 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

Measuring Time

Time is measured horizontally

1 mm = 0.04 sec = 40 ms

5 mm = 0.2 sec = 200 ms

25 mm = 1 sec

Measuring Amplitude

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.  