An Electrocardiogram (ECG or EKG) is a non-invasive diagnostic tool that records the heart’s electrical activity. By placing electrodes on the body, the test records the voltage changes that occur as the heart muscle depolarizes and repolarizes during each heartbeat. This process creates a wave tracing that allows medical professionals to assess the rhythm, rate, and overall electrical health of the heart. The ECG is frequently used when symptoms like chest pain, palpitations, or shortness of breath are reported. Deviations from the expected pattern on this tracing are considered abnormalities, signaling that the heart’s electrical system may be functioning outside of normal parameters.
Reading the Normal ECG Tracing
A healthy heart operates under a predictable electrical cycle known as normal sinus rhythm, which originates in the sinoatrial (SA) node, the heart’s natural pacemaker. On the ECG tracing, this cycle is represented by the P wave, the QRS complex, and the T wave.
The P wave is the first deflection and signifies atrial depolarization, the electrical signal spreading through the upper chambers, causing them to contract. Following the P wave is the PR segment, which represents the time the impulse pauses in the atrioventricular (AV) node before continuing. The duration of the PR interval, from the start of the P wave to the start of the QRS complex, is normally between 0.12 and 0.20 seconds.
The QRS complex is a sharp, larger deflection representing ventricular depolarization, the electrical activation of the heart’s main pumping chambers. This event triggers the contraction of the ventricles, and its normal duration ranges from 0.06 to 0.10 seconds. Finally, the T wave follows the QRS complex and represents ventricular repolarization, the electrical recovery phase where the ventricles reset for the next beat. Normal sinus rhythm heart rate is defined as being between 60 and 100 beats per minute.
Abnormalities in Heart Rate and Rhythm
Abnormalities in heart rate and rhythm, collectively known as arrhythmias, show up as deviations in the rate, regularity, or timing of the P-QRS-T sequence. Bradycardia is defined as a heart rate below 60 beats per minute in an adult. Conversely, Tachycardia is diagnosed when the heart rate exceeds 100 beats per minute.
Rhythm disturbances often involve the atria, such as Atrial Fibrillation (A-fib), characterized by chaotic electrical activity in the upper chambers. On the ECG, A-fib is identified by the absence of distinct P waves, an irregularly irregular rhythm, and fine fibrillatory waves along the baseline. The uncoordinated firing of the atria prevents an organized contraction and leads to an erratic ventricular response.
A related condition is Atrial Flutter, typically caused by a rapid, organized electrical circuit, usually in the right atrium, producing an atrial rate around 300 beats per minute. The resulting ECG tracing often displays a distinctive “sawtooth” pattern of flutter waves. The ventricles cannot keep pace with this rate, leading to a fixed conduction ratio, such as 2:1 or 4:1 block, where only every second or fourth atrial impulse is transmitted.
Abnormalities in Electrical Conduction and Structure
Changes in the shape and duration of the QRS complex, ST segment, and T wave often indicate issues concerning the heart muscle’s health or electrical wiring. The ST segment, the line between the end of the QRS complex and the start of the T wave, is important for diagnosing insufficient blood flow, known as myocardial ischemia. When a heart attack is occurring, a significant upward displacement of this segment, called ST elevation, suggests a complete blockage of a coronary artery.
Downward displacement of the ST segment, or ST depression, can also signal ischemia or may be a reciprocal change to ST elevation in other leads. The T wave can also be altered, with T wave inversion being a common sign of ischemia, though it can be associated with non-ischemic conditions like ventricular hypertrophy. Hypertrophy, the abnormal thickening or enlargement of the heart muscle, especially the left ventricle, causes the QRS complexes to appear abnormally large or tall.
This enlargement alters the electrical axis and can lead to secondary changes in the ST segment and T wave, where their direction is opposite to the main QRS deflection. Conduction Blocks, such as Bundle Branch Blocks (BBBs), are visible as an abnormally wide QRS complex, lasting longer than 0.10 seconds. This widening occurs because the electrical impulse is delayed or blocked from traveling down one of the main pathways, forcing the ventricles to depolarize more slowly.
What an Abnormal ECG Indicates
An abnormal ECG tracing is rarely the final diagnosis; rather, it functions as a screening tool pointing toward the need for further medical evaluation. The significance of an abnormal finding is always assessed within the context of the individual’s symptoms, medical history, and physical examination. Some variations, such as minor changes caused by certain medications or temporary electrolyte imbalances, may be benign or easily reversible.
Any finding that suggests a structural issue, such as signs of past or current heart attack, or a potentially dangerous arrhythmia, necessitates immediate follow-up. The next step often involves consultation with a cardiac specialist, or cardiologist, to determine the clinical meaning of the tracing. To gain a more comprehensive understanding, the physician may order additional diagnostic tests.
Common follow-up tests include an echocardiogram, which uses ultrasound to visualize the heart’s structure and pumping function. A Holter monitor records the ECG continuously over 24 to 48 hours to capture intermittent rhythm problems. A stress test may also be performed, recording the ECG while the patient exercises to check for signs of ischemia that appear under physical exertion. These subsequent tests help correlate the electrical findings from the ECG with the heart’s mechanical function, guiding the appropriate course of treatment.