Inclusion Criteria:  Patients with suspected brain death undergoing apnea test for detecting the absence of spontaneous breathing  Exclusion Criteria:  age under 18 pregnancy pulmonectomy, lung resection in the past medical history clinically end stage chronic obstructive pulmonary disease (COPD) severe hemodynamic instability (vasopressor refractory shock) severe bullous emphysema and/or spontaneous pneumothorax in the past medical history chest drainage in situ due to pneumothorax and/or bronchopleural fistula