Renal, Fluids, Electrolytes & Acid-Base · Topic 22 of 22

Respiratory Acidosis & Alkalosis

Renal & electrolytes · acid–base

Respiratory Acidosis & Alkalosis

PaCO₂ moves the pH. Fix the ventilation, and judge acute versus chronic by how far bicarbonate has moved per 10 mmHg.

Key numbers

PaCO₂ up = acidosis; PaCO₂ down = alkalosisPaCO₂ 35–45 mmHg

Acidosis: HCO₃⁻ +1 acute, +4 chronic↑ 1 acute · ↑ 4 chronic

Alkalosis: HCO₃⁻ −2 acute, −5 chronic↓ 2 acute · ↓ 5 chronic

01 · CalculateAcidosis: HCO₃⁻ +1 acute, +4 chronic

Respiratory acidosis: bicarbonate rises about 1 mEq/L per 10 mmHg PaCO₂ rise acutely, about 4 mEq/L once renal compensation is complete (3–5 days). More means added metabolic alkalosis; less, added metabolic acidosis.

02 · CalculateAlkalosis: HCO₃⁻ −2 acute, −5 chronic

Respiratory alkalosis: bicarbonate falls about 2 mEq/L per 10 mmHg PaCO₂ fall acutely, about 5 mEq/L after 2–3 days of renal compensation. More means added metabolic acidosis; less, added metabolic alkalosis.

03 · DoTreat respiratory acidosis by restoring ventilation

Adequate ventilation alone corrects respiratory acidosis: reverse the cause (naloxone for opioid overdose, bronchodilators for obstruction) and provide noninvasive positive-pressure ventilation or intubation as needed.

What it is

Core idea

PaCO₂ up = acidosis; PaCO₂ down = alkalosis must knowRespiratory acidosis is a primary rise in PaCO₂ from alveolar hypoventilation; respiratory alkalosis is a primary fall in PaCO₂ from hyperventilation. Normal PaCO₂ is about 35–45 mmHg; the kidneys compensate by retaining or excreting bicarbonate.

Causes & risk factors

Why it happens

Hypoventilation causes: COPD, sedatives, neuromuscular weaknessCommon causes of acute respiratory acidosis: COPD or severe asthma exacerbation, opioid or benzodiazepine overdose, neuromuscular weakness (Guillain-Barré, myasthenia gravis, botulism) and obstructive sleep apnea or obesity hypoventilation.

Hyperventilation causes: hypoxemia, sepsis, salicylates, liver diseaseRespiratory alkalosis follows hypoxemia (pneumonia, pulmonary embolism, altitude), sepsis, early salicylate toxicity, cirrhosis, pregnancy, pain, anxiety, CNS disease and ventilator overbreathing.

Presentation

What you'll see

Acute hypercapnia: confusion, asterixis, CO₂ narcosisAcute or worsening hypercapnia causes headache, confusion, drowsiness and stupor (CO₂ narcosis), with tremor, myoclonus, asterixis and papilledema; cerebral vasodilation raises ICP. Chronic hypercapnia is often well tolerated.

Acute hypocapnia: paresthesias and carpopedal spasmAcute respiratory alkalosis causes light-headedness, confusion, circumoral and peripheral paresthesias, cramps and syncope; carpopedal spasm reflects a fall in ionized calcium.

Diagnosis & monitoring

Work-up

Neuromuscular weakness: follow vital capacity and NIFIn neuromuscular weakness the ABG can stay normal until late: vital capacity below 10–15 mL/kg or inability to generate a negative inspiratory force of 15 cmH₂O signals imminent ventilatory failure.

Do right now

In this order

First moves

Treat respiratory acidosis by restoring ventilation

→

No sodium bicarbonate for respiratory acidosis

→

Lower chronic hypercapnia slowly on the ventilator

Why each step

Treat respiratory acidosis by restoring ventilationAdequate ventilation alone corrects respiratory acidosis: reverse the cause (naloxone for opioid overdose, bronchodilators for obstruction) and provide noninvasive positive-pressure ventilation or intubation as needed.

No sodium bicarbonate for respiratory acidosisSodium bicarbonate is almost always contraindicated in respiratory acidosis because it can cause paradoxical CNS acidosis; a possible exception is severe bronchospasm, where it may improve beta-agonist response.

Lower chronic hypercapnia slowly on the ventilatorCorrect chronic hypercapnia over hours: rapid PaCO₂ lowering unmasks the retained bicarbonate, causing posthypercapnic alkalosis and an abrupt rise in CNS pH that can trigger seizures and death.

Management

Orders

Bilevel NIV first for COPD hypercapnic failureNoninvasive positive-pressure ventilation is the preferred initial support for acute-on-chronic hypercapnic failure in COPD; intubate for worsening mental status, fatigue or deteriorating gases despite NIV.

Treat the cause; no paper-bag rebreathingRespiratory alkalosis itself is not life-threatening, so treat the cause rather than the pH; paper-bag rebreathing can cause hypoxia and can blunt compensation for an unrecognized metabolic acidosis.

The rules

Know these

Exclude PE before blaming anxietyDo not attribute hyperventilation to anxiety until serious causes are excluded: check oxygenation and the A–a gradient, and strongly consider pulmonary embolism, which often presents without hypoxemia.

Acute or chronic respiratory acidosis?

Commonly confused

HCO₃⁻ rise per 10 mmHg PaCO₂	About 1 mEq/L
pH	Clearly acidemic
Symptoms	Headache, confusion, somnolence, CO₂ narcosis
Marker of acute-on-chronic failure	—
Correction	Restore ventilation promptly

Commonly confused

Tell apart

Salicylates: respiratory alkalosis plus gap acidosisSalicylate overdose directly stimulates the respiratory center (respiratory alkalosis) while uncoupling oxidative phosphorylation (anion gap metabolic acidosis) — a classic mixed disorder.

More detail

Reference

Show 6 reference facts

Severe acidemia (pH < 7.2) from hypercapnia causes pulmonary vasoconstriction, systemic vasodilation, reduced myocardial contractility, hyperkalemia, hypotension and arrhythmias.

In total apnea PaCO₂ rises about 3–6 mmHg per minute, faster than any renal compensation.

Increased CO₂ production (fever, sepsis, burns, hyperthyroidism, carbohydrate overfeeding) causes hypercapnia only when the patient cannot raise minute ventilation to match.

Pseudorespiratory alkalosis: in ventilated patients with poor perfusion (cardiogenic shock, CPR) the arterial gas shows alkalosis while venous blood is acidotic with high lactate; treat the hemodynamics.

Progesterone drives chronic respiratory alkalosis in pregnancy; PaCO₂ can fall to about 28 mmHg in the third trimester without needing treatment.

Respiratory alkalosis shifts potassium and phosphate into cells (mild hypokalemia and hypophosphatemia) and lowers ionized calcium.

Pearl

Boston 1-4-2-5: per 10 mmHg PaCO₂, bicarbonate rises 1 then 4 in acidosis and falls 2 then 5 in alkalosis.

Sources · Merck Manual Professional — Respiratory Acidosis (Mar 2025) · Merck Manual Professional — Respiratory Alkalosis (updated Jul 2026) · Merck Manual Professional — Acid-Base Disorders (table: Primary Changes and Compensations) · Merck Manual Professional — Ventilatory Failure (Jun 2026) · Tucker AM, Johnson TN. Breathing and balance: clinical insights and management strategies of respiratory acid-base disorders. Nutr Clin Pract 2025 · Merck Manual Professional — Treatment of Acute COPD Exacerbation (ventilatory support) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).