An electrocardiogram (ECG) is a graphic recording that traces the electrical activity generated by the heart muscle over time. This non-invasive diagnostic tool captures the sequence of depolarization and repolarization that drives the heart’s pumping action. Interpreting an ECG strip involves recognizing patterns and deviations from a normal baseline to understand the heart’s rhythm and condition.
Establishing the Foundation: The ECG Grid and Wave Components
The ECG is printed on specialized graph paper marked with a grid of small and large squares. The standard recording speed is twenty-five millimeters per second, which dictates the time scale used for all measurements.
The horizontal axis of the grid represents time, where each small square measures 0.04 seconds, and consequently, each large square measures 0.20 seconds. The vertical axis, in contrast, represents voltage or amplitude, allowing for the measurement of the strength of the electrical signal. This vertical axis is typically calibrated so that ten small squares equal one millivolt of electrical potential.
The flat line between deflections is the isoelectric line, representing the period when no significant electrical current is flowing. The first deflection is the P wave, which signifies the electrical activation, or depolarization, of the atria. This wave is usually small and rounded, reflecting the impulse traveling from the sinoatrial node.
Following the P wave is the QRS complex, a sharp sequence of deflections representing the depolarization of the ventricles, the heart’s main pumping chambers. Because the ventricular muscle mass is larger than the atria, the QRS complex displays a significantly greater amplitude than the P wave. The final major component is the T wave, which reflects the electrical recovery, or repolarization, of the ventricles.
Step 1: Calculating Heart Rate and Assessing Regularity
The first step in analyzing any ECG strip is determining the heart rate and assessing the overall regularity of the rhythm. The heart rate, expressed in beats per minute, provides an immediate indication of whether the heart is beating too fast, too slow, or within a normal range. The method used for calculation depends entirely on whether the rhythm appears regular or irregular.
For rhythms that are demonstrably regular, the “300-rule” method offers a quick and effective estimation of the heart rate. This method involves locating an R wave that falls directly on a heavy line of the grid and then counting the number of large squares until the next R wave appears. By dividing 300 by the counted number of large squares, a close approximation of the heart rate is obtained.
If the rhythm is irregular, the 300-rule cannot be accurately applied because the R-R interval varies significantly across the strip. Instead, the “6-second method” must be employed for a more reliable estimation of the average rate. This technique requires identifying a 6-second segment of the tracing, which corresponds to thirty large squares on the standard speed paper.
Once the 6-second segment is identified, the number of QRS complexes within that period is counted, and this total is then multiplied by ten. Multiplying the count by ten provides the average rate of beats per minute, offering a practical assessment when the rhythm is disorganized.
Assessing the regularity, or rhythm, of the heart involves measuring the interval between consecutive R waves, known as the R-R interval. A rhythm is considered regular when the distance between successive R waves is consistent throughout the entire strip. Small variations are expected, but significant differences suggest an irregular rhythm.
To confirm regularity, one can measure the R-R interval using calipers or by marking the distance on a piece of paper and comparing it across multiple cycles. If the intervals are consistently different, the rhythm is labeled irregular, which then necessitates using the 6-second rate calculation method.
Step 2: Detailed Analysis of Waveforms and Intervals
After establishing the rate and regularity, a systematic examination of the morphology and duration of the individual waves and intervals provides deeper diagnostic clues. This step uses the grid measurements to assess the timing and sequence of the heart’s electrical events.
The P wave is analyzed first, focusing on its presence, shape, and its relationship to the QRS complex. A normally conducted impulse originating in the sinus node results in a P wave that is upright in most leads and precedes every QRS complex. The duration of the P wave should typically be less than 0.12 seconds, or no more than three small squares wide, reflecting rapid atrial depolarization.
Following the P wave, the PR interval measures the time from the beginning of atrial depolarization to the beginning of ventricular depolarization. This measurement starts at the beginning of the P wave and ends at the beginning of the QRS complex. A normal PR interval falls within the range of 0.12 to 0.20 seconds, spanning three to five small squares on the grid.
The QRS complex analysis focuses on the duration. The QRS duration represents the time required for the impulse to spread through the ventricles via the specialized conduction system. A normal QRS complex is narrow, measuring less than 0.12 seconds, or less than three small squares wide, indicating fast and efficient ventricular activation.
If the QRS complex is wider than three small squares, it suggests a delay in ventricular conduction, possibly due to a block in the bundle branches or an impulse originating outside the normal pathways. The specific shape of the complex, including the presence or absence of Q, R, and S waves, can also indicate issues such as prior myocardial damage or hypertrophy. The amplitude of the QRS complex is assessed vertically to determine the voltage.
The final measurement is the QT interval, which encompasses the entire period of ventricular electrical activity, from the start of the QRS complex to the end of the T wave. This interval reflects both ventricular depolarization and repolarization, and its duration is sensitive to changes in heart rate. For a basic assessment, the QT interval should generally be less than half of the preceding R-R interval.
Identifying Basic Arrhythmias
The most important baseline rhythm to establish is the Normal Sinus Rhythm (NSR), which serves as the standard against which all other rhythms are compared. NSR is defined by a heart rate between 60 and 100 beats per minute, a regular rhythm, and the presence of a P wave that precedes every QRS complex, with a consistent PR interval.
When the analyzed rate falls outside the normal range, the rhythm is labeled accordingly, provided all other sinus rhythm criteria are met. Sinus Tachycardia is identified when the heart rate is greater than 100 beats per minute, maintaining a regular rhythm originating from the sinus node. The P waves remain present and normal in appearance.
Conversely, Sinus Bradycardia is diagnosed when the heart rate is less than 60 beats per minute, while still maintaining a regular rhythm with normal PQRST morphology. This slower rate indicates a decreased firing frequency of the sinus node, although the conduction pathway remains intact and functional.
A different category of rhythm disturbance is Atrial Fibrillation, which is characterized by a rapid, disorganized electrical activity in the atria. The hallmark of Atrial Fibrillation is an irregularly irregular rhythm, meaning the R-R intervals are constantly changing without any discernible pattern. This chaotic activity results in the complete absence of distinct, uniform P waves.
Instead of clear P waves, the tracing may show fine, erratic baseline oscillations known as fibrillatory waves, or sometimes the baseline appears simply wavy. The QRS complexes in Atrial Fibrillation are typically narrow because the impulse travels correctly once it passes through the atrioventricular node. The identification hinges on the combination of the highly irregular ventricular rate and the lack of organized atrial activity.