obesity hypoventillation syndrome

Question 1

diagnostic criteria?

Answer 1

obesity with BMI = or more than 30 kg/m2, awake daytime hypercapnia ( PaCo2 > 45 mmHg)
no alternate cause of hypoventilation.

Question 2

workup?

Answer 2

ABG on room air ( hypercapnia, normal A-a gradient).

No intrinsic pulmonary disease on CXR. A restrictive pattern on PFTs. Normal TSH. polysomnography.

Question 3

Treatment?

Answer 3

Nocturnal positive pressure ventilation as first-line therapy.
weight loss ( bariatric surgery in select cases).
Avoidance of sedative medications.
respiratory stimulants ( ex: acetazolamide ) as last resort.

Question 4

pt with OHS also have?

Answer 4

coexisting obstructive sleep apnea with freq apneic events and daytime hypersomnolence.

Question 5

other features?

Answer 5

dyspnea, polycythemia, respiratory acidosis with compensatory metabolic alkalosis, pulmonary hypertension, and cor pulmonale.

Question 6

mechanisms behind hypoventilation and subsequent hypercapnia?

Answer 6

obesity reduces chest wall and lung compliance, leading to a decrease in tidal volumes and total lung capacity and an increase in airway resistance. higher levels of the ventilatory drive are required to maintain normocapnia, inability to exhale excess co2 during the day due to persistent resistance as a result co2 accumulates during the night with subsequent chronic respiratory acidosis.
Renal Hco3 exertion is decreased as a compensatory mechanism that blinds the respiratory ventilatory response to increased co2 and leads to hypoventilation.