Authors
Corlin Jewell, MD
William Burns, MD
Editors
Kristen Grabow Moore, MD, MEd
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FoEM ECG I: Unit Summary Unit 0: How to Read an ECG |
Last Revision Date: Spring 2024
Creative Commons © Foundations of Medical Education, Inc.
*The following are general teaching points and guidelines. Always consider institutional policies and individual patient factors when making any clinical decisions.
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ECGs are a critical diagnostic tool in the emergency department. Accurate interpretation guides diagnosis, disposition, and therapeutic interventions. ECG interpretation mastery requires deliberate practice to guide the development of pattern recognition as well as comfort with a standardized interpretation framework. The “standard” ECG interpretation framework is to assess rate, rhythm, axis, intervals, hypertrophy, and ischemia. An alternative that provides increased focus on the specific aspects of ECG interpretation is the Rule of Fours approach. The Rule of Fours is adapted below from Life in the Fast Lane & Dr. Gerard Fennessy (@doctorgerard). It differs from the “standard” framework because it focuses on the patient’s clinical picture while they interpret the ECG as well as much more complete approach to interpretation that offers distinct advantages when identifying complex rhythms. Early learners benefit from a more complete framework with the Rule of Fours when they are approaching a new ECG for the first time; similarly, experienced clinicians are able to leverage the same framework when confronted by an ECG that requires more than pattern recognition to correctly interpret. Rule of Fours “Standard” Four Initial Features: Rate History/Clinical Picture Rhythm Rate Axis Rhythm Intervals Axis Hypertrophy Four Waves: Ischemia P Waves Q/R/S Waves T Waves U Waves Four Intervals/Segments: PR Interval QRS Width ST Segment QT Interval
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FoEM ECG I - How to Read an ECG |
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60 yo F with no known PMHx with chest pain. |
HR: 45 BP: 160/110 RR: 18 O2 Sat: 96% |
Example Case |
ECG and magnifications throughout the unit courtesy of William Burns, MD
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History/Clinical Picture —60 yo F with no known PMHx with chest pain. Rate�Option 1: Count each QRS complex in rhythm strip and multiply by 6 (ECG is 10 sec. long). Ex: 6 x 6 = 36. Option 2: “Rule of 300.” For regular (and only regular) rhythms count large boxes between QRS complexes and estimate. *ECGs with a normal rate will have QRS complexes occurring between every 3-5 large boxes. 1 Box = 300/1 = 300 bpm 5 Boxes = 300/5 = 60 bpm 2 Boxes = 300/2 = 150 bpm 6 Boxes = 300/6 = 50 bpm 3 Boxes = 300/3 = 100 bpm 7 Boxes = 300/7 = 43 bpm 4 Boxes = 300/4 = 75 bpm 8 Boxes = 300/8 = 37 bpm
Rhythm�Sinus Rhythm is defined by morphologically identical P waves with a constant PR interval before every QRS. |
Rule of Fours: Four Initial Features |
Each large box is 5 small boxes
Is this a sinus rhythm?
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Axis—Modern ECG machines are generally quite good at determining the axis value but it is still important to know how axis deviation is defined. Axis can be often be manually determined by evaluating whether leads I & aVF are positive, equiphasic or negative. However when the axis is unclear, like the difference between pathologic and physiologic left axis deviation the tie breaker is lead II.
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Machine read was 41° on this ECG. Also, using the table
above, the axis is normal (between 0° and 90°) because I,
II, & aVF are all positive
Axis Images Courtesy of Corlin Jewell, MD
Rule of Fours: Four Initial Features |
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P Waves Are P waves present? Yes, P waves are present.
Morphology—do all P waves look the same or do they vary? All look the same. Are the P waves normal—do they look enlarged (> 1.5 mm tall in V1-6 or > 2.5 mm in any other lead) or are they peaked? Yes, they look normal. Q, R, S Waves Low voltage R waves? (V1-6 R waves are all less than 10 mm tall or I/II/III R waves are all less than 5 mm tall)? No High voltage R waves (are the R waves in V1-6 excessively tall)? No |
S
R
Q
S
R
R
S
Rule of Fours: Four Waves (or complexes) |
P
Note that this P wave is NOT followed by a QRS complex
P
P
P
Q waves
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Negative deflection that precedes an R wave
Pathological Q waves indicate current or prior MI.
Q
Q
Images courtesy of Life in the Fast Lane
Rule of Fours: Four Waves (or complexes) |
R waves
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First upward deflection after the P wave; represents early ventricular depolarization
Poor R wave progression is described with an R wave ≤ 3 mm in V3 and is caused by:
Dominant R wave in V1
Dominant R wave in aVR
R Wave Progression Image Courtesy of William Burns, MD; Other Images Courtesy of Life in the Fast Lane
Normal R wave progression:
V1
V2
V3
V4
V5
V6
Rule of Fours: Four Waves (or complexes) |
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T Waves Ventricular repolarization following QRS complex; typically concordant with QRS deflection Inversion? (normally inverted in V1 and aVR, can be inverted in III if QRS complexes are also negative) No.
Peaked/Hyperacute? (no universal definition; often disproportionate with corresponding QRS complex) No
Flattened? (typically occurs before inversion) No |
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Hyperacute T waves
Rule of Fours: Four Waves (or complexes) |
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U Waves
Present? Not on our original ECG; below is an example of a different ECG with prominent U waves
Rule of Fours: Four Waves (or complexes) |
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PR Interval (Green Arrow) Measured from the beginning of the P wave to the beginning of the Q wave (not the R wave) Normal width (120-200 ms)? No, the PR intervals for the p waves that are followed by a QRS complex is ~320 ms. Some �p waves are NOT followed by a QRS, meaning that they do not have a PR interval.
QRS Complexes (Blue Arrow) Measured from the beginning of the Q wave to the end of the S wave Normal width (70-100 ms)? Yes, the QRS complexes are ~94 ms
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Example:
Wide QRS Complex�(~130 ms)
Clinical Pearl:
Certain critical diagnoses can affect morphology, including Wolff-Parkinson-White (WPW); depicted here with a wide, upsloping QRS complex
ECG above courtesy of Shanna Jones, MD
Note that this�p wave does NOT conduct; therefore cannot calculate PR interval
ECG above courtesy of Dr. Corlin Jewell
Rule of Fours: Four Intervals (or segments) |
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QT Interval (Orange line)
arrhythmias. Life in the Fast Lane provides a great review on ways to calculate the QTc.
Often easiest to look at the machine read, but take caution it may be unreliable at high or low heart rates. |
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R-R Interval
QT Interval
Clinical Pearl:
Can visually estimate if QT is > 1/2 R-R interval (shown above)
Rule of Fours: Four Intervals (or segments) |
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ST Segments Any ST elevation? Yes, nearly 1 mm in III & aVF, subtle (~0.5 mm) in II Any ST depression? Yes, aVL (~0.75 mm) and V1 (~1 mm) as very subtle (< 0.5 mm) in V2 Measuring ST changes: Measured as vertical distance from the Isoelectric line (green dotted line) to the “J” point (pink dotted line) - Isoelectric line lies along TP segment between the preceding T and P waves - J point is the beginning of ventricular repolarization (end of the S wave and beginning of the ST segment) |
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Example from another ECG with more obvious ischemic ST elevation
Rule of Fours: Four Intervals (or segments) |
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Extra Content: Deep Dive on Measuring Complicated QT Intervals |
Sometimes P waves can obscure the true end of the T wave. If the end of the T wave is not clear and you would like to double check the machine measurement of the QT interval you can use the maximum slope method The maximum slope method defines end of T wave as the point of intersection between: Isoelectric line (blue line) The line created by the maximum slope of the T wave (red line). |
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End of the T wave
QT Interval = ~400 ms (10 small boxes)