Canonical Question
i Pregnancy Physiology
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
Sakurai 2016
Exam appearances
| Exam | Exact wording | Relationship | 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). | historical_member | 50.00% |