Alveolar-arterial gradient and risk of hypoxemia in morbidly obese patients the day after laparoscopic bypass surgery: sustained effect of Boussignac CPAP?

Background and Goal of Study: Morbidly obese patients develop more atelectasis during anesthesia which frequently persist twenty four hours after surgery and have and increased risk of desaturation in this period. Our aim was to evaluate the impact of immediate post extubation use of Boussignac Continuous Positive Airway Pressure (CPAP) on alveolar-arterial gradient (A-a gradient) and the prevalence of post-operative hypoxemia 24h after Roux-en-Y laparoscopic gastric bypass. 

Materials and methods: A prospective study was conducted after hospital ethics committee approval. Patients with body mass index>35Kg/m2, age 18-65 years and American Society of Anesthesiologists class 1-3 were included. Exclusion criteria: preexisting lung parenchyma disease, chronic obstructive pulmonary disease, moderate to severe asthma, congestive heart failure, hemoglobin concentration lower than 7g/dl, language barriers or severe psychiatric disorder. Patients had the same anesthetic and analgesic protocols. Patients signed consent the day before surgery and were randomly assigned to receive Boussignac CPAP or venturi mask which were used immediately after extubation in the operating room and maintained during two hours in the recovery room. Hypoxemia (PaO2 <60mmHg) and A-a gradient were evaluated in the preoperative period (T0), 1h(T1) and 24h(T24) after extubation. Statistical analysis: categorical variables are presented as frequency and percentage and continuous variables are presented as mean+standard deviation; student´s t and chi-squared tests were used. A p-value< 0.05 was considered statistically significant.

Results and discussion: 
11 patients were enrolled in each group. No differences in demographic and preoperative data. In the preoperative evaluation and at T1 there were no PaO2 values <60mmHg in both groups. At T24 no patient in the Boussignac group and 36% of patients in the venturi group had a PaO2 <60mmHg. 
There were no differences in pre-operative A-a gradient values between groups. After extubation it was significantly (temos valor de p??) lower in the Boussignac group at T1 (148,9±75,9 versus 211,4±18,5) and T24 (27,8±7,9 versus 37,7±6,4). 

Conclusions
In line with our study results it seems that Venturi group patients are more prone to have hypoxemia. A lower A-a gradient after extubation in the Boussignac group may be an indicator of a better ventilation/perfusion matching and suggest a positive effect of CPAP in the prevention of additional atelectasias after surgery.

Alveolar-arterial gradient and risk of hypoxemia in morbidly obese patients the day after laparoscopic bypass surgery: sustained effect of Boussignac CPAP?