Respiratory

• The respiratory system undergoes significant changes to meet the increased oxygen requirements of pregnancy and to facilitate gas exchange between the maternal and fetal blood
• 
• The main respiratory changes include:
  • Increased oxygen consumption: The amount of oxygen used by the body per minute increases by 15% to 20% during pregnancy. This is due to increased metabolic rate (the rate at which chemical reactions occur in the body) and increased fetal oxygen demand. The oxygen consumption peaks at 32 weeks of gestation and then remains stable until term. The normal range of oxygen consumption in pregnancy is 250 to 300 mL/min.
  • Increased tidal volume: The amount of air breathed in or out per breath increases by 30% to 40% during pregnancy. This is due to increased progesterone levels that stimulate the respiratory center in the brain and increase the sensitivity to carbon dioxide. The tidal volume peaks at 28 weeks of gestation and then remains stable until term. The normal range of tidal volume in pregnancy is 500 to 700 mL.
• • Decreased functional residual capacity: The amount of air remaining in the lungs after a normal expiration decreases by 20% during pregnancy. This is due to elevation of the diaphragm (the muscle that separates the chest and abdomen) by the gravid uterus and increased chest wall compliance (the ease of expansion of the chest). The functional residual capacity decreases progressively throughout pregnancy and reaches its lowest point at term. The normal range of functional residual capacity in pregnancy is 1.5 to 2 L.
  • Increased minute ventilation: The amount of air breathed in or out per minute increases by 40% to 50% during pregnancy. This is due to increased tidal volume and increased respiratory rate (the number of breaths per minute). The minute ventilation peaks at 28 weeks of gestation and then remains stable until term. The normal range of minute ventilation in pregnancy is 7 to 10 L/min.
  • Decreased partial pressure of carbon dioxide: The pressure exerted by carbon dioxide in the arterial blood decreases by 15% during pregnancy. This is due to increased minute ventilation that removes more carbon dioxide from the body. The partial pressure of carbon dioxide reaches its lowest point at mid-pregnancy and then remains stable until term. The normal range of partial pressure of carbon dioxide in pregnancy is 27 to 32 mmHg.
  • Increased partial pressure of oxygen: The pressure exerted by oxygen in the arterial blood increases by 5% during pregnancy. This is due to increased minute ventilation that brings more oxygen into the body. The partial pressure of oxygen reaches its highest point at mid-pregnancy and then remains stable until term. The normal range of partial pressure of oxygen in pregnancy is 95 to 105 mmHg.
  • The respiratory changes result in increased alveolar ventilation (the amount of air reaching the alveoli or air sacs in the lungs) and decreased physiological dead space (the amount of air that does not participate in gas exchange) that enhance gas exchange between the maternal and fetal blood
  • The respiratory changes may cause symptoms such as dyspnea (difficulty breathing), nasal congestion, epistaxis (nosebleed), or voice changes that are normal or abnormal depending on the severity, duration, and frequency