Electrocardiography (ECG) is an essential diagnostic tool used to assess cardiac function. ECG readings provide valuable insights into heart rate, rhythm, and overall cardiac health. Mastering ECG interpretation is crucial for diagnosing arrhythmias, myocardial infarctions, and other cardiac conditions. This guide explores how to read an ECG, interpret normal and abnormal findings, and recognize key cardiac conditions. By the end, you’ll have a solid foundation for analyzing ECGs with confidence.
How to Read an ECG
Normal ECG
A normal ECG consists of consistent waveforms, a regular rhythm, and expected intervals. Confirming normal ECG readings involves checking heart rate, rhythm, cardiac axis, and wave morphology. The presence of normal P waves, PR intervals, QRS complexes, and ST segments indicates a well-functioning heart.
Heart Rate
What is a Normal Adult Heart Rate?
The normal resting heart rate for an adult ranges from 60-100 bpm. Bradycardia occurs when the heart rate is below 60 bpm, while tachycardia is diagnosed when the heart rate exceeds 100 bpm.
Regular Heart Rhythm
To determine a regular heart rhythm, examine the R-R intervals—if they are consistent, the rhythm is regular.
Heart Rate Calculation Example
Heart rate can be calculated using the formula:
Irregular Heart Rhythm
When the R-R intervals vary significantly, an irregular rhythm is present. Common causes include atrial fibrillation and ectopic beats.
Heart Rate Calculation Example for Irregular Rhythm
For irregular rhythms, count the number of QRS complexes in a 6-second strip and multiply by 10. This provides an estimate of the heart rate.
Heart Rhythm
Identifying the heart rhythm involves evaluating the P wave, PR interval, and QRS complex morphology. Recognizing abnormalities in these waveforms can help diagnose conduction disorders and arrhythmias.
Cardiac Axis
Normal Cardiac Axis
The normal cardiac axis ranges from -30° to +90° and is determined by evaluating limb leads I, II, and III.
Right Axis Deviation
Right axis deviation (RAD) occurs when the QRS axis is greater than +90°. It is associated with conditions such as right ventricular hypertrophy and pulmonary hypertension.
Left Axis Deviation
Left axis deviation (LAD) is defined by a QRS axis less than -30° and can be seen in left ventricular hypertrophy and left anterior fascicular block.
P Waves
P waves should be upright in leads I, II, and aVF and inverted in aVR. The presence of abnormal P waves suggests atrial enlargement or ectopic atrial activity.
PR Interval
The PR interval represents the time between atrial and ventricular depolarization. Normal PR interval duration ranges between 120-200 ms (3-5 small squares).
Prolonged PR Interval (>0.2 seconds)
A prolonged PR interval suggests first-degree AV block. The delayed conduction may be due to aging, medications, or intrinsic conduction disease.
Types of Heart Block
- First-degree heart block: PR interval >0.2s, no dropped beats.
- Second-degree heart block (Type 1): Progressive PR prolongation leading to dropped QRS.
- Second-degree heart block (Type 2): Fixed PR intervals with occasional dropped QRS complexes.
- Third-degree heart block (Complete heart block): No association between P waves and QRS complexes.
Tips for Remembering Heart Block Types
- “Longer, longer, drop = Wenkebach” (Type 1)
- “Some P waves don’t get through = Type 2”
- “P waves and QRS dissociate = Complete block”
Shortened PR Interval
A shortened PR interval (<0.12s) may indicate Wolff-Parkinson-White (WPW) syndrome, which is characterized by an accessory pathway known as the Bundle of Kent.
QRS Complex
Width
A normal QRS width is <120ms (3 small squares). A widened QRS suggests a bundle branch block or ventricular conduction delay.
Bundle Branch Block
- Right bundle branch block (RBBB): rSR’ pattern in V1.
- Left bundle branch block (LBBB): Broad, notched R waves in V6.
Height
Tall QRS complexes suggest ventricular hypertrophy. Criteria such as the Sokolow-Lyon index can be used to confirm left ventricular hypertrophy.
Morphology
- Delta wave: Slurred upstroke of QRS (seen in WPW syndrome).
- Q-waves: Pathological Q-waves suggest prior infarction.
- R and S waves: Used to assess chamber enlargement.
J Point and ST Segment
- ST elevation: Indicates acute myocardial infarction (STEMI).
- ST depression: Suggests ischemia.
T Waves
- Tall T waves: Can indicate hyperkalemia.
- Inverted T waves: Found in ischemia or strain patterns.
- Biphasic T waves: May suggest ischemia.
- Flattened T waves: Associated with hypokalemia.
U Waves
U waves may be seen in hypokalemia or bradycardia.
Documenting Your Interpretation
A systematic approach ensures thorough documentation. Consider the following:
- Heart rate: Regular or irregular?
- Heart rhythm: Sinus, atrial fibrillation, etc.?
- Cardiac axis: Normal, left, or right deviation?
- Intervals: PR, QRS, QT durations?
- Wave morphology: Normal or abnormal findings?
- Final interpretation: Summarize key findings.
Normal ECG Example
A normal ECG should show:
- HR: 60-100 bpm
- Rhythm: Sinus
- Axis: Normal (-30° to +90°)
- PR interval: 120-200ms
- QRS width: <120ms
- ST segment: No elevation or depression
- T waves: Upright (except in aVR, V1)
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