EKG Basics

I’m assuming you learned some EKGs at the end of your second year, before you started clinicals. But now it’s April and you may have forgotten. So let’s review some basics and get you started on reading EKGs better.

How to Read EKGs

You’re going to want to bookmark Life in the Fast Lane’s ECG Basics, because it’s the best resource for students. You need to be systematic, just like reading a chest x-ray. You should always follow a pattern, not let your eyes flit around randomly. That pattern is going to start with rate, rhythm, intervals, axis, all the other leads, then end with considering “oh-no’s.”

It will help to pull up a “normal” EKG, so look at this normal sinus EKG from Life in the Fast Lane. While you’re there, review the characteristics of normal sinus rhythm on EKG. You can even skip the rest of this page and go straight to their “Rule of Fours,” which is a better version of what I wrote here, but I’m going to publish because I’ve already done all the work. Then come back for links to specific oh-no’s to review when you’re solid on basics.


Look at lead II, at the bottom of the page, and count out your rate. Don’t use the ECG machine’s auto-generated number.

  • The tiny boxes are 40 msecs, and five of them make a “big” box of 200 msecs
  • The “big boxes” on an EKG can be counted across between R-waves, which I prefer (300,150,100,75,60,50…)


  • Is the rhythm regular? Or irregular?
  • Is there a P before every QRS? A QRS after each P?
  • Is the QRS appropriately narrow?
  • Are there any missing/dropped P waves?


  • Is your PR 120-200 msec?
  • QRS 70-100?
  • Is the QTc (corrected QT) appropriate or prolonged? (Per LITFL QTc is prolonged if > 440ms in men or > 460ms in women, however my pocketbooks from residency have 440-470 for men, and 460-480 for women, presumably because so many of our patients were on QT prolonging meds and we had to alter our cut-offs for panicking.)

Axis (Axii?)

To consider axis you are looking at the QRS in leads I and avF, and comparing if the “point up” or deflect down.

I prefer the quadrant method, of comparing lead 1 and avF. If the QRS in both is positive (points up) there’s no deviation, and it’s a normal axis.

If there’s anything else, it’s time to review LITFL’s axis interpretation page.

All the Other Leads

Now look at V1-V6 and look for at ST segments and T waves. What are you looking for? Inverted or peaked T waves, ST segment depressions or elevations. Anything that looks different or weird.

The Oh-No’s!

Pacer spikes, hypertrophy (left or right?), ischemia/myocardial infarction (STEMI or NSTEMI?), electrolyte imbalances (esp hyperkalemia), pericarditis and more. I could attempt to teach you all this, or work smarter instead of harder and send your straight to LITFL again. Seriously, their ECG basics is wonderful – now go look at their excellent Clinical Interpretation section at the end of the basics page, and get learning.

Study hard and stay safe,

Pixie written in signature style


Pixie is happiest reading with a cup of tea in hand. She enjoys women’s health, adolescent medicine, painting and polymer clay. For more info, see her much longer bio on the author page.

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