Table 1:
Respiratory changes during normal pregnancy show the respiratory changes during normal pregnancy.
| Variable | Normal values | |
|---|---|---|
| Pregnant (updated) | Non-pregnant women | |
| Lung volumes and ventilation | ||
| Respiratory rate, mL/min | No change/minimal increase | 14–20 |
| Tidal volume, mL | Increased (30–35% at term) | 500–600 |
| Minute ventilation, mL | Increased (40–50% at term) | 5,000–8,000 |
| Inspiratory capacity, mL | Increased (by 10–15% at term) | 1,900–2,400 |
| Vital capacity, mL | Unchanged/minimal decrease | 2,800–3,200 |
| Inspiratory reserve volume, mL | Increased (by 5–8% at term) | 1,800–2,200 |
| Expiratory reserve volume, mL | Decreased (10–30% at term) | 700–900 |
| Residual volume, mL | Decreased (15–25% at term) | 1,000–1,200 |
| Functional residual capacity, mL | Decreased (15–25% at term) | 1,600–2,000 |
| Total lung capacity, mL | No change/minimal (4–6%) decrease | 4,000–4,400 |
| Spirometry | ||
| Forced expiratory volume in 1 s (FEV1) | No change | 2,500–3,250 |
| Forced vital capacity (FVC) | No change | 3,250–3,750 |
| (FEV1)/FVC | No change | >70% |
| Blood gases | ||
| pH | Slight respiratory alkalosis (7.4–7.47) | 7.35–7.45 |
| Bicarbonate, mmol/L | Decreased by 10–15% (18–22) | 22–26 |
| Arterial pO2, mmHg | Increased by 10–12% (97–98) [12] | 90–100 |
| Arterial pCO2, mmHg | Decreased by 15–30% (28–32) | 35–40 |
| Base excess | No change | +2 to −2 |
Recent literature has demonstrated that although the respiratory rate does not change significantly during pregnancy compared to the nonpregnant state (an average of 15 beats/min), the pO2 decreases from a median of 98% at 12 weeks of gestation, to reach a minimum of 97% at 40 weeks of gestation. This small decrease in pO2 differ from previously published reports that suggest moderate reduction in pO2 in the second and third trimesters of pregnancy