increased QRS voltage, such as when the S wave depth in V1 added to the R wave height in V5 or V6 exceeds 35 millimeters. This increased muscle mass often causes the electrical vector to shift, leading to the Left Axis Deviation identified during the axis analysis. Recognition of specific arrhythmias depends on the findings from the rate and rhythm section combined with waveform analysis. Atrial fibrillation is characterized by an irregularly irregular R-R interval and the absence of distinct P waves, which are replaced by chaotic electrical activity.