Here is a summary of all the rules. Discussions for the practice cases will refer to the rule numbers listed here:
- Upright P waves (in lead II) are from the SA node 95% of the time
- Inverted P waves are from the atria, AV node, or ventricles
- No P waves rules out the SA node
- A normal PR interval rules out the AV node
- A normal PR interval rules out the ventricle
- Wavy baseline with P waves and QRS complexes = no fibrillation
- Wavy baseline with no P waves = atrial fibrillation (probably) = atrial origin
- Wavy baseline with no QRS complexes = ventricular fibrillation = ventricular origin
- Narrow QRS rules out the ventricles
- Wide QRS could be caused by diseased purkinje fibers
- An atrial rate > 200 rules out the SA node
- A ventricular rate > 200 rules out the SA node and atria
- More P waves than QRS complexes = conduction block
- P waves not all the same size and shape = 2 or more origins for the P waves
- P waves + QRS complexes present but absent PR interval = 2 origins
Is your head swimming with all these rules?
Here is your chance to practice. This might be ugly at first so don't panic. Click on the ask/comment button if you need human help. We will try to reply within 12 hours.
Read the discussions carefully for detailed explanations. Every case has a discussion that explains the ECG. Good luck!