I will be writing several articles in the next few weeks that pay particular attention to the pulmonary complications of pregnancy. There are major physiologic changes occurring in pregnancy that pertain specifically to the lungs and chest wall. These alterations are normal and are a result of the effects of the hormonal changes noted in pregnancy. In addition mechanical and circulatory changes occur that effect the respiratory system. A common example of these changes in pregnancy is an increase in minute ventilation. This is the amount of air that you breathe per minute. The increase occurs because of an increase in the volume of the average breath that is taken.
A hormone called relaxin alters the ligamentous attachment of the rib cage and changes the movement of the diaphragm to accommodate the enlarging uterus. These anatomic changes cause a reduced ability for the chest to expand. As a result of these anatomic changes and the increase in minute ventilation, about 75% of women report shortness of breath with activity by 30 weeks of gestation. This sensation is often referred to as the “physiologic dyspnea (shortness of breath) of pregnancy”. In order to differentiate the normal physiologic shortness of breath from abnormal shortness of breath due to disease, requires an understanding of the physiologic changes of the respiratory system during pregnancy. Surprisingly, many tests, such as spirometry, remain normal during pregnancy, but pregnancy alters oxygen levels and blood gas analysis. Look for future articles on pregnancy and pulmonary disease.