Cardiac structure and function during human pregnancy

Abstract

© 2006 Dr. Dominica ZentnerThis thesis explored, by non-invasive assessment, the cardiovascular function of volunteer participants during the menstrual cycle and pregnancy. In doing so it addresses the potential influences of hormonal variation on the heart and blood vessels. In the case of the post-ovulatory luteal phase of the menstrual cycle and early pregnancy, the predominant hormonal change is increased progesterone. This hormonal similarity raises questions about the degree to which cardiovascular changes might also qualitatively resemble one another in these 2 circumstances, possibly providing a physiological "bridge" between pre- and early pregnancy. This thesis also examines the changes in cardiovascular structure and function between early and late pregnancy, a period of growth for the fetus and a time during which the mother shows features of insulin resistance that might be influenced by pre-pregnancy maternal characteristics such as body weight. This provides an opportunity to assess the relative strength of association between these factors (fetal and maternal) and cardiovascular adaptation. Cardiovascular characteristics were measured longitudinally during the menstrual cycle both before (follicular phase) and after (luteal phase) ovulation. Careful timing of measurement ensured that estradiol levels were stable between the follicular and luteal phase and that progesterone levels increased markedly after ovulation. Nine healthy women had measurements made in two separate cycles that were averaged to minimise individual variation. Although weight did not alter between the follicular and luteal phases, hemoglobin and hematocrit decreased significantly, suggesting an increase in intravascular volume. Supine systolic and diastolic blood pressures were lower during the luteal phase. There was a borderline increase in cardiac output over this period, suggesting a reduction in total peripheral resistance to account for the fall in supine blood pressure. Evidence of homeostatic responses to this vasodilatation included the suggestion of increased sympathetic reactivity (higher blood pressure on standing) and reduced parasympathetic responses (failure to reduce blood pressure with carotid pressure), increased renin and reduced atrial natriuretic peptide. The studies in pregnancy involved 100 healthy women who were assessed in early pregnancy (average of 16 weeks gestation). Thirty two of these women had repeat cardiovascular assessments in late pregnancy (average of 37 weeks gestation). Reflecting an exaggeration of the changes observed between the follicular and luteal phases of the menstrual cycle, in early pregnancy hemoglobin, hematocrit and blood pressure were lower than observed in the women (matched for age, body weight) in the menstrual study. It was observed that the cardiovascular system in early pregnancy was characterised by tachycardia that required a period of rest to reach basal levels. These levels were lower than previously reported. Also in contrast to previous studies, although median cardiac output in early pregnancy was slightly greater than in the menstrual study, this was not statistically significant. The most significant original finding was that at term, a decrease was observed in both systolic and diastolic left ventricular function as measured using modern tissue Doppler methods. These unexpected findings, in otherwise well women, suggest that the stress of pregnancy on the maternal heart is greater than previously appreciated. The diminished functional capacity of the heart was more marked in overweight women. These findings might also provide a clue to the (rare) complication of peripartum cardiomyopathy, a condition known to occur more frequently in overweight/obese women. Findings from this thesis provide new insights into the physiological changes of the cardiovascular system during the menstrual cycle and pregnancy. The studies were undertaken after rest, but in pregnancy especially it would be of interest to examine cardiac performance during activity. Further studies in complicated pregnancies might also clarify whether cardiovascular characteristics are qualitatively or quantitatively different to normal pregnancies. Nevertheless, these studies, which provide normative data on modern cardiovascular phenotypes, might find clinical utility particularly for women during pregnancy complicated by pre-existing cardiac disease or by medical maternal complications of pregnancy. Finally, in a more general sense, the demonstration that maternal weight impacts on cardiovascular function adds strength to the current concerns about excess weight in our community