How to Read an Electrocardiogram
The first thing to check for when reading an Electrocardiogram is if it is correctly performed.
For that, we must check the speed at which the paper moves and the amplitude values–on a standard EKG, speed is 25 mm per second and the amplitude value, 1mV per 10 mm (see EKG Paper), that all 12 Leads are present, and that the EKG doesn’t show too many artifacts which could make our reading difficult.
If that is not the case, the Electrocardiogram must be performed again, if possible.
Assuming the Electrocardiogram is well done, we start our reading, always following the same sequence so we don’t miss anything:
Heart Rate must be determined on every Electrocardiogram, at least know if the EKG has a tachycardia, bradycardia or normal normal Heart Rate. This allows you to move to the next step.
Next thing to check is: Is the QRS complex rhythm regular or not? To know, we must see a similar distance between QRS complexes. When in doubt, use a compass or ruler.
Then we must determine whether the Electrocardiogram returns a report of Normal Sinus Rhythm. To do that, we must ascertain whether each cardiac cycle has a sinus P Wave (Read Normal Sinus Rhythm) always followed by a QRS complex.
The PR interval (normal between 0.12-0.20 seconds) should be measured from the beginning of the P wave to the beginning of the QRS.
The QT interval should be measured from the beginning of the QRS complex to the end of the T wave.
The QT interval varies depending on heart rate, so to determine whether is normal or not it should undergo adequate rate correction. The corrected QT interval or QTc interval is normal between 350 ms and 454 ms (Read QT Interval).
This is one of the hardest Electrocardiogram reading steps. We can quickly determine it is normal if leads I and aVF are positive (Read Heart Axis).
ST Segment Abnormalities
Now is the time to look at the ST segment, the scary line that warn us of coronary artery disease.
The ST segment is the line between the end of the QRS complex and the beginning of the T wave. It must be isoelectric. To find out if we are confronting a ST segment elevation/depression we must compare it to the PR segment or, when in doubt, to the TP segment, the one between the previous cycle’s T wave and the actual cycle’s P wave (Read ST Segment Abnormalities).
Check all waves and intervals:
To end our analysis we must check every ECG wave and interval not evaluated by now. For example: Bundle Branch Block, P wave or T wave abnormalities (Read Abnormal waves and intervals).