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Part 5: Rate calculation
Let us begin this section with a story.
Once upon a time, there was a fresh faced FY1 on a cardiology rotation.
The cardiologist was in a GREAT mood, and wanted to test our knowledge.
So they started soft and easy, and of course gave us an ECG to look at.
The first question was:
"How do you calculate the rate?"
The FY1 in question very quickly answered: "It says the rate in the top corner!"
That FY1 was the author.
I am a firm believer that a picture says a thousand words, so i will simply yield the floor to the following:
And from that day, was the FY1 forever scarred and learned that relying upon machine interpretation is not going to land you within a cardiologists good graces.
Admittedly, chatGPT and other various AI equivalents weren't around yet, but the premise stands.
Everybody lies. Machines included.
That said, there is one part of the machine ECG that must be interrogated, as follows:
This is the idea that the "standard rhythm strip" is calibrated to a specific rate and voltage.
Standard ECG settings use a paper speed of 25 mm/second (time on X-axis)
and 10 mm/mV (voltage on Y-axis),
This results in one small square being 0.04s and 0.1mV/mm.
One large square, being made up of 5 small squares, therefore yields a value of 0.2s and 0.5mV/mm.
This calibration, and thus squares, is generally what all cardiology/ECG definitions are based upon.
For example, using the ECG above - Broad complex is defined as >0.12s.
Given the human brain is poor at seeing in units of time, we instead utilise units of shape.
We can therefore see that the example ECG above has both
narrow complexes
(<0.12s, <3 small squares)
and
broad complexes
(>0.12s, >3 small squares)
What this also means is that the typical ECG set to this calibration will run over 10 seconds.
Therefore, because our modern rely upon babylonian mathematics, who felt 60 was a much nicer number to work with than 100, a regular(ish) heart rate can be calculated by multiplying the amount of QRS complexes by 6.
As a small detour into history, during the French Revolution there was an attempt to bring in decimal-based time keeping.
As you may have guessed by the fact that the world speaks English, and not baguette, this did not entrench for various reasons.
Given the reader is here to learn about ECG's and not history, i will simply skip to the ending where Napoleon's attempts at world domination did not come to fruition.
This also means that the score card also reads
Ancient Sumerians/ Babylonians: 1
Napoleon: 0
(As an aside, given modern infrastructure world wide relies upon the 60s model, changing this would probably cause the world to implode)
Back to our ECG's:
Hopefully the reader is not a surgeon, and therefore knows how to count.
The ECG above has 13 complexes on the long strip.
13x6 = 98. Job done.
The problem is when the rhythm is not regular all the way through.
For example, the broad complexes above are clearly running quite fast.
Thus, the alternative and personally preferred method is that of
R-R Interval / 300.
In other words, how many large boxes are between each peak?
In the example above, this seems to be between 1-1.5.
Plugging this back into our equation:
1/300
to
1.5/300
Yields a HR variance during the abnormal rhythm of around 200-300bpm.
That is the end of this section, which probably leaves the reader pondering why I feel surgeons cannot count.
The answer is very simple.
Ask anybody who has been in an operating theatre who does the swab count at the end, and the answer will the professional entrusted to do basic arithmetic.
I.e: Not the surgeon.
Click for part 6:
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