(3) If the R to R interval (RRI) is inconsistent and the rhythm is irregular, the
heartbeats are premature. A basically regular rhythm occurs when a heartbeat or two
interrupt a regular rhythm; for example, ectopic (heartbeats originating from an
abnormal place; that is, from somewhere other than the S-A node) or grouped
heartbeats. A regularly irregular rhythm is a pattern of irregularity. A totally irregular
rhythm has no pattern at all.
Figure 2-8. Irregular rhythm. (file: 5712-8.pcx)
Rate for regular heartbeat rhythms can be calculated in two ways.
(a) Method I. The most accurate method is to count the number of
small squares between the two R waves and divide that number into 1500.
(b) Method II. The faster method is to count the number of large
squares between R waves and divide that number into 300. This method is useful
during an emergency. This method does not require a 6-second strip. Also, it allows
rapid rate calculation using simple memorization of rate for large boxes. See figure 2-9.