300 beats
per minute. If the 
R-peak matched every other dark vertical line
it would be 300/2 or 150. If every third line, 100; every fourth
line 75 and every fifth line 60. 
Ordinarily, the heart rate can be determined by taking the
patient's pulse or by auscultation of the heart with a
stethoscope. Some premature beats (ectopic) or very rapid beats
cannot be detected via the pulse or a stethoscope, but they will
register on an ECG strip. When evaluating an ECG, the first item
to determine is heart rate. Locate a reference point P-wave,
R-wave etc. which falls on a dark line on the ECG paper. If the
peak of an R-wave (or any other reference point), falls on each
dark vertical line on a strip, that would indicate a rate of 5
large boxes per second times 60 seconds per minute or 300 beats
per minute. If the
R-peak matched every other dark vertical line
it would be 300/2 or 150. If every third line, 100; every fourth
line 75 and every fifth line 60.
At the top of each ECG strip is a series of 
marks or dots.
The distance between dots represents 3 seconds. So if we count
the number of R-peaks between alternate dots (6 seconds) and
multiply by 10, we have the heart rate. This is especially useful
when the rate is very slow or irregular.