Internal Medicine

0521779407-C02 CUNY1086/Karliner 0 521 77940 7 June 4, 2007 20:53

Chronic Respiratory Acidosis 351

Chronic Respiratory Acidosis............................

F. JOHN GENNARI, MD

history & physical

■CO 2 retention for >3–4 days

■May be asymptomatic

■Exertional dyspnea

■Right-sided heart failure (cor pulmonale)

■Cardiac arrhythmias

■Confusion, fine tremor

■Abnormal chest examination, peripheral edema, papilledema when

severe

tests

Laboratory

■Arterial blood gases are diagnostic – PCO2 >45 mmHg, pH <7.40 (see

below for rule of thumb for [HCO3−])

Imaging

■Chest X-ray, CT of chest if necessary

differential diagnosis

■Conditions to Distinguish from Chronic Respiratory Acidosis

➣Metabolic alkalosis

➣Mixed disorders

Acute on chronic respiratory acidosis

Acute respiratory acidosis plus metabolic alkalosis

■Rule of Thumb:

➣Expected [HCO3−] in chronic respiratory acidosis:

➣[HCO3−](expected)=24 mEq/L+0.4 (PCO2−40, mmHg)

Observed [HCO3−] should be within 4 mEq/L of the expected

value in uncomplicated chronic respiratory acidosis.

■Causes of Chronic Respiratory Acidosis

➣Chronic obstructive pulmonary disease (most common cause)

➣CNS depression

➣Sleep apnea

➣Obesity-hypoventilation syndrome

➣Neuromuscular impairment (see under acute respiratory acido-

sis)

➣Restrictive lung diseases