Respiratory Physiology — Pregnancy Spirometry

Current · V5 (2025) → C8.iii Historical · V4 (2023) → F9.i, V1.i 1 exam appearance

2012A Q13

Exam question

Draw (include labels and normal values) a normal spirometry trace in a young adult (20% of marks). Describe the changes seen in the spirometry, and the respiratory system overall, in pregnancy at term (80% of marks).

CICMWrecks answer

Master answer

Normal Spirometry trace

Volumes	per kg	70kg Adult
Inspiratory reserve volume	45ml/kg	3 L
Tidal volume	7ml/kg	500 ml
Expiratory reserve volume	15ml/kg	1.1 L
Residual volume	15ml/kg	1.1 L
Total Lung Capacity	82ml/kg	5.7 L
Vital Capacity	67ml/kg	4.6 L
Functional Residual Capacity	30ml/kg	2.2 L

In Pregnancy

Anatomical

Diaphragm ascends ~4cm due to foetus
AP and lateral diameters of thorax increase 2~3cm due to relaxin effect on chest
wall
Thoracic circumference increases 5~7cm (AP and lateral diameters increased 2~3cm)
Airway dilatation → decreased resistance (35%) and increased deadspace (45%)
Increased cardiac output, intravascular volume, and decreased oncotic pressure
- Increased plasma volume → increased pulmonary blood volume → decreased compliance
- Venous engorgement → Upper airway oedema

Mechanical

Lung volume changes occur from 20 weeks
TLV decreases 5% from baseline
FRC decreased 20% due to both ERV and RV
Significant postural change in FRC of 70%
Progesterone → increased sensitivity to CO2 → Minute ventilation increases 50%
- TV increases (25%)
- RR increases
- Results in hypocarbia and compensated respiratory alkalosis
  - pCO2 ~ 26-32mmHg
  - HCO3 ~ 18-20mmol/l
- Further increase in MV during contractions
Inspiratory capacity increased by 10%
Closing capacity unchanged
Increased O2 consumption during labour (60%)

After birth

FRC and RV return to normal within 48 hours
TV returns to normal within 5 days

Respiratory physiology of pregnancy — CICMWrecks original diagram

Source: Modified from: Prowse CM, Gaensler EA. Respiratory and acid-base changes during pregnancy. Anesthesiology 1965; 26:381 · Source/permission verification pending.

Sakurai 2016

Answer by Sakurai

Past papers

Exam appearances

1 appearance

Exam	Exact exam wording	Candidate success
2012A Q13	Draw (include labels and normal values) a normal spirometry trace in a young adult (20% of marks). Describe the changes seen in the spirometry, and the respiratory system overall, in pregnancy at term (80% of marks).	50%