The effects of exercise during pregnancy on haemodynamics

During pregnancy the maternal body undergoes significant physiological changes, with some of the most profound changes occurring within the cardiovascular system. Many of the cardiovascular changes occur within the first month or two of pregnancy including an increase in blood volume, cardiac output, heart rate and stroke volume, which are all associated with a decrease in vascular resistance and systemic vascular tone. Maladaptive changes to these maternal haemodynamic processes can occur during gestation, increasing the risk of gestational hypertensive conditions. Regular exercise performed during pregnancy has been shown to reduce the risk of developing perinatal gestational hypertensive conditions. Further evidence on the exact parameters of exercise needed to explain these beneficial responses is required, within both uncomplicated and at-risk pregnancies.

Chapter 1 provides a general review of the literature on exercise in pregnancy, and introduces the thesis aims. The topics covered in the general literature review include: gestational hypertensive conditions of pregnancy and the short to long term maternal and foetal consequences; exercise guidelines in pregnancy; arterial function in pregnancy and the effects of exercise on arterial function in non-pregnant populations. The detailed methods for Studies 2, 3, 4 and 5 are outlined in Chapter 2. Further methods applicable to each of the specific studies are reported in the relevant chapters.

Chapter 3 comprises a systematic review and meta-analysis investigating the effects of aerobic and resistance exercise on blood pressure in uncomplicated and at-risk pregnancies. The findings showed that compared to usual care, aerobic and/or resistance exercise performed throughout uncomplicated pregnancy had no influence on blood pressure. In women who are at risk of, or diagnosed, with gestational hypertensive conditions during pregnancy, moderate to vigorous exercise during pregnancy improves blood pressure outcomes. Higher risk pregnancies may reduce their risk of future cardiovascular complications through regular exercise training during pregnancy.

Chapter 4 includes a case study which aimed to observe the acute effects of a submaximal graded exercise test on arterial stiffness and blood pressure longitudinally throughout pregnancy. A healthy pregnant woman was recruited from five weeks of gestation to perform weekly submaximal graded exercise tests on the treadmill until 35 weeks of pregnancy. An aged matched non-pregnant control was recruited to perform weekly – fortnightly sessions over 30 weeks. The primary outcome measures included: arterial stiffness measured by pulse wave velocity (PWV) and pulse wave analysis (PWA), blood pressure (BP), and mean arterial pressure (MAP). The findings showed a gradual decrease in resting arterial stiffness throughout pregnancy, with the greatest response in PWV following exercise seen in the first trimester. Resting systolic (SBP), diastolic (DBP) and MAP all followed similar trends decreasing from trimester 1 (T1) to trimester 2 (T2), before increasing again in trimester 3 (T3). The findings lead us to speculate that the ability of the maternal cardiovascular system to adapt to submaximal exercise may decrease as gestation progresses, which led to the design of Chapter 5.

A cross-sectional analysis of 34 pregnant participants is included in Chapter 5, in which the immediate effects of vigorous intensity exercise at various gestational weeks on vascular function is assessed. Baseline submaximal testing data from the participants in Chapters 6 and 7 were analysed. Based on the findings from this study, it is suggested that the maternal body is able to adapt to stressful stimuli, with the cardiovascular system recovering quickly (within 10 minutes) following vigorous intensity exercise. Further research in pregnancy is warranted on the acute effects of vigorous exercise on arterial stiffness given the varied responses seen in this study.

Chapter 6 was designed to assess the feasibility of vigorous intensity interval training (VIIT), along with resistance training (RT) and reformer Pilates (PIL) in pregnancy. Most of the available research to date on the acute effects of exercise in pregnancy has focused on one type or intensity of exercise (e.g. walking/cycling at moderate intensity). Given the varied benefits derived from a balanced exercise program incorporating aerobic and resistance exercise, it is important to understand the acute physiological effects of these different modes. Pilates was identified as a low to moderate intensity mode of exercise which has a paucity of evidence in pregnancy despite its increase in popularity over the past decade. The three modes of exercise in this study proved feasible in the second and third trimesters of uncomplicated pregnancies. All participants were able to achieve the prescribed intensities for VIIT, RT and PIL, with VIIT perceived as more difficult later in gestation despite similar cardiovascular responses between modes. This could be due to an increase in fatigue and discomfort associated with progressing gestation rather than an increased cardiovascular response.

A longitudinal crossover trial was employed in Chapter 7 which aimed to observe the effects of VIIT, RT and PIL on arterial stiffness and BP completed weekly throughout pregnancy. We specifically wanted to observe whether the acute response to these three modes of exercise changed as pregnancy progressed. Resting measures of arterial stiffness, BP and HR followed trends previously outlined in longitudinal studies of pregnancy. PWV was significantly higher immediately and 10 minutes following VIIT and RT, but only immediately post Pilates, indicating a greater response with more intense exercise. Similarly, significant increases in MAP and BP were seen following VIIT and RT but not Pilates. Despite these differences in arterial pressure, along with a greater increase in MHR with more intense exercise, foetal heart rate (FHR) responses remained similar and within normal clinical limits regardless of type of exercise or exercise intensity. The findings in Chapter 7 support VIIT as an apparently safe mode of exercise throughout gestation, with 229 VIIT sessions in 20 participants closely monitored across the course of the study.

A summary of the outcomes from all studies is provided in Chapter 8. This chapter discusses the practical implications of the thesis findings and makes recommendations for future research in this area. The thesis findings indicate that acute cardiovascular responses to exercise change as gestation increases. Further longitudinal research is imperative throughout pregnancy to facilitate the development of trimester-specific guidelines for exercise. While vigorous exercise seems to be safe and achievable during gestation, incorporating shorter bouts of higher intensities may enhance enjoyment and therefore adherence in pregnancy. Nonetheless, it is worth noting that it is hypothesised that engaging in higher intensity exercise during the later stages of pregnancy may not yield significantly greater benefits compared to moderate intensity workouts, due to alterations in resting cardiovascular measures.