This chapter is not relevant to any recent part of CICM Second Part Exam, probably because the whole topic of pregnancy-associated physiological changes has migrated to the Primary Exam section at some stage in 2007-2008. Before that exam existed, the college had asked about this in Question 18 from the first paper of 2006, but since then there have been no SAQs on it. As such, all of this sort of material has been transferred to the Part One section, leaving only a brief summary to assist with revision. 

Thus:

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-valve-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 excursion 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%.

All of this can probably be demonstrated in a big confusing diagram.

changes in respiratory volumes during pregnancy

 

References

Oh's Intensive Care manual:

Chapter 64   (pp. 684) General  obstetric  emergencies by Winnie  TP  Wan  and  Tony  Gin

Chapter 65   (pp. 692) Severe  pre-existing  disease  in  pregnancy by Jeremy  P  Campbell  and  Steve  M  Yentis

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.

Zwillich, C. W., et al. "Effects of progesterone on chemosensitivity in normal men." The Journal of laboratory and clinical medicine 92.2 (1978): 262-269.

Moore, LORNA GRINDLAY, et al. "Maternal hyperventilation helps preserve arterial oxygenation during high-altitude pregnancy." Journal of Applied Physiology 52.3 (1982): 690-694.

Levinson, Gershon, Sol M. Shnider, and John L. Steffenson. "Effects of maternal hyperventilation on uterine blood flow and fetal oxygenation and acid-base status." Anesthesiology 40.4 (1974): 340-347.

Huch, Renate. "Maternal hyperventilation and the fetus." (1986). J.Perinat.Med 14 (1986) 3

Fadel, Hossam E., et al. "Normal pregnancy: a model of sustained respiratory alkalosis." Journal of Perinatal Medicine-Official Journal of the WAPM 7.4 (1979): 195-201.

Jensen, Dennis, et al. "Physiological mechanisms of hyperventilation during human pregnancy." Respiratory physiology & neurobiology 161.1 (2008): 76-86.

AWE, ROBERT J., et al. "Arterial oxygenation and alveolar-arterial gradients in term pregnancy." Obstetrics & Gynecology 53.2 (1979): 182-186.

Milne, J. A. "The respiratory response to pregnancy." Postgraduate medical journal 55.643 (1979): 318-324.

Madsen, Hans, and Jøm Ditzel. "Red cell 2, 3-diphosphoglycerate and hemoglobin-oxygen affinity during normal pregnancy." Acta obstetricia et gynecologica Scandinavica 63.5 (1984): 399-402.