Guidelines May Speed Obesity Hypoventilation Syndrome Diagnosis

Nancy A. Melville

August 28, 2019

Obesity hypoventilation syndrome (OHS) is a serious yet commonly misdiagnosed condition. New recommendations from a panel of the American Thoracic Society (ATS) offer clinical guidance on screening and treatment for the condition, which is likely to become more prevalent as obesity rates rise.

"Despite the availability of effective therapies, most patients with OHS remain undiagnosed and untreated until late in the course of the disease when they present to high-acuity settings with acute-on-chronic hypercapnic respiratory failure or, alternatively, when ambulatory care is escalated to include evaluation by pulmonary or sleep specialists," the authors state in the guideline, which was published in the August issue of the American Journal of Respiratory and Critical Care Medicine.

"During this delay, patients with OHS use more healthcare resources than eucapnic patients of comparable obesity," write Babak Mokhlesi, MD, a pulmonologist and sleep specialist who is a professor of medicine and director of the Sleep Disorders Center at the University of Chicago, Illinois, and colleagues. "Unfortunately, OHS is misdiagnosed even in patients with severe obesity who are hospitalized with hypercapnic respiratory failure. Early recognition and effective treatment are important in improving morbidity and mortality."

OHS is defined as the combination of obesity (body mass index [BMI] ≥ 30 kg/m2) and daytime hypercapnia (PaCO2 ≥ 45 mmHg at sea level) while awake, after excluding other causes of hypoventilation. The condition is estimated to affect as many as 8% to 20% of severely obese patients (with a BMI > 40 kg/m2) who have been referred to a sleep clinic for evaluation of sleep disordered breathing.

The syndrome is the most severe form of obesity-related respiratory conditions and is linked to an increased risk of mortality, chronic heart failure, and other comorbidities.

For the recommendations, ATS convened a panel of 18 experts in specialties ranging from pulmonology, sleep medicine, and respiratory therapy to critical care, pulmonary hypertension, and weight reduction. A key issue addressed in the recommendations involves OHS screening, during which most diagnosis mishaps occur.

Standard practice requires two tests for OHS diagnosis: a sleep study (polysomnography or respiratory polygraphy) to establish the presence of sleep disordered breathing and a measurement of arterial blood gases during wakefulness to establish hypercapnia. However, patients with sleep-disordered breathing typically do not have arterial blood gas measured, which commonly leads to delays in OHS diagnosis.

Therefore, to simplify diagnosis, the panel recommends the use of a serum bicarbonate level below 27 mmol/L as a cut-off to exclude the diagnosis of OHS when the suspicion for OHS is not very high (< 20%). Arterial blood gas measurement should still be used when there is a strong suspicion of OHS.

"The recommendation of using [the cut-off of] 27 mmol/L is a practical one," said Frances Chung, MBBS, a professor in the Department of Anesthesiology and Pain Medicine, University of Toronto, Ontario, Canada, who was not involved in the guideline panel.

"OHS is difficult to diagnose as it involves using arterial blood gas to measure PaCO2 level in the blood. [Furthermore], an arterial puncture is very painful," she told Medscape Medical News.

The recommendation "can exclude those who are unlikely to have OHS [and] also direct those who are most likely to have OHS to have arterial blood gas measurement," Chung said. "This is a new recommendation that should be adopted."

Severe obesity is a major risk factor for OHS, and Chung agreed that improvements in accurate diagnoses are needed.

"OHS is vastly undiagnosed," Chung said. "These patients have higher mortality and morbidity. There should be increasing awareness of this issue among patients and healthcare workers."

COPD Confusion Common

Patients with OHS are commonly mislabeled as having hypercapnia related to chronic obstructive pulmonary disease (COPD), Mokhlesi explained.

"In part, clinicians in the emergency departments or intensivist or hospitalists are used to seeing more cases of COPD exacerbation presenting with acute-on-chronic hypercapnic respiratory failure, and therefore, they may be more biased to label patients with OHS as COPD exacerbation," Mokhles told Medscape Medical News.

The recommendation should help distinguish the two conditions.

"If serum bicarbonate is below 27 mmol/L, then the chances of having OHS are extremely low," Mokhlesi said. "However, if serum bicarbonate is elevated, the diagnosis needs to be confirmed with a blood gas."

PAP in the Treatment of OHS

The ATS panel of experts in specialties ranging from pulmonology and sleep medicine to critical care and weight reduction also addressed treatment decisions in its systematic review of the evidence.

For stable ambulatory OHS, they say use of positive airway pressure (PAP) during sleep is acceptable.  

However, as many as 70% of patients with stable, ambulatory OHS have concomitant severe obstructive sleep apnea, defined as an apnea-hypopnea index above 30 events/hour. For these patients, the panel recommends a first-line treatment of continuous positive airway pressure (CPAP) over noninvasive ventilation.

"In patients with OHS who are outpatients and are stable in whom the sleep study confirms coexistence of severe obstructive sleep apnea (OSA), we recommend starting treatment with CPAP rather than fancier, more complex and more expensive noninvasive ventilation modalities," Mokhlesi said.

Discharge of Suspected OHS Patients

Regarding the important concern of an increased short-term mortality risk in the discharge of hospitalized patients with respiratory failure suspected of having OHS, the panel recommends that discharge with noninvasive ventilation is acceptable until patients are able to undergo outpatient diagnostic procedures.

PAP titration should be performed in the sleep laboratory, ideally within the first 3 months of hospital discharge, the panel recommends.

"Our panel of experts also recognized that it may not be realistic or feasible for all patients to get noninvasive ventilation before hospital discharge," Mokhlesi noted.

"Therefore, we recommend at a minimum that patients suspected of having OHS undergo full evaluation in the sleep laboratory within 3 months of hospital discharge."

Weight-Loss Interventions

Finally, to tackle obesity, the underlying cause of OHS, the panel recommends interventions that produce a sustained weight loss of 25–30% of body weight, including a potential role of bariatric surgery.

"This level of weight loss is most likely required to achieve resolution of hypoventilation," the panel says. "Those who have no contraindications may benefit from being evaluated for bariatric surgery."

With Research Limited, Quality of Evidence Is Low

The guidelines come with the important caveat that the quality of evidence in the systematic review is described as "very low," primarily because of a limited number of research studies, Mokhlesi said.

Clinicians, however, can improve diagnosis of OHS by simply paying attention to key signs and symptoms.

"These patients tend to be severely obese, sleepy during the day, they may have lower extremity edema, shortness of breath with exertion, high blood pressure, be loud snorers, and bedpartners may describe witnessed apneas during sleep," Mokhlesi said.

Mokhkesi has served as an expert witness for the Roetzel and Andress Law Firm. Disclosures of the other panel members listed in the guideline. Chung has reported no relevant financial relationships.

Am J Respir Crit Care Med. 2019;200:e6-e24. Full text

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