Critical Care Medicine-Pulmonary Disorders>>>>>Sleep Apnea
Question 5#

A 43-year-old female is referred to your clinic by her primary care physician after undergoing a polysomnographic study, which supports a diagnosis of OSA (Apnea-Hypopnea Index [AHI] 25). Her BMI is 36 kg/m2 , her BP is 135/70, HR 82 bpm, and SpO2 88% on room air. She presents you with a recent arterial blood gas on room air (pH 7.35, pCO2 51 mm Hg, pO2 60 mm Hg, and HCO3 − 31 mEq/L), chest x-ray (which is reported as normal), and spirometry (showing a restrictive picture). She denies ever smoking or taking recreational drugs and only takes simvastatin for her high serum cholesterol levels (now under control).

What is the MOST likely diagnosis?

A. Chronic obstructive pulmonary disease (COPD)
C. Overlap syndrome
D. OSA and obesity hypoventilation syndrome (OHS)

Correct Answer is D


 Correct Answer: D

OHS is defined as a combination of obesity (BMI >30 Kg/m2 ) and daytime hypercapnia (PaCO2 >45 mm Hg) in the absence of other causes that could account for awake hypoventilation, such as lung or neuromuscular disease. The patient in the presented case meets diagnostic criteria for both OSA (positive PSG with AHI 25) and OHS (PaCO2 51) (1, 2) (option D).

The patient’s spirometry showing a restrictive, rather than obstructive, picture and the absence of smoking history make the diagnosis of COPD unlikely (option A). Overlap syndrome is the combination of COPD and OSA and, for the same reason, is unlikely (option C). Finally, the patient has a clear laboratory diagnosis of OSA, as stated in the description, but option D is a more complete explanation of the clinical picture than option B. 


  1. Piper AJ, Grunstein RR. Obesity hypoventilation syndrome: mechanisms and management. Am J Respir Crit Care Med. 2011;183(3):292-298.
  2. Raveedran R, Wong J, Singh M, et al. Obesity hypoventilation syndrome, sleep apnea, overlap syndrome: perioperative management to prevent complications. Curr Opin Anaesthesiol. 2017;30(1)146-155.