Obesity hypoventilation syndrome

Question 1

what is the definition of OHS?

Answer 1

combination of BMI 30+, sleep disordered breathing, and daytime hypercapneia pCO2 >45

Question 2

what percentage of OHS patients have OSA?

Answer 2

90%

Question 3

what is the definitive test for OHS and what will it show

Answer 3

ABG awake on room air - pCO >45

Question 4

A patient has a HCO3 of 26, what is the chance they have OHS?

Answer 4

97% NPV if < 27

Question 5

what are the 3 mechanisms by which OHS occurs

Answer 5

1) obesity related changes in respiratory system - excess adipose reduces lung volume (FRC and ERV). There is mechanical stress on the diaphragm. Increased gas trapping. Increased atelectasis at the bases.

2) central hypoventilation - REM hypoventilation (2/2 atonia) eventually induces a secondary depression of respiratory centers leading to daytime hypoventilation.. Mediated via leptin pathway.

3) OSA.

Question 6

what is the therapy for OHS?

Answer 6

PAP therapy. the modality depends on the phenotype.

central hypoventilation may benefit more from BiPAP. OSA may benefit more from CPAP. In the ERS Masa et al 2019 review on OHS, the data they review does not demonstrate superiority for either modality.