Of physiological lung changes, unctional residual capaciy (PRe) decreases by approximately 20 to 30 percent or 400 to 700 mL during pregnancy (Fig. 4-12). his capacity is composed of expiratory reserve volume-which drops 15 to 20 percent or 200 to 300 mL-and residual volume-which decreases 20 to 25 percent or 200 to 400 mL. FRe and residual volume decline progressively across pregnancy due to diaphragm elevation. Signiicant reductions are observed by the sixth month. Inspiratory capaciy, the maximum volume that can be inhaled from FRe, rises by 5 to 10 percent or 200 to 350 mL during pregnancy. Total lung capaci-the combination of FRe and inspiratory capacity-is unchanged or decreases by less than 5 percent at term Decreased plasma osmolality also results in less respiratory depression (Moen, 2014). his provides an additional mechanism for the increased minute ventilation seen in pregnancy, and one that is not dependent on progesterone.
Regarding pulmonary function, peak expiratory low rates rise progressively as gestation advances (Grindheim, 2012). Lung compliance is unafected by pregnancy. Airway conductance is increased and total pulmonary resistance reduced, possibly as a result of progesterone. The maximum breathing capaciy andorced or timed vital capaciy are not altered appreciably. It is unclear whether the critical closing volume-the lung volume at which airways in the dependent parts of the lung begin to close during expiration-is higher in pregnancy (Hegewald, 2011). Pulmonary function with a singleton pregnancy does not signiicantly difer from that with twins (MAulife, 2002; Siddiqui, 2014). Importantly, the greater oxygen requirements and perhaps the increased critical closing volume imposed by pregnancy make respiratory diseases more serious.
Demir and colleagues (2015) studied nasal physiology in 85 pregnant women. Although the minimal cross-sectional area decreased between the irst and third trimesters, subjective reports ,/ 103.5° ""
II , " \\ . (37 weeks) : (Hegewald, 2011).
The respiratory rate is essentially unchanged, but tidal volume and resting minute ven as pregnancy advances. Kolar zyk and coworkers (2005) .. -� is-7 em .--_ - reported significantly greater mean tidal volumes-0.66 to 0.8 LI min-and resting minute ventilations-10.7 to 14.1 Llmin-compared with those of nonpregnant women. The elevated minute ventilation is caused by several factors. These include enhanced respira ,;, . ,----------.---.A..,, .I \IIII,\\ ..<: ::: --------------- toy drive primarily due to the stimulatory action of proges terone, low expiratory reserve
Chest wall measurements in nonpregnant (left) and pregnant women (right). The subcostal angle increases, as does the anteroposterior and transverse diameters of the chest wall and chest wall circumference. These changes compensate for the 4-cm elevation of the diaphragm volume, and compensated respiso that total lung capacity is not significantly reduced. (Redrawn from Hegewald MJ, Crapo RO: ratory alkalosis (Heenan, 2003). Respiratory physiology in pregnancy. Clin Chest Med 32(1 ):1,2011.) 5 4 TLC 2 FVC FAC RV Not pregnant Pregnant (7-9 mos.) 4 FVC 3 2 RV