Understanding EKG
EKG Leads
The normal electrocardiogram
12 step approach to EKG interpretation
Rate
Rhythm
Axis
P wave
PR interval
PR segment
QRS complex
QRS amplitude
QT interval
ST segment
T wave
U wave
EKG Leads
The normal electrocardiogram
12 step approach to EKG interpretation
Rate
How to calculate the rate from first principles?
First find the speed of the EKG paper strip. It is normally set at 25 mm/s as shown above (bottom left). This means it traverses 25 mm in one second.
Let us practice some simple algebra:
If we get one QRS complex for every large box, this
implies that you have 1 complex per 0.2 or 1/5 seconds or 5
complexes per 1 second or (5 x 60 = 300 complexes) per 60 seconds or
1 minute: => 300/min
If we get one QRS complex for every 2 large boxes,
this implies that you have 1 complex per 0.4 or 2/5 seconds or 5/2
complexes per 1 second or (5/2 x 60 = 150 complexes) per 60 seconds
or 1 minute: => 150/min
If we get one QRS complex for every 3 large boxes,
this implies that you have 1 complex per 0.6 or 3/5 seconds or 5/3
complexes per 1 second or (5/3 x 60 = 100 complexes) per 60 seconds
or 1 minute: => 100/min
If we get one QRS complex for every 4 large boxes,
this implies that you have 1 complex per 0.8 or 4/5 seconds or 5/4
complexes per 1 second or (5/4 x 60 = 75 complexes) per 60 seconds
or 1 minute: => 75/min
If we get one QRS complex for every 5 large boxes,
this implies that you have 1 complex per 1 or 5/5 seconds or 5/5
complexes per 1 second or (5/5 x 60 = 60 complexes) per 60 seconds
or 1 minute: => 60/min
If we get one QRS complex for every 6 large boxes,
this implies that you have 1 complex per 1.2 or 6/5 seconds or 5/6
complexes per 1 second or (5/6 x 60 = 60 complexes) per 60 seconds
or 1 minute: =>50/min
As you can see, there is a pattern: To find the rate you divide the
number of boxes into 300 = 300/(# of large boxes)
If the pattern is regular in rhythm (see below), you can apply the
above formula to get the rate. If the distance between the
complexes is not a whole number of large boxes, then you can
approximate it by dividing 300 into the fraction (e.g. 300/2.5 =
120).
From the strip above, it is a little less than 3 large boxes, so we know the rate is a little higher than 100/min.
If the pattern is irregular, the above rule cannot be used.
If you glance at the EKG strip above, you will see I have marked off
the strip for every 5 large boxes or 1 second intervals, and as you
can see, a strip runs for 10 seconds.
To calculate the rate, you count the number of complexes in the 10
seconds and then multiply by 6 to get the number of complexes in 60
seconds or 1 minute to get the average heart rate.
This can also be done even if the rhythm is regular.
For the strip above, there are 18 complexes in the rhythm strip (the lowest continuous reading above), so there are 18 x 6 = 108/min, which is what was expected.
Rhythm
Once you have calculated the rate, we can now determine the rhythm.
First determine if there is a P wave before each QRS complex. If there is a P wave before each complex, then the patient is said to be in sinus rhythm as the sinoatrial node is setting the pace for the heart and hence the rhythm.
If the rate calculated above is between 60 and 100,
the patient is said to be in NORMAL SINUS RHYTHM.
If the rate calculated above is less than 60
the patient is said to be in SINUS BRADYCARDIA.
If the rate calculated above is greater than 100,
the patient is said to be in SINUS TACHYCARDIA.
If there is no P wave before each QRS complex and the patient has an irregularly irregular rhythm, the patient is said to have ATRIAL FIBRILLATION.
If there are multiple P waves in a saw tooth pattern, or one P wave per each large box, then the patient is said to have ATRIAL FLUTTER.
Axis
The axis is the direction of the resultant summation of forces as the ventricles contract. To determine this, we use the principle that any vector can be resolved into 2 perpendicular components, and vice versa. That is if we have 2 perpendicular components of a vector, we know the direction of the resultant vector.
From above, the two perpendicular axes that would be used are I and AvF.
If I and AvF both have a net positive amplitude, then the axis is between 0  90 degrees, and hence is said to have normal axis.
If I has a net positive amplitude and aVF has a net negative amplitude, then the net deflection would lie between 0 and 90 degrees.
To determine if it is less than 30 degrees (green area above in the left upper quadrant) or more than  30 degrees (the red area above in the left upper quadrant), we need to look at a lead that is perpindicular to the 30 degress line, and lead II fits the bill.
So we now turn to lead II, and if it is net positive, then the axis is less than 30 degrees (green), and hence normal axis. If is net negative, then it is more than 30 degrees (red), and hence the patient is said to have left axis deviation.
The other leads are straightforward.
If lead I is net negative, and lead AvF is net positive, then the patient is said to have right axis deviation.
If lead I is net negative and lead AvF is net negative, then the patient is said to have extreme right axis deviation.
The importance of knowing the axis of the heart, is that abnormal deviation of the resultant voltages are seen in certain heart conditions and may provide clues to underlying heart abnormalities.
This is summarised in the table below:
Axis  Lead I  Lead AvF  Lead II  Differential Diagnoses 
Normal  +ve  +ve  Normal ( 0  90 degress)  
Normal  +ve  ve  +ve  Normal (30  0 degrees) 
Left axis deviation  +ve  ve  ve  Left ventricular hypertrophy Left bundle branch block Left anterior fascicular block 
Right axis deviation  ve  +ve  Right ventricular hypertrophy Right bundle branch block 

Extreme right axis  ve  ve  Dextrocardia Reversal of leads 
References
http://en.wikipedia.org/wiki/Electrocardiography
http://medictests.com/ecgcheatsheet/
http://www.bem.fi/book/15/15.htm