ECG Interpretation Basics, Animation.

ECG Interpretation Basics, Animation.

Lead II is most popular among the 12 leads. This is because the net movement of the heart’s
impulses is toward lead II, making it the best general view. Unless otherwise specified, we will be looking
at lead II. Our analysis will include the following:
For heart rate: Identify the QRS complex – usually the biggest on an ECG; count the number of
small squares between two consecutive QRS complexes and calculate the heart rate with
this formula. If this number is variable, count the number
of QRS complexes on a 6 second strip and multiply by 10. A normal heart rate is between 60 and 100
beats per minute. For rhythm: measure the intervals between
the R waves. If these intervals vary by less than 1.5 small
squares, the rhythm is regular; if the variation is greater than 1.5 small squares, the rhythm
is irregular. P wave represents depolarization of the atria
initiated by the SA node. Presence of a normal P wave therefore indicates
sinus rhythm. P waves are most prominent in leads II, III,
aVF and V1. Absence of P waves indicates non-sinus rhythms. Absence of P waves and presence of irregular
narrow QRS complexes are the hallmark of atrial fibrillation. The baseline may be undulating or totally
flat. A sawtooth pattern instead of regular P waves
signifies atrial flutter. These are called flutter waves. The number of flutter waves preceding a QRS
complex corresponds to number of atrial contractions to one ventricular contraction. P wave is the summation of 2 smaller waves
resulting from depolarization of the right atrium followed by that of the left atrium. Normal P waves are rounded, smooth and upright
in most leads. In V1, P wave is biphasic, with an initial
positive deflection corresponding to activation of the right atrium, and a subsequent negative
deflection, resulting from activation of the left atrium. Unusual morphology of P waves is indicative
of atrial enlargement. In right atrial enlargement, depolarization
of the right atrium lasts longer than normal and its waveform extends to the end of that
of the left atrium. This results in a P wave that is taller than
normal, more than 2.5 small squares. Its duration remains unchanged, less than
120ms. In V1, this is seen as a taller initial positive
deflection of the P wave, more than 1.5 small squares. Right atrial enlargement is usually due to
pulmonary hypertension. In left atrial enlargement, depolarization
of the left atrium lasts longer than normal. This results in a wider P wave, of more than
3 small squares. The waveform may also be notched. In V1 the negative portion of P wave is deeper
and wider. Left atrial enlargement is commonly due to
mitral stenosis. P-wave inversion in the inferior leads indicates
a non-sinus rhythm. When this happens measure the PR interval. If the PR interval is less than 3 small squares,
the rhythm is started in the AV junction – AV nodal junctional rhythm. If the PR interval is more than 3 small squares,
the origin of the rhythm is within the atria – ectopic atrial rhythm. The PR interval is measured from the start
of the P wave to the start of the QRS complex and reflects the conduction through the AV
node. A longer than normal PR interval signifies
an abnormal delay in the AV node, or an AV block. A consistent long PR interval of more than
5 small squares constitutes first-degree heart block. A progressive prolongation of PR interval
followed by a P wave WITHOUT a QRS complex is the hallmark of second-degree AV block
type I. A shorter than normal PR interval, of less
than 3 small squares, signifies that the ventricles depolarize too early. There are 2 scenarios for this to happen:
– Pre-excitation syndrome: presence of an accessory pathway bypassing the AV node. – AV nodal (junctional) rhythm: Non-sinus
rhythm initiated from around the AV node area instead of the SA node. In this case, P waves are either absent or
inverted in the inferior leads. The QRS complex represents depolarization
of the ventricles. A normal QRS complex is narrow, between 70
and 100 ms. A wider QRS complex, resulting from an abnormally
slow ventricular depolarization, may be caused by:
– A ventricular rhythm: rhythms originated from ectopic sites in the ventricles. OR
– An impaired conduction within the ventricles in conditions such as bundle branch block,
hyperkalemia or sodium-channel blockade. A QRS complex wider than three small squares
despite sinus rhythm is the hallmark of bundle branch block. When bundle branch block is suspected, check
leads V1 and V6 for characteristic patterns of the QRS complex.

22 Replies to “ECG Interpretation Basics, Animation.”

  1. This video explains basic steps in reading an ECG strip. In real life, it may be more complicated as many conditions may happen at one time. You need to know about the cardiac conduction system ( and the 12-lead ECG system ( to understand this video. For details of the mentioned conditions click on the “i” icon at the top right corner for links. You can find them here as well:
    As with all our videos, this is for educational purposes only – it should NOT be used as guide for diagnosis in real life. Happy learning!

  2. Thanks for this awesome video. Can you explain hyperkalemia / hypercalcemia ?
    OR at least , their effects on the heart ?

  3. Thank you for watching! Follow us on Twitter for instant notification of our new videos!

  4. You've got to be kidding me: You put all this effort into making an educational video and then you let a good damn text to speech program read the voiceover? I can't stand listening to this, it makes my ears bleed and my heart ache over so much wasted video potential…

  5. If this video is helpful to you, please consider supporting our next projects. As a token of our appreciation, we also offer early access to our videos and free image downloads in return, please check us out here:

  6. You may find these ECG/EKG guides/tools helpful:

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