Re-examining the relationship between low-to-high-frequency ratio and cardiac autonomic balance and regulation: A focus on systolic time intervals

While converging evidence supports the high frequency (HF) component of heart rate variability (HRV) as an index of parasympathetic (PNS) activity, continuing debate over the validity of the low frequency (LF) component of HRV as an index of sympathetic (SNS) activity calls into question the LF/HF ratio as an index of autonomic nervous system balance. We recently showed a strong relationship and good agreement between the LF/HF ratio and impedance derived measures of cardiac autonomic balance (CAB) and regulation (CAR), suggesting that the LF/HF ratio may indeed serve as a good surrogate index of both ANS balance and regulation. However, research has not yet considered how this relationship may change as a function of using different systolic time intervals, including both pre-ejection period (PEP) and left ventricular ejection time (LVET), in the calculation of CAB and CAR. Thus, under resting-baseline conditions, the present study reexamined the direct association between LF/HF ratio and both CAB and CAR, calculated differently using both PEP and LVET as indices of SNS activity, in a sample of 50 apparently healthy participants (78% female, mean age = 18.80 years). CAB and CAR scores were calculated using both PEP and LVET, and measures of PNS-mediated HRV via standard time- and spectral-analytic techniques. Both greater CAB scores and lower LF/HF ratios reflect greater PNS activity relative to SNS activity, whereas greater CAR scores reflect greater activity of both the PNS and SNS, whereas. Correlational analyses revealed a significant and negative relationship between the LF/HF ratio and both CAB_LVET (r = -.432, p = .002) and CAR_PEP (r = -.335, p =.019). No significant relationship was found between CAB_PEP (r = -.226, p = .118) and CAR_LVET (r = -.118, p = .419). Bland-Altman plot analyses showed excellent agreement between the LF/HF ratio and both CAR_PEP and CAB_LVET. Our data suggest that while the exact nature of LF-HRV component remains controversial, in a resting state, the LF/HF ratio is indeed related to and may serve as a useful proxy of cardiac autonomic balance, in addition to regulation. However, the strength of this relationship may vary as a function of the systolic time interval used to calculate impedance-based CAB and CAR measures. Overall, the LF/HF ratio may differentially predict CAB and CAR based on systolic time intervals, and future investigations are needed to understand possible physiological mechanisms, such as the cardiac effector tissue associated with the sinoatrial node (LVET) versus atrioventricular node (PEP), underlying these pattern of results.