The Influence of Oral Contraceptives on the Autonomic Nervous System

This thesis aims to determine the effects of oral contraceptives (OC) on resting autonomic function, autonomic reflex activation, and cerebral blood flow (CBF). The first study systematically reviewed the effects of OC on any aspect of autonomic function. A search strategy was applied to several databases, and 6,148 citations were retrieved. Forty studies were included and grouped by measurement of autonomic function investigated. Physiological responses to isolated reflex activation (i.e., the chemoreflex, mechanoreflex and metaboreflex) were influenced by OC; however, the effect of OC on resting autonomic indices and response to autonomic stressors was less consistent. These inconsistencies may be caused by hormone dosage within OC formulations or stressor intensity. The second study investigated the influence of OC on the cardiorespiratory response to metabo- and mechanoreflex activation in the arm and leg. Two minutes of isometric handgrip or plantarflexion exercise-induced similar increases in blood pressure (BP) and ventilation (VE) in both OC and non-OC users (NOC). While both exercise modalities increased VE, neither OC nor NOC exhibited a sustained increase in VE during 3 minutes of arm or leg post-exercise circulatory occlusion. All women increased BP and VE during 3 minutes of arm or leg passive movement. Considering that all women had a cardiorespiratory response to mechanoreflex but not metaboreflex activation, we suggest that the mechanoreflex may drive VE during exercise in women. The final study investigated the influence of OC and the menstrual cycle on CBF and cerebral autoregulation (CA) during 5 minutes of hypercapnia (5%). Regardless of menstrual or pill phase, all women improved high frequency and very low frequency dynamic CA (dCA) during hypercapnia, although low frequency dCA decreased in the high hormone phase of NOC and the low hormone phase of the pill cycle. During hypercapnia, the presence of endogenous hormones attenuated dCA, while the chronic use of exogenous hormones (i.e., OC) chronically attenuated dCA (i.e., when hormones were not present). The findings of this thesis shed light on the complex influence of OC on autonomic function, CBF and different physiological stressors, emphasizing the need for further research to fully elucidate its impact. There is a further need to control for menstrual cycle, OC formulation, dose, progestin generation and/or type when studying factors affecting autonomic physiology.