Normal physiological changes of pregnancy
• Elevated serum progesterone levels stimulate respiratory drive and lead to an increased tidal volume and raised minute ventilation, with only a modest increase in O2 consumption. The subsequent fall in maternal pCO2 facilitates foetal CO2 transfer across the placenta; any cause of maternal hypercapnia leads quickly to foetal respiratory acidosis. Respiratory rate is unaffected by pregnancy. Elevation of the diaphragm occurs due to the enlarging uterus, leading to a reduced functional residual capacity (FRC), although diaphragm function is normal and VC is unaffected. Peak flow and FEV1 are unaffected by pregnancy
• Increased cardiac output occurs due to an increase in heart rate (HR) (by about 15 beats/min) and stroke volume; peripheral resistance falls. BP is reduced in the first and second trimesters by 10–20mmHg but is normal at term. Peripheral pulses tend to be increased in volume. Dependent oedema is common. Third heart sound and ejection systolic murmurs are commonly heard. May hear venous hums in the neck
• Raised levels of coagulation factors and impaired fibrinolysis, combined with venous stasis, result in a 5-fold increased risk of venous thromboembolism (VTE)
• Upper airway oedema, particularly in the setting of pre-eclampsia, may predispose to upper airways obstruction during sleep, but rarely frank OSA. OSA tends to occur in obese women and may be associated with impaired foetal growth and pre-eclampsia. Snoring in pregnancy is a poor predictor of OSA.
Causes of breathlessness in pregnancy
Causes are listed in Box 10.1. In general, breathlessness may be due to:
• Normal physiological changes of pregnancy. Up to 70% of pregnant women experience a degree of breathlessness, perhaps as a result of the increase in ventilation. Tachypnoea is a useful sign, as it is abnormal in pregnancy and suggests an underlying disease process
• New disease process. PE is the commonest and is a major cause of maternal death. Other rare, but serious, causes include amniotic fluid embolism and ARDS
• Exacerbation of chronic respiratory or cardiac disease. Asthma is the commonest. Unsuspected underlying disease may present for the first time in pregnancy, e.g. structural heart disease such as mitral stenosis, LAM. PHT is associated with a particularly poor prognosis during pregnancy. Patients with ILD and VC <1L should also consider avoiding pregnancy. In patients with CF, the presence of PHT or FEV1 <60% predicted are associated with a poor outcome.
Liaise with your obstetrics team, as well as with paediatricians and anaesthetists, if delivery is approaching. Management of specific conditions is discussed in the individual disease chapters in Part 2.
The following investigations may be affected by the pregnancy itself:
• ABGs Normal maternal pO2 >13.3kPa and pCO2 3.7–4.3kPa.A compensatory fall in serum bicarbonate (to 18–22mmol/L) occurs, resulting in an average pH of 7.44. During the third trimester, perform ABGs in an upright position, as pO2 may be 2.0kPa lower when supine. A–a gradient is unaffected during pregnancy, except when supine near term
• Blood tests In normal pregnancy, WCC, platelets, ESR, D-dimers, and fibrinogen are usually raised, and serum creatinine levels reduced. CRP is not significantly affected. D-dimer is increased from about 6 weeks’ gestation to 3 months post-partum
• CXR may show increased pulmonary vasculature due to normal increase in cardiac output. Required for diagnosis of pneumonia and pneumothorax. With abdominal shielding, the radiation doses to mother and baby are negligible, and CXR should be performed if clinically indicated. Lateral CXR carries a greater radiation exposure and should be avoided
• Further investigation to exclude PE should be guided by local policy. Many experts recommend bilateral leg vein ultrasound (US) first; if an asymptomatic deep vein thrombosis (DVT) is confirmed in the setting of clinical features, suggestive of PE, then treatment may be started without the need for radiation exposure from further imaging. V/Q scans are associated with a higher radiation dose to the foetus and, as such, a slightly higher risk of childhood cancer, whereas CTPA carries a greater maternal radiation dose and, in the setting of the hormonal changes within the breast during pregnancy, leads to an increased risk of breast cancer in the mother. The ventilation component of a V/Q scan can often be omitted during pregnancy, reducing the radiation dose. CTPA can identify other pathology, e.g. aortic dissection.