An Electrocardiogram (ECG or EKG) is a quick, non-invasive medical test that records the heart’s electrical activity using electrodes placed on the skin. The heart generates electrical impulses that coordinate the contractions of its chambers, and the ECG machine translates this activity into a wave-like tracing. This recording provides a visual representation of the heart rate and rhythm. The purpose of the test is to assess the heart’s electrical conduction system and identify potential issues like previous heart attacks or rhythm disturbances.
Decoding the ECG Waveform and Grid
The foundation of any ECG report is the grid paper, which serves as a standardized map for measuring time and voltage. The horizontal axis represents time, and the vertical axis represents electrical voltage or amplitude. The paper is marked with small and large squares.
A standard recording speed of 25 millimeters per second ensures consistency. Horizontally, each small square represents 0.04 seconds, and five small squares (one large square) represent 0.20 seconds. Vertically, each small square represents 0.1 millivolts (mV) of electrical potential. These grid measurements provide the necessary scale to judge if the heart’s electrical events fall within normal parameters.
The tracing is composed of three main waves and a complex, known as the PQRST sequence. The P wave represents atrial depolarization, the electrical activation that causes the atria to contract. Following the P wave is the QRS complex, which signifies ventricular depolarization, triggering the powerful contraction of the main pumping chambers. The T wave represents the repolarization, or electrical recovery, of the ventricles.
Understanding Key Intervals and Rate Calculations
Measuring the intervals between waves determines the precise timing of electrical events. These intervals represent the time required for the impulse to travel through the heart’s conduction system. The PR interval spans from the beginning of the P wave to the start of the QRS complex, reflecting the time taken for the impulse to travel from the atria to the ventricles. A normal duration is typically between 0.12 and 0.20 seconds (three to five small squares).
The QRS duration measures the width of the QRS complex, indicating the time the electrical signal takes to spread through the ventricles. A normal QRS duration is narrow, lasting 0.10 seconds or less (about two and a half small squares). The QT interval extends from the start of the QRS complex to the end of the T wave, representing the total time for ventricular activation and recovery. Since this interval varies with heart rate, it is often expressed as the corrected QT interval (QTc) for accurate assessment.
Heart rate is calculated using the R-R interval, the distance between two consecutive R waves. For a regular rhythm, a rapid method divides 300 by the number of large squares between R waves. A more precise method divides 1500 by the number of small squares. If the rhythm is irregular, count the QRS complexes within a six-second strip (30 large squares) and multiply that number by ten.
Identifying Common Cardiac Rhythms and Abnormalities
Wave shapes and numerical measurements are synthesized to determine the heart’s rhythm and identify deviations from normal. The baseline is Normal Sinus Rhythm (NSR), characterized by a heart rate between 60 and 100 beats per minute, with a P wave preceding every QRS complex. In NSR, the electrical impulse originates in the sinoatrial node, and the rhythm is regular, with consistent P-P and R-R intervals.
Heart rate deviations are categorized as Sinus Tachycardia or Sinus Bradycardia. Sinus Tachycardia is a regular rhythm exceeding 100 beats per minute, often a physiological response to factors like exercise or fever. Conversely, Sinus Bradycardia is a regular rhythm below 60 beats per minute, which may be a normal finding in conditioned athletes or result from medical conditions.
Simple rhythm disturbances include premature beats, such as Premature Atrial Contractions (PACs). A PAC appears as an early P wave with an altered shape, followed by a QRS complex. Another common finding is First-Degree AV Block, characterized by a consistently prolonged PR interval greater than 0.20 seconds. This indicates a delay in the electrical signal traveling to the ventricles, though every atrial impulse still conducts.
Contextualizing the Report and Next Steps
The ECG provides a fundamental assessment of the heart’s electrical activity, but it has inherent limitations. Since the ECG is a snapshot in time, it may not capture intermittent or sporadic rhythm abnormalities occurring outside the brief recording period. Therefore, a normal ECG result does not definitively rule out heart disease, especially if the patient continues to experience symptoms.
The report must be interpreted within the larger clinical context, including the patient’s medical history, physical examination, and current symptoms. Abnormal findings, such as patterns suggesting chamber enlargement or previous injury, often require confirmation using imaging techniques like an echocardiogram. The ECG focuses solely on electrical function and provides no direct information about the heart’s structure.
If the initial ECG suggests an abnormality or if symptoms are intermittent, follow-up tests are necessary for a comprehensive view. A common next step is a Holter monitor, a portable device that records the heart’s electrical activity continuously for 24 to 48 hours to capture transient events. Another follow-up procedure is a stress test, which records an ECG while the patient exercises to see how the heart performs under increased demand. Ultimately, the ECG is a valuable screening tool, but any interpretation or diagnosis requires the expertise of a medical professional.