Postoperative Hypoxemia in Obese Patients

Postoperative atelectasis is one of the most common pulmonary complications in surgical patients, and a fair majority of studies have suggested that postoperative atelectasis is harmful. It increases the risk of hypoxemia and forms the pathophysiologic basis for other postoperative pulmonary complications. Atelectasis can last for several days after surgery impairing respiratory function, and ultimately delaying patient discharge.

Obese patients are more likely than non-obese patients to develop atelectasis that resolves more slowly. This is because of a marked impairment of the respiratory mechanics (decreased chest wall and lung compliance and decreased functional residual capacity) promoting airway closure with reduction of the oxygenation index (Pao2/ PAo2) to a greater extent than in healthy-weight +subjects . Also the weight of the abdomen makes diaphragmatic excursions more difficult, especially when recumbent or supine, which is intensified in the setting of diaphragmatic paralysis associated with neuromuscular blockade.

Although there have been many studies regarding ventilatory techniques to reduce postoperative pulmonary complications, only a few studies have focused on the period of recovery from anesthesia. The benefits obtained from the protective ventilation techniques may be lost during this emergence process. Whalen et al. found that recruitment maneuver and the application of positive end-expiratory pressure (PEEP) improved intraoperative oxygenation, but the effect dissipated promptly after extubation. Many studies have observed the development of atelectasis during the emergence period. Furthermore, it is estimated that the emergence period contributes to approximately 39% of the total amount of postoperative atelectasis.

Currently, we allow patients to breathe spontaneously and assist their respiration intermittently during the transition from controlled ventilation to spontaneous respiration while assessing whether the patients have enough power to breathe without assistance. However, patients who are spontaneously breathing remain under the influence of residual anesthetic agents and neuromuscular blockers and may not have restored their functional residual capacity, subsequently developing atelectasis. In addition, pain-induced respiratory restriction or respiratory muscle fatigue during spontaneous respiration may increase the risk of atelectasis.

Pressure support ventilation is widely used for weaning from mechanical ventilation in the intensive care unit (ICU) and is recently available in anesthesia machines. Pressure support ventilation applies a fixed amount of pressure the physician selects to the patients throughout each breath to augment their own respiration and is one of the most comfortable ventilation modes for patients. In these aspects, pressure support ventilation during recovery from anesthesia may reduce postoperative atelectasis compared to spontaneous respiration with intermittent manual assistance. To date, few studies have assessed the effect of pressure support ventilation on postoperative atelectasis.

Moreover, laparoscopic surgery are associated with a higher risk of postoperative atelectasis due to the high intra-abdominal pressure which pushes the diaphragm upward and subsequently results in the collapse of the alveoli.

Our hypothesis is that pressure support ventilation will reduces the postoperative hypoxemia and atelectasis compared to spontaneous respiration with intermittent manual assistance during anesthetic emergence in obese patients undergoing laparoscopic surgery.

Aim of the study:

The aim of our study is to assess the possible superiority of pressure support ventilation compared to spontaneous respiration with intermittent manual assistance to reduce postoperative hypoxemia and atelectasis during anesthetic emergence in obese patients undergoing laparoscopic surgery.

Inclusion criteria

• ASA I-III patients.
• aged from 18-60 years old.
• body mass index :(BMI) ≥ 35 Kg/m2 scheduled for laparoscopic bariatric surgery.

Exclusion criteria

• age ≤18 or ≥ 60 years.
• pregnant females.
• underlying lung pathology, moderate to severe impairment of RFT, previous lung surgery, pneumothorax and pleural effusion.