Respiratory Acidosis

When CO₂ climbs because the lungs can't blow it off, pH drops fast — but the cause determines whether you grab a bronchodilator or call a rapid response for impending respiratory failure.

Respiratory acidosis occurs when alveolar ventilation is inadequate to eliminate CO₂, causing PaCO₂ to rise above 45 mmHg and pH to fall below 7.35. The core problem is always hypoventilation — anything that depresses the respiratory drive, obstructs airways, or impairs gas exchange at the alveolar level. Acute causes include opioid overdose, anesthesia recovery, pneumothorax, and severe asthma exacerbation. Chronic causes include COPD, obesity hypoventilation syndrome, and neuromuscular diseases like myasthenia gravis or ALS. In acute respiratory acidosis, pH drops sharply because the kidneys haven't had time to compensate; expect HCO₃⁻ to rise only ~1 mEq/L per 10 mmHg rise in PaCO₂ (e.g., pH 7.28, PaCO₂ 58, HCO₃⁻ 26). In chronic respiratory acidosis, renal compensation begins within hours but reaches full effect over 3–5 days, with HCO₃⁻ rising ~3.5 mEq/L per 10 mmHg PaCO₂ increase, bringing pH near-normal despite persistently elevated PaCO₂ (e.g., pH 7.35, PaCO₂ 58, HCO₃⁻ 32). Nursing assessment priorities include monitoring respiratory rate, depth, and pattern; oxygen saturation; and level of consciousness. Rising CO₂ causes CO₂ narcosis — progressive somnolence, confusion, and eventually coma. For the client with COPD, high-flow oxygen can worsen CO₂ retention through suppression of hypoxic ventilatory drive and V/Q mismatch changes; titrate to SpO₂ 88–92% using low-flow oxygen. Interventions focus on restoring ventilation: repositioning, incentive spirometry, bronchodilators, suctioning, naloxone for opioid-induced hypoventilation, or mechanical ventilation when conservative measures fail.

Key Distinctions

Don't confuse acute respiratory acidosis (low pH, high CO₂, normal HCO₃⁻) with chronic compensated (near-normal pH, high CO₂, high HCO₃⁻) — the bicarbonate tells you the timeline. Students mix up respiratory acidosis with metabolic acidosis — when you see a low pH, check PaCO₂ first before looking at HCO₃⁻ to determine respiratory versus metabolic origin. Drowsiness in respiratory acidosis signals worsening, not improvement — CO₂ narcosis is a red flag, not reassurance.

Clinical Pearl

Think "can't blow off CO₂" — if the lungs aren't ventilating, carbon dioxide piles up like exhaust in a garage with the door closed.