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The rate of paper (i.e. of recording of the EKG) is 25 mV/s which results in:

The voltage recorded from the leads is also standardized on the paper where 1 mm = 1 mV (or between each individual block vertically) This results in:


Heart rate calculation:

Normal range at rest is between 60-100 beats per minute (bpm).


The basic way to calculate the rate is quite simple. You take the duration between two identical points of consecutive EKG waveforms such as the R-R duration. Take this duration and divide it into 60. The resulting equation would be:

Rate = 60/(R-R interval)

A quicker way to obtain an approximate rate is

  1. to go by  RR or PP interval. If it is 1 big box (0.2 secs) then the rate is 60/0.2 = 300 bpm. The rest of the sequence would be as follows.
    • 1 big box = 300 beats/min (duration = 0.2 sec)
    • 2 big boxes = 150 beats/min (duration = 0.4 sec)
    • 3 big boxes = 100 beats/min (duration = 0.6 sec)
    • 4 big boxes = 75 beats/min (duration = 0.8 sec)
    • 5 big boxes = 60 beats/min (duration = 1.0 sec)
  2. Count the number of RR intervals between two Tick marks (6 seconds) in the rhythm strip and multiply by 10 to get the bpm. This method is more effective when the rhythm is irregular.


Rhythm can be quite variable. It could be


Normal sinus rhythm (NSR): indicates that the rate is between 60 and 100, inclusive, and that the P waves are identifiable and are of the same morphology throughout. The RR interval or PP intervals between beats are same.

Sinus arrhythmia: There is a cyclical acceleration of heart rate with inspiration and slowing  with expiration. The beat to beat interval is slightly different.The rhythm is regularly irregular, in the sense that there is a pattern to irregularity.  This is termed sinus arrhythmia.


P wave

Represents discharge of SA node and depolarization of both atria





QRS complex is a series of wave forms following P wave.

Naming convention:


Duration: 0.08-0.12 seconds (2-3 horizontal boxes)

Contour is same between beats



Delay in conduction through the ventricles leads to prolongation of QRS complex


Change of contour between beats suggests ectopic foci

Abnormal  but constant contour suggests

  • Bundle branch blocks
  • Drug toxicity
  • Electrolyte imbalance


Q wave


Usually very small or absent

Normal in III and AVR .


A Q wave is significant if it is greater than 1 box wide (0.04 secs) in leads other than III and AVR

Greater than 1/3 the amplitude of the QRS complex.

Greater than 1/4th of R wave

Abnormal Q waves: indicate presence of infarct


T wave

First upward deflection after QRS complex. Represents: ventricular repolarization


In general, T waves are in the same direction as the largest deflection of the QRS (normally the R wave).

Negative in AVR

Inverted T waves in precordial leads V1, V2, V3 can be seen in normal, young athletes

Low T voltage changes may occur in the absence of any heart disease at all.


T wave changes can be primary or secondary. 

Primary T wave change refer to abnormal repolarization

Secondary T wave changes are caused by QRS changes.  T wave changes caused by bundle branch block or ventricular hypertrophy are secondary.

Tall peaked T waves

Electrolyte imbalance =  Hyperkalemia causes tall peaked T waves.  overall maximum of 15 mV but this is not sensitive.  T wave looks like an isosceles triangle.

Low voltage T waves

  • Hypokalemia causes low voltage T waves and prominent U waves.  T waves less than 1mV in the limb leads and less than 2mV in the precordial leads.

  • low T voltage and sagging or flattened ST segments.  these changes may occur in the absence of any heart disease at all.

Inverted T waves


 U wave

What it represents is not certain.

This upright wave, when present, follows the T wave.


The presence of  U waves may indicate Hypokalemia.

Hypokalemia is associated with flat T waves, U waves. U waves taller than T waves.


PR interval

Represents: atria to ventricular conduction time (through His bundle)  It includes P wave and PR segment.

Normal duration: 0.12-2.0 seconds (3-5 horizontal boxes). This is measured from the onset of the P wave to the onset of the QRS complex regardless if the initial wave is a Q or R wave.


Abnormal duration:


If the PR interval is greater than 0.2 sec, then an AV block is present. There are several types of AV blocks:


A PR interval that is <0.12 sec (when associated with a prolonged QRS) should prompt evaluation for Wolff-Parkinson-White Syndrome (WPW).
<0.12 sec when associated with prolonged QRS should prompt evaluation for Wolff-Parkinson syndrome (WPW).


ST segment

Represents early phase of repolarization of ventricles.

Begins at the end of S wave and ends at the beginning of T wave.

In normal situations, it serves as the isoelectric line from which to measure the amplitudes of the other waveforms.

ST segments are usually isoelectric and normal.

When examining the ST segment, evaluate elevations or depressions 0.06 seconds after the J point (since the ST segment can at times be sloping).


This segment is important in identifying pathology such as myocardial infarctions (elevations) and Ischemia (depressions).

ST segment elevation

The location of the ST elevations on the EKG can help to identify a location of the infarct:

ST segment depression


QT  and QTc (corrected QT) interval

QT represents the duration of activation and recovery of the ventricular muscle.

This duration varies inversely with the heart rate

Since the duration of QT varies inversely with the heart rate, the QT is not used, but rather the corrected QT is.

QTc interval

QTc = QT + 1.75 (ventricular rate - 60)


The normal QTc is approximately 0.41 seconds. It tends to be slightly longer for females and increases slightly with age.


Prolonged QT

Shortened QT