God Knows
God Knows
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1 second
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ECG basics: there are 4 components:
p wave, the QRS complex, the t-wave, and the u-wave.  We will leave out discussion of the u-wave at this time.  This is the electrical pattern that causes the mechanical action of the heart to beat.  These are the components we will monitor and measure.  

The smallest boxes represent 0.04 seconds.  Five of these make up a larger box that represents 0.20 seconds.  Five of the larger boxes represent 1 second, see left.  
Each strip will be shown as a 6 second strip as shown below.  From red mark to red mark is 1 second 
(5 larger boxes).  

Next picture shows the mechanical heart showing the equivalent in the electrical pattern.
o    n    e     s    e    c    o    n    d
0.20 sec
0.04 s
0.20 sec
0.20 sec
Enlarged for you to see.
SA Node starts the pacemaker beat.  This is the start of the P-wave.  The PR interval is from the SA node through the AV node and stops at the Bundle Branches.  The QRS complex 
shows the ventricular beat 
through the Bundle 
Branches to the perkinje
fibers.  The T wave is the 
repolarization of the 
ventricules.  This is why we
do not want to see another
QRS complex on the T wave.

You may or may not see a 
U-wave.  Best seen in
V3, under normal 
conditions.  If prominent, it can indicate some disease processes like hypokalemia.
The Rhythms Page has basic information to read a strip and a link to the Rhythm Test.  The Rhythm Test will let you identify the proper rhythm and then the rhythm will tell you which algorithm to follow. 
SA Node
AV Node
Left Bundle Branch
Right Bundle Branch
Perkinje Fibers
Perkinje Fibers
Lead I 
QRS complex pointing up.
Lead II 
Lead MCL1 
Lead III
Next is a diagram showing placement of leads and what you should expect to see on the ECG strip.
QRS complex pointing down.
QRS complex pointing up.
QRS complex pointing up.