How can we optimise oxygenation in this population?
Pregnancy has multiple anatomical and physiological changes that have implications for the intensivist. Early hormonal effects result in a hyperdynamic circulation. A decrease in systemic vascular resistance leads to a compensatory increase in stroke volume and heart rate to maintain arterial pressure and uteroplacental oxygenation. Plasma volume increases by 40-50%, with red blood cell mass increasing by only 20%. The resulting dilutional anaemia impairs tissue oxygenation further. At 38-40 weeks there is around a 30% decrease in cardiac output on moving from a lateral to supine position. As the uteroplacental circulation possess no powers of autoregulation, any reduction in blood pressure will result in hypoperfusion and foetal acidaemia.
To impair matters further, global oxygen requirement is increased by 60% during pregnancy. An overall increase in minute volume aims to mitigate this, resulting in a respiratory alkalosis. As the uterus expands, it encroaches onto the diaphragm, reducing functional residual capacity (up to 30% when supine). Parturients therefore become hypoxaemic very quickly during episodes of apnoea.
Ventilatory strategies and prone positioning
Current critical care management of these patients is largely as it would be for any non-pregnant patient with a few added considerations.
Ideally, ventilation for pregnant patients should target a relative hypocapnia while still aiming to maintain lung-protective tidal volumes and plateau pressures. When severe lung pathology prevents this, mild hypercapnia is generally acceptable. However, hypercapnia for prolonged periods has been shown to reduce uterine blood flow and increase foetal ICP in animal models.
Achieving plateau pressures of less than 30 cm H2O may be challenging due to pressure from a gravid uterus. Oesophageal pressure monitoring is an option to help determine accurate transpulmonary pressure by distinguishing from chest wall pressure.
It is common to employ high PEEP strategies when managing ARDS. The known physiologic effects of this include decreased preload and cardiac output. This is of particular importance in a patient with a gravid uterus and significant risk of aortocaval compression. PEEP greater than 10 cm H2O should be used with caution. Left lateral tilt or manual uterine displacement should be used to mitigate the physiological consequences of aortocaval compression at all times if greater than 20 weeks gestation.
Deepened sedation and paralytics to improve oxygenation are not contraindicated in pregnancy, but may need to be given in larger doses owing to the increased volume of distribution in the obstetric patient (6).
Prone positioning in intubated patients with ARDS has proven benefits for oxygenation and mortality. However, pregnant women were excluded from the trials that produced these results. Pregnancy in the second and third trimester has long been thought of as a contraindication to proning, with only a few published case studies showing its safety (7). Tolcher et al (6) have recently published their own local guideline for proning both awake and intubated pregnant patients.