Inclusion Criteria:  American Society of Anesthesiologists' physical status grades I, II, and III.  Exclusion Criteria:  Patient refusal. Psychiatric diseases. Body Mass Index > 35 Kg/m2. Previous intrathoracic procedures. History of severe obstructive pulmonary disease. History of severe restrictive lung disease. Pulmonary arterial hypertension ( systolic pulmonary arterial pressure >40 mmHg). Pregnancy.