One small box is 0.04 seconds. To get a heart rate, usually expressed as "per minute", divide 300 by the number of LARGE boxes between QRS wave peaks. A large box is 0.2 seconds. (Math: one minute = 60 seconds. One second = 5 x 0.2 seconds per large box, thus 60s x 5 boxes per second = 300 LARGE boxes per minute) which also happens to be the upper limit of normal for the PR interval used in determining the presence of primary AV block. One can also memorize the rate for the number of large boxes, rather than doing the math: 1 = 300; 2 = 150; 3 = 100; 4 = 75; 5 = 60. If you have more boxes than that, or less, you'd better page me rather than worrying about math!
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artifact in ECG
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EKG paper is a grid where time is measured along the horizontal axis, of EKG graph paper and where to measure the components of the EKG wave form.
The Einthoven triangle is an equilateral triangle with the heart at its centre and the limbs (left arm, right arm and legs) at the apices. This triangle forms the basis for the theory behind how the ECG functions.