Functional residual capacity (FRC) and pulmonary gas exchange decrease during induction of general anaesthesia due to closure of airways and genesis of atelectasis. It has been demonstrated that there is a correlation between body constitution and airway closure under general anaesthesia with mechanical ventilation. In the supine position, FRC and respiratory compliance decrease more with an increased body mass index (BMI). In morbidly obese patients general anaesthesia and paralysis lead to even more atelectasis and an increased risk of hypoxemia. In about 90 % of all patients ventilated without positive end-expiratory pressure (PEEP) atelectases are formed, which contribute to impairment of gas exchange. Atelectasis formation can be reduced by PEEP but increasing intrathoracic pressure with PEEP, impairs the venous return to the heart, which reduces stroke volume and cardiac output.