HONG KONG RESPIRATORY MEDICINE

A website for HK Thoracic Society, ACCP HK & Macau Chapter

2003 Dec - What are the physiological changes of respiratory system during pregnancy?

Thomas Li, Intensive Care Unit, Prince of Wales Hospital

Anatomical changes
Elevated plasma oestrogen level during pregnancy causes airway mucosal oedema. Large airways are dilated. There is increase in subcostal angle, anterior-posterior diameter and the circumference of the chest. The diaphragm is pushed upward by the gravid uterus during the third trimester.

Physiological changes
Functional residual capacity decreases by 30 % at the third trimester because of the upward displacement of diaphragm by the gravid uterus. This is particularly severe in supine position. Tidal volume and minute ventilation increase by nearly 40 % at third trimester. There is little change in respiratory rate. Vital capacity and forced expiratory volume in 1 second are not changed significantly during pregnancy. Arterial pO2 and pH increases slightly. Arterial pCO2 decreases slightly. The hyperventilation is believed to be due to the effect of increased plasma progesterone level on the central chemoreceptors in medulla. The respiratory responses to hypoxaemia and hypercarbia are augmented. The oxygen consumption increases progressively during pregnancy. It is about 30 % above non-pregnant level at full term.

Clinical significances of changes
Because of the reduction of funtional residual capacity (the immediately available oxygen store), pregnant women are very susceptible to hypoxaemia during anaesthesia.

Dyspnoea is a very common "symptom" of pregnancy even before the diaphragm is pushed upward by the uterus. This is believed to be due to the sensitization of the central chemoreceptors by progesterone.

Reference
Lumb AB. Nunn 's applied respiratory physiology, 5th edition, Butterworth Heinemann, 13:319-320.