The heart has 2 atria and 2 ventricles.

      Their rates (beats/minute) may differ (as in atrial fibrillation, atrial flutter, etc.).

     
Ventricular rate (QRS rate) is regarded as the heart rate.

     
Atrial rate (P wave rate) is NOT regarded as the heart rate.

  If the heart rhythm is regular, then ( at a paper speed of 25mm/second):

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

  If the heart rhythm is regular, then (at a paper speed of 25mm/second):

      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.

 
Sometimes, ECG computer's software may calculate the heart rate erroneously.

      Especially, when
QRS complexes and T waves have similar amplitudes ECG software may

      fail to discriminate them, and may
inadvertently report the heart as doubled.




ECG 1.
A QRS complex on a thick line is taken as reference and thick lines to the next QRS complex on a thick line are counted:
300-150
. The heart rate is 150 /minute.
This calculation method is valid only at a recording (paper) speed of 25mm/second.





ECG 2.
A QRS complex on a thick line is taken as reference and thick lines to the next QRS complex on a thick line are counted:
300-150-100
. The next QRS complex is on the third thick line. The heart rate is 100 /minute.
This calculation method is valid only at a recording (paper) speed of 25mm/second.





ECG 3.
A QRS complex on a thick line is taken as reference and thick lines to the next QRS complex on a thick line are counted:
300-150-100-75
. The next QRS complex is between the next third and fourth lines.
The heart rate is
between 75-100/minute.
This calculation method is valid only at a recording (paper) speed of 25mm/second.





ECG 4.
A QRS complex on a thick line is taken as reference and thick lines to the next QRS complex on a thick line are counted:
300-150-100-75-60-50-43
. The next QRS complex is between the next sixth and seventh lines.
The heart rate is
between 43-50/minute.
This calculation method is valid only at a recording (paper) speed of 25mm/second.





ECG 5. The ECG above shows the onset of
Adenosine effect in a patient with atrial flutter
Adenosine injection results in atrioventricular (AV) block and heart rate (ventricular rate) slows down.
When
heart rate (ventricular rate) slows down, flutter waves become clearly visible.
This is atrial flutter.
Ventricular rate (QRS rate) is regarded as heart rate.
Atrial rate (P wave rate) is not regarded as heart rate.

Click here for a more detailed ECG





ECG 6. Above is an ECG from a 75 years-old woman with hypertension, COPD, and chronic renal failure.
When the above ECG was recorded, her serum potassium level was measured as 8.6 mmol/L (hyperkalemia).
She was also under Digoxin therapy.
Nodal rhythm is seen. P waves are not visible.
QRS complexes are widened and the T waves in right precordial leads are relatively prominent.
This ECG was recorded at a standart calibration of 25 mm/second.
Hyperkalemia may result in tall, pointed T waves.
In the above ECG, many leads show QRS complexes and T waves with similar amplitudes.
Because of this, the ECG software counts
QRS complexes and T waves as seperate QRS complexes,
which results in
inadvertently doubling the heart rate as 62/minute.
The actual heart rate in this patient is 31/minute.

Click here for a more detailed ECG