Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (612 page)

   Mechanical overventilation, cardiopulmonary bypass

   Diagnostic Findings

   Acute hypocapnia: Usually only a modest decrease in plasma HCO
₃
⁻
concentrations due to conversion to CO
₂
and marked alkalosis

   Chronic hypocapnia: Usually only a slight alkaline pH (not usually >7.55)

RESPIRATORY ACIDOSIS

   Laboratory findings differ in acute and chronic conditions.

   Acute Respiratory Acidosis

   Caused by decreased alveolar ventilation impairing CO
₂
excretion:

   Cardiopulmonary (e.g., pneumonia, pneumothorax, pulmonary edema, foreign body aspiration, laryngospasm, bronchospasm, mechanical ventilation, cardiac arrest).

   CNS depression (e.g., general anesthesia, drugs, brain injury, infection).

   Neuromuscular (e.g., Guillain-Barré syndrome, hypokalemia, myasthenic crisis).

   Acidosis is severe (pH 7.05–7.10), but HCO
₃
⁻
concentration is only 29–30 mmol/L.

   Severe mixed acidosis is common in cardiac arrest, when respiratory and circulatory failure causes marked respiratory acidosis and severe lactic acidosis.

   Chronic Respiratory Acidosis