If the heart rhythm is regular, then:

      Heart rate =
1500 / the number of small squares between two consecutive R waves.

  If the heart rhythm is regular, then:

      Heart rate =
300 / the number of large squares between two consecutive R waves.

  If the heart rhythm is irregular (as in atrial fibrillation, or in atrial flutter with variable AV block)

      we can only roughly estimate the heart rate.

      In such a situation, the number of QRS complexes during a 6 second period (30 large squares) is counted.

      If this number is multiplied by 10, we get an approximate heart rate in one minute.

  The heart rhythm is irregular in patients with atrial fibrillation.

      When such a patient is monitorized, the heart rate depicted on the monitor will change continuously.

      This continuous change in heart rate results from abrupt changes in RR intervals.

      A heart rate monitor measures RR intervals very frequently and the instantenous heart rate will be

      derived from these continuously changing RR intervals which will obviously result in a continously

      changing heart rate.

      In such patients, the sudden changes observed on a heart rate monitor does not necessitate to be panicked.