An electrocardiogram (ECG) is a brief, non-invasive test that records the electrical activity generated by the heart as it beats. This electrical signal controls the heart’s rhythm, and the ECG provides a snapshot of how this system is functioning. Doctors use the ECG for routine screening, investigating symptoms like chest pain or palpitations, and monitoring known heart conditions. An “abnormal” ECG means the tracing deviates from a defined range of normal electrical patterns, suggesting a need for further medical attention. The result is rarely a final diagnosis, but rather a prompt to explore the underlying cause of the electrical change.
What Defines an Abnormal ECG Reading
An abnormal ECG is identified by deviations in three primary areas: the heart rate, the rhythm’s regularity, and the morphology of the electrical waves and intervals. The typical adult resting heart rate falls between 60 and 100 beats per minute (bpm). A rate consistently above 100 bpm is defined as tachycardia, while a rate below 60 bpm is classified as bradycardia.
Rhythm is analyzed by looking at the regularity of the R-R intervals, the time between successive heartbeats. An irregular rhythm, where the intervals are inconsistent, suggests an abnormality in the heart’s natural pacing system. The presence or absence of the P wave (atrial contraction) and its relationship to the QRS complex (ventricular contraction) are essential for determining the electrical origin of the beat.
The intervals and wave morphology provide detail about the electrical signal’s journey through the heart muscle. The PR interval, normally between 0.12 and 0.20 seconds, measures the time for the impulse to travel from the atria to the ventricles. A prolonged PR interval suggests a delay in conduction, often at the atrioventricular (AV) node.
The QT interval represents the total time for the ventricles to depolarize and repolarize. A corrected QT interval (QTc) that is too long or too short indicates a heightened risk for dangerous heart rhythms. Changes in the ST segment, which connects the QRS complex and the T wave, are significant, as displacement above or below the baseline can signal a lack of oxygen to the heart muscle.
Common Cardiac Conditions Implied by Abnormalities
Changes in the heart’s rate and rhythm often point to various arrhythmias, which are electrical disturbances. An “irregularly irregular” rhythm without discernible P waves is the hallmark of Atrial Fibrillation (Afib), where the upper chambers quiver instead of contracting effectively. Heart block, characterized by a prolonged PR interval or dropped QRS complexes, indicates a conduction problem between the atria and ventricles.
Abnormalities in the ST segment and T wave are frequently associated with inadequate blood flow or damage to the heart muscle. ST-segment elevation, where the segment is lifted above the baseline, can indicate a complete blockage of a coronary artery, known as ST-elevation myocardial infarction (STEMI). Conversely, ST-segment depression, where the segment dips below the baseline, often suggests subendocardial ischemia, meaning the inner layer of the heart muscle is not receiving enough oxygen. New, deep Q waves can also signify a prior heart attack where a section of heart muscle has been replaced by scar tissue.
Changes in the QRS complex voltage or the electrical axis can suggest structural changes. Left ventricular hypertrophy (LVH), often caused by long-standing high blood pressure, causes the left ventricle to thicken. This increase in muscle mass amplifies the electrical signal, resulting in unusually tall R waves and deep S waves. The strain on the heart muscle can also cause secondary changes, such as ST-segment depression and T-wave inversion.
Non-Cardiac Causes of ECG Deviations
An abnormal ECG does not exclusively indicate primary heart disease; systemic and external factors can also disrupt the heart’s electrical balance. Electrolyte imbalances, particularly in potassium and calcium, are common causes of ECG deviations. Low potassium levels (hypokalemia) can cause ST-segment depression and a prominent U wave, while high potassium (hyperkalemia) can result in tall, peaked T waves.
Calcium levels also influence electrical activity. High calcium (hypercalcemia) shortens the QT interval, and low calcium (hypocalcemia) prolongs it. These mineral disturbances affect the heart muscle’s ability to repolarize correctly.
Many common prescription and over-the-counter medications are known to alter the heart’s electrical properties. Certain antiarrhythmic drugs, some antibiotics, and antidepressants can prolong the QTc interval, increasing the risk for a serious ventricular arrhythmia called Torsades de Pointes.
Technical or situational factors can also lead to a misleading reading. Examples include poor positioning of the electrodes, patient movement during the recording, or heightened sympathetic nervous system activity caused by acute anxiety.
Follow-Up and Next Diagnostic Steps
Receiving an abnormal ECG result requires a personalized follow-up plan to determine its clinical significance. Since the standard ECG provides only a 10-second snapshot of heart activity, a doctor may order a Holter monitor. This portable device is worn for 24 to 48 hours to capture intermittent rhythm abnormalities that the initial static ECG might have missed, such as infrequent palpitations.
To assess the physical structure and function of the heart, an Echocardiogram may be performed. This ultrasound test provides moving images of the heart chambers, valves, and muscle walls. It allows doctors to measure muscle thickness and evaluate the heart’s pumping ability.
If the abnormality is related to blood flow that only occurs with exertion, a stress test may be necessary. This involves monitoring the heart’s electrical activity and function while the patient walks on a treadmill or receives medication to simulate exercise.
The ECG interpretation is integrated with the patient’s symptoms, medical history, and risk factors. These further diagnostic steps help confirm whether the ECG deviation represents a benign electrical variation, a systemic issue like an electrolyte imbalance, or a serious underlying cardiac condition.