Physiology of gas exchange during anaesthesia

Mechanical ventilation is a prerequisite for many surgical interventions. Furthermore, during states of severe gas exchange disturbance or impaired neurological conditions with the threat of aspiration or cardiovascular instability, it is a life-saving intervention on every ICU. Even the induction of anaesthesia disturbs the physiological lung function, due to changes in chest wall mechanics and diaphragm relaxation, generating atelectases, gas exchange disturbance and ventilation-perfusion mismatch. Additionally, the application of positive pressure to lung structures elicits ventilator-induced lung injury, with the severity of injury dependent on the applied volume, peak pressures and levels of positive end-expiratory pressure. Although these pathophysiological changes may be of minor importance for the majority of ventilated patients in the operating room, these mechanisms may harm patients during surgical interventions with the need for one-lung ventilation or with underlying co-morbidities such as chronic obstructive pulmonary disease (COPD) or acute respiratory distress syndrome (ARDS). This review provides an outline of the major components of the pathophysiological changes associated with general anaesthesia and describes the additional risks in patients with COPD and ARDS as common co-morbidities in every hospital.