Patients requiring one lung ventilation (OLV) for open thoracic surgery will be ventilated (breathing performed by a breathing machine) during anesthesia using a lung protective ventilation strategy (small breath volumes at 6ml/kg). During thoracic surgery the anesthesiologist is able to ventilate only one lung by inserting a special breathing tube, allowing the surgeon to operate on the non ventilated (diseased) lung. In a randomized trial two interventions used to improve blood oxygen levels during one lung ventilation will be compared . The two interventions are:

1. Continuous Positive Airway Pressure (CPAP) applied to the non ventilated (non breathing) lung and
2. Positive End Expiratory Pressure following a lung Recruitment Maneuver (RM-PEEP) to the ventilated (breathing) lung.

CPAP is performed by applying a steady flow of oxygen to the non ventilated (non breathing) lung at a continuous gentle pressure of 5cmH20.

To perform a Recruitment Maneuver (RM) the anesthesiologist inflates the ventilated (breathing) lung with oxygen, holding the breath for 25 seconds so all the lung is opened up. Immediately after the recruitment maneuver PEEP will be applied. PEEP is an action which also helps keep the lung open, maintaining the benefits achieved by the RM. It is performed by adjusting settings on the ventilator (breathing machine). The ventilator creates and applies a gentle pressure (5cmH20) to the ventilating lung at the end of each breath.

The outcome measure will be the oxygen content in blood (PaO2), measured in mmHg, using blood sample analysis.

The null hypothesis is that compared to CPAP, RM-PEEP does not significantly increase the oxygen content of blood during OLV when using a lung protective ventilation strategy.

Patients requiring one lung ventilation (OLV) for open thoracic surgery will be ventilated intra-operatively using a lung protective ventilation strategy (small tidal volumes {Vts} at 6ml/kg Ideal Body Weight {IBW}). In a randomized, crossover trial Continuous Positive Airway Pressure (CPAP) to the non ventilated lung or a Recruitment Maneuver (RM) followed by the application of Positive End Expiratory Pressure (PEEP) (acronym RM-PEEP) to the ventilated lung will be applied and blood oxygenation (PaO2) measured by arterial blood gas sampling to determine which intervention has the most beneficial effect on PaO2 (CPAP or RM-PEEP).

CPAP will be applied at a pressure of 5cmH2O by a CPAP breathing circuit (designed for the purpose and commonly used in anesthetic practice). The PEEP will be applied at a pressure of 5cmH20 by the operating room (OR) anesthetic machine. The RM will involve a valsalva maneuver, held for 5 seconds at a pressure of 25cmH20, again performed using the OR anesthetic machine.

Null hypothesis: Compared to CPAP, RM-PEEP does not significantly increase PaO2 or reduce the incidence of hypoxia (oxygen blood saturation less than or equal to 90%), when employing a lung protective ventilation strategy.

This study is based on our previous research (citation 12, Badner et al) in which we compared CPAP to PEEP alone (omitting the recruitment maneuver). Here it was noted that CPAP to the non ventilated lung improved oxygenation more than PEEP to the ventilated lung (even though PEEP is an easier modality to provide), when employing a lung protective ventilation strategy.

Inclusion Criteria:

1. Age 18 or older
2. Competent to give consent to enroll in study
3. Booked for scheduled open thoracotomy which involves wedge resection, lobectomy or pneumonectomy requiring OLV
4. American Society of Anesthesia physical status score (ASA) 1-4

Exclusion Criteria:

1. Unable to give consent
2. Pregnant women
3. Inability to insert an arterial line
4. Presence of other significant pulmonary impairment (PaO2 on room air <50mmHg, PaCO2 >50mmHg or known pulmonary hypertension (mean PAP>25mmHg)
5. Presence of significant cardiovascular disease
6. Altered liver function (Child Pugh scale ≥B)
7. Patients with bullous lung disease. -