Permissive hypercapnia with and without expiratory washout in patients with severe acute respiratory distress syndrome

Background: permissive hypercapnia is a ventilatory strategy aimed at avoiding lung volutrauma in patients with severe acute respiratory distress syndrome (ARDS). Expiratory washout (EWO) is a modality of tracheal gas insufflation that enhances carbon dioxide removal during mechanical ventilation by reducing dead space. The goal of this prospective study was to determine the efficacy of EWO in reducing the partial pressure of carbon dioxide (Pa-CO2) in patients with severe ARDS treated using permissive hypercapnia. Methods: Seven critically ill patients with severe ARDS (lung injury severity score, 3.1 +/- 0.3) and no contraindications for permissive hypercapnia were studied. On the first day, hemodynamic and respiratory parameters were measured and the extent of lung hyperdensities was assessed using computed tomography. A positive end-expiratory pressure equal to the opening pressure identified on the pressure-volume curve was applied. Tidal volume was reduced until a plateau ah-way pressure of 25 cm H2O was reached. On the second day after implementation of permissive hypercapnia, EWO was instituted at a flow of 15 1/min administered during the entire expiratory phase into the trachea through the proximal channel of an endotracheal tube using a ventilator equipped with a special flow generator. Cardiorespiratory parameters were studied under three conditions: permissive hypercapnia, permissive hypercapnia with EWO, and permissive hypercapnia. Results: During permissive hypercapnia, EWO decreased Pa-CO2 from 76 +/- 4 mmHg to 53 +/- 3 mmHg (-30 %; P < 0.0001), increased pH from 7.20 +/- 0.03 to 7.34 +/- 0.04 (P < 0.0001), and increased PaO2 from 205 +/- 28 to 296 +/- 38 mmHg (P < 0.05). The reduction in Pa-CO2 was accompanied by an increase in end-inspiratory plateau pressure from 26 +/- 1 to 32 +/- 2 cm H2O (P = 0.001). Expiratory washout also decreased cardiac index from 4.6 +/- 0.4 to 3.7 +/- 0.3 1.min(-1) m(-2) (P < 0.01), mean pulmonary arterial pressure from 28 +/- 2 to 25 +/- 2 mmHg (p < 0.01), and true pulmonary shunt from 47 +/- 2 to 36 +/- 3% (P < 0.01). Conclusions: Expiratory washout is an effective and easy-to-use ventilatory modality to reduce Pa-CO2 and increase pH during permissive hypercapnia. However, it significantly increases airway pressures and lung volume through expiratory now limitation, reexposing some patients to a risk of lung volutrauma if the extrinsic positive end-expiratory pressure is not substantially reduced. (Key words: Hypercapnia. Lungs. Respiratory failure. Mechanical ventilation, lung: acute respiratory distress syndrome).