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The prevalence of asthma in women of childbearing age is increasing and asthma is the most common pre-existing medical disorder encountered in pregnancy. Management during pregnancy should include reassurance regarding the safety of medications used to control asthma. The biggest danger to the mother and her fetus comes from poorly controlled or under treated disease.
Changes in respiratory function during pregnancy
Normal pregnancy is associated with a 20% increase in oxygen consumption and a 15% increase in the maternal metabolic rate. This extra demand is achieved via a 40–50% increase in resting minute ventilation, resulting mainly from a rise in tidal volume rather than respiratory rate. This hyperventilation causes the arterial oxygen tension (Pao 2) to increase and arterial carbon dioxide tension (Paco 2) to fall, with a compensatory fall in serum bicarbonate to 18–22 mmol/l. A mild respiratory alkalosis is therefore normal in pregnancy (arterial pH 7.44).
Up to 75% of women experience a subjective feeling of breathlessness at some time during pregnancy, possibly due to an increased awareness of the physiological hyperventilation. This is most common in the third trimester and may lead to diagnostic confusion. In late pregnancy the diaphragmatic elevation caused by the enlarging uterus leads to a decrease in functional residual capacity, but diaphragm excursion is unaffected so vital capacity is unchanged. There is no change in peak expiratory flow rate (PEFR) or forced expiratory volume in one second (FEV1) in pregnancy. However, the fall in functional residual capacity (FRC) may exacerbate hypoxaemia because of premature airway closure when acute asthma complicates pregnancy.1
Effect of pregnancy on asthma
Literature addressing the effect of pregnancy on asthma is conflicting, with no consistent trend to improvement or worsening of disease severity. Discrepant results between studies relate to methodology (such as whether studies are retrospective or prospective) and whether the outcome measure is objective or relies …
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