Electrocardiogram Interpretation in 10 Simple Steps
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Article at a Glance
- On an ECG grid, one small box represents 0.04 seconds while one large box represents 0.20 seconds.
- Electrocardiogram irregularities may indicate conditions such as myocardial infarction and atrial fibrillations.
- Clinicians will learn the 10 steps to interpret an electrocardiogram.
Electrocardiogram Interpretation
There are 10 easy steps to interpret an electrocardiogram (ECG) rhythm.
- Know the measurements
- Count the heart rate
- Determine rhythm regularity
- Evaluate the P wave
- Measure the PR interval
- Measure the QRS interval
- Look at the ST segment
- Evaluate the T wave
- Measure QT interval
- Interpret the strip
An electrocardiogram records the electronic signals of the heart.
Related Video – Systematic Approach to ECG Interpretation
Know your Measurements
The ECG grid is separated into big and small boxes. Each small box is a measurement of 0.04 seconds, while each big box is 0.20 seconds. Each big box is a 5 x 5 grid of small boxes.
1 small box = 0.04 seconds
1 large box = 0.20 seconds
Count the Heart Rate
There are black line markings at the top of the ECG strip. To measure six seconds on the ECG strip, clinicians choose a segment between three of these line markings at the top.
Next, providers count the R-R complexes within the three line markings. That measurement is the number of heartbeats per six seconds.
Lastly, clinicians multiply the number by 10 to get the heart rate per minute.
For example, if there are eight R-R complexes within a six-second strip, the heart rate is 80 beats per minute.
The electrical activity of the heart is denoted by the letters P, Q, R, S, T, and U.
Read: Electrolytes Explained
Determine if the Rhythm is Regular
Clinicians use a piece of paper to mark the R-R complexes. They go across the ECG strip and check if the distance between each R-R complex is the same. If the R-R complexes are evenly spaced, the rhythm is regular.
Alternatively, if someone has atrial fibrillation or dysrhythmia, the R-R complexes will be scattered, indicating irregular rhythm.
Evaluate the P-wave
Clinicians first look for the presence of a P wave before checking to see if there’s one for every complex.
Next, providers check if the P wave is upright and normally shaped. If yes, the patient has a sinus rhythm.
Measure the PR Interval
The PR interval is the time from the onset of the P wave to the start of the QRS complex. The PR interval represents the conduction time through the AV node. A normal PR interval is 120–200 milliseconds.
Clinicians count the number of small boxes between the start of the P wave to the beginning of the QRS complex. The PR interval should be less than 0.20 seconds. Typically, the P wave is between 3–5 small boxes in length. If the PR interval is longer than 0.20 seconds, a first-degree AV block is present.
Measure the QRS Interval
Physicians start counting where the Q leaves the baseline and stops when the S wave returns to baseline. A normal QRS interval is between 0.08 and 0.10 seconds. An interval between 0.10 and 0.12 seconds is slightly prolonged.
Any QRS complex longer than 0.12 seconds (three small boxes) indicates a bundle branch block, an electrical conduction problem through the bundle branches. It does not mean a physical block is present.
Look at the ST Segment
The ST segment is in between the end of the S wave to the beginning of the T wave. If the ST segment is depressed or elevated, it indicates ischemia or infarction.
Evaluate the T wave
Similar to the P wave, providers ensure there is an upright T wave for every complex. An inverted T wave indicates some degree of ischemia.
Measure the QT Interval
The QT interval is the start of the Q wave to the end of the T wave, representing the time it takes for the ventricles to depolarize and repolarize. The QT interval should not be longer than 0.50 seconds (500 milliseconds).
If a patient has a prolonged QT interval, they may be susceptible to fatal dysrhythmias. Some medications prolong the QT interval, such as clarithromycin and ketoconazole.
Interpret the ECG Strip
Finally, clinicians evaluate the overall ECG strip.
The ECG strip at the top shows bradycardia, while the one at the bottom displays tachycardia.
Summary
The ECG grid contains large and small boxes. Large boxes represent 0.20 seconds, while the small boxes represent 0.04 seconds.
Clinicians perform 10 steps to interpret an ECG rhythm. They must know measurements, count the heart rate, determine rhythm regularity, evaluate the P wave, measure the PR interval, measure the QRS interval, observe the ST segment, evaluate the T wave, and interpret the ECG strip.
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