An Electrocardiogram (ECG or EKG) is a non-invasive diagnostic test that records the electrical activity generated by the heart as it contracts. This procedure involves placing small electrodes on the skin of the arms, legs, and chest to detect these electrical impulses. A healthcare provider may order an ECG during a routine physical examination, as a pre-surgical screening tool, or to investigate symptoms such as dizziness, chest discomfort, or palpitations. An abnormal ECG indicates a deviation from the expected electrical pattern, requiring further investigation rather than serving as a final diagnosis.
What an ECG Measures
The heart’s function is driven by a precise sequence of electrical signals that coordinate the muscle contractions necessary for pumping blood. This electrical impulse normally originates in the sinoatrial (SA) node, the heart’s natural pacemaker. The ECG machine translates this electrical flow into characteristic wave patterns visible on the tracing.
The tracing begins with the P wave, which represents the electrical activation (depolarization) of the atria. Immediately following is the QRS complex, reflecting the depolarization of the ventricles, the heart’s main pumping chambers. The final component is the T wave, which indicates ventricular repolarization, the electrical recovery phase.
These three components—P wave, QRS complex, and T wave—must occur in a specific pattern, regularity, and duration to be considered a normal sinus rhythm. Any deviation in the shape, timing, or amplitude of these waves suggests an electrical or structural issue within the heart.
Abnormalities Related to Heart Rate and Rhythm
One of the most common groups of ECG abnormalities involves deviations in the heart’s rate and rhythm, collectively known as arrhythmias. The normal adult heart rate typically falls between 60 and 100 beats per minute. A persistent heart rate below this range is termed bradycardia, suggesting the heart’s natural pacemaker is firing too slowly or the electrical signal is being delayed.
Conversely, a sustained heart rate above 100 beats per minute is called tachycardia, which can result from a normal physiological response (like stress or fever) or a pathological issue. Certain tachycardias, such as supraventricular tachycardia (SVT), originate above the ventricles. Ventricular tachycardia, originating in the lower chambers, is often more serious and can compromise the heart’s ability to circulate blood effectively.
Irregular rhythms represent another significant category, where the heart’s electrical signals fire chaotically or out of sequence. Atrial fibrillation (AFib) is a frequent example, characterized by rapid, disorganized electrical activity in the atria that replaces the distinct P wave with a chaotic, wavy baseline. This results in an “irregularly irregular” rhythm in the ventricles, which can lead to blood pooling and an increased risk of stroke.
Conduction defects, where the electrical signal is blocked or slowed, also manifest as rhythm abnormalities. A Bundle Branch Block (BBB) occurs when one of the main pathways supplying electricity to the ventricles is interrupted. This forces the electrical impulse to detour, prolonging the time it takes for the ventricles to depolarize, resulting in a widened QRS complex on the ECG tracing.
Abnormalities Indicating Muscle Damage or Structure Issues
Beyond issues of timing and rhythm, an abnormal ECG can point to physical changes or damage to the heart muscle (myocardium). Ischemia, a lack of sufficient blood flow and oxygen, is often indicated by a depression of the ST segment, the line between the QRS complex and the T wave.
If the blockage is prolonged, it can lead to myocardial infarction (a heart attack), where a portion of the heart muscle dies. Acute heart attacks are frequently identified by a distinctive elevation of the ST segment (STEMI), indicating a complete blockage of a coronary artery. Damaged tissue eventually loses its ability to conduct electricity normally, resulting in a pathological Q wave.
Structural changes, such as the thickening or enlargement of a chamber wall, also alter the electrical signature. Ventricular hypertrophy, often a response to chronic strain from conditions like high blood pressure, causes the heart muscle mass to increase. This greater mass generates a larger electrical signal, appearing on the ECG as increased voltage, specifically taller R waves and deeper S waves.
The increased muscle mass in hypertrophy can also alter the direction of the heart’s electrical spread, leading to a shift in the electrical axis. These voltage and axis changes suggest the heart has had to work harder for a sustained period, leading to physical remodeling.
Follow-up and Next Steps After an Abnormal Reading
Receiving an abnormal ECG result does not automatically confirm a serious heart condition, as the tracing is merely a snapshot of the heart’s electrical state at a single moment in time. The physician must correlate the ECG findings with the patient’s age, medical history, and current symptoms. For instance, some ECG patterns considered abnormal in a sedentary adult may be a normal variant in a highly trained athlete.
Factors outside the heart can influence electrical activity and produce a misleading tracing. Imbalances in electrolytes (such as potassium or calcium) or the side effects of certain medications can significantly alter the heart’s electrical currents. Technical errors or physiological changes like obesity can also mimic true abnormalities, requiring the clinician to consider non-cardiac causes.
To diagnose the underlying issue, a physician recommends further diagnostic tests. An Echocardiogram (ultrasound) is often the next step to visualize the heart’s structure and assess its pumping function. For intermittent rhythm disturbances, a Holter monitor may be prescribed, recording electrical activity continuously over 24 to 48 hours.
A Stress Test evaluates the heart’s electrical and blood flow response to physical exertion. The ECG guides the selection of these subsequent tests needed to reach an accurate diagnosis and determine treatment.