Fluid, Electrolyte & Acid-Base · Topic 5 of 23
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: read the gas in three calls
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A textbook respiratory acidosis with no patient attached yet. Read it in three calls, then load a patient.
Make the three calls first. What to look for, what to do and what to expect ordered fill in for the disorder you find.
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Respiratory acidosis
Breathing too little. CO₂ stays in the blood, the pH falls — and the patient gets sleepier as it climbs.
Slow, shallow breathing → CO₂ builds up → acidosis
Example · opioid overdose, RR 6: pH 7.26 · PaCO₂ 60 · HCO₃⁻ 26 — acute, the kidneys haven't caught up. Illustration: real PaCO₂ depends on breath depth as well as rate.
mmHg · normal 35 – 45 · with a pH below 7.35 (normal 7.35 – 7.45)
Too little air moving in and out, so CO₂ builds up — every cause ends here
Drowsiness is CO₂ sedating the brain, not rest — report new drowsiness or confusion
What causes it
Watch forEvery cause ends in hypoventilation
Acute-on-chronic: a patient who already retains CO₂ (e.g., COPD) gets an acute problem such as pneumonia, and the pH falls further.
What you'll see
SignsLabs & diagnostics
CheckPaCO₂ above 45 with pH below 7.35
Expected compensation: HCO₃⁻ rises about 1 mEq/L per 10 mmHg PaCO₂ rise when acute, and about 3.5 – 4 mEq/L per 10 mmHg when chronic.
Red flags
Act nowTreatment: help them breathe
VentilationImprove the ventilation — sodium bicarbonate is almost always contraindicated (it does not remove CO₂)
Reverses opioid-induced hypoventilation.
Shorter-acting than many opioids — watch for re-sedationOpens obstructed airways.
For an alert patient who can protect the airway; intubation if BiPAP fails.
Target SpO₂ 88 – 92% for a chronic CO₂ retainer (e.g., COPD).
SpO₂ near 99% = too muchMore detail
Nursing priorities
In orderRespiratory or metabolic acidosis?
ROME — Respiratory Opposite, Metabolic Equal
| pH | pH below 7.35 |
|---|---|
| PaCO₂ | High PaCO₂ (above 45) — the cause |
| HCO₃⁻ | Normal at first; above 26 when chronic — the kidneys hold on to it |
| Which way | PaCO₂ moves opposite to the pH (Respiratory Opposite) |
| Breathing | Slow, shallow breathing — hypoventilation |
| Potassium | Usually only mildly high |
| Classic causes | Opioid overdose, COPD, severe asthma |
| Fix | Improve the breathing — naloxone, bronchodilator, BiPAP |
Teach your patient
DischargeRespiratory acidosis: CO₂ trapped by breathing too little. A patient who is getting sleepier is getting sicker, not resting — report it.
Sources · OpenStax Medical-Surgical Nursing 10.4, OpenStax Fundamentals of Nursing 20.2, StatPearls on respiratory acidosis, ABG analysis and hypercapnia, Merck Manual Professional: respiratory acidosis, Open RN Nursing Skills 11.2, the British Thoracic Society emergency oxygen guideline and Orr et al., opioids and respiratory consequences. Typical adult values — follow your protocol.
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 patient 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.
Knowledge Check
3 quick questions on the must-knows for this topic.
What is the core problem driving respiratory acidosis?
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Go further
- SimulatorAcid–Base & ABG SimulatorBuild a gas. Watch compensation happen. Move CO₂ or bicarbonate, then read the gas in three calls: pH, cause, compensation.
- SimulatorFluid & Electrolyte SimulatorChange one lab. Watch the whole patient respond. Sodium, potassium, calcium, magnesium, pH and volume, all linked — with a live ECG and an IV pole where every bag runs.
- GuideRespiratory vs Metabolic Acidosis and Alkalosis: The 4 Disorders With Causes, Findings and What the Nurse DoesTwo organs, two directions. Respiratory disorders are CO₂ problems that belong to the lungs and happen in minutes; metabolic disorders show up in the bicarbonate and come from everywhere else. Each of the four with its causes, what you see, what the nurse does and what the provider orders — in a table, then patient by patient, with each one loaded in a live gas.
- GuideABG NCLEX Questions: The 6 Patterns the Exam Keeps TestingThe NCLEX rarely asks you to name a gas. It asks what the gas means and what the nurse does. Name the gas with the normal-pH trap, which patient is at risk, what the nurse does first, what the provider will order, the electrolyte that moves with the pH, and the finding that means it is getting worse — with a live gas for the anticipate-the-order pattern.
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