Wednesday, October 15, 2014

Electricardiogram (ECG) Interpretation

To interpet the ECG, a good understanding of the electrophysiology of the heart is required.

Important thing to remember, that the ECG is generally recorded at a speed of 25mm/second. This means that
a big box,  which had a 5mm length is covered in 0.2 second. And the 5 small boxes each present in a big box are 1 mm in length and are covered in 0.04 second each.

Therefore in order to calculate heart rate, we need to know the amount of area covered by the ECG strip in 60 seconds. This will be 25*60 = 1500 mm. Thus the number of RR intervals in this 1500 mm will be heart rate or alternatively the number of RR intervals in 25mm strip can be multiplied by 60 or 300/number of big boxes between RR interval

Systematic analysis of the ECG:
1. Analyse the rate - of the atria and the ventricles
2. Analyse the rhythm - is it regular, is it regularly irregularly or irregularly irregular.
3. Analyse the PR interval (0.2 sec, less than one big box). Is it constant?
4. Analyse the RR interval
5. Analyse the p wave- is there a p wave?, is there a p wave before every QRS complex ?, is there a QRS complex before every p wave?
6. QRS interval length is not more than 0. 12 seconds i.e., not more than 3 little boxes, is there a delta wave?

Normal Sinus Rythm - this should be interpret systematically
1. RYTHM: Look at the RR interval , is it of the same length between complexes if yes then is the Rythm is constant?
2. HEART RATE: The heart rate can be calculated by dividing 300 by the number of large boxes between RR interval. Therefore if there are 5 big boxes between RR waves then the rate is 60 and if there are 4 big boxes then the rate is 75. However if the boxes counted are 4 big boxes plus 3 small boxes then the heart rate will be between 60 and 75 i.e. 67
3. Is there a p wave for each QRS and QRS complex for each p wave.
4. The QRS complex should not be more than 3 small boxes i.e., 0.12 seconds.

If the heart rate is calculated to be less than 60 it means the patient is suffering from bradycardia. If p waves preceed the QRS complex then this means that the Sinus node is driving the QRS complex there it is in Sinus Rythm.
Causes for sun us bradycardia are:
increased vagal tone, SA node disturbances, drugs, also seen in athletes
No treatment is required for bradycardia unless it is also causing ventricular irritability or hypotension.

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