One-lung ventilation, even in patients with healthy weight, causes an impairment in gas-exchange and respiratory mechanics. We hypothesized that oxygenation during the one-lung ventilation in obese patients would be improved by applying inverse-ratio ventilation, reducing atelectasis in the dependent lung and resulting in reduced shunt. Because of the restrictive ventilatory effects of obesity, these patients often show a decreased functional residual capacity and decreased expiratory reserve volume, leading to an overall decreased tidal volume. These reduction leads to arterial hypoxemia, V-Q mismatch and Rt to Lt shunting. There are few studies on the one lung ventilation in obese patients about this prolonged inspiratory time ventilatory method. We plan to investigate the effect of this ventilation method in obese patients during one lung ventilation.

Inclusion Criteria:

1. Above 40 years of age.
2. American Society of Anesthesiologists (ASA) Physical Status I, II, III.
3. Obesity (BMI >25 kg/ m2 )
4. thoracic surgical procedure

Exclusion Criteria:

1. severe functional liver or kidney disease
2. diagnosed HF ( NYHA class >3)
3. reduced pulmonary diffusion capacity < 80%