Pregnancy and the lungs

Respiratory problems are common in pregnancy and it is worth noting that in the most recent Confidential Enquiry into Maternal Deaths (1994–96), 53.7% of direct deaths were as a result of respiratory problems excluding seven other deaths from indirect causes (see table 1). Some women will have pre-existing conditions such as asthma, tuberculosis, cystic fibrosis, and less commonly restrictive lung diseases or lung transplant. Others may have an acute illness like pneumonia, pneumothorax, or more serious conditions such as pulmonary embolism or adult respiratory distress syndrome (ARDS) complicating pregnancy. Although a team approach is essential, a well informed obstetrician can make a major contribution to the wellbeing and safety of both mother and fetus. This article intends to provide an overall review of various respiratory conditions which obstetricians may encounter and help in their management. Apart from a decrease in functional residual capacity (FRC) secondary to a decrease in the expiratory reserve volume, pregnancy does not effect the lung volumes.3 5 This fall in FRC begins from the fifth month of pregnancy and by term the FRC is reduced by 10%–20%. Large airway function is not usually impaired by pregnancy, and forced expiratory volumes and their ratios are unaffected.5 The total pulmonary resistance may be decreased due to relaxation of the smooth muscle in the tracheobronchial tree under hormonal influence. Diffusing capacity of the lungs for carbon monoxide remains normal or decreases during the second half of pregnancy.