Here are the practical EKG parameters to remember when doing 12 lead EKG interpretation.
EKG Measurements and Calibration
The smallest box on the paper = 1 mm on all sides
A big box containing 5 small boxes = 5 mm on all sides
Time is measured horizontally
1 mm = 0.04 sec = 40 ms
5 mm = 0.2 sec = 200 ms
25 mm = 1 sec
Electrical amplitude is measured vertically
Calibration can be changed on EKG machines, but the standard calibration is 10 mm = 1 mV
5 mm = 0.5 mV
1 mm = 0.1 mV
Determining Heart Rate
Find two consecutive R waves. Count the number of 5 mm boxes between the waves.
Take 300 divided by the number of boxes.
300/#big boxes between R waves = heart rate
A normal rate = between 3-5 big boxes
Normal Wave Interval Values
PR Interval: <0.21 sec (5 small boxes)
Measured from beginning of P wave to the beginning of the QRS complex.This is the time it takes for the impulse to travel from the SA node, through the atria, and to the distal third of the septum.
QRS Complex: <0.12 sec (3 small boxes)
Measured from the beginning of the complex to the end of the complex. Time of depolarization of the rest of the septum and both ventricles completely.
QT Interval: <0.43 sec when corrected
Measured from the beginning of the Q wave to the end of the T wave. It is the total time for the depolarization and repolarization of the septum and the ventricles. The QT interval varies with heart rate (see below).
ST Interval: 0.08 - 0.12 sec
Begins at the end of the S wave through the beginning of the T wave. This is the period of time when no electrical activity of the heart can occur, like a forced resting period.
Calculating the QT Interval
When the heart rate is greater than 60 beats per minute, the measured QT interval must adjusted through a calculation. When calculated, the QT interval is known as QTc.
The QT interval is adjusted by 0.02 seconds for every 10 beats per minute above 60 the heart rate is at. So take the difference between the heart rate and 60, divide that by 10, multiply by 0.02 and add that to the measured QT interval.
QTc = QT + 0.02*((HR-60)/10))
Estimating the Electrical Axis of the Heart
To estimate the electrical axis of the heart, only look at the limb leads (I, II, III, aVL, aVR, aVF) in the frontal plane.
Step 1. Determine the Dominant Quadrant Using Lead I and aVF
- Determine the overall positive or negative amplitude of the QRS complex in leads I and aVF
- Match the polarity of the two leads with the appropriate quadrant in the image
Step 2. Find the Isoelectric Limb Lead
This is the lead with the lowest amplitude when you sum the positive and negative deflections of the QRS complex.
Step 3. Find the Limb Lead Perpendicular to the Isoelectric Lead
Using your knowledge of the limb lead placement on the frontal plane in the image below, choose the lead that is perpendicular to the isoelectric lead. The value of that lead within the dominant quadrant is the axis value.