Autonomic reactivity to pain throughout the menstrual cycle in healthy women.

INTRODUCTION

We previously demonstrated that only men showed a significant correlation between heart rate (HR) and pain. Other authors also found sex differences in the autonomic and cardiovascular reactivity to pain, and sex hormones have been proposed to be partly responsible for these differences. However, no previous studies were done to examine if the autonomic and cardiovascular reactivity to pain vary across the menstrual cycle (MC).

METHODS

Thirty-two healthy women were randomly tested 3 times across their MC (days 1-3, days 12-14 and days 19-23). The painful stimulus consisted of a 2 min cold pressor test (CPT) (immersion of the arm in cold noxious water at 12 degrees C). HR and blood pressure were recorded before and during the immersion using an ECG which also allowed us to measure heart rate variability (HRV).

RESULTS

Pain ratings during the CPT did not vary across the MC (P = 0.14). HRV (sympathetic and parasympathetic indicators) and blood pressure (systolic and diastolic) analysis showed that women had similar cardiovascular reactivity to pain throughout their MC. However, we found that the correlation between HR and pain ratings during the CPT varied across the MC, where there was a significant positive relationship between HR and pain (r = 0.36, P < 0.05) only during the menstrual phase.

INTERPRETATION

These results add to our previous finding but tend to show that sex hormones have minimal influence on autonomic reactivity. Moreover, the great variability in intra- and inter-subject reactivity to pain does not allow us to predict the autonomic and cardiovascular reactivity to pain women will show throughout the MC.