![]() ![]() As the amount of dead space remains the same, the overall result is of increased alveolar ventilation by 30-50%. This effect is prompted by progesterone as well as increased production of CO 2. In the first trimester, there is an increase in minute ventilation via increased tidal volume and a slight increase in respiratory rate. The FRC remains below pre-pregnancy values for 1-2 weeks after delivery. In the supine position, FRC can be less than closing capacity, resulting in closure of small airways. The decrease in FRC can be worsened by assuming the supine position, the effect of which can be somewhat ameliorated by increasing the head of the bed by 30 degrees. Functional residual capacity steadily decreases during the latter half of pregnancy to 80% of pre-pregnancy volume due to the cephalad movement of the diaphragm this decrease is made up of decreases to the RV and ERV. Tidal volume increases by 45% while inspiratory reserve volume decreases these changes result in increased inspiratory capacity by 15% while expiratory reserve volume decreases. Overall, the changes in pregnancy are reciprocal as there is a finite total lung capacity. Total lung capacity and residual volume are slightly decreased. Items that remain unchanged on PFTs include: the shape of flow-volume loops, FEV1, and FEV1:FVC. Progressive increase in ligamentous laxity peaks at 37 weeks due to the hormone relaxin, which increases the AP and transverse diameters of the lower rib cage while decreasing the height of the chest cavity due to elevation of the diaphragm.Īs the chest wall expands to a greater resting diameter due to the effect of relaxin, a greater percentage of the work of inspiration is imparted to diaphragmatic excursion.
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