An electrocardiogram (ECG or EKG) is a non-invasive diagnostic tool that records the electrical activity of the heart. Physicians use this recording to assess the heart’s rhythm, rate, and the efficiency of its electrical conduction system.
The normal electrical sequence of a heartbeat is represented by a repeating pattern of waves labeled P, QRS, and T. The P wave corresponds to the electrical signal that initiates the contraction of the upper chambers, the atria (atrial depolarization). Following this is the QRS complex, which signifies the rapid depolarization of the lower, larger chambers, the ventricles, leading to their powerful contraction. Finally, the T wave represents ventricular repolarization, the electrical recovery phase. An ECG abnormality is a deviation from the expected shape, timing, or regularity of this standard P-QRS-T cycle.
Issues with Heart Rate and Rhythm
Abnormalities related to the speed and regularity of the heart are called arrhythmias, often involving beats that are too fast, too slow, or disorganized. A heart rate consistently above 100 beats per minute is known as tachycardia, while a rate below 60 beats per minute is termed bradycardia. These rate issues can be benign, such as a slow rate in a highly conditioned athlete, or they can signal an underlying structural or electrical problem.
One of the most frequently encountered rhythm disorders is Atrial Fibrillation (Afib), which is the most common sustained arrhythmia. On an ECG, this condition is characterized by an “irregularly irregular” rhythm, meaning the time between successive heartbeats is completely erratic. The atria, instead of contracting efficiently, quiver chaotically, leading to a loss of the distinct P wave on the recording.
The disorganized electrical activity in Afib causes the heart’s lower chambers to receive a rapid, disorganized barrage of signals, resulting in the irregular ventricular rate. Afib prevalence increases dramatically with age. This chaotic rhythm significantly increases the risk of stroke and heart failure, making its detection on an ECG a serious finding.
Other common rhythm disturbances involve premature contractions, often described by patients as “skipped beats” or palpitations. These are known as premature ventricular complexes (PVCs) or premature atrial complexes (PACs). PVCs show up on an ECG as an early, wide, and bizarre-looking QRS complex not preceded by a P wave. While occasional premature beats are common and usually harmless, frequent occurrences can sometimes indicate underlying heart strain or may be a precursor to more severe arrhythmias.
Indicators of Heart Muscle Damage
The ECG is a powerful tool for detecting physical stress or damage to the heart muscle (ischemia or infarction). Changes in the ST segment are the primary indicators for these issues; the ST segment is the normally flat, isoelectric line between the QRS complex and the T wave.
ST segment depression, where the line dips below the baseline, often suggests subendocardial ischemia, meaning the inner layer of the heart muscle is not receiving enough oxygen. This often occurs during periods of high cardiac demand or in the context of unstable angina. Conversely, ST segment elevation, where the line rises above the baseline, indicates a severe, acute myocardial infarction affecting the full thickness of the heart wall.
Another finding suggesting prior significant muscle damage is the appearance of pathological Q waves, which represent an electrical “hole” in the heart tissue. These Q waves are abnormal because the infarcted, dead scar tissue can no longer conduct electricity. They appear as a negative deflection that is wider or deeper than normal. Pathological Q waves generally develop hours to days after a major infarction and typically remain on the ECG indefinitely as a sign of permanent scarring.
Problems with Electrical Pathway Conduction
Some common ECG abnormalities involve a delay or block in the specialized wiring that carries the electrical signal through the ventricles, known as conduction blocks. The main pathways are the right and left bundle branches; a delay in either is termed a Bundle Branch Block (BBB). A BBB forces the electrical impulse to travel through slower muscle tissue, leading to a delayed and asynchronous activation of the ventricles. This results in a distinctive widened QRS complex, lasting longer than 120 milliseconds on the ECG.
Delays can also occur at the junction between the upper and lower chambers, specifically at the atrioventricular (AV) node, leading to an AV block. The mildest form is a First-Degree AV Block, which is characterized by a prolonged PR interval, exceeding 200 milliseconds. This interval represents the time it takes for the electrical signal to travel from the atria to the ventricles.
In a First-Degree AV Block, every atrial impulse successfully reaches the ventricle, meaning every P wave is followed by a QRS complex, but the travel time is simply extended. While this type of block is often asymptomatic and usually does not require treatment, it indicates a slowed conduction. More severe, higher-degree AV blocks involve dropped beats, where some atrial signals fail to reach the ventricles entirely.
When to Seek Medical Attention
While some findings, such as an isolated premature beat or mild bradycardia in an athlete, may be normal variations, others demand immediate attention. The clinical significance of an ECG change is always determined alongside a patient’s reported symptoms and medical history. Any new occurrence of chest pain, severe shortness of breath, sudden fainting (syncope), or persistent palpitations should prompt an immediate medical assessment. Specifically, ST segment elevation suggests an ongoing, acute heart attack requiring emergency intervention. A newly identified, fast, and irregular rhythm like Atrial Fibrillation also needs prompt evaluation due to the associated risk of stroke.