Rate, Rhythm and Sinus

Counting the rate off the grid

Because the paper speed is fixed, the spacing between beats tells you the heart rate directly. The quickest method counts large squares between two beats. One large square is 0.20 seconds, so five of them make a full second and 300 of them make a minute. Find an R wave that lands on a heavy line, then count large squares to the next R: divide 300 by that number and you have the rate.

Rate from large squares between consecutive R waves:

  Large squares apart   Heart rate (bpm)
  -------------------   ----------------
        1                 300
        2                 150
        3                 100
        4                  75
        5                  60
        6                  50

Worked example (irregular or slow rhythm):
  Count QRS complexes on a 6-second rhythm strip = 8 beats.
  8 beats x 10 = 80 bpm.

  (A 6-second strip is 30 large squares wide;
   counting beats x 10 works even when the rhythm is irregular.)

What makes a rhythm 'sinus'

A normal heart is driven by its natural pacemaker, the sinoatrial node, and the resulting pattern is called sinus rhythm. It has a tidy signature you can verify by eye. First, every QRS is preceded by a P wave — the atria always fire before the ventricles. Second, the P wave points the right way (upright in the lead that looks along the normal direction of spread). Third, the rhythm is regular and the rate sits in a normal band. When all three hold, the heart's own boss is in charge.

When the rate is normal we call it sinus rhythm, usually about 60 to 100 beats per minute at rest. Faster sinus is sinus tachycardia — a normal response to exercise, fever, fear or pain. Slower sinus is sinus bradycardia, which is common and healthy in athletes and during sleep. The key point is that sinus describes who is driving; tachy or brady only describes the speed.