A short glance at ARDS – an old disease and new facts

A short glance at ARDS – an old disease and new facts

  • Issue 65

Davide Chiumello

Since its first description, the acute respiratory distress syndrome (ARDS) has been redefined several times in order to ameliorate the accuracy of the clinical diagnosis. ARDS is characterized by a non-cardiogenic pulmonary edema with bilateral chest x-ray opacities, reduction in lung compliance, and hypoxemia. However, independent from the different proposed definitions, the hallmark of ARDS is the arterial hypoxemia refractory to oxygen therapy, due to pulmonary shunt. Two thresholds for severe hypoxemia (PaO2/FiO2 lower than 150 or 100 mmHg) have been proposed; both of them are associated with the highest mortality (up to 45%), duration of mechanical ventilation, and risk of ventilator-induced lung injury (VILI). It can be found in up to 20-30% of the patients. According to the recent Berlin definition, ARDS is characterized by an inflammatory lung edema of recent onset, causing severe respiratory failure that requires invasive or non-invasive ventilation. Classically, the increase in lung edema (i.e., lung weight) and pleural pressure raising the hydrostatic pressure transmitted throughout the lung, reduce the lung gas volume and promote the development of non-aerated regions (consolidated or atelectatic), mainly in the more dependent lung regions. The increasing knowledge of ARDS pathophysiology through the years has led to suggest the application of a lung protective ventilatory strategy, which in addition to insuring adequate oxygenation (PaO2 between 60 and 80 mmHg), should minimize the VILI. Unfortunately, a completely “safe” lung protective ventilation does not exist, and the ventilatory support should be individualized according to the best compromise among respiratory mechanics, recruitability, gas exchange, and haemodynamics.

Although the standard supportive treatment remains mechanical ventilation (non-invasive and invasive with intubation), possible adjuvant therapies can be considered. In the moderate-to-severe form or in the presence of other organ failure, non-invasive ventilatory support presents high risk of failure; thus in those cases the risk/benefit of delay of mechanical ventilation should be carefully evaluated. The possible use of non-invasive ventilation (NIV) in patients with ARDS, although this could reduce the intrapulmonary shunt and decrease the work of breathing, is still controversial due to the high risk of failure and to a possible delay in starting invasive mechanical ventilation. The last consensus conference on NIV pointed out that: “Larger controlled studies are required to determine the potential benefit of adding NIV to standard medical treatment in the avoidance of endotracheal intubation.” A possible alternative to NIV could be the application of the high flow nasal cannula (HFNC) system, which can deliver a very high, heated, and humidified oxygen flow through the nose. The HFNCs are able to increase the end-expiratory lung volume, reduce the work of breathing, and improve CO2 clearance and oxygenation. In addition to these beneficial effects, and contrary to NIV, the HFNC, which does not require any nasal or mask interface, significantly improves the long-term tolerance and use.

The use of non-invasive ventilator supports, the management of airway in patients with low arterial saturation, and the evaluation of the patients with ARDS were all discussed during the last Euroanaesthesia Congress in London, with interesting implications for future studies.