The effects of abdominal opening on respiratory mechanics during general anaesthesia for open bariatric surgery in morbidly obese patients.

Background: Morbid obesity BMI 40 kg mL-2 is associated with several changes in anatomy and physiology of the respiratory system. Respiratory work is increased whereas compliance of the chest wall and lungs is decreased. Increased intra-abdominal pressure compresses the diaphragm and reduces its movability. Abdominal opening should influence respiratory mechanics.

Methods: Forty-seven morbidly obese patients (BMI 49.54 ± 7.21 kg m-2) scheduled for elective bariatric surgery (open Roux-en-Y-Gastric Bypass - RYGB) were included in the study and divided into 3 groups: 40< BMI <50, 50< BMI <60 and BMI >60. Measurements were performed during general anaesthesia at two time points: before (T1) and after abdominal opening (T2). All patients were ventilated using the volume control mode.

Results: Preoperative spirometry revealed significantly decreased FEV 1% in the BMI >60 group. The conditions of mechanical ventilation during general anaesthesia deteriorated with an increasing BMI. After abdominal opening,the mechanical ventilation conditions improved: increased lung compliance and decreased airway pressures. The difference was statistically significant in the entire population studied. After dividing the population into groups, however, the difference was no longer significant in patients with BMI >60. The most significant difference in peak and plateau pressures after abdominal opening was observed in patients with BMI <50.

Conclusion: Abdominal opening improves the conditions of mechanical ventilation in morbidly obese patients; when BMI is over 60 kg m-2, this improvement may not be relevant.