An electrocardiogram (ECG or EKG) is a non-invasive diagnostic tool that records the heart’s electrical activity. It translates these impulses into a visual representation on a strip or digital display, providing a detailed graph of the heart’s rhythm and electrical function for evaluation.
Understanding the ECG Grid
An ECG strip is printed on specialized grid paper with horizontal and vertical lines for precise measurements. The horizontal axis measures time, while the vertical axis represents voltage or amplitude. This grid uses small and large squares as standardized units for interpreting electrical signals.
Each small square measures 1 millimeter by 1 millimeter. Horizontally, a single small square represents 0.04 seconds, indicating the duration of an electrical event. Vertically, each small square corresponds to 0.1 millivolts, showing the signal’s strength.
Five small squares make up one large square, creating a 5mm by 5mm grid. Horizontally, each large square represents 0.20 seconds (5 x 0.04 seconds). Vertically, a large square signifies 0.5 millivolts (5 x 0.1 millivolts). These standardized measurements allow for consistent analysis of the heart’s electrical activity.
Components of the ECG Waveform
The ECG waveform is composed of distinct deflections and segments, each reflecting specific electrical events within the heart’s cardiac cycle. The typical waveform begins with the P wave, followed by the QRS complex, and concludes with the T wave.
The P wave is the first small, rounded upward deflection. It represents atrial depolarization, the electrical activation of the atria that causes them to contract and pump blood into the ventricles. A normal P wave measures less than 0.11 seconds in duration and less than 0.25 millivolts in amplitude.
Following the P wave, the QRS complex appears as a larger set of deflections. This complex signifies ventricular depolarization, the electrical activation of the ventricles leading to their contraction and the ejection of blood. The QRS complex consists of three waves: the Q wave (downward), the R wave (large upward), and the S wave (downward after R). Its duration is between 0.06 and 0.10 seconds.
The T wave is a rounded, upward deflection that follows the QRS complex. This wave represents ventricular repolarization, the electrical recovery of the ventricles as they relax and prepare for the next beat. The T wave has a broader and more rounded appearance than the P wave.
Beyond individual waves, intervals on the ECG strip measure the time between specific electrical events. The PR interval measures the time from the beginning of atrial depolarization to the beginning of ventricular depolarization, ranging from 0.12 to 0.20 seconds. This interval includes the P wave and the segment before the QRS. The QT interval measures the total time for ventricular depolarization and repolarization, extending from the start of the QRS complex to the end of the T wave. Its duration varies with heart rate but is less than 0.44 seconds in adults.
Interpreting Basic Rhythms and Rate
Determining heart rate from an ECG strip involves counting ventricular contractions. For regular rhythms, the “300 method” is common: locate an R wave on a heavy grid line, count large squares to the next R wave, and divide 300 by that number (e.g., 1 large square = 300 bpm, 2 = 150 bpm, 3 = 100 bpm).
An alternative for regular rhythms involves counting the small squares between two consecutive R waves and dividing 1500 by that number. For instance, if there are 20 small squares between R waves, the rate is 1500/20 = 75 beats per minute. This method offers greater precision for regular rhythms.
For irregular rhythms, where the distance between R waves varies, a different approach is used. One common technique involves counting the QRS complexes within a 6-second strip (30 large squares) and multiplying that count by 10. For example, if you count 7 QRS complexes in a 6-second segment, the estimated heart rate is 70 beats per minute.
Assessing rhythm regularity involves examining the R-R intervals, the distance between consecutive R waves. If the R-R intervals are consistent, the rhythm is regular. If they vary significantly, the rhythm is irregular. Observing these patterns helps differentiate between various cardiac electrical activities.
Recognizing Common Variations
Deviations from a normal ECG pattern can signal underlying cardiac issues. These observations are not diagnostic but indicate a need for professional medical evaluation.
One common variation relates to heart rate. Tachycardia refers to a heart rate faster than the normal range, exceeding 100 beats per minute in adults. On an ECG strip, this appears as R waves that are closer together, indicating more frequent electrical impulses.
Conversely, bradycardia describes a heart rate slower than normal, below 60 beats per minute in adults. An ECG strip showing bradycardia will have R waves spaced further apart, reflecting fewer electrical activations per minute. Both conditions can be a physiological response or indicate a medical concern.
Significant irregularities in the R-R intervals also represent a common variation. An irregular rhythm suggests that the electrical impulses are not originating or propagating consistently, leading to an unpredictable heart beat pattern.