Is ABG Necessary to Diagnose Hypercarbic Respiratory Failure?

Clinicians managing patients in the hospital often rely on arterial blood gases (ABGs), which measure arterial CO₂ levels (PaCO₂), to diagnose hypercarbic respiratory failure. However, in this review from the Journal of Hospital Medicine's series, “Things We Do for No Reason,” authors make a convincing case for using venous blood gases (VBGs) — rather than ABGs — as the initial screening test (J Hosp Med 2025 Apr 4; [e-pub]).

Compared with ABGs, VBGs have a slightly lower pH (mean difference, 0.03) and bicarbonate level (1–2 mmol/L). Venous CO₂ (PvCO₂) levels are, on average, ≈5 mm Hg higher than PaCO₂ levels in ABGs, although more variation is seen from patient to patient. Nevertheless, a PvCO₂ <45 mm Hg is an exceptionally good test to rul…