One of the first aspects of analyzing an ECG is determining the rate. When doing this, think about the following three things:
Atrial rate
Ventricular rate
Regularity
Atrial Rate
This is the time rate of the p waves.
In the example below, p waves occur at regular intervals roughly every 21 mm, or 840 ms.
This translates to an atrial rate of 71 bpm.
Ventricular Rate
This is the rate of the QRS complexes, and equivalent to the heart rate.
In the same example, QRS complexes also occur regularly every 21 mm, equating to a ventricular rate of 71 bpm.
Regularity
Sometimes, the atrial or ventricular beats do not occur at regular intervals.
Regularly irregular: refers to a rate that is not regular, but repeats in a predictable pattern.
Irregularly irregular: refers to a rate that is not regular and not periodic or predictable.
Note: While regularity usually means “QRS regularity”, there is value to determining “p wave regularity” as well.
Regularly Irregular
Consider the example below.
There is a regular atrial rate with a constant p-p interval of 13 mm.
There is an irregular ventricular rate with non-constant R-R intervals. However, the R-R intervals have a pattern of 15 mm, then 14 mm, then 23.5 mm, then back to 15 mm and so on.
Due to this predictable, repetitive pattern, we can call this a regularly irregular rate.
Irregularly Irregular
Consider the example below.
This is an example of an irregular ventricular rate with no identifiable pattern and seemingly random R-R intervals.
Hence, this is an irregularly irregular rate.
Methods for Easy Rate Determination
Counting the millimeters between R waves and converting to bpm can become tedious.
There are two faster methods of determining rate:
Method 1: Counting QRS’s.
Method 2: The 300-150-100 rule.
Counting QRS's
By counting all the QRS complexes in a 10-second EKG strip and multiplying by 6, you can determine the beats-per-minute.
This is especially useful when the rate is irregular due to the great degree of beat-to-beat fluctuation in R-R intervals. It gives us a better sense of the “average” R-R interval over the 10s strip.
In this example, the heart rate is roughly 11 x 6 = 66 bpm and regular.
The 300-150-100 rule
The basis of the 300-150-100 rule is memorizing the bpm corresponding to specific R-R intervals, and estimating in-between values.
Taking the same example as before, let’s try the 300-150-100 rule.
This method works best on regular rates, which this is.
Best to start on an R wave that lands directly on a big block.
In the example below, the R-R intervals are roughly between 4 and 5 big blocks wide.
This places the heart rate at somewhere between 60 and 75 bpm, according to the 300-150-100 rule.
The R-R interval is closer to 5 big blocks wide, so it must be closer to 60 bpm than 75 bpm.
We can estimate 65 bpm. This is very close to the 66 bpm from the other method.