An electrocardiogram, commonly known as an EKG or ECG, is a simple, non-invasive test that records the electrical activity of the heart. The visual output is a tracing printed on specialized grid paper, which can appear complex at first glance. Understanding how to read this tracing involves learning the language of the electrical waves and the grid measurements. This guide provides the foundational knowledge necessary to appreciate the components of a normal EKG.
The EKG Grid and Measurement Basics
The paper used for an EKG tracing is a standardized grid that allows for precise measurement of time and voltage. The horizontal axis measures time, while the vertical axis measures the amplitude, or voltage, of the electrical signals. This standardization is fundamental to interpreting the tracing accurately.
The grid is composed of small squares, each measuring 1 millimeter by 1 millimeter. With the standard paper speed set to 25 millimeters per second, each small square horizontally represents 0.04 seconds. Five small squares form a larger, bolder square, representing a time span of 0.20 seconds. Vertically, one small square represents 0.1 millivolts (mV), allowing clinicians to gauge the strength of the heart’s electrical impulses.
Proper calibration ensures these measurements are consistent. A standard calibration mark should show a vertical deflection of 10 small squares (1.0 mV) over a duration of 5 small squares (0.20 seconds). This framework is the essential ruler for all subsequent EKG measurements.
Understanding the Waveforms (P, QRS, T)
The tracing on the EKG paper is a sequence of characteristic deflections, known as waveforms, that correspond to the electrical events of a single heartbeat. A normal cardiac cycle features three primary waveforms: the P wave, the QRS complex, and the T wave. Each wave represents a specific phase of depolarization (electrical activation leading to contraction) or repolarization (electrical recovery).
The P wave is the first small, rounded upward deflection, signifying atrial depolarization. This impulse originates in the sinoatrial (SA) node and spreads through the atria, causing the upper chambers to contract. A P wave typically lasts no more than 0.12 seconds, or three small squares.
Following the P wave is the QRS complex, a sharp, larger set of waves representing ventricular depolarization. This complex indicates the electrical activation of the ventricles, the heart’s main pumping chambers. It is composed of the negative Q wave, the positive R wave, and the negative S wave. The QRS complex is narrow, typically lasting 0.06 to 0.10 seconds, or two to two-and-a-half small squares.
The final component is the T wave, a broad, rounded deflection that represents ventricular repolarization, the electrical recovery phase. This wave is generally slightly asymmetrical and usually follows the same direction as the preceding QRS complex. Atrial repolarization also occurs but is obscured by the much larger QRS complex.
Interpreting Intervals and Segments
Beyond the individual waveforms, the timing and position of the electrical events are measured using intervals and segments, which represent the duration of conduction between different parts of the heart. These measurements provide insights into the functional integrity of the heart’s electrical system. Intervals include at least one wave and the intervening straight line, while segments are the straight lines between waves.
The PR interval measures the time from the beginning of atrial depolarization (P wave) to the start of ventricular depolarization (QRS complex). This duration reflects the time taken for the electrical impulse to travel through the atria and pause briefly at the atrioventricular (AV) node before moving to the ventricles. A normal PR interval falls between 0.12 and 0.20 seconds, or three to five small squares, and deviations from this range can indicate issues with conduction speed through the AV node.
The QRS duration measures the entire QRS complex, representing the total time for the electrical signal to spread through the ventricles. A duration exceeding 0.10 seconds suggests a delay or abnormality in the ventricular conduction pathway, such as a bundle branch block. A widened QRS complex is a sign of impaired electrical signaling.
The ST segment is the flat line connecting the end of the QRS complex to the beginning of the T wave. It represents a period where the entire ventricle is depolarized but not yet repolarized. This segment should align with the isoelectric baseline. Deviation of the ST segment, either elevated or depressed, is a significant finding that can indicate myocardial ischemia, or lack of blood flow to the heart muscle.
The QT interval measures the total duration of ventricular activity, from the start of the QRS complex to the end of the T wave, encompassing both depolarization and repolarization. Because this interval is highly dependent on heart rate, it is often corrected (QTc) to assess its true duration, with a normal value typically being 0.44 seconds or less. An abnormally prolonged QT interval can increase the risk of serious ventricular arrhythmias.
Analyzing Heart Rate and Rhythm
The final steps in reading an EKG involve synthesizing the measurements to determine the heart’s rate and assessing the regularity of its rhythm. The heart rate, measured in beats per minute (bpm), can be quickly estimated using the large squares on the grid when the rhythm is regular. Since there are 300 large squares in one minute, a simplified method involves counting the number of large squares between two consecutive R waves and dividing 300 by that number.
For example, if two R waves are separated by three large squares, the heart rate is calculated as 100 bpm (300 divided by 3). This method provides a rapid approximation, but it is reliable only when the beats are evenly spaced. For irregular rhythms, the preferred technique is to count the number of R waves within a six-second strip, which is 30 large squares, and multiply that count by 10 to estimate the average rate.
Rhythm analysis involves checking two primary characteristics: the regularity of the beats and the relationship between the waves. Normal sinus rhythm (NSR) is the standard healthy rhythm, defined by a regular rate between 60 and 100 bpm. In NSR, every QRS complex is preceded by a uniform P wave, and the PR interval must be constant. This demonstrates that the electrical impulse originated correctly in the SA node and followed the normal conduction pathway.