Patients were allocated to five groups according to the preoperative BMI : underweight ( BMI < 20 kg/m2 ) ; normal weight ( BMI 20.0-24 .9 kg/m2 ) ; overweight ( BMI 25.0-29 .9 kg/m2 ) ; obese ( BMI 30.0-34 .9 kg/m2 ) ; and morbidly obese ( BMI > 34.9 kg/m2 ) .