How to Interpret an ECG Systematically | Clinical Medicine

TL;DR

Interpret an ECG systematically by checking the rate, rhythm, axis, intervals, P waves, QRS complex, ST segment, and T waves in that order. Calculate heart rate by counting R waves on the rhythm strip and multiplying by six, or dividing 300 by the boxes between R waves; above 100 beats per minute is tachycardia and below 60 is bradycardia. Read on to distinguish common tachycardias, bradycardias, and AV blocks.

Transcript

H what's up Ninja nerds in this video today we're going to be talking about ECG interpretation every clinician should have a very good systematic approach to how to read an ECG let's get into it one of the approaches that I think is pretty good from a lot of the textbooks and I think also maybe has some degree of clinical practice for myself is I l... Read More

Key Insights

• A systematic ECG approach follows a fixed order: determine rate, then rhythm, axis, intervals, P waves, QRS complex, and finally the ST segment and T waves, so no finding is missed on any tracing.
• Heart rate is calculated by counting R waves in the rhythm strip and multiplying by six, or by dividing 300 by the number of boxes between one R wave and the next, since one box represents 300 beats per minute.
• Rate classification is simple: greater than 100 beats per minute is tachycardia, 60 to 100 is a normal heart rate, and less than 60 is bradycardia.
• Tachycardias are sorted by QRS width and R-to-R regularity. Narrow means less than 120 milliseconds (three little boxes), and regular means the R-to-R distance is identical down the whole strip.
• Narrow regular tachycardias include sinus tachycardia (upright P waves, inverted in aVR, each P followed by QRS), 2-to-1 atrial flutter (flutter waves, rate around 150), and paroxysmal supraventricular tachycardia (no visible P waves, can reach 170s-180s).
• Narrow irregular tachycardias include atrial fibrillation (fibrillatory waves, no P waves), atrial flutter with variable block (inconsistent 2:1 or 3:1 conduction), and multifocal atrial tachycardia (three or more differently shaped P waves, common in COPD).
• Wide regular tachycardia suggests ventricular tachycardia until proven otherwise, while wide irregular points to polymorphic VT with prolonged QT (torsades de pointes) or ventricular fibrillation, the most dangerous rhythm.
• The PR interval distinguishes AV blocks: greater than 200 milliseconds constant is first-degree, progressively lengthening then a dropped QRS is Mobitz I (Wenckebach), and a constant prolonged PR with dropped QRS is Mobitz II.
• The QT interval matters because a prolonged QT raises the patient's risk of torsades de pointes, where the tracing gives a classic ribbon-like appearance as QRS complexes widen and narrow.
• The ST segment and T wave are assessed last to look for evidence of myocardial ischemia or infarction, completing the systematic read of the tracing.

Questions & Answers

Q: What is the systematic approach to ECG interpretation?

Start with the heart rate, then evaluate the rhythm, axis, intervals, P waves, QRS complex, ST segment, and T waves. The rhythm is categorized as narrow regular, narrow irregular, wide regular, or wide irregular. The final ST-segment and T-wave assessment looks for evidence of myocardial ischemia or infarction.

Q: How do you calculate heart rate on an ECG?

One method is to count the R waves in the rhythm strip and multiply by six; 11 R waves gives about 66 beats per minute. Alternatively, divide 300 by the number of boxes between two R waves. For example, 300 divided by about 4.5 boxes gives roughly 67 beats per minute.

Q: What ECG heart rates indicate tachycardia and bradycardia?

A heart rate greater than 100 beats per minute is tachycardia. A rate from 60 to 100 beats per minute is normal, while a rate below 60 beats per minute is bradycardia. This initial classification guides the subsequent rhythm assessment.

Q: How do you distinguish narrow regular tachycardias?

A narrow regular tachycardia has a QRS under 120 milliseconds, or three small boxes, with consistent R-to-R intervals. Sinus tachycardia has upright P waves, inverted P waves in aVR, and a QRS after every P wave. Two-to-one atrial flutter shows flutter waves and a rate around 150, while paroxysmal supraventricular tachycardia has no visible P waves and may reach the 170s or 180s.

Q: What rhythms cause narrow irregular tachycardia?

The listed possibilities are atrial fibrillation, atrial flutter with variable block, and multifocal atrial tachycardia. Atrial fibrillation has fibrillatory waves without obvious P waves, while variable atrial flutter alternates between conduction patterns such as 2:1 and 3:1. Multifocal atrial tachycardia has three or more differently shaped P waves and is common in patients with COPD.

Q: What does a wide-complex tachycardia indicate?

A wide QRS is greater than 120 milliseconds, or three small boxes. A wide regular tachycardia suggests ventricular tachycardia until proven otherwise. Wide irregular rhythms include polymorphic ventricular tachycardia with prolonged QT, called torsades de pointes, and ventricular fibrillation.

Q: How does the PR interval distinguish AV blocks?

A constant PR interval greater than 200 milliseconds without a dropped QRS indicates first-degree AV block. Progressive PR lengthening followed by a dropped QRS indicates Mobitz I, or Wenckebach. A constant prolonged PR interval with a dropped QRS indicates Mobitz II.

Q: Why are the QT interval, ST segment, and T wave checked?

The QT interval is assessed because prolongation raises the patient's risk of torsades de pointes. Torsades can produce a ribbon-like tracing as the QRS complexes widen and narrow. The ST segment and T wave are checked last for evidence of myocardial ischemia or infarction.

Summary & Key Takeaways

• Every clinician should use a systematic approach to reading an ECG. The order is: determine the rate, then the rhythm (narrow or wide, regular or irregular), the axis for any deviation, the intervals (especially PR and QT), the P wave, the QRS complex, and finally the ST segment and T wave.

• Heart rate is found on the rhythm strip using the R-wave method: count the R waves and multiply by six. Alternatively, divide 300 by the number of boxes between two R waves. Rates above 100 are tachycardia, 60-100 are normal, and below 60 are bradycardia.

• Tachycardias are classified by QRS width and R-to-R regularity, generating differentials from sinus tachycardia and atrial flutter to atrial fibrillation, multifocal atrial tachycardia, ventricular tachycardia, torsades de pointes, and ventricular fibrillation. Bradycardias are worked up via the PR interval to distinguish sinus bradycardia from first-degree, Mobitz I, and Mobitz II AV blocks.