Abstract
The acute respiratory distress syndrome (ARDS) was first described in 1967 by Ashbaugh and since then therapy has been primarily supportive consisting of oxygen, mechanical ventilation and positive end expiratory pressure (PEEP).1Despite this, mortality has remained high and has varied according to underlying aetiology. The mortality associated with trauma is 38%, with aspiration 60% and is 60%, 68% and 86%, respectively for extra abdominal, intra-abdominal and opportunistic infection. The prognosis may be improving with time, though this is less apparent in certain of the associated conditions. Three new concepts have stimulated considerable research and debate. Firstly, ARDS may not be one disease, in that at least two different forms exist, primary, due to pulmonary disease and secondary, due to extrapulmonary disease. If so, the current definition of ARDS is inadequate and cannot dictate management or predict prognosis. Secondly, there is evidence that ventilatory technique may influence the degree of lung injury and that this may be ameliorated by pressure and volume limitation and the application of PEEP sufficient to ensure lung patency. Thirdly, and perhaps most importantly, secondary ARDS is not a homogeneous disease and that at least three different pulmonary zones exist which respond differently to ventilation. These are: (1) aerated zones susceptible to barotrauma; (2) consolidated air spaces that are not recruitable; (3) atelectatic zones with interstitial oedema that are recruitable. Primary ARDS does not respond to PEEP with an increase in compliance, rather with stretching phenomena as manifested by a decrease in compliance of the total respiratory system and the lung. Conversely, re-inflation of atelectatic zones (recruitment) can occur in those with secondary ARDS in response to PEEP or change in body position and this may be associated with an increase in compliance. Recruitment of these areas could significantly improve hypoxia, reduce ventilatory pressures and allow rapid reduction in FiO2. Atelectasis is predominantly dependent and has been elegantly described by Gattinoni utilizing computed tomography. Animal studies have shown that shear forces generated during repetitive opening and closing of lung units may be associated with lung injury and that PEEP, which ''stents'' alveoli open, dramatically reduces ventilator induced lung injury (VILI). In the light of the above, a number of manoeuvres have been developed to capitalize on the fact that recruitment of posterior atelectasis improves oxygenation, and may also reduce VILI. These are: (1) PEEP; (2) prone positioning; (3) ventilator controlled recruitment manoeuvres; (4) high frequency oscillatory ventilation with the open lung approach; (5) partial liquid ventilation.