 The authors to measure passive tidal volumes generated during external chest compressions in order to determine whether chest compressions alone without any mechanical ventilatory support in cardiopulmonary resuscitation can provide adequate ventilation. The authors will study 25 volunteers who meet the following inclusion criteria: age 18-55 years, ASA I-II, candidates for surgical operation, who will receive general anesthesia. Patients who will be operated in the chest, who present musculoskeletal diseases, cardiopulmonary or vascular acute or chronic diseases, who had history of pneumothorax, of thromboembolism, osteoporosis, menopause, history of current rib or sternal or clavicle fractures, will be excluded. All eligible patients will enter the study and will receive general anesthesia and intubation according to treating doctors decision. 