Attending rounds: How to use acetazolamide to correct hypercapnia in OSA and COPD?

A 68 yo CF with morbid obesity, OSA and hypercapnia is admitted to the hospital. ABG is shown below:


ABG

What is the abnormality on ABG?

ABG shows chronic (compensated) primary respiratory acidosis, with metabolic alkalosis. Click here for MedCalc: Acid-Base Calculator.

The patient refuses to wear her BiPAP at home and in the hospital. She has episodes of confusion with elevation of PaCO2 which resolve when PaCO2 is decreased with BiPAP.

Will she benefit from acetazolamide (Diamox)?

A therapeutic trial of acetazolamide is probably indicated in attempt to control hypercapnia in this patient.

Acetazolamide, an inhibitor of carbonic anhydrase, stimulates the ventilatory drive by inducing metabolic acidosis.

Patients receiving who are able to increase their ventilation will decrease their PaCO2 because their increased ventilatory drive will surpass their impaired excretion PCO2. In contrast, patients who are unable to augment their ventilation (eg, those with severe COPD) may develop a severe acidosis on acetazolamide.

Acetazolamide reduced the AHI and destaturation index but the effect is modest. It can not be recommended for the routine care of patients with OSA.

Acetazolamide may play a role in the treatment of acute metabolic alkalosis caused by diuresis or steroids in patients with hypercapnic COPD. A short course may correct the underlying metabolic alkalosis that may be the cause of worsening hypercapnia. Acetazolamide has not been demonstrated in large controlled trials to have significant benefit for the long-term management of hypercapnia in COPD patients.

When used used as respiratory stimulant in COPD and OSA (unlabeled use), acetazolamide dose is 250 mg twice daily. It may induce agranulocytosis and monitoring of CBC is needed with prolonged use.

References:
Acid-base Balance Cases and Calculators. Clinical Cases and Images - Blog.
Use of Acetazolamide in COPD Patients. Medscape.
Disorders of ventilatory control. UpToDate, 15.3.
Carbonic anhydrase inhibitors for hypercapnic ventilatory failure in chronic obstructive pulmonary disease. Cochrane Reviews.
Google Books: 1 and 2.

Related:
A challenging ABG: question and answer. DB’s Medical Rants, 02/2008.

Updated: 03/07/2008