Final answer:
The 6-second method is used to calculate both regular and irregular heart rhythms, as it allows health professionals to measure heart rate even when there's irregularity in the beats.
Step-by-step explanation:
The method that can be used to calculate both regular and irregular heart rhythms is the 6-second method. This is a technique used to measure heart rate from an electrocardiogram (ECG) by counting the number of QRS complexes in a 6-second period and multiplying this number by 10 to estimate the beats per minute (bpm). The 6-second method is particularly useful for irregular rhythms, where methods like the 1500 method or the 300 method, which are based on regularity, may not be accurate.