The goal of this Randomized Clinical trial is to to investigate if the use of Air Pressure Release Ventilation in morbidly obese patients undergoing open heart surgery will improve post operative pulmonary outcomes 60 Patients will be randomized into two groups according to the mode of ventilation used into: Group A: airway pressure release ventilation (APRV) group (30 Patients) Group B: Standard (control) group (30 Patients) Post-operatively, Patients will be ventilated with conventional Synchronized Intermittent Mandatory Ventilation (SIMV) volume control mode

Obesity is an ongoing worldwide problem, and as such, weight-related issues arise especially in the intensive care unit (ICU). Morbid obesity adversely affects physiology; restricted thoracic movement and increased intra-abdominal pressure leads to reduced lung volumes and compliance resulting frequently in atelectasis, ventilation-perfusion mismatch.

Cardiac surgical patients are at particular risk for PPCs. The use of cardiopulmonary bypass (CPB) is responsible for a systemic inflammatory response and oxidative stress, leading to pulmonary ischemia-reperfusion injury. Mechanical ventilation frequently is disrupted during CPB, thus inducing atelectasis. Some proposed mechanisms are leukocyte activation, fluid accumulation in the lung and atelectasis.

Atelectasis is a highly prevalent pulmonary complication in patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) and an important cause of postoperative hypoxemia. Pulmonary collapse occurs early after the induction of anesthesia and persists for several days postoperatively. Studies based on thoracic computed tomography (CT) have shown that pulmonary collapse is mainly distributed to the dependent regions close to the diaphragm and may encompass up to 35% of the overall lung parenchyma which predispose to pneumonia.

This impairment of lung function, particularly oxygenation of the blood, is a common and potentially serious complication after cardiac surgery. It has been shown to persist for as long as one week after the operation.

In obese patients, the functional residual capacity (FRC) and lung compliance are reduced, and thus the elastic work of breathing (WOB) is elevated. By applying continuous positive airway pressure (CPAP), the FRC is restored, and inspiration starts from a more favorable pressure-volume relationship, facilitating spontaneous ventilation, and improves oxygenation.

APRV was originally described as a mode to treat acute lung injury in patients and attempt to maintain the level of airway pressure without reducing cardiac function, delivering mechanical breaths without excessive airway pressure and allowing unrestricted spontaneous ventilation. 6 APRV is essentially a high-level continuous positive airway pressure (CPAP) mode that is terminated for a very brief period of time. It is this short release period that allows carbon dioxide to be cleared. The lengthy time during which the high-level CPAP is present results in substantial recruitment of alveoli of markedly different regional time constants, at rather low gas flow rates and lower airway pressures. The establishment of intrinsic PEEP by the short release time enhances oxygenation. Carbon dioxide clearance is aided by recruitment of the patient's lung at close to total lung capacity; elastic recoil creates large volume gas flow during the release period.

Recent advances in the understanding of the respiratory physiologic alterations in the obese patient suggest that ''open lung'' ventilation approaches such as APRV recruit lung alveolar volumes without exacerbating lung injury and maintain acceptable gas exchange. Given the spontaneous nature of the mode, it is hypothesized that there should be no need for continuous infusions of neuromuscular blocking agents in patients placed on this mode of ventilation.

This may result in a shorter duration of intensive care unit (ICU) stay. Furthermore, because patients may be ventilated at lower airway pressures than are required with cyclic ventilation, there may be a reduced need for pressor support of hemodynamics to ensure oxygen delivery which is favorable in cardiac surgical patients.

So, the investigators hypothesize that APRV may be a preferred mode of ventilation in morbidly obese patients after cardiac surgery.

Inclusion Criteria:

• Age ≥18 y.
• Scheduled for open heart Surgery with cardiopulmonary bypass (CPB) pump.
• Class III Obesity (Morbid obesity): BMI ≥ 40.0 kg/m².

Exclusion Criteria:

• Patients with obstructive lung disease (asthma or chronic obstructive pulmonary disease).
• Pneumothorax or Surgical emphysema.
• Mechanical ventilation prior to the operation.
• Patient Refusal.