pulmonary function test changes in pregnancy
- related: PFT and lung functions
- tags: #literature #boards
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PFT changes:
- decrease ERV, RV, FRC, TLC, IC
- unchanged: VC
- increased: TV, minute ventilation CO:
- increased: stroke volume, heart rate, CO
- decrease: PVR, SVR
- decrease: systolic and diastolic pressure acid base
- mild respiratory alkalosis from increased ventilation
- bicarb decreased in compensation
The major change in pulmonary function testing in pregnancy is a progressive decrease in the expiratory reserve volume by 8% to 40% and a decrease in residual volume by 7% to 22%, both changes caused by the enlarging uterus and diaphragmatic elevation. Because of these changes, functional residual capacity is, in turn, decreased by 10% to 25% by the third trimester of pregnancy. Total lung capacity may decrease slightly, inspiratory capacity increases, and vital capacity does not change significantly during pregnancy. Tidal volume increases significantly during pregnancy by 150 mL to a final value of 450 to 600 mL, or a 30% to 50% increase in the average patient. Because of the increase in tidal volume, there is a significant increase in resting minute ventilation from 6 L in the nonpregnant state to 9 L at full term (or 20%-50% above baseline). Gas exchange in pregnancy is characterized by a mild compensated respiratory alkalosis secondary to an increase in minute ventilation out of proportion to maternal needs. Thus, normal PaCO2 values are lower (28 to 32 mm Hg) and serum bicarbonate values are compensatorily decreased to 18 to 21 mEq/L. Normal pH is slightly alkalemic at 7.40 to 7.45. Total blood volume and plasma volume increase up to 35% to 50% of normal, peaking in the mid-third trimester with a lesser increase in RBC volume (approximately 20%-40%). There is a significant increase in cardiac output (by 30%-30%) due to increases in both heart rate and stroke volume, beginning in the first trimester and peaking about the 25th-32nd week. There is a decrease in both systemic vascular resistance and pulmonary vascular resistance.1
Gas exchange in pregnancy is characterized by a mild compensated respiratory alkalosis secondary to an increase in minute ventilation. Thus, normal Paco2 values are lower, 28 to 32 mm Hg, and serum bicarbonate is compensatorily decreased to 18 to 21 mEq/L (18 to 21 mmol/L) (choice C is correct). Normal pH is mildly alkalemic at 7.40 to 7.45 (choice D is incorrect). Pao2 is slightly elevated, 100 to 105 mm Hg, with a slight decrease at term to 100 mm Hg. There is normally a 5 to 10 mm Hg elevation in alveolar-arterial gradient above baseline, especially in the supine position. Oxygen consumption increases by 20% to 30% during pregnancy, with a concomitant increase in carbon dioxide production by 30% to 35%. These changes are due to increased maternal and fetal metabolic requirements and increases in work of breathing and cardiac output.
The major change in pulmonary function testing in pregnancy is a progressive decrease in the expiratory reserve volume by 8% to 40% and a decrease in residual volume by 7% to 22%, both changes due to the enlarging uterus and diaphragmatic elevation. Because of these changes, functional residual capacity is, in turn, decreased by 10% to 25% by the third trimester of pregnancy. Total lung capacity may decrease slightly, inspiratory capacity increases, and vital capacity does not change significantly during pregnancy (choice B is incorrect). The decrease in residual volume with a relatively maintained total lung capacity results in a low ratio of residual volume to total lung capacity. Closure of the small airways during normal tidal breathing due to the reduction in functional residual capacity can result in changes in ventilation/perfusion matching and gas exchange. Tidal volume increases significantly during pregnancy by 150 mL to a final value of 450 to 600 mL, or a 30% to 50% increase in the average patient. This is most likely due to a direct progesterone-mediated increase in central respiratory drive and enhancement of the hypercapnic ventilatory drive. There is little or no change in respiratory rate during pregnancy, and tachypnea is an unusual finding. Because of the increase in tidal volume, there is a significant increase in resting minute ventilation from 6 L in the nonpregnant state to 9 L at full term (or 20% to 50% above baseline). There are no significant changes in spirometry, including FEV1, FVC, and FEV1/FVC ratio, or airway resistance, during pregnancy. Although lung compliance does not change significantly, there is a reduction in total respiratory compliance owing to a reduction in chest wall compliance because the diaphragm is elevated due to uterine enlargement by the third trimester. Diffusing capacity of the lung for carbon monoxide may increase slightly in the first trimester followed by a slight decrease later in pregnancy, likely due to alterations in pulmonary vascular volume.
The cardiovascular system probably undergoes the most significant changes during pregnancy, including an increase in cardiac output, beginning in the first trimester and peaking at about the 25th to 32nd week at 30% to 50% above normal. This is due to both a change in heart rate as well as stroke volume, and a decrease in systemic vascular resistance, partially due to shunting of blood to the low-resistance placental bed and perhaps due to increased levels of vasodilator mediators. Pulmonary vascular resistance also decreases. Both systolic and particularly diastolic pressures are reduced. Postural hypotension may be apparent, particularly in the third trimester when the uterus can compress the inferior vena cava, impeding venous return to the heart.