Acute lung injury (ALI) is a syndrome of lung inflammation and increased permeability that is not explained by left atrial or pulmonary capillary hypertension (although they may coexist). The previous term, adult (now acute) respiratory distress syndrome (ARDS), is now reserved for the most severe form of ALI.
Both conditions are characterised by their acute onset, bilateral diffuse infiltrates on chest radiography not explained by cardiac failure or volume overload, pulmonary artery wedge pressure <18 mmHg or absence of clinical evidence of left atrial hypertension, and arterial hypoxaemia resistant to oxygen therapy alone (ratio of arterial partial pressure/ inspired fractional concentration of oxygen < 39.9 kPa [300 mmHg] for ALI, < 26.6 kPa [200 mmHg] for ARDS). Other features include reduced respiratory compliance and lung volumes, increased work of breathing, ventilation-perfusion mismatch and increased shunt.
ALI or ARDS associated with pulmonary oedema is a common feature of deaths associated with pregnancy, and has received special attention in past reports of the Confidential Enquiries into Maternal Deaths; mortality associated with ARDS is elevated in the pregnant population and has been reported as being between 10% and 40%. It has been suggested that ALI may be more likely in the pregnant state, possibly as result of the physiological changes of pregnancy, especially the increased cardiac output, lower colloid osmotic pressure and leaky capillaries. It is also suspected that aggressive fluid therapy, especially in obstetric haemorrhage and pre-eclampsia, has led to many cases of ALI. In the case of haemorrhage, this may be related to rapid transfusion in the presence of high circulating levels of catecholamines and a relatively constricted pulmonary circulation; in pre-eclampsia, over-emphasis on treating oliguria by ‘pushing fluids’ may lead to pulmonary oedema and ALI.
Problems and special considerations
The causes of ALI in pregnancy are generally the same as those in the non-pregnant state, with sepsis the most common cause, although pre-eclampsia, haemorrhage, aspiration of gastric contents, presence of a dead fetus or retained products of conception and amniotic fluid embolism are also important. Use of p2-agonists in premature labour may also contribute by causing pulmonary oedema (although whether this in itself leads to ALI and ARDS is uncertain). It is important to consider other causes of respiratory failure, especially peripartum cardiomyopathy, as this is more common than ARDS.
The increased demands on the maternal cardiovascular and respiratory systems make the obstetric patient with ALI less able to cope with hypoxaemia, especially in the third trimester. The decreased functional residual capacity increases the likelihood of airway closure and ventilation-perfusion mismatch. However, established ALI has a similar mortality in both pregnant and non-pregnant women.
The fetus is particularly at risk from hypoxaemia, and this is compounded by any associated cardiovascular instability and the risk of aortocaval compression.
Management options
Management requires early referral to the intensive care unit and involves increasing levels of respiratory support as ALI increases in severity; non-invasive ventilatory support may be considered in pregnancy, but if there is no clinical improvement, invasive ventilation using lung protective strategies may be necessary. Extracorporeal membrane oxygenation (ECMO) has been used successfully in pregnant patients with ARDS and refractory hypoxaemia. Management of the predisposing condition should continue as for any acutely ill patient, and a conservative fluid strategy is usually recommended. The fetus should be monitored for signs of distress that might prompt delivery. The physiological changes of pregnancy pose particular problems for the critically ill obstetric patient (see Chapter 155, Critical care in pregnancy).
Key points
• Obstetric patients may be especially prone to acute lung injury.
• Acute lung injury is a common feature in deaths caused by pre-eclampsia, sepsis and massive obstetric haemorrhage.
• The increased physiological demands of pregnancy make obstetric patients especially susceptible to hypoxaemia.
Further reading
Duarte AG. ARDS in pregnancy. Clin Obst Gynecol 2014; 57: 862-70.
Schwaiberger D, Marcz M, Menk M et al. Respiratory failure and mechanical ventilation in the pregnant patient. Crit Care Clin 2016; 32: 85-95.