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.

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.