Respiratory failure may be classified as hypercapnic or hypoxemic.
Hypercapnic respiratory failure is defined as an arterial PCO2 (PaCO2 ) greater than 45mmHg.
Hypoxemic respiratory failure is defined as an arterial PO2 (PaO2 ) less than 55 mmHg when the fraction of oxygen in inspired air (FiO2) is 0.60 or greater.
In many cases, hypercapnic and hypoxemic respiratory failure coexist.
Distinctions between acute and chronic respiratory failure are summarized in the table below.
In general, acute hypercapnic respiratory failure is defined as a PaCO2 greater than 45 mmHg with accompanying acidemia (pH less than 7.30). The physiological effect of a sudden increase in PaCO2
depends on the prevailing level of serum bicarbonate anion. In patients with chronic hypercapnic respiratory failure e.g. COPD, a long-standing increase in PaCO2 results in renal compensation and an increased serum bicarbonate concentration. A superimposed acute increase in PaCO2 has a less dramatic effect than does a comparable increase in a patient with a normal bicarbonate level.
Distinction between acute and chronic hypoxemic respiratory failure may not be readily made on the basis of arterial blood gas values only. The presence of markers of chronic hypoxemia (e.g., polycythemia or cor pulmonale) provides clues to a long-standing disorder, whereas abrupt changes in mental status suggest an acute event.