The Curbsiders podcast

#269 Obesity Hypoventilation Syndrome (OHS) and the Didgeridoo with Dr Aneesa Das

April 19, 2021 | By


Does obesity hypoventilation syndrome (OHS) give you respiratory distress? Are you baffled by bilevel? The wait is over! Learn all the ins and outs about OHS from Dr. Aneesa Das, @AneesaDas,  a sleep specialist and pulmonologist at The Ohio State University! You’ll learn tips and tricks regarding the diagnosis and management of OHS, the important role PCPs can play, and why on earth we’re discussing the didgeridoo!

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  • Written and Produced by: Cyrus Askin, MD
  • Infographic: Cyrus Askin, MD
  • Cover Art: Kate Grant MBChb, MRCGP
  • Hosts: Cyrus Askin, MD; Matthew Watto MD, FACP; Paul Williams MD, FACP; Stuart Brigham MD
  • Reviewer: Leah Witt, MD
  • Editor: Matthew Watto, MD (written materials); Clair Morgan of
  • Guest: Aneesa Das, MD

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Show Segments

  • Intro, disclaimer, guest bio
  • Guest one-liner
  • Picks of the Week
  • Case from Kashlak
  • Basic risk factors for OHS and red flags
  • That pathophysiologic trifecta
  • Physical Exam & History – OHS
  • Surveys and specific testing
  • Labs and other tests (PFTs, Echo, and more)
  • OSA, OHS, and Central Sleep Apnea
  • Treating OSA: CPAP vs Bilevel
  • The importance of weight loss
  • Keys for inpatient management
  • Perioperative considerations
  • Take-home points
  • Outro

Obesity Hypoventilation Pearls

  1. Obesity + daytime hypercapnia (awake ABG with pCO2> 45 mm) + sleep-disordered breathing without an alternative explanation for hypoventilation = OHS
  2. OHS and OSA are often comorbid conditions, and therapies targeting OSA often resolve the OHS as well
  3. While CPAP is often sufficient to treat OHS (generally, when OSA is also present), some patients do need bilevel PAP to more directly support ventilation and reduce work of breathing
  4. Oxygen therapy is not a substitute for PAP therapy!
  5. Adherence is critical – having a CPAP or Bilevel PAP on the nightstand is no good if it doesn’t get used!
  6. The primary care provider is critical! As a PCP you can reinforce adherence, follow up on results of therapy and most importantly, work with your patient towards a strategy for weight loss – the most important part of OHS treatment!

Obesity Hypoventilation Syndrome (OHS) In-Depth Show Notes 

History, symptoms, and more

  • OHS Definition: Obesity (BMI ≥30) + daytime hypercapnia (ABG with pCO2 greater than or equal to 45 mmHg) + sleep-disordered breathing, without another alternative explanation for hypoventilation [Masa 2018]
    • So… BMI ≥30 is a risk factor!
    • Diagnosis of exclusion! If your patient has COPD, Dr. Das explains that they technically cannot meet diagnostic criteria of OHS… but clinical judgement is key to really sort through these challenging cases!
  • Symptoms: per Dr. Das, symptoms can be “relatively silent” so need to maintain a high index of suspicion. Ask your patient about:
    • Dyspnea with exertion
    • Headache: specifically morning headaches– may be secondary to retained carbon dioxide
    • Other sleep-disordered breathing questions: Snoring? Poor quality sleep? Mental cloudiness? 
    • STOP-BANG or Epworth Sleepiness Scale
      • These are classically for obstructive sleep apnea (OSA), but given concurrence of the two diseases, these can be very helpful for OHS too 
    • Dr. Das specifically recommends the FOS-Q that can be useful both during diagnosis and to track improvements (Example – Here)
    • Prior sleep studies, if none done, the patient will almost certainly need one
  • Things to look (or test) for:
    • Central obesity (increased waist to hip ratio, increased neck circumference) 
    • Airway exam (Mallampati Score of 3 or 4)
    • Signs of cor pulmonale (lower extremity edema, clubbing)
    • A serum bicarbonate less than 27 may help you screen low-to-moderate probability OHS patients (<20%), Dr. Das says this has good negative predictive value and can spare the patient an ABG [2019 ATS OHS GuidelineMokhlesi 2019]
    • Dr. Das reminds us that PFTs are important: need to rule out obstructive lung disease on PFTs (if present, can be the cause of hypoventilation). In OHS, PFTs can demonstrate extrinsic restriction (reduced TLC, preserved DLCO. Extrinsic restriction from obesity shows low FRC and very low ERV. In contrast neuromuscular disease (which can also cause hypoventilation, and needs to be ruled out to confirm OHS diagnosis) has increased FRC, ERV, and RV. Need a PFT refresher? Check this out!
    • In-Lab Sleep Study: better to evaluate OHS and titrate PAP treatment and/or oxygen if needed. Home sleep tests are great for diagnosing OSA in the general population and these patients can try auto-PAP (insurance coverage for test can be more challenging, depending on payer). See here for more info /  a useful reference
    • Other considerations per Dr. Das include
      • Chest X-Ray 
      • Transthoracic echocardiogram to identify comorbid cardiac disease– pulmonary hypertension is a particularly frequent comorbidity
      • Check daytime oxygen saturation – if in the low/mid 90s, that may raise concern

The pathophysiologic trifecta [Masa 2018]

  • Leptin resistance (leptin being a powerful ventilatory stimulus
  • Extrathoracic chest wall restriction (central obesity) –> PFT changes as discussed above
  • Airway obstruction (OSA) – 70-90% overlap between OHS and OSA [Lacedonia 2017]

Getting into the weeds: Central & Peripheral Sleep Apnea

  • Peripheral” Sleep Apneas:
    • Obstructive Sleep Apnea: Must have at least 5 apneas or hypopneas to meet criteria for OSA
    • Obesity Hypoventilation Syndrome: some patients may have NO apneas or hypopneas 
      • Up to 90% of OHS patients will meet OSA criteria… but that means 10% won’t
  • Central Sleep Apnea: often overly brisk ventilatory response to small changes in CO2 – example, Cheyne-Stokes Breathing in heart failure, or periodic breathing in opioid use or high altitude  [Eckert 2007]

Treatment Considerations

  • The 2019 ATS guidelines cite that “more than 70% of patients with OHS also have severe OSA” and thus, those patients with findings consistent with both OHS and OSA, who are stable/ambulatory patients – especially with AHI >30 should be trialed on CPAP first, as opposed to starting with Bilevel
  • Oxygen monotherapy is not appropriate and may result in worsening hypercapnia
  • Goals of treatment:
    • Improvement/normalization of the pCO2
    • Improvement/normalization of the pH
    • Improvement/normalization of the serum bicarbonate, often elevated in these patients before initiation of therapy.

CPAP, Bi-Level, APAP… What does it all mean?!

  • PAP: Positive airway pressure – comes in three basic flavors, APAP (automatic or auto-titrating PAP), CPAP and Bi-Level (often referred to as BiPAP)
  • CPAP: continuous positive airway pressure – Dr. Das: “think of it as an airway stent”
    • Increases pressure at mouth and nose to keep the airway open
  • Bi-Level=BIPAP: two different pressures, a type of non-invasive ventilation (NIV)
    • Expiratory (E-PAP): Like CPAP
    • Inspiratory (I-PAP): Initiates when the patient attempts to breathe in– critical for ventilation
    • Delta pressure: the difference between IPAP and EPAP
    • Example – Bi-Level at 12/8 would be a constant pressure of 8 cm H2O (E-PAP), with an additional 4 H2O (the “delta”)  to “assist” the patient every time they try to take a breath (resulting in the I-PAP of 12 cm H2O)
    • Benefit? The E-PAP keeps the airways stented open while the I-PAP kicks in when the patient inhales to assist with work of breathing/increase tidal volume
    • For OHS: the delta pressure can be quite high, e.g, greater than 8 cm H2O
    • Don’t get confused…Bilevel settings are not perfectly analogous to pressure support on a ventilator! Pressure Support, a term reserved for ventilators, in the example above would refer to the “4”  or the “delta” between the expiratory pressure (which on a ventilator is PEEP) and the inspiratory pressure
  • Oxygen? Dr. Das explains that sometimes patients may need oxygen initially. However, after treatment with PAP they may no longer need oxygen as their lungs start to work better.
    • OHS is not a set-it-and-forget-it diagnosis – treatment needs to be revisited periodically, and changes may be needed during the course of one’s clinical journey

How can the Primary Care Provider help

  • The underlying issue in OHS is the O – i.e, the Obesity
    • Dr. Das: Obesity is a disease much like cancer
  • The PCP ise instrumental in promoting weight loss / weight management either through lifestyle modifications, medications or referral for bariatric surgery 
    • The ATS recommends measures to achieve sustained weight loss to the tune of  25-30% [Mokhlesi 2019]– WOW that can be tough!
  • A good medication reconciliation is very important – opioids, benzodiazepines and sleep aides can all exacerbate OHS (increased hypoventilation!) and a vigilant PCP should eliminate these if possible
  • Follow up, make sure your patient is actually adherent to PAP!
  • Perioperative Considerations?
    • First and foremost, perioperative adherence to PAP therapy is critical.Patients should bring their device with them to the hospital for post-operative use
    • The pre-op team/ anesthesiologists need to know about the OHS diagnosis, as this may alter the perioperative anesthetic plan

OHS management in the inpatient setting

  • Men tend to have more obstructive sleep apnea, while women and men are equally likely to develop obesity hypoventilation syndrome [Palm 2016]
    • Women may be more likely to present later in their course and thus, present with more advanced disease / a need to be hospitalized
  • Per ATS: patients admitted with concern for OHS that have not yet had the diagnosis confirmed should be started on NIV and actually discharged on NIV while awaiting ambulatory sleep testing and PAP titration (which should occur within 3 months of discharge) [Mokhlesi 2019]
    • Patients often will ultimately end up on CPAP once their acute presentation has been remedied with NIV
  • The key for an inpatient admission for OHS is to get control of the acidosis

Take Home Points

  •  Obesity is a disease!he higher the BMI the greater the risk of OHS (and other obesity-related comorbidities)
  • The greater the AHI (in OSA) the greater the risk of OHS
  • Oxygen is not a substitute for PAP– VENTILATION is key
  • Adherence, adherence adherence! The best thing any PCP can do is to ask about adherence frequently, and help address issues (e.g. follow up with sleep, patient calls DME company)


  1. Cyrus’s pick of the week: Untitled Art
  2. Paul’s pick of the week: Cyberpunk 2077 (despite its limitations)
  3. The didgeridoo and OSA
  4. Link to buy a didgeridoo on Amazon… here!
  5. The 2019 American Thoracic Society (ATS) Guidelines on Obesity Hypoventilation Syndrome

*The Curbsiders participates in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising commissions by linking to Amazon. Simply put, if you click on our links and buy something we earn a (very) small commission, yet you don’t pay any extra.


Listeners will explain the basic pathophysiology, diagnosis, and management of obesity hypoventilation syndrome with a goal of understanding the chronic management of these patients, and how it differs from acute management in the hospital setting.

Learning objectives

After listening to this episode listeners will…  

  1. Recognize historical and physical exam findings that may be suggestive of obesity hypoventilation syndrome (OHS).
  2. Appreciate the pathophysiology of OHS and common concomitant disease states.
  3. Master the work up for OHS and the diagnostic criteria required to make the diagnosis.
  4. Learn how to counsel patients with OHS prior to initiating therapy.
  5. Gain insights regarding the therapies available for OHS.
  6. Recognize the interplay between OSA and OHS.
  7. Appreciate the potential benefits of weight loss and/or weight-loss surgery in context of OHS.
  8. Appreciate the role of the primary care physician in the management of OHS.



Dr Das reports no relevant financial disclosures. The Curbsiders report no relevant financial disclosures. 


Askin CA, Das A, Williams PN, Brigham SK, Watto MF. “269 Obesity Hypoventilation Syndrome”. The Curbsiders Internal Medicine Podcast. April 19, 2021.

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The Curbsiders are partnering with VCU Health Continuing Education to offer FREE continuing education credits for physicians and other healthcare professionals. Visit and search for this episode to claim credit.

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