Normal Respiratory Parameters in Pregnancy
Respiratory rate remains unchanged throughout pregnancy and should stay below 20 breaths per minute; any respiratory rate exceeding 20 breaths per minute warrants investigation for pathology. 1
Respiratory Rate
- Normal respiratory rate is unaltered during pregnancy and remains at the same level as non-pregnant women 1
- A respiratory rate greater than 20 breaths per minute should prompt evaluation for a pathological cause rather than being attributed to normal pregnancy 1
- This unchanged respiratory rate persists despite significant increases in minute ventilation, which is achieved through increased tidal volume rather than increased breathing frequency 1, 2
Oxygen Saturation
- Oxygen saturation remains unchanged throughout all trimesters of pregnancy 1
- Normal oxygen saturation should be maintained at the same levels as non-pregnant women (typically >96%) 1
- Pregnant women are at increased risk for rapid desaturation during apnea or hypoventilation due to decreased functional residual capacity (reduced by 10-25%) and increased oxygen consumption (20-33% above baseline by third trimester) 1, 3, 4
- Arterial oxygen saturation monitoring by pulse oximetry is recommended during labor and delivery, particularly in women with respiratory disease 1
Breath Sounds
- Normal breath sounds should remain clear and unchanged during pregnancy 1
- Any abnormal lung sounds (crackles, wheezes, decreased breath sounds) require investigation and should not be attributed to normal pregnancy 3
- The presence of abnormal breath sounds alongside dyspnea, particularly with tachypnea at rest, persistent tachycardia, or oxygen desaturation, mandates urgent workup to exclude conditions like pulmonary embolism 3
Important Physiological Context
- While respiratory rate and oxygen saturation remain normal, minute ventilation increases by 20-40% above baseline by term, driven by progesterone-mediated increased respiratory center sensitivity 3, 5
- This increased minute ventilation is accomplished through increased tidal volume (rising from approximately 563 ml to 715 ml at term), not increased respiratory rate 2
- The result is a mild, fully compensated respiratory alkalosis that is normal during pregnancy 1
- Functional residual capacity decreases by 10-25% as the enlarging uterus elevates the diaphragm, creating vulnerability to rapid oxygen desaturation if ventilation is compromised 1, 3, 4
Clinical Red Flags
- Tachypnea at rest (>20 breaths/min), oxygen desaturation, persistent tachycardia, or abnormal lung sounds require immediate investigation 3
- Severe dyspnea at rest, chest pain, syncope, hemoptysis, or palpitations mandate urgent workup 3
- The combination of reduced functional residual capacity and increased oxygen consumption means pregnant patients can develop hypoxemia rapidly 1