An electrocardiogram (ECG or EKG) is a non-invasive medical test that records the heart’s electrical activity. This electrical activity drives heart muscle contractions, enabling blood circulation. An ECG captures these impulses over time, providing a visual representation of how the heart is functioning and serving as a diagnostic tool for heart health.
Deciphering the ECG Tracing
An ECG tracing is printed on grid paper, with the horizontal axis measuring time and the vertical axis measuring voltage. Each small square represents 0.04 seconds horizontally and 0.1 millivolts vertically. Larger squares, composed of five small squares, represent 0.20 seconds horizontally and 0.5 millivolts vertically.
The tracing displays waves, intervals, and segments, each corresponding to specific electrical events in the heart. The P wave is the first small, rounded upward deflection, signifying atrial depolarization (electrical activation causing atrial contraction). The QRS complex, a larger and sharper deflection after the P wave, represents ventricular depolarization (electrical activation leading to ventricular contraction). Atrial repolarization, the atria’s electrical relaxation, occurs during the QRS complex but is typically hidden. The T wave, a rounded deflection after the QRS complex, indicates ventricular repolarization (electrical resetting and relaxation of the ventricles).
Intervals and segments between these waves provide additional information. The PR interval measures the time from the P wave’s beginning to the QRS complex’s start, reflecting impulse travel from atria through the atrioventricular (AV) node to ventricles. The ST segment is the flat line connecting the QRS complex’s end to the T wave’s beginning, representing the period when ventricles are fully depolarized and beginning to repolarize. The QT interval encompasses the QRS complex, ST segment, and T wave, measuring total ventricular depolarization and repolarization time.
Assessing Heart Rate and Rhythm
Determining heart rate from an ECG tracing is a key interpretation step. For regular rhythms, find an R wave that aligns with a thick grid line. Count the number of large squares until the next R wave, then divide 300 by that number for the heart rate in beats per minute (bpm). For instance, two large squares between R waves equals approximately 150 bpm.
For irregular rhythms, the 6-second method is useful. Locate a 6-second segment on the ECG strip, which typically spans 30 large squares. Count the QRS complexes (R waves) within this segment and multiply by 10 for the heart rate in bpm. A normal resting heart rate for adults is usually 60-100 bpm.
Assessing heart rhythm involves observing heartbeat regularity. A regular rhythm has consistent R-R intervals (distances between consecutive R waves). An irregular rhythm shows noticeable variations. Normal sinus rhythm, the heart’s natural rhythm, is characterized by a regular rate (60-100 bpm), each QRS complex preceded by a normal P wave, and a constant PR interval. This indicates electrical impulses originate from the sinoatrial node and conduct normally.
Recognizing Basic Patterns
ECG tracings reveal patterns suggesting deviations from typical heart rhythm. A visually “fast” heart rate (R waves close together) indicates tachycardia. Conversely, a “slow” rate (R waves widely spaced) suggests bradycardia.
Rhythm variations are also noticeable. An irregular rhythm displays inconsistent R-R intervals, with some beats appearing closer and others farther apart. This contrasts with the predictable, even spacing of a regular rhythm. Wave shapes and presence also offer insights. A consistently absent P wave before a QRS complex, or unusual P wave shapes, can signal altered atrial impulse origin or conduction.
The QRS complex can exhibit distinct patterns. An unusually wide complex suggests prolonged ventricular depolarization, while a narrow complex indicates rapid ventricular depolarization. Similarly, T waves that are unusually tall, flattened, or inverted deviate from the typical rounded, upright appearance. These observations indicate potential differences from a normal ECG pattern.
Practical Steps for Simplified Interpretation
A structured method aids basic ECG interpretation:
Ensure proper recording: Check for standard 25 mm/second speed and voltage calibration for accurate measurements.
Determine heart rate: Use methods like counting large squares between R waves for regular rhythms or the 6-second method for irregular ones.
Evaluate rhythm: Observe spacing between QRS complexes to determine regularity (consistent spacing) or irregularity (varying distances).
Examine P waves: Confirm their presence before every QRS complex and consistent shape to verify atrial activity.
Assess QRS complexes: Note their width (narrow or wide) and consistent shape across the tracing.
Review T waves and ST segments: Check if T waves appear typical in shape and direction, and if the ST segment is at the baseline.
This simplified approach provides a framework for understanding an ECG. Remember, this basic interpretation is for general understanding and does not replace professional medical diagnosis.