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:
ECG rate calculation Large square method
Large square method: Divide 300 by the number of large squares between R-R interval. Useful for regular rhythms
  • 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:
ECG Rate calculation Small square method
Small square method: Divide 1500 by number of small squares between R-R interval.
  • 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
ECG RATE rhythm strip 10 seconds 300 1500 rules ECG
Comparison of three methods: The R wave method is often easiest as a quick calculation

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:

ECG RATE 25 versus 50 mm sec show p waves flutter
At 50mm/sec, P waves are more clearly discernible in a 2:1 ratio to QRS complexes

C. Paper speed: 10mm/sec
  • 1mm (small square) = 0.1 sec (100ms)
  • 5mm (large square) = 0.5 sec (500ms)

References

Advanced Reading

Online

Textbooks


LITFL Further Reading

Cite this article as: Mike Cadogan and Robert Buttner, "ECG Rate Interpretation," In: LITFL - Life in the FastLane, Accessed on November 21, 2022, https://litfl.com/ecg-rate-interpretation/.

ECG LIBRARY

Electrocardiogram

Associate Professor Curtin Medical School, Curtin University. Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

6 Comments

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