ECG Interpretation | How to Interpret An ECG in 30 Seconds

ECG Interpretation | How to Interpret An ECG in 30 Seconds. ECG is the graphical representation of electrical potentials produced when the electric current passes through the heart. Before interpreting an ECG one must know details about the ECG paper, standardization and different waves in ECG. It is a matter of experience and pattern  interpretation. This article deals with easy interpretation of an ECG.

ECG papers

  • ECG paper is a graphical papers which has an X-axis and a Y—axis
  • X—axis indicates time in second
  • Y—Axis indicates amplitude/ Voltage in millivolt
  • It consists of 2 type of square small @ large
  • Small square is  1mm
  • Large square is 5mm
  • 1 large square = 5 small square


Standard voltage of ECG recording is 1mv

  • 1Mv = 10 mm / 10 small square = 2larg square
  • Before interpreting the ECG first look for the voltage is 2 large square or not
  • If voltage is 1 large square is the ECG is half voltage ECG
  • Then voltage of P_QRST will Actually be double of what are u seeing

Heart rate

If rhythm is regular-


Heart Rate = ————————————————————————

                            Numbers of large square between two R-R interval

If rhythm is regular-

It calculate the number of R in  6 sec or 30 large square and multiply it with 10

If rhythm is irregular : numbers of R in 30 large square   × 10

ECG Leads

Lead is nothing but like CC Camera.  If you place 4 or 6 CC camera in 4 or 6 corner of your room and u sit in the middle of the room, the camera placed front of you it will take the picture of your front view and the camera placed behind of you will take the posterior view of your body. The side cameras will take left or right lateral surface of your body.  Like this differents lead take picture of different  parts (surface ) of the heart .such as  inferior lead  show the inferior surface of  the heart

The standard ECG has 12 leads

1) Three bipolar standard limb leads- I, II and III.

2) Three unipolar augmented limb leads – aVL, aVF and aVR.

3) Six chest leads – V1, V2, V3, V4, V5 and V6.

In this picture we can see

  • Lead I is most close proximity to lead  AVL
  • Lead III is close proximity to lead AVF
  • Lead II is opposite to  lead AVR
  • Any change(like T –inversion ) in lead I will be found in lead AVL , and
  • Change in lead III will be found in AVF
  • What you get in lead II (positive R) will be opposite in lead AVR(negative R)

In this picture we can also see that II, III, AVF see the heart from inferior or ground so take picture of the inferior surface of the heart. That s why the are called inferior lead. Lead I and AVL are taking the picture from the side of body see the lateral part of the heart  that’s why  lead I and lead AVL are called Lateral lead

ECG Interpretation in 30 Seconds

1st  seeIn lead IIRhythm

If Regular —go next step

If irregular –it  will be AF ,

·         To confirm look for

Absent of p wave –if absent –then u r 100%

·         If u confused about P wave present or not then look for

PR interval ,In sinus rhythm P  is Isomorphic &  PR  is fixed before all R in lead II

Next seeHeart Rate in lead II

If heart rate is less 40

·       exclude complete heart block

If your find  ECG that

R-R interval  equal and P-P interval equal but PR interval is not fixed


Try to find following :

·         T inversion ,

·         ST elevation

·         Q wave


1st  see ———–AVL , I  —————acute lateral MI

Then ————-II , III ,  AVF —–acute inferior MI

Then ————-V1 to   V6 ————– acute extensive ant MI

Then————–V 1—V 4—-Acute  antero-septal MI

Then ————-I, AVL , V5-V6 acute antero-lateal MI

Next see V 5

Tall R— LVH

 (if with T- inversion or ST –depression then strain pattern  )

Abnormal   R or M pattern — — LBBB

Next see V 2

Tall R— RVH

(if with T- inversion or ST –depression then strain pattern  )

Abnormal   R  or  RBBB

Next seeI . II. IIICompare height of R

 to see axis deviation

Left axis…. R (+) in  I  &   R (- ) II and III

Right axis…. R (-) in  I  &   R (+ ) II and III


Next seeV 1 to  V 6

If R  in V 6   <   S in V 6     —– poor progression of  R wave



Dr. Shahidullah Shamol (FCPS in Medicine)

Assistant Professor, Department of Medicine

Mymensingh Medical CollegeTimes New Roman

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