BACKGROUND: Cardiac autonomic activity quantified by heart rate variability (HRV) has been proposed to alter across the menstrual cycle (MC), however, most prior research fails to confirm MC phase using female sex hormones. Therefore, the purpose of this study was to examine the relationship between changes in estrogen (E2) and progesterone (P) with resting HRV in women throughout the MC. METHODS: Naturally cycling females (n=20; age=21±3yrs, ht=163.0±4.5cm; mass= 63.2±11.4kg) collected daily saliva and resting HRV across two MC. Saliva was assayed for free E2 and P while HRV was collected in a supine position using a heart rate monitor and a phone application. During each MC, a blood draw was performed at the laboratory 5-8 days following a positive ovulation test and assayed for serum P. Ovulation was confirmed using a combination of ovulation tests, serum P, and salivary P profiles. Anovulatory cycles (n=6) were excluded from analysis and 18 of the 20 participants completed collection across both MC, therefore a total of 32 cycles were analyzed. HRV analysis was performed on the cleanest 5-min of each segment where the root mean square of successive R-R intervals (rMSSD), high frequency (HF), and sample entropy (SampEn) was assessed. All sex hormones and HRV metrics were log transformed (ln), and mixed-effects models were used to examine the relations between P and E2 on HRV. Model parameters included an interaction term of day (based on ovulation) with salivary E2 and P, respectively. Linear and quadratic time (i.e., day) were examined. RESULTS: An ANOVA revealed a quadradic term (x2) was appropriate for the model with lnrMSSD (p<0.05) but not lnHF or lnSampEn (p>0.05). A main effect of E2 and P was observed for lnSampEn (E2= -2.97e-02, p = 0.01; P=1.619e-02, p=0.03) but not lnrMSSD (E2 = 1.79e-02, p=0.55; P=-1.62e-02, p=0.94) or lnHF (E2 = -007, p=0.88; P =-0.03, p=0.34). Across both MC, E2 negatively influenced lnrMSSD (Day*E22 = -3.479e-04, p = 0.03), however, no other interaction effects were observed for E2 or P on lnrMSSD, lnHF, or lnSampEn (p>0.05). CONCLUSION: Given the influence of E2 on rMSSD across the MC, it should be measured and controlled for in females if tracking changes across time. Across the MC, HF and SampEn do not appear to be affected by either E2 or P, but both E2 and P are associated with SampEn. Further studies should confirm these findings for additional HRV metrics.

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