Lessons · Medicine foundations · heart rate from a rhythm strip
Heart rate off a rhythm strip: 300 over the large squares
On standard paper the rate is 300 divided by the number of large squares between two beats, or 1500 divided by the number of small squares.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A strip is handed over at two in the morning with the machine's printed rate cut off at the edge of the paper. Counting squares takes four seconds and needs nothing but your eyes, and it is the one thing everybody in the room can do without a device.
How to think about it
Find two neighbouring R waves, the tall narrow spikes. Count the large squares in the gap between them, not the waves themselves. Divide 300 by that count. If the gap is not a whole number of large squares, count the small squares instead and divide 1500 by that, because five small squares make one large.
Worked example
Four large squares between two R waves: 300 / 4 = 75 beats per minuteFewer squares means a faster rate, because the beats are closer together.
Three large squares: 300 / 3 = 100 beats per minuteOne square less, and the rate rises by twenty-five.
The same four-square gap counted small: 1500 / 20 = 75 beats per minuteTwenty small squares are four large ones, and the two methods agree, which is the check.
Your turn
There are five large squares between two R waves on a standard strip. Write the line that gives the rate.
300 / 5 = beats per minute
Solve one, graded on the server
The trap
Counting the R waves rather than the gaps between them. Five spikes have four gaps, and using five gives a rate a quarter too slow every single time.