An electrocardiogram (ECG) is a non-invasive test that records the heart’s electrical activity. The standard 12-lead ECG uses electrodes placed on the limbs and chest to create 12 different perspectives, or “leads,” to view the electrical current. This comprehensive view allows for the systematic assessment of the heart’s rhythm, rate, and the timing of its electrical events. Following a consistent, step-by-step methodology helps professionals interpret the complex waveform tracings.
Establishing Rate and Rhythm
The initial step in systematic ECG analysis is determining the heart’s rate and assessing the regularity of its beat. A normal heart rate falls between 60 and 100 beats per minute. Rates above 100 beats per minute are termed tachycardia, and rates below 60 beats per minute are known as bradycardia.
For regular rhythms, the heart rate can be quickly estimated using the “rule of 300,” dividing 300 by the number of large squares between two consecutive R waves. For irregular rhythms, a more accurate method involves counting the number of R waves over a six-second strip (30 large squares) and multiplying that number by ten.
Rhythm analysis focuses on the consistency of the beat, measured by the distance between consecutive R waves (the R-R interval). If these intervals are equal, the rhythm is regular. If they vary without a pattern, the rhythm is irregularly irregular, often associated with Atrial Fibrillation. To confirm a normal sinus rhythm, which originates in the Sinus Node, a P-wave must consistently precede every QRS complex.
Detailed Analysis of Waveforms and Intervals
After assessing rate and rhythm, the next step is the detailed measurement of the duration and amplitude of individual waveforms and intervals. The P wave represents the electrical activation of the atria (depolarization). It should normally last less than 0.12 seconds and have an amplitude not exceeding 2.5 millimeters. This wave is typically upright in Lead II.
The PR interval measures the time for the electrical impulse to travel from the atria through the AV node to the ventricles. Measured from the start of the P wave to the start of the QRS complex, its normal duration is between 0.12 and 0.20 seconds. A consistently prolonged PR interval suggests a delay in conduction through the AV node.
The QRS complex represents the rapid electrical activation of the ventricles (depolarization). The complex’s duration should be narrow, typically less than 0.12 seconds. It is composed of the Q wave (downward deflection), the R wave (upward deflection), and the S wave (second downward deflection).
A normal Q wave is short, less than 0.04 seconds in duration, and small in amplitude, representing the initial activation of the ventricular septum. Another element is the R-wave progression across the precordial leads (V1-V6), where the R wave should gradually increase in height from V1 through V5.
The ST segment and T wave reflect the recovery phase, or repolarization, of the ventricles. The ST segment connects the end of the QRS complex to the beginning of the T wave. It should be isoelectric, resting on the same baseline as the PR interval. The T wave is typically rounded and should be oriented in the same direction as the preceding QRS complex in most leads.
The QT interval measures the total time required for the ventricles to complete both depolarization and repolarization, from the start of the QRS complex to the end of the T wave. Because this interval is influenced by heart rate, it must be corrected (QTc). A corrected normal duration is generally less than 440 milliseconds for males and less than 460 milliseconds for females.
Interpreting Lead Placement and Electrical Axis
The 12 leads are grouped to provide specific spatial views of the heart, following the temporal measurements. Six limb leads (I, II, III, aVR, aVL, aVF) view the heart in the frontal plane. The other six are the precordial leads (V1 through V6), placed across the chest to view the heart in the horizontal plane.
These leads are grouped anatomically to localize findings:
- Leads II, III, and aVF view the inferior wall.
- Leads I, aVL, V5, and V6 view the lateral wall.
- Leads V1 and V2 view the septal area.
- Leads V3 and V4 view the anterior wall.
This organization allows a physician to connect an electrical abnormality to a specific region of the heart muscle.
The electrical axis represents the mean direction of the electrical current flowing through the ventricles during depolarization. This axis is determined using the limb leads, specifically Lead I and Lead aVF, via the quadrant approach. The normal electrical axis is directed downward and to the left, falling between 0 degrees and +90 degrees.
The axis is considered normal if the QRS complex is predominantly positive (upward deflection) in both Lead I and Lead aVF. If Lead I is positive but Lead aVF is negative, the axis is shifted to the left (Left Axis Deviation). Conversely, a negative Lead I and a positive Lead aVF indicate a shift to the right (Right Axis Deviation).
What Specific Findings Indicate
Systematically measuring ECG components identifies deviations from normal values, pointing toward underlying physiological issues. Deviations in heart rate, such as sustained tachycardia or bradycardia, suggest an imbalance in the heart’s pacing function. Issues with rhythm regularity, such as the chaotic electrical activity seen in Atrial Fibrillation, indicate a problem with the orderly sequence of contractions.
Abnormalities in measured intervals and segments often indicate conduction delays or muscle damage. A prolonged PR interval suggests a first-degree heart block, indicating a delay in the signal passing through the AV node. The elevation or depression of the ST segment is a primary indicator of potential myocardial ischemia or injury. While this systematic method provides the foundation for interpretation, a definitive diagnosis requires a full assessment alongside the patient’s overall clinical presentation.