ECG Facts

One big box (5mm) = 0.20 sec
One small box (1mm) = 0.04 sec
3 small boxes = 0.12 sec

The ECG paper runs at 25 mm/sec through the ECG printer.

P wave < 0.11 sec
PR interval ≤ 0.20 sec. Actually, the normal range is 0.12-0.20 sec.  PR is prolonged if it’s more than one big box or five small boxes.
QRS interval≤ 0.10 sec. Some sources like the ACLS algorithms say QRS ≤ 0.12 (i.e. ≤  3 small boxes).
QTc interval 0.33-0.47

Approach to Reading ECG

It’s vital to use a systematic approach. Mine is Rate Rhythm I AM QRST or Rate Rhythm I AM PQRST
-Rate – Is it fast or slow (i.e. tachycardia or bradycardia?)
-Rhythm (? relationship between P and QRS) – Regular / Irregular
-Intervals –   QRS – Wide or narrow? PR interval prolonged/shortened? QT interval prolonged? NB: Intervals are the same in every lead. They are looking at the electrical activity.
-Axis (? LAD or RAD) 
-Morphology (? LAA and or RAA? LVH and/or RVH)
-QRST changes (? Q waves, poor R-wave progression V1-V6, – Ischemia/Infarction present? – ST changes (ST elevation/depression), T-wave changes?

Mnemonic: Rate, Rhythm, I AM QRST: Rate, Rhythm, Intervals, Axis, Morphology, QRST changes.

Rate, rhythm, and intervals: Use one lead to examine them, e,g, Lead II if you have a 12 lead ECG
V1-V3 is the best place to look at QRS morphology.
Axis – Use I & aVF or I & II. See explanation here.
Chamber abnormality and QRST changes. You have to scan through all the 12 leads to look for these since they reflect specific morphological changes in the heart. E.g. Inferior MI will show in the inferior leads.

Electrophysiologist, Dr. Bhakta, approaches ECGs clinically to get the most urgent things first before focusing on other details. He asks 4 questions in order.

  1. Rate – Is it fast or slow (i.e. tachycardia or bradycardia?)
  2. QRS – Wide or narrow?
  3. Regular / Irregular
  4. Ischemia/Infarction present ? (ST wave changes, T wave changes?)

Wide complex tachycardias are impacting the pumping of the heart because it’s the ventricles that are that are pumping fast.

While AV block is first examined by determining the length of the PR interval, bundle branch blocks are first examined by determining the length of which electrocardiographic entity? The QRS complex.


An ECG is a Voltmeter

An ECG machine is a voltmeter (galvanometer). It is used to record the electrical impulses that stimulate the heart. The electrodes record a potential difference. The needle (or pen) of the ECG is deflected a given distance depending upon the voltage measured.

The ECG is a plot of voltage on the vertical axis against time on the horizontal axis.

ECG leads are vectors. “The term ‘lead’ in electrocardiography refers to the 12 different vectors along which the heart’s depolarization is measured and recorded.” Wikipedia

Heart axis: Play with the axis software on this site.

The right leg electrode is used for grounding where the voltage is zero.

P-QRS-T is a good way to keep tap of the intervals. The PR or PQ interval and the ST interval. The PR=PQ segment. They are often called the PR segment because the Q wave is usually missing.

P waves from the same focus look the same in a given lead ( i.e. when looked at in the same lead). If P waves have different shapes in the same exact lead, they are certainly coming from different foci. The same lead is like one camera positioned at a specific angle, pointing to the heart. The camera doesn’t move and should take the same exact picture if the electrical activity is coming from the same focus. A change in the shape of the p wave means a change in focus. *Also, each focus always acts the same way. It doesn’t change.

In sinus arrhythmia, the rate usually gradually slows and then gradually speeds up. This is due to breathing and the effects of the sympathetic and parasympathetic system. In sinus arrhythmia, there is a normal, extremely minimal increase in HR during inspiration and an extremely minimal decrease in HR during expiration.  The slight increase in heart rate is due to inspiration-activated sympathetic stimulation of the SA Node. The slight decrease in HR is due to expiration-activated parasympathetic inhibition of the SA Node. The sympathetic NS stimulate when you suck in air and parasympathetic stimulate when you push out air.

Segments vs. Intervals in an ECG. E.g. The PR segment vs. the PR interval.

segmentsA segment in an ECG is the region between two waves. PR segment starts at the end of the P wave and ends at the start of the QRS complex. The ST segment starts at the end of the QRS wave and ends at the start of the T wave. The TP segment is between the end of the T wave and the beginning of the next P wave; It is the true isoelectric segment in the ECG. ** With segments, you talk about morphology: elevation or depression or progression of segments.

An interval in an ECG is a duration of time that includes one segment and one or more waves. The PR (or PQ) interval starts at the start of the P wave and ends at the start of the QRS. It denotes the conduction of the impulse from the upper part of the atrium to the ventricle. The QRS interval covers the QRS complex from beginning to end. [The QRS complex is also covers an interval]. The QT interval starts at the start of the QRS and ends at the end of the T wave. It denotes the electrical systole of the heart. **Intervals are only described based on their duration of time. You speak of it as a duration and so cannot talk about the morphology or depression or elevation of an interval.Segments and Intervals on ECG

Calculating the Heart Rate

There are several methods for determining heart rate.

Method #1: The number of QRS complexes per 6-second strip times 10. Count the number of QRS complexes over a 6-second interval. Multiply by 10 to determine heart rate. This method works well for both regular and irregular rhythms.

Method #2: Count the number of small boxes for a typical R-R interval. Divide 1500 by this number to determine heart rate.

Method #3: Identify an R-wave that is on a line. Use that as the start R-wave and then count success big boxes from the start as 300, 150, 100, 75, 60, 50. Below 50, use the formula given in method #2.


ECG Tips from an Indian Physician

The Eyeball Technique: 10 Rules for rapid ECG Reading.

  1. Heart Rate: 3 to 5 rule & big box rule
  2. P wave: Lead II taller than I
  3. AV Conduction 1: > one large square rule
  4. AV Conduction 2: Who is related to whom?
  5. QRS duration: Little Box 3 rule?
  6. Wide QRS: Find the Traffic Block – Where are the rabbit ears? Right or left?
  7. QRS Axis: Look at where I and aVF are pointing
  8. QT interval: 50% rule.
  9. R & T wave: Who is taller in V1 or V6?
  10. ST segment: 1mm x 2 rule



1. Heart Rate. Normal heart rate falls within a range of 3 to 5 big boxes. That corresponds to 60 to 100 bpm if you use the formula in method #2 above. Use this 3 to 5 big box rule to know if the heart rate is normal or not. If it’s normal, there is no need to calculate it when working on the ward.

  • 1=300
  • 2=150
  • 3=100
  • 4=75
  • 5=60
  • 6=50

In essence, this is an application of method #3 of determining heart rate.

2. P Wave: Is Lead II taller than lead I? I.e. Is the P wave in lead II taller than p wave in lead I? It should be for it to be normal. Why?

QRS axis: Look at where I and aVF are pointing. If they point in the same direction, then the axis is normal. If the point towards each other, then they are Reaching out to each other which is Right axis deviation. If they point away from each other, then they are Leaving each other, which is Left axis deviation.

AV Block

AV Block

Systematic approach of ECG

Personal Quick Reference Sheets from Dale Dubin’s Rapid EKG Interpretation.

A systematic approach is vital to ECG interpretation. Always follow the same steps for every ECG you look at. Don’t skip around. Your speed will increase without decreasing your efficiency.

Things to check with EVERY EKG you read!

  1. Rate and Rhythm.
  2. Axis and Intervals
    1. PR Interval. It is increased consistently in first degree AV block; Progressively increases in each series of cycles with Wenkeback; totally variable in 3rd degree AV block; and decreased in WPW and LGL
    2. P without QRS response. Wenkeback and Mobitz 2nd degree AV blocks; 3rd degree AV block has independent atrial and ventricular rates.
    3. QRS width (interval). Normal is less than 3 small squares. A diagnosis of Bundle Branch Block is mainly based on widened QRS (3 small squares (0.12 secs) or more.)
  3. Hypertrophy and Enlargement
    1. Sum of S in V1 plus R in V5. Mere observation is enough. If more than 7 large boxes (35 mm) is L.V.H.
  4. Infarction

ECG Websites