Figure 1.

Illustration of the ambiguity in the LF-HF ratio through a class of possible relationships between the low- (LF) and high-frequency bands (HF) in heart rate variability (HRV), given in Equation (1). The metrics P₁ to P₅ represent possible sympathovagal balance relationships and are designed so that every metric P_i increases with the activity of the sympathetic nervous system (SNS), and decreases with an increase in the activity of the parasympathetic nervous system (PNS). However, observe that for a constant LF_p/HF_p ratio, which is believed to indicate a constant sympathovagal balance, the curves for various P_i, i = 1, 2, …, 5 in Equation (1) behave quite differently, which demonstrates the inadequacy of any 1D metrics for the assessment of stress.