ECG calipers are an important tool for interpreting results. When measuring the R-R interval, the ECG operator places one point of the calipers on the peak of an R wave, and the other at the peak of the next R wave. In doing this, the operator can now see if the R-R interval is equidistant across the whole rhythm strip. If it is, the rhythm is said to be regular. If it is not, then the pattern of irregularity must be determined (see Figure 2.7): 

• Regularly irregular: there is a pattern to the irregularity 
• Basically regular: the rhythm is regular, but there is a beat or two that interrupts the regularity
• Totally irregular: no pattern exists at all

Figure 2.9 A Regularly Irregular Heart Rhythm

Frequent premature atrial complexes (PACs) followed by brief pauses.

If the rhythm is irregular, the R-R intervals are measured with calipers, and the operator looks for patterns of irregularity. These irregularities may include an ectopic beat (a regular rhythm with a disrupting beat) or a group of regular beats combined with an ectopic beat. An irregular ECG is totally irregular if there is no pattern to the irregularity.

Slow heart rates are an exclusion. Often, a pattern of irregularity consisting of one small square is seen in patients with slow heart rates. These patterns may still be regarded as regular.