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The 6 Second Rule:

All but very slow heart rates can be determined by counting the number of cycles that occur within 6 seconds, and then multiplying that number by 10.  With very slow bradycardias, counting the number of cycles within 6 seconds may not be enough.

At the standard chart speed of 25mm/sec, five 0.5 cm blocks span 1 second (30 blocks/6 seconds).

At a paper speed of 50mm/sec, 10 blocks span 1 second (60 blocks/6 seconds).

For convenience, most ECG charts mark off every 7.5 cm along the upper margin.  These marks correspond to a 3 second interval at a chart speed of 25mm/sec, and a 1.5 second interval at a speed of 50mm/sec.

Ex:


At a paper speed of 25mm/sec, a 6 second strip is selected between the black arrows.  The number of heart cycles counted is 7.  HR= 7 x 10= 70 bpm.

 

Another method for determining the heart rate is "the rule of 300."

Take the number of “big boxes” between neighboring QRS complexes, and divide this into 300.  The result will be approximately equal to the rate

 

Although fast, this method only works for regular rhythms and at a paper speed of 25mm/sec.

Ex:

What is the HR?

At a paper speed of 25mm/sec, what is the HR? 300/6= 50bpm

 

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