If a different paper speed is used, calculations will have to be modified appropriately.

A. The standard paper speed is 25mm/sec:

• 1mm (small square) = 0.04 sec (40ms)
• 5mm (large square) = 0.2 sec (200ms)

B. Paper speed: 50mm/sec

• 1mm (small square) = 0.02 sec (20ms)
• 5mm (large square) = 0.1 sec (100ms)

Why use 50 mm/second?

Doubling the standard rate can reveal subtle ECG findings hidden at the slower rates

C. Paper speed: 10mm/sec

• 1mm (small square) = 0.1 sec (100ms)
  
• 5mm (large square) = 0.5 sec (500ms)

Estimate the rate

At a paper speed of 25 mm/second

• 1 SMALL square = 0.04 seconds
• 5 SMALL squares = 1 LARGE square = 0.2 seconds
• 5 LARGE squares = 1 second
• ECG rhythm strip:
  • = 250 SMALL squares = 50 LARGE squares = 10 seconds
• To calculate beats per minute (bpm):
  • 1500 SMALL squares = 300 LARGE squares = 1 minute

There are multiple methods to estimate the rate:

We can calculate the beats per minute (bpm) by dividing 1500 by the number of SMALL squares between two R waves (R-R interval = one beat)

We can calculate the beats per minute (bpm) by dividing 300 by the number of LARGE squares between two R waves (R-R interval = one beat)

REGULAR rhythms

• Rate = 300 / number of LARGE squares between consecutive R waves.

Very FAST rhythms:

• Rate = 1500 / number of SMALL squares between consecutive R waves.

SLOW or IRREGULAR rhythms:

• Rate = Number of R waves X 6
• The number of complexes (count R waves) on the rhythm strip gives the average rate over a ten-second period. This is multiplied by 6 (10 seconds x 6 = 1 minute) to give the average Beats per minute (bpm)
  

Example of 1500 (small squares) versus 300 (large square) method

Now adding the R wave (10 second rhythm strip)

Note:

• Calculate atrial and ventricular rates separately if they are different (e.g. complete heart block)
• The machine reading can also be used and is usually correct — however, it may occasionally be inaccurate in the presence of abnormal QRS/T-wave morphology, e.g. may count peaked T waves as QRS complexes or miss QRS complexes with reduced amplitude.

Interpretation (adults)

• Normal: 60–100 beats/min
• Tachycardia: >100 beats/min
• Bradycardia: <60 beats/min

Normal Heart Rates in Children

• Newborn: 110 – 150 bpm
• 2 years: 85 – 125 bpm
• 4 years: 75 – 115 bpm
• 6 years+: 60 – 100 bpm

References

• Lin M. Trick of the Trade: Speed up ECG paper rate to differentiate tachycardias
• Larkin J. ECG of the Week – 20th August 2012
• Accardi A, Miller R, Holmes J. Enhanced diagnosis of narrow complex tachycardias with increased electrocardiograph speed. J Emerg Med. 2002;22(2):123-126. [PMID 11858914]
• Gaspar J, Body R. Best evidence topic report. Differential diagnosis of narrow complex tachycardias by increasing electrocardiograph speed. Emerg Med J. 2005;22(10):730-732. [PMC1726576]

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest

Advanced Reading

• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
• Wagner GS. Marriott’s Practical Electrocardiography 12e
• Chan TC. ECG in Emergency Medicine and Acute Care
• Rawshani A. Clinical ECG Interpretation
• Mattu A. ECG’s for the Emergency Physician
• Hampton JR. The ECG In Practice, 6e