Mechanical ventilation during anaesthesia: Pathophysiology and clinical implications

SummaryAnaesthesia produces muscle relaxation and consequently reduces lung volumes, especially the functional residual capacity. This leads to repeated closure of small airways and constitution of atelectases. Repeated closure of small airways and atelectases not only alter gas exchanges but also contribute to ventilator-induced lung injury. Over the last decade, accumulating experimental and clinical data encourage to revise ventilation of anaesthetized patients. Alveolar collapse can be prevented as soon as the induction of anaesthesia by positioning the patient in head-up position, applying a continuous positive airway pressure, and lowering the inspired oxygen fraction. During mechanical ventilation, positive end-expiratory pressure becomes the cornerstone of the prevention of alveolar de-recruitment. Despite these measures, atelectases do develop in some circumstances and need to be reversed by recruitment manoeuvres. In addition, the extent of the tidal volume should be decreased to limit pulmonary overdistension and subsequently ventilator-induced lung injury. The extubation phase is also critical since hypoventilation and re-occurrence of atelectases secondary to the administration of pure oxygen can occur during the transition between controlled and spontaneous ventilation. The efficient measures recommended during the induction of anaesthesia should be also applied during extubation. An assisted mode of ventilation could be beneficial to smoothen this transition phase.