Hormonal and mechanical factors during pregnancy cause major physiological and anatomical changes in the respiratory system.
Hormonal changes result in capillary engorgement and swelling of the mucosa of the nose, oropharynx, larynx and trachea. This may be exacerbated by pre-eclampsia or fluid overload. As a result, smaller tracheal tubes may be required for intubation and bleeding may be precipitated by the use of nasal tubes, nasogastric tubes or oral/nasal airways. The enlarging uterus displaces the diaphragm upwards, but the volume of the thoracic cage remains the same due to an increase in both the anterio-posterior and transverse chest diameters.
There is a progressive increase in oxygen consumption in pregnancy caused by the increased metabolic needs of the mother and fetus oxygen consumption increase by 20-30% at term. During Labour, it is increased by 60% as a result of the increased cardiac and respiratory workload. This is compensated for by the effect of progesterone stimulating respiration and enhancing the response of the respiratory centre to carbon dioxide (CO2). There is a 45-50% increase in minute ventilation caused by an increase in tidal volume increases from 500ml to 700ml. during labour, ventilation may be further accentuated in response to pain or anxiety. Ventilation in pregnancy is greater than the body's demand for oxygen and Co2 production, which cause Co2 to be washed out of the lungs creating a respiratory alkalosis with paCo2 falling from 5 to 4 kpa. This mild alkalaemia improves oxygen release to the fetus. Although the minute ventilation increases during pregnancy and labour, this increase is trivial in comparison to the increases which occur during exercise i.e a 10-fold increase. This considerable respiratory reserve explains why respiratory failure due to chronic respiratory disease is uncommon in pregnancy.
The enlarging uterus causes a 4cm elevation of the diaphragm, but the total lung capacity only decreases by 5% because the lower ribs flare out, increasing the transverse and antero-posterior diameters of the chest. Despite the upward displacement of the diaphragm, it moves with greater excursion in the pregnant woman. The functional residul capacity (FRC), hich is the volume of air remaining in the lung at the end of a normal breath, decreases by 20% as pregnancy progresses due to the increased intra-abdominal pressure and upward displacement of the diaphragm. Closing capacity (lung volume at which airway closure occurs) can encroach on the FRC, but remains less than the FRC while the patient is in the upright position. During the second half of pregnancy, closing capacity is greater than FRC in 50% of pregnant woman when she lies down. This causes airway closure in the dependant part of the lung, an increases in ventilation/ perfusion (V/Q) mismatch and predisposes to hypoxia.
Clinical Implications of Respiratory Changes
The combination of increased oxygen consumption and decreased FRC means that a pregnant mother who is rendered apnoeic at term, will desiderate much more quickly than her non-pregnant counterpart. This fall in blood oxygen saturation will be worse in women with twin or triplet pregnancies and the morbidly obese patient. Good preoxygenation is therefore essential in all pregnant women before general anesthesia. Airway management is more challenging in pregnancy. Laryngoscopy may be difficult due to weight gain and breast engorgement.
Instrumentation of the airway can cause profuse bleeding from the nose or oropharynx, especially when there is fluid overload or oedema associated with pre-eclampsia, making intubation even more difficult.
An increasing number of women with respiratory disease are becoming pregnant: the incidence of asthma is increasing and pulmonary tuberculosis remains a problem in developing countries. Respiratory disease may be exacerbated by the increased respiratory demands of pregnancy and labour, although asthma often improves during pregnancy. Women with severe respiratory disease should be seen early in pregnancy and have pulmonary function tests. Women with mild disease can be treated as normal. In patients with moderate to severe disease, epidural anesthesia can help to reduce the stress of labour. If available, pulse oximetry monitoring is useful for labour in women with respiratory disease. For caesarean section, spinal anesthesia avoids the depressant effects of general anesthetic drugs but care must be taken to avoid a high spinal block.