An electrocardiogram (ECG or EKG) is a non-invasive test that provides a graphical representation of the heart’s electrical activity. Electrodes placed on the chest and limbs record the tiny electrical impulses that drive the cardiac cycle, capturing the sequential depolarization and repolarization of the heart muscle.
The standard diagnostic tool is the 12-lead ECG, which uses ten electrodes to create twelve distinct electrical perspectives, or “views,” of the heart. These views are grouped into six limb leads and six precordial (chest) leads, offering a comprehensive three-dimensional picture of electrical flow. Systematically analyzing these views allows for the identification of conditions ranging from rhythm disturbances to heart muscle damage.
The Anatomy of the ECG Tracing
The electrical activity recorded by the ECG is displayed as a wave tracing on a grid, where the horizontal axis measures time and the vertical axis measures voltage. The standard paper speed means each small square represents 0.04 seconds and each large square represents 0.20 seconds. Recognizing the components of this tracing is the foundation for interpretation.
The P wave represents the depolarization of the atria as the electrical impulse spreads from the sinoatrial (SA) node. Following the P wave, the PR interval measures the time from the start of atrial depolarization to the start of ventricular depolarization. The normal PR interval is between 0.12 and 0.20 seconds, representing the delay as the impulse travels through the atrioventricular (AV) node.
The QRS complex signifies the rapid depolarization of the left and right ventricles. The Q wave is the first downward deflection, the R wave is the first upward deflection, and the S wave is the first downward deflection after the R wave. The normal QRS duration is typically less than 0.12 seconds, reflecting the speed of electrical conduction through the specialized Purkinje fibers.
The ST segment follows ventricular depolarization and represents the period when the ventricles are fully depolarized. This segment is normally isoelectric (on the baseline); any measurable deviation can indicate injury to the heart muscle. The cardiac cycle concludes with the T wave, which represents the repolarization of the ventricles.
Establishing Rate and Rhythm
The initial steps in systematic ECG interpretation involve determining the heart rate and identifying the fundamental rhythm. Heart rate is expressed in beats per minute (bpm); a normal resting rate generally falls between 60 and 100 bpm.
For a regular rhythm, the rate can be quickly estimated using the “300 method,” which involves dividing 300 by the number of large squares between two consecutive R waves. For example, four large squares separating two R waves yields an estimated rate of 75 bpm. A more precise method for any rhythm is to count the number of QRS complexes within a 6-second strip (30 large boxes) and multiply that number by 10.
After establishing the rate, the next step is to determine the rhythm’s origin and regularity. A normal sinus rhythm requires that every QRS complex is preceded by a P wave. This P wave must also be upright in leads I and II, confirming the electrical impulse originated in the SA node.
Regularity is assessed by measuring the R-R interval across the strip. Consistent R-R intervals indicate a regular rhythm and a stable pacemaker source. If the intervals vary significantly, the rhythm is irregular, requiring investigation for causes such as atrial fibrillation or premature beats.
Evaluating Electrical Axis and QRS Morphology
Evaluating the electrical axis involves determining the average direction of the electrical spread through the ventricles in the frontal plane. This direction normally points downward and to the left, falling within a range of -30 to +90 degrees. Deviations from this range can suggest structural changes, such as ventricular hypertrophy, or specific conduction system blocks.
A rapid method for approximating the axis involves examining the QRS complexes in Lead I and Lead aVF.
Axis Deviation
If the QRS complex is predominantly positive in both Lead I and Lead aVF, the axis is considered normal. A positive Lead I with a negative Lead aVF suggests Left Axis Deviation (LAD), which may occur with conditions like left anterior fascicular block. Conversely, a negative Lead I with a positive Lead aVF suggests Right Axis Deviation (RAD), which can be seen in cases of right ventricular hypertrophy.
QRS Morphology
The QRS morphology—the shape and duration of the complex—provides information about the speed of ventricular conduction. The normal QRS duration is typically 80 to 110 milliseconds. A widened QRS complex, defined as 120 milliseconds or greater, indicates a delay in the electrical signal’s spread through the ventricles.
This widening is often the hallmark of a bundle branch block, where an impaired conduction pathway forces the impulse to spread slowly through the muscle tissue. The specific pattern of the widened QRS complex in the chest leads helps distinguish between a Left Bundle Branch Block and a Right Bundle Branch Block.
Searching for Signs of Injury or Ischemia
The final step focuses on identifying changes that suggest current or past damage to the heart muscle, known as myocardial ischemia or infarction. Ischemia occurs when blood flow is restricted, and this state is often reflected in the T wave or the ST segment. Ischemia can manifest as symmetrical T wave inversions or a depression of the ST segment below the baseline.
An ST segment elevation is a sign of acute injury, such as an ST-Elevation Myocardial Infarction (STEMI). This occurs when the isoelectric baseline is noticeably lifted, indicating a full-thickness injury to the heart muscle. These changes are typically localized to specific leads corresponding to the area of the heart deprived of blood flow.
Evidence of previous heart muscle death (infarction) is often found in the presence of pathological Q waves. While small Q waves are normal, a pathological Q wave is typically wider than 0.04 seconds or deeper than one-third the height of the subsequent R wave. These abnormal deflections represent electrically silent scar tissue, helping determine the location of the prior heart attack.