ECG Basics

Electrocardiography is a fundamental part of cardiovascular assessment. It is an essential tool for investigating cardiac arrhythmias and ischemia. Just because electrocardiography is a basic skill in EM doesn’t mean that our skills should be basic…we must be the EXPERTS! Below is a summary of some ECG basics including:

  • ECG Measurements
  • ECG Rate
  • ECG Axis

ECG Measurements

For the purpose of this post, the duration of waveforms will be expressed as 0.04 sec (40 msec) = 1mm = 1 small square and the amplitude of waveforms will be expressed as 0.1 mV = 1mm = 1 small square.

ECG Voltage

ECG Rate


ECG Rate

ECG Rate

1. The Cardiac Ruler or Sequence Method: Count the number of big boxes between R waves and count using the following numbers: 300-150-100-75-60-50.  This can only be used on regular rhythms and not on irregular rhythms

Cardiac Ruler Method

2. The Six Second Method:  Get 6 seconds of ECG tracing (i.e. 30 big boxes) and count the number of R waves that appear within that 6 second period and multiply by 10.  Another version of this is the 10 second method (i.e. 50 big boxes) and count the number of R waves that appear within that 10 second period and multiply by 6.  This is a great method for slow or irregular rhythms

The Six Second Method


The Six Second Method


3. The 300 Method: Count the number of large boxes between 2 successive R waves and divide by 300 to obtain heart rate.

4. The 1500 Method: Count the number of small boxes between two successive R waves and divide this number into 1500 to obtain heart rate.  This works well for faster heart rates.


ECG Axis

ECG Axis

 The definition of axis is an average of all electrical signals from the heart, indicating the average direction of electrical depolarization. To determine the heart axis you look at the extremity leads only (not V1-V6).

ECG Axis

  • Normal Axis = -30 to 90 degrees
  • Left Axis Deviation (LAD) = -30 to -90 degrees
  • Right Axis Deviation (RAD) = 90 t0 180 degrees
  • Extreme Axis = -90 to -180 degrees

ECG Axis

A simple way to look at this is to look at the main direction of your QRS in leads I and aVF.  Lead I represents a left sided lead and lead aVF represents a right sided lead. You can use a quick look test to guestimate your axis.

Quick Look Test

Quick Look Test for ECG Axis

In reality though, normal axis is -30 to 90 degrees not 0 to 90 degrees.  The easiest way to account for this is if your QRS vector is up in lead I and down in aVF, then next look at lead II.  This lead is exactly 90 degrees from lead aVL.  If the QRS vector is up in II then you have a normal axis.  If the QRS is down in lead II, then you have LAD.  This is represented in the table below.

Summary of ECG Axis


The electrocardiogram is one of the simplest and basic cardiac investigations available. It can provide a wealth of useful information, and remains an essential part of the assessment of patients. After reading this post, you should be comfortable with ECG measurements, rate, and axis. There is much more to reading ECGs, but hopefully this is a good start to some ECG Basics.

Cite this article as: Salim Rezaie, "ECG Basics", REBEL EM blog, February 15, 2014. Available at:
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Salim Rezaie

Emergency Physician at Greater San Antonio Emergency Physicians (GSEP)
Creator & Founder of REBEL EM

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10 thoughts on “ECG Basics”

  1. @ Salim – Why is -30 degrees a normal axis – yet -31 degrees starts LAHB (left anterior hemiblock)? Transition between “normal” and LAHB is perhaps more gradual than stated above. Life is easier (esp. if teaching beginners) to simply call “normal” between 0 to +90 degrees – and you can tell in 2 seconds if the axis is normal or not simply by looking just at leads I and aVF (they’ll both be positive if the axis is normal). I find it easiest to distinguish between LAD (Left Axis Deviation) and LAHB (Left Anterior HemiBlock) – by determining if the axis is more negative than -30. This is EASY – IF you have LAD, check lead II – and if more neg than pos you have LAHB. So – if the axis is ~ -20 degrees – then you have LAD but not LAHB (which means that the axis is not quite “normal” – but it is not enough of a left axis shift to be pathologic = not leftward enough to indicate LAHB).

    • Hello Ken,
      Great point. Transition between “normal” and LAHB can be more gradual. So to be more clear:

      Distinguishing between LAD (Left Axis Deviation) and LAHB (Left Anterior HemiBlock) can be determined by…if the axis is more negative than -30.

      If lead II more neg = more neg than -30 degrees = LAHB
      If not more neg than -30 degrees (i.e. approx -20 degrees) = LAD but no LAHB

      TY Ken.


  2. Mr. Rezaie, is it possible for an axis deviation to be normal in inferior infarct patients? MI occurred 3 months prior. Thank you.

  3. Left axis deviation (³ -30 degrees) is the most common “abnormality” in adults occurring in over 8%. It can be part of the criteria for LVH but in isolation it has little significance.

    Marked LAD (³ -45 degrees) is called left anterior hemiblock or left anterior fascicular block.

    If left axis deviation is present, make sure that the patient does not have hypertension in multiple clinical circumstances including under stress. If noted to be borderline, this may prove to be an indication for ambulatory BP monitoring. Also, check for borderline indicators of LVH (i.e., the voltage criteria and left atrial enlargement). Note whether diagnostic inferior Q waves are present since an inferior MI can cause LAD.
    Thank you..

  4. It is very helpful for starters and even for patients, at least they will understand bit about their tests. Thank you for sharing this very very informative post.


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