# 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.

## Annotated example

**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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ECG LIBRARY

Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Co-creator of the LITFL ECG Library. Twitter: @rob_buttner

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Ma il genere cosa si usa il 25mm o il 50mm?

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