An electrocardiogram (ECG or EKG) is a simple, non-invasive test that measures the electrical activity of the heart. The procedure involves placing small electrodes on the chest and limbs to record the impulses that cause the heart muscle to contract and relax. Healthcare providers use an ECG to diagnose issues with heart rhythm, electrical conduction, or structural damage. While an abnormal result can be concerning, it does not automatically signal a severe health emergency. The test is a snapshot of heart function, and an unexpected finding often signals the need for further investigation.
Understanding the ECG Baseline
A normal ECG tracing reflects the heart’s organized electrical pathway, which starts in the sinoatrial node, the natural pacemaker. The resulting waveform is a predictable sequence of peaks and valleys, commonly described as the P-QRS-T complex. The P wave represents the electrical impulse spreading through the upper chambers (atria), causing them to contract.
The largest feature is the QRS complex, which signifies the rapid electrical activation and contraction of the lower chambers (ventricles). This ventricular depolarization is responsible for pumping blood out to the body and lungs. Following this complex, the T wave represents the electrical recovery, or repolarization, of the ventricles as they prepare for the next beat.
An abnormal ECG reading is any deviation from this expected pattern of timing, shape, or amplitude. This deviation suggests that the heart’s electrical signal is being delayed, rerouted, or originates from an unexpected location. The exact nature of the change provides specific clues about where the problem lies within the heart’s electrical system or muscle structure.
Categories of Abnormal ECG Readings
One of the most immediate abnormalities an ECG can detect relates to the heart’s rate, which is typically between 60 and 100 beats per minute in a resting adult. Tachycardia is an abnormally fast heart rate, while bradycardia is a rate that is too slow. These rate issues can be temporary, caused by stress or medication, or they may indicate a deeper underlying condition.
Rhythm irregularities, known as arrhythmias, show up as disorganized electrical activity. Atrial Fibrillation (A-Fib) is a common arrhythmia that appears as a chaotic, rapid twitching of the atria, leading to an irregular ventricular rhythm. Other rhythm disturbances, such as premature ventricular contractions (PVCs) or premature atrial contractions (PACs), are brief, extra beats that interrupt the normal sequence.
Conduction disorders involve a delay or blockage of the electrical signal as it travels through the heart’s wiring system. Atrioventricular (AV) blocks occur when the signal transmission between the atria and ventricles is slowed or completely interrupted, potentially leading to a dangerously slow heart rate. Bundle branch blocks show up as a widened QRS complex, indicating the electrical impulse is delayed in reaching one of the ventricles.
The ECG is also capable of revealing issues related to the heart muscle’s structure or blood supply. A lack of oxygen to the heart muscle, called ischemia, can cause characteristic changes in the ST segment (the flat line between the QRS complex and the T wave). If the heart muscle is actively being damaged, such as during a heart attack (myocardial infarction), the ECG may show a significant elevation of the ST segment.
Changes in the QRS complex voltage or the direction of the T wave can indicate structural changes. Ventricular hypertrophy, a thickening of the heart muscle often due to long-standing high blood pressure, can cause the electrical signals to have a greater amplitude.
Interpreting the Significance of an Abnormal Result
The finding of an abnormal ECG is not always indicative of serious heart disease, as the test is highly sensitive to various internal and external factors. Certain medications, including some antidepressants and blood pressure drugs, can alter the heart’s electrical activity. Furthermore, imbalances in electrolytes such as potassium, calcium, and magnesium can disrupt electrical conduction, causing irregular patterns that are often easily corrected.
Non-cardiac factors like significant anxiety or physical stress can temporarily cause a faster or irregular heart rhythm captured on the test. Technical errors, such as a misplaced electrode, or simple anatomical variations can sometimes mimic a problem on the tracing. For example, Early Repolarization is a common finding in healthy young adults and athletes, yet it is technically flagged as an abnormality.
The clinical context is paramount in determining the true significance of any abnormal tracing. A single finding in isolation, such as a slightly slow rate in a marathon runner, may be considered a normal variation. However, the same finding in a person with symptoms like dizziness or fainting warrants immediate and thorough investigation. The ECG must always be interpreted alongside the patient’s age, symptoms, medical history, and physical examination.
Indicators that prompt urgent follow-up include acute ST elevation, which is a strong sign of an ongoing heart attack, or the new onset of a high-grade AV block. New, significant rhythm disturbances like certain types of ventricular tachycardia also require prompt attention due to the risk of sudden cardiac events. The ECG serves as an initial screening tool, and its abnormality means a deeper look is necessary to understand the cause and determine the appropriate next steps.
Next Steps Following an Abnormal ECG
Once an abnormal ECG is identified, the next step is often to utilize more advanced diagnostic tools to confirm or rule out an underlying condition. An Echocardiogram (Echo) is a common follow-up test that uses ultrasound to create moving pictures of the heart’s structure and function. This test checks the size of the heart chambers, the function of the valves, and the overall pumping strength of the muscle.
If the abnormality is intermittent or related to the heart’s rhythm, a Holter monitor or event recorder may be prescribed. This portable device is worn for 24 hours up to several weeks, continuously recording the heart’s electrical activity. This allows physicians to capture brief arrhythmias that were missed during the initial ECG. For abnormalities that only appear with physical exertion, a stress test is performed, which involves monitoring the ECG while the patient walks on a treadmill or pedals a stationary bike.
Blood work is also frequently ordered to check for markers of heart muscle damage, such as troponin, or to assess the balance of electrolytes necessary for electrical stability. The ECG is fundamentally a screening test, and these subsequent investigations provide the comprehensive data needed to make a definitive diagnosis and management plan.