Sympathetic nervous system regulation of cardiovascular function: effect of acute and chronic exercise on the normative adaptations in healthy pregnancy
Pregnancy is associated with profound changes to the sympathetic nervous system control of blood pressure while maintaining blood pressure itself. This involves a coordinated increased muscle sympathetic nerve activity (MSNA) alongside reduced sympathetic baroreflex gain (BRG) and neurovascular transduction (NVT) that has been observed in the third trimester (TM3) of normotensive pregnancy. The mechanisms by which blood pressure is raised during stress in pregnant women are also altered.
... e has been shown to have many health benefits in pregnancy including a 40% reduction in the risk of developing gestational hypertension. In fact, exercise is recommended for all pregnant women who do not have explicit absolute contraindications. These recommendations include a combination of aerobic and resistance exercise to achieve 150-minutes of moderate intensity activity per week. We know very little about the mechanisms behind which exercise is beneficial in reducing hypertension risk in pregnancy, and the studies here aim to address these questions from the perspective of the sympathetic nervous system. Aims and hypotheses: The aim of our first study was to addresses whether there are differences between pregnant and non-pregnant women in the blood pressure and MSNA response during acute exercise (i.e. isometric handgrip; IHG). We hypothesized that the blood pressure response to IHG would not be different, but that pregnant women in TM3 would have a greater MSNA response to elicit this (i.e. blunted NVT). The aim of our second study was to identify whether a structured aerobic exercise program could alter the sympathetic regulation of blood pressure. We hypothesized that exercise would attenuate the rise in MSNA (and subsequent blunting of BRG and NVT) and would augment the decrease in arterial stiffness (pulse wave velocity; PWV) and other positive vascular changes associated with healthy pregnancy (i.e. changes in carotid iii distensibility). We also hypothesized that an aerobic exercise program would reduce the blood pressure and MSNA response to cold pressor test (CPT). Methodology: Two studies were used to test the hypotheses. First, a cross sectional study comparing the response to two minutes of isometric handgrip (IHG) and two minutes of postexercise circulatory occlusion (PECO) between pregnant women in TM3 and non-pregnant women. In this study we measured heart rate, blood pressure, MSNA, and blood samples to measure catecholamine (norepinephrine and epinephrine) concentrations. Second, we conducted a randomized controlled trail (prenatal exercise and cardiovascular health; PEACH study). We enrolled women into exercise (3-4x/week 25-40minutes, moderate intensity, aerobic exercise, for 14±1 weeks) or control (standard care) groups and we tested their neurovascular health before (16-20weeks) and after (34-36weeks) the intervention. The neurovascular assessment involved measuring resting heart rate, blood pressure, MSNA, PWV, and blood vessel diameter and flow in the carotid and superficial femoral arteries. We determined from these resting measures of sympathetic baroreflex gain (BRG) and neurovascular transduction (NVT), central and peripheral PWV, carotid artery distensibility measures, and femoral blood flow resistance (FVR) and conductance (FVC). In addition to resting measures, we also conducted a three-minute-long CPT (hand in ice water) where we measured the heart rate, blood pressure, MSNA, and the reactivity of the superficial femoral artery (FVR and FVC). Results: Compared to non-pregnant women, here we show that MSNA reactivity during IHG and PECO is not different in TM3 of normotensive pregnancy. Next, we showed that an aerobic exercise intervention resulted in attenuation of the rise in MSNA (and blunting of NVT) in pregnancy. This occurred without differences in resting blood pressure, carotid artery iv distensibility, FVR, FVC, PWV, concentrations of sex hormones, metabolic markers, or angiogenic factors. Aerobic exercise intervention also did not alter the blood pressure, MSNA or superficial femoral artery response to CPT during pregnancy. Discussion: Pregnancy is associated with changes in blood pressure regulation independent of exercise. However, aerobic exercise between TM2 and TM3 can alter the trajectory of the rise in basal sympathetic activity within a group of normotensive pregnant women. More research is needed to elucidate if greater positive changes, including those of the vasculature (e.g. carotid distensibility and PWV) can be impacted with a longer duration intervention (i.e. starting before or earlier in pregnancy) or in women who are at higher risk for the development of gestational hypertension. v Preface This thesis is original work by Rachel Skow. There are two projects, of which this thesis is a part, having received research ethics approval from the University of Alberta Research Ethics Board: 1) project name: "The effects of pregnancy on blood pressure control during handgrip exercise"