Grading obesity hypoventilation syndrome severity

To the Editors:

Obesity hypoventilation syndrome (OHS) is commonly defined as a combination of obesity (body mass index (BMI) >30 kg·m⁻²), waking arterial hypercapnia (arterial carbon dioxide tension (P_a,CO2) >6.0 kPa (45 mmHg)) and sleep-disordered breathing. Essential to the diagnosis is exclusion of other causes of alveolar hypoventilation 1. The lack of a standardised definition of OHS in general, and of OHS–obstructive sleep apnoea relationships in particular, leads to confusion.

One of the main aspects that has not been clarified is the assessment of OHS severity. This appears to be directly related to the degree of hypercapnia, the degree of hypoxaemia and the presence of complications 2, 3. Nevertheless, defined criteria to quantify OHS severity do not exist in the literature. The question is: are all OHS cases supposedly severe? We think there are several approaches for determining the level of OHS severity.

One approach would be to grade OHS severity according to the degree of impairment of functional respiratory parameters, such as hypercapnia or hypoxaemia. For example, a patient with a P_a,CO2 of 6.1–8.0 kPa (46–60 mmHg) could be considered as mild, 8.1–10.6 kPa (61–80 mmHg) as moderate, and >10.6 kPa (80 mmHg) as severe.

One could also grade OHS severity according to the BMI or spirometric findings. For example, a patient with a BMI of 30–40 kg·m⁻² could be considered as mild, 40–50 kg·m⁻² as moderate and >50 kg·m⁻² as severe. Based on spirometric findings, the OHS could be classified as severe if a pulmonary function test reveals a severe restrictive impairment.

Another approach to scoring OHS severity could be based on polysomnographic findings, such as the percentage of time spent with arterial oxygen saturation <90%, the respiratory disturbance index or the apnoea/hypopnoea index. The presence of complications could also be taken into account. In this case, we could grade as severe OHS patients with pulmonary hypertension, cor pulmonale, left ventricular failure, polycythaemia or a history of intensive care unit hospitalisations.

A quite different approach could be based on an “asthma-control” strategy for OHS patients. We could look at OHS not only in terms of severity but also in terms of response to treatment. We could consider OHS patients as controlled, partly controlled or uncontrolled. Attaining optimal OHS control would be an important goal of all physicians attending to those with OHS. But what is a well-controlled OHS patient? In our opinion, OHS could be considered well controlled when there is: 1) absence of symptoms; 2) no nocturnal or early morning awaking; 3) good tolerance to noninvasive ventilation; 4) absence of respiratory insufficiency; and 5) appreciation by the patient and their physician that the OHS is well controlled.

It seems reasonable to suggest that management of obesity hypoventilation syndrome patients could be different according to the severity of the disease and that not all obesity hypoventilation syndrome patients have the same level of severity. Perhaps a multifactorial approach, involving several aspects concerning the severity of obesity hypoventilation syndrome, would be necessary. Tables 1⇓ and 2⇓ show proposals for classification based on functional parameters or disease control. The strategy of increasing treatment until control is achieved could be a new approach to obesity hypoventilation syndrome management.

Statement of interest

None declared.