An electrocardiogram (ECG) is a non-invasive medical test that provides a visual representation of the heart’s electrical activity. It helps healthcare professionals assess heart function by revealing if the heart is beating too fast, too slow, or with an irregular rhythm.
The Normal Heartbeat on an ECG
To understand irregular heartbeats, it is helpful to first recognize a normal heart rhythm on an ECG. A healthy heart beats regularly, originating from the sinoatrial (SA) node, often called the heart’s natural pacemaker. This normal pattern, known as normal sinus rhythm, typically has a heart rate between 60 and 100 beats per minute.
A normal ECG tracing consists of distinct waves and complexes, each representing a specific electrical event in the heart. The P wave signifies the contraction of the upper chambers (atria). The QRS complex, a sharp deflection, indicates the contraction of the lower chambers (ventricles). The T wave represents ventricular electrical recovery. In a normal ECG, these waves appear consistently with regular spacing, reflecting the heart’s synchronized and efficient pumping action.
Recognizing Deviations: The Arrhythmia Basics
An arrhythmia is any deviation from the heart’s normal rhythm, meaning the heart may beat too quickly, too slowly, or irregularly. These irregularities arise from problems within the heart’s electrical conduction system, which coordinates each heartbeat. On an ECG, arrhythmias manifest as changes in the rate, rhythm, or the shape and timing of the P, QRS, or T waves.
Abnormalities can involve heart rate. Tachycardia is a heart rate exceeding 100 beats per minute in adults. Bradycardia is a heart rate below 60 beats per minute. Rhythm abnormalities involve irregular patterns, such as erratic or skipped beats, where consistent spacing between heartbeats is lost. Conduction abnormalities reflect issues with how electrical signals travel through the heart’s pathways, altering the appearance, duration, or sequence of the P wave, QRS complex, or T wave on the ECG.
Key Arrhythmia Signatures on ECG
Specific arrhythmias present with distinct patterns on an ECG, allowing healthcare providers to identify them. Atrial fibrillation (Afib) is a common arrhythmia characterized by an “irregularly irregular” rhythm. On an ECG, Afib typically lacks distinct P waves, replaced by chaotic, rapid electrical activity (fibrillatory waves) as the atria quiver instead of contracting effectively. QRS complexes usually appear narrow, but their timing is uneven.
Sinus tachycardia and sinus bradycardia represent heart rate abnormalities while maintaining a normal electrical sequence. In sinus tachycardia, the ECG shows a regular rhythm with a heart rate over 100 beats per minute. P waves, QRS complexes, and T waves are present and normal, though the P wave may be hidden by the preceding T wave due to the rapid rate. Sinus bradycardia displays a regular rhythm with a heart rate under 60 beats per minute. Here, P waves, QRS complexes, and T waves are also present and normal, but the space between beats is prolonged.
Premature ventricular contractions (PVCs) appear on an ECG as an early, wide, unusually shaped QRS complex interrupting the normal rhythm. These extra heartbeats originate from the lower chambers, causing the electrical signal to spread differently. A PVC is usually followed by a pause before the normal rhythm resumes, which can feel like a “skipped beat.” The QRS complex during a PVC is broad, often exceeding 120 milliseconds, compared to the narrow QRS complexes of normal beats.
Importance of Medical Interpretation
While understanding basic ECG patterns provides insight, professional medical interpretation is essential for accurate diagnosis and appropriate management of heart conditions. An ECG is a diagnostic tool, but it is only one component of a comprehensive medical assessment. Healthcare professionals consider symptoms, medical history, physical examination findings, and other diagnostic tests alongside the ECG.
Automated ECG analyses exist, but an experienced clinician should always confirm them to avoid misdiagnosis. Different arrhythmias require specific treatments. A complete understanding of the heart’s electrical activity is necessary to determine the most effective course of action. Consult a doctor if there are concerns about heart health or ECG results, as self-interpretation cannot replace expert medical judgment.