Heart rate variability (HRV) is the temporal variation
between sequences of consecutive heart beats. On a
standard electrocardiogram (ECG), the maximum
upwards deflection of a normal QRS complex is at the
peak of the R-wave (Figure 1), and the duration between
two adjacent R-wave peaks is termed as the R-R interval.
The ECG signal requires editing before HRV analysis can
be performed, a process requiring the removal of all non
sinus-node originating beats. The resulting period
between adjacent QRS complexes resulting from sinus
node depolarizations is termed the N-N (normal-normal)
interval. HRV is the measurement of the variability of the
N-N intervals (Reed 2005).