What changes occur in respiratory physiology during pregnancy?

Respiratory Physiology Changes During Pregnancy

Functional residual capacity decreases by approximately 20% during pregnancy, making option (b) the most accurate statement, though the magnitude stated (50%) is exaggerated.

Analysis of Each Statement

Statement A: Tidal Volume Remains the Same

This is incorrect. Tidal volume significantly increases during pregnancy:

• Tidal volume rises from approximately 563 ml in non-pregnant women to 632 ml in early pregnancy, peaking at 715 ml in late pregnancy 1
• This represents a substantial increase of approximately 25-30% above baseline 1

Statement B: Functional Residual Capacity Decreases by Approximately 50%

This statement is partially correct in direction but overstated in magnitude:

• Functional residual capacity does decrease during pregnancy, but by approximately 20%, not 50% 2
• The decrease is primarily due to the growing uterus causing upward pressure on the diaphragm 3, 4
• Despite this reduction, vital capacity and measures of forced expiration remain well preserved 2

Statement C: Inspiratory Capacity Rises by 5-10%

This is correct and represents an accurate physiological change:

• Inspiratory capacity increases during pregnancy as the diaphragm increases its contribution to both spontaneous and maximal breathing 4
• The diaphragmatic contribution to tidal volume and inspiratory capacity increases progressively through pregnancy 4

Statement D: Respiratory Rate Increases

This is incorrect. Respiratory rate remains essentially unchanged:

• Respiratory rate does not change significantly during pregnancy 1
• Breath frequency remains nearly at the same level throughout all trimesters 5
• The increased minute ventilation during pregnancy is achieved through increased tidal volume, not respiratory rate 1, 5

Key Physiological Changes to Remember

Minute ventilation increases substantially:

• Rises from 9.4 L/min in non-pregnant state to 10.5 L/min by 8-11 weeks, reaching 12.6 L/min in late pregnancy 1
• This increase is driven by increased tidal volume, not respiratory rate 1, 5

Alveolar ventilation increases markedly:

• Increases from 3.4 L/min in non-pregnant women to 6.2 L/min in early pregnancy, peaking at 6.7 L/min at term 1

Compensated respiratory alkalosis develops:

• Alveolar carbon dioxide tension falls from 4.6 kPa (34 mmHg) to 4.0 kPa (30 mmHg) in early pregnancy 1
• This represents a major physiologic adaptation 2

Clinical Pitfalls

The growing uterus causes mechanical changes but not true restriction:

• Despite upward pressure on the diaphragm, forced vital capacity remains normal (101% predicted) 4
• Reduced rib cage expansion occurs, but the diaphragm compensates by increasing its contribution 4
• Diaphragm thickness is maintained throughout pregnancy, suggesting a conditioning effect 4

Position matters significantly:

• The supine position should be avoided after 20 weeks as the gravid uterus can compress the inferior vena cava, decreasing venous return and cardiac output 3, 6
• Left lateral positioning optimizes cardiac output 7