ECG Rate Interpretation

Understanding paper speeds

• Paper output speed is the rate at which the ECG machine produces a trace
• Standard output is 25mm per second
• If a different paper speed is used, standard rate calculations will have to be modified appropriately (see other examples below)

The standard paper speed is 25mm/sec:

• 1 SMALL square (1mm) = 0.04 sec (40ms)
• 5 SMALL squares (5mm) = 1 LARGE square = 0.2 sec (200ms)
• 5 LARGE squares = 1 second

At standard paper speed of 25mm/sec, the rhythm strip comprises of:

• 250 SMALL squares = 50 LARGE squares = 10 seconds

Before calculating rate in beats per minute (bpm), we should understand that a rhythm strip recorded for 1 minute will therefore compromise:

• 1500 SMALL squares = 300 LARGE squares = 1 minute

Calculating rate

There are three main methods of calculating ECG rate. There is no specific best method, and preference varies between clinicians. However, certain methods may be better suited for rhythms such bradyarrhythmias or tachyarrhythmias.

1) Large square method

• Recall above that 300 large squares is equal to 1 minute at a paper speed of 25mm/sec
• We can thus calculate bpm by dividing 300 by the number of LARGE squares between each R-R interval (space between two consecutive R waves = one beat)
• For example, two large squares between each R-R interval implies a rate of 150 bpm, three implies a rate of 100 bpm and so forth:

• Useful as quick calculation for regular rhythms at regular rate

2) Small square method

• Similar to above, except 1500 is divided by the number of SMALL squares between consecutive R waves
• For example, 10 small squares between R-R interval implies a rate of 150 bpm, 15 implies a rate of 100 bpm, and so forth:

• Useful for very fast regular rhythms, as likely to provide more accurate rate than large square method

3) R wave method

• Rate = Number of R waves (rhythm strip) 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)
• Useful for slow and/or irregular rhythms
  

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: 50 – 100 bpm
• Tachycardia: > 100 bpm
• Bradycardia: < 50 bpm

Normal Heart Rates in Children

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

Other paper speeds: 50mm/sec

Doubling the standard rate will cause the ECG to appear drawn out or wider complex than 25mm/sec paper speeds

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

The rhythm strip will thus comprise 5 seconds total capture compared to the standard 10 seconds.

Why use 50 mm/second?

Doubling the standard rate can reveal subtle ECG findings hidden at the slower rates, in particular atrial flutter waves in a 2:1 block:

C. Paper speed: 10mm/sec

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

References

• Accardi A, Miller R, Holmes J. Enhanced diagnosis of narrow complex tachycardias with increased electrocardiograph speed. J Emerg Med. 2002;22(2):123-126
• 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.
• Lin M. Trick of the Trade: Speed up ECG paper rate to differentiate tachycardias

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Zimmerman FH. ECG Core Curriculum. 2023
• Mattu A, Berberian J, Brady WJ. Emergency ECGs: Case-Based Review and Interpretations, 2022
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Hampton J, Adlam D. The ECG Made Practical 7e, 2019
• Kühn P, Lang C, Wiesbauer F. ECG Mastery: The Simplest Way to Learn the ECG. 2015
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

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

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