Question 18

Explain the respiratory changes that occur at term in pregnancy.

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College Answer

Good answers addressed the following in an ordered way: anatomic changes (including to airway, thoracic dimensions, dead space, airways resistance); what changes there are to respiratory volumes, capacities, and compliance; what happens to oxygen consumption and oxygen tension; and the acid-base changes that occur. Good answers didn't just list these changes, but also provided an explanation for them. Few candidates mentioned the changes in oxygen tension and oxygen consumption, and why these occurred.

Discussion

Respiratory Changes that Occur in Pregnancy
Pregnancy-related changes	Effect of these changes
Airway function and structure
Mucosal oedema in the upper airway	Greater resistance to flow Bag-valme-mask ventilation becomes more difficult Greater risk of airway obstruction with sedation
Structural properties of the chest wall and lung volumes
Anatomical changes	The diaphragm is pushed up by 4cm Diaphragmatic excusion increases by about 2cm Rib cage expands: subcostal angle of the ribs at the xiphoidal level increases from 68.5° at the beginning of pregnancy to 103.5° at term Anatomical dead space increases by about 445% due to increased airway diameter late in pregnancy
Lung volumes	Tidal volume increases by ~ 30-50% Respiratory rate increases to 15-17 Minute volume increases by 20-50%. TLC is reduced by about 5% IC is increased by about 10% FRC is decreased by about 20%
Compliance	Chest wall compliance decreases due to increased fat and abdominal content Lung compliance remains the same
Resistance	Resistance to air flow in the lower airways increases in early pregnancy, and decreases in late pregnancy FEV1/FVC is stable over the course of pregnancy
Gas exchange and gas transport
Blood gas tension	PaCO₂ decreases PaO₂ increases
Oxygen-carrying capacity	Maternal erythrocytes produce higher levels of 2,3-DPG p50 remains normal (by the combined effects of 2,3-DPG increase and alkalosis)
Control of ventilation
Central respiratory control	Progesterone-associated chronic respiratory alkalosis develops (due to increased sensitivity to CO₂)
Respiratory workload and demand
Demands on the respiratory system	Increased body mass (increased weight plus one extra organism on board) = increased total body oxygen demand (by about 21%) and increased ventilatory requirements for the clearance of the excess CO₂ During labour, the oxygen consumption increases by about 60%.

References

Yeomans, Edward R., and Larry C. Gilstrap III. "Physiologic changes in pregnancy and their impact on critical care." Critical care medicine 33.10 (2005): S256-S258.

Hegewald, Matthew J., and Robert O. Crapo. "Respiratory physiology in pregnancy." Clinics in chest medicine 32.1 (2011): 1-13.

LoMauro, Antonella, and Andrea Aliverti. "Respiratory physiology of pregnancy: physiology masterclass." Breathe 11.4 (2015): 297-301.

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