Obesity Hypoventilation Syndrome and Anesthesia Considerations Roop Kaw, MD a, *, Maged Argalious, MD, MBA b , Loutfi S. Aboussouan, MD b , Frances Chung, MBBS c INTRODUCTION Small cohort studies have reported the prevalence of obesity hypoventilation syndrome (OHS) in the general population to be around 0.3% to 0.4%. 1 Among the only surgical series to report preva- lence of OHS is a recent report from premeno- pausal women presenting for bariatric surgery with 8% prevalence of OHS. 2 Among patients with known obstructive sleep apnea (OSA) the re- ported prevalence of OHS is between 10% and 20% and is known to increase with obesity to as high as 50% as the body mass index (BMI) ex- ceeds 50 kg/m 2 . 3 OHS is often undiagnosed or un- dertreated before elective noncardiac surgery and is usually associated with many medical comor- bidities. This article describes and examines many important steps and strategies that can improve perioperative outcomes in this high-risk population. IMPORTANT PATHOPHYSIOLOGIC CONSIDERATIONS Effect of Obesity, Supine Posture, and Anesthesia on Lung Function Functional residual capacity (FRC) can be expo- nentially reduced in the supine position and Tren- delenburg positions as a function of increasing BMI. Reductions as low as 51% have been described in morbidly obese patients undergoing jejunoileal bypass surgery. 4 With a decreased FRC, the closing volume, at which dependent al- veoli start collapsing, may start encroaching on the normal tidal volume excursions, resulting in increased airway closure such that well-perfused basal areas may be closed at normal tidal breaths, thereby creating hypoxemia caused by a mismatch between ventilation and perfusion (Fig. 1). 5 The FRC reduction may last up to 7 days after surgery and recovery of spirometric Disclosure: None. a Cleveland Clinic Lerner College of Medicine, Departments of Hospital Medicine and Outcomes Research Anesthesia, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, OH 44195, USA; b Cleveland Clinic, 9500 Euclid Avenue, Cleveland, OH 44195, USA; c Department of Anesthesia, University Health Network, University of Tor- onto, Toronto Western Hospital, 399 Bathurst Street, Toronto, ON, Canada * Corresponding author. E-mail address: kawr@ccf.org KEYWORDS  Obesity hypoventilation syndrome  Perioperative  Anesthesia  Surgery  Positive airway pressure KEY POINTS  Obesity hypoventilation is often undiagnosed before surgery, hence appropriate suspicion and preparedness are required.  Preoperative identification of undiagnosed compensated respiratory acidosis and obstructive sleep apnea are key.  Primary treatment is positive airway pressure therapy, and appropriate sleep referral may be mandated before major surgery. Sleep Med Clin 9 (2014) 399–407 http://dx.doi.org/10.1016/j.jsmc.2014.05.005 1556-407X/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved. sleep.theclinics.com