An electrocardiogram, or ECG, is a non-invasive medical test that records the heart’s electrical activity. Healthcare providers use it to assess heart health, diagnosing and monitoring various conditions. The ECG machine detects tiny electrical changes as the heart muscle depolarizes and repolarizes with each heartbeat, translating these impulses into a visual graph. This graph provides a snapshot of the heart’s rate, rhythm, and overall electrical function.
The Basics of a Normal ECG
A normal ECG trace exhibits a consistent and predictable pattern. Each heartbeat is represented by a sequence of distinct waves. The P wave, a small upward deflection, signifies the electrical activation of the atria. Following the P wave, the QRS complex, a sharp, larger deflection, indicates the electrical activation of the ventricles. The T wave, another upward deflection, represents the electrical recovery of the ventricles.
Specific time intervals between these waves provide additional information about the heart’s conduction system. The PR interval measures the time from atrial to ventricular activation, while the QT interval reflects the total time for ventricular depolarization and repolarization. A healthy heart produces an ECG with a regular rhythm, consistent intervals between beats, and uniform P, QRS, and T wave shapes. This pattern serves as a baseline for identifying deviations that might suggest a heart issue.
When Heart Rhythms Go Awry
An irregular ECG often points to abnormalities in the heart’s rhythm, known as arrhythmias. These disturbances alter the regular spacing or rate of heartbeats. Tachycardia, for instance, appears as a heart rate above 100 beats per minute, characterized by rapid and often narrow QRS complexes. Conversely, bradycardia shows a heart rate below 60 beats per minute, with slow and more widely spaced QRS complexes.
Some rhythm irregularities involve an unpredictable heartbeat. Atrial fibrillation (AFib) is a common arrhythmia where the atria quiver chaotically instead of contracting in a coordinated manner. On an ECG, this appears as an irregularly irregular rhythm with an absence of distinct P waves, replaced by rapid, disorganized “fibrillatory” waves. Premature ventricular contractions (PVCs) and premature atrial contractions (PACs) are instances where extra beats interrupt the normal rhythm. PVCs originate from the lower chambers, appearing as early, wide, and often bizarre-looking QRS complexes, typically followed by a brief pause. PACs arise from the upper chambers, presenting as an abnormally shaped P wave that occurs earlier than expected, usually followed by a normal QRS complex.
Signs of Other Heart Conditions
Beyond rhythm disturbances, an ECG can reveal morphological irregularities in wave shapes and segments, indicating other heart conditions. Myocardial ischemia, where blood flow to the heart muscle is reduced, can manifest as ST segment depression, appearing atypically low below the baseline. In contrast, a heart attack (myocardial infarction) often causes ST segment elevation, significantly raised above the baseline, indicating acute injury. Inverted T waves and pathological Q waves can also signal myocardial ischemia or previous heart damage. Inverted T waves may indicate ischemia. Pathological Q waves suggest a prior myocardial infarction, reflecting areas of electrically inactive scar tissue.
Enlarged heart chambers, such as ventricular hypertrophy, can also alter the ECG’s appearance. In left ventricular hypertrophy, electrical signals from the thickened heart muscle result in taller R waves and deeper S waves, reflecting increased muscle mass. Conduction blocks, which impede electrical signal transmission, can prolong the PR interval (first-degree AV block) or widen the QRS complex (bundle branch blocks), indicating a delay or interruption. These changes provide visual cues for diagnosing various heart issues.
Interpreting Your ECG Results
An irregular ECG tracing requires careful evaluation by a medical professional. It is not a definitive diagnosis, but an indication that further investigation may be necessary. The complexity of ECG interpretation means self-analysis is not advisable. Various factors, including symptoms, medical history, and other clinical findings, must be considered alongside ECG results for an accurate assessment.
Often, an irregular ECG prompts additional diagnostic tests to determine the underlying cause. These may include blood tests, an echocardiogram to visualize heart structure and function, or a stress test to evaluate heart activity during exertion. Consulting a healthcare provider is important if there are concerns about ECG results or if symptoms suggestive of a heart condition are present.