Primary CO2 retention or loss from ventilation derangements; pH compensated by renal HCO3 handling.
Also known as: respiratory acidosis, respiratory alkalosis, hypercapnia, hyperventilation syndrome, CO2 retention
Overview
Respiratory acidosis: primary increase in PaCO2 (>45 mmHg) with decreased pH (<7.35), from alveolar hypoventilation. Respiratory alkalosis: primary decrease in PaCO2 (<35 mmHg) with increased pH (>7.45), from alveolar hyperventilation. Each can be acute (minutes-hours, minimal renal compensation) or chronic (>2-3 days, full renal compensation).
Epidemiology
Respiratory acidosis: common in COPD exacerbations, severe asthma, opioid overdose, and neuromuscular disease. Respiratory alkalosis is among the most common acid-base disorders in hospitalized patients — present in sepsis, pneumonia, anxiety, pulmonary embolism, salicylate toxicity, hepatic failure, and high-altitude exposure.
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Question 1RenalMedium
A 24-year-old woman is brought to the emergency department after an argument with her partner, reporting rapid breathing, light-headedness, and tingling around her mouth and in both hands. She has no medical history, takes no medications, and denies fever, chest pain, or recent travel. On exam she is afebrile, with a heart rate of 104/min, respiratory rate of 32/min, and SpO2 of 99% on room air; flexion of the wrist and fingers (carpopedal spasm) is noted. Arterial blood gas on room air shows pH 7.52, PaCO2 27 mmHg, PaO2 105 mmHg, and HCO3 21 mEq/L, with a normal A-a gradient. Which of the following best explains these findings?
AAcute respiratory alkalosis
BAcute respiratory acidosis
CChronic respiratory alkalosis
DChronic respiratory acidosis
Reveal answer & full explanation
Correct answer: A — Acute respiratory alkalosis
AAcute respiratory alkalosis✓
BAcute respiratory acidosis
CChronic respiratory alkalosis
DChronic respiratory acidosis
Why Acute respiratory alkalosis is correct
The ABG shows alkalemia (pH 7.52) driven by a primary fall in PaCO2 (27 mmHg) from alveolar hyperventilation, the defining pattern of respiratory alkalosis.
HCO3 of 21 fits the expected acute compensation: acute respiratory alkalosis lowers HCO3 about 2 mEq/L per 10 mmHg drop in PaCO2 (here PaCO2 fell ~13 from 40, predicting HCO3 ~22), confirming an acute rather than chronic process.
Perioral and finger paresthesias plus carpopedal spasm reflect an acute drop in ionized calcium as pH rises, a hallmark of acute hyperventilation triggered by anxiety after organic causes are excluded.
Why the others are wrong
Chronic respiratory alkalosis: renal adaptation over days would lower HCO3 much more (about 4 to 5 per 10 mmHg drop in PaCO2, to roughly 14 to 15 here); the acute onset and HCO3 of 21 point to an acute disorder.
Acute respiratory acidosis: this would show a LOW pH with a HIGH PaCO2 from hypoventilation; here the pH is high and PaCO2 is low, the opposite pattern.
Chronic respiratory acidosis: this also produces a low or near-normal pH with elevated PaCO2 and a compensatory HIGH HCO3 (the COPD pattern); it does not fit an alkalemic patient with a low PaCO2.
Question 2RenalMedium
A 64-year-old man with severe COPD comes to clinic for a routine visit. He is at his baseline, with mild chronic dyspnea on exertion but no fever, increased cough, or change in sputum. He uses home oxygen at 2 L/min. Vital signs are stable and he is alert. An arterial blood gas and metabolic panel show: pH 7.36, PaCO2 62 mmHg, HCO3 34 mEq/L, anion gap 10. Which of the following findings best supports a diagnosis of chronic (compensated) respiratory acidosis rather than an acute process?
AArterial blood pH value of 7.36
BArterial PaCO2 value of 62 mmHg
CSerum anion gap of 10 mEq/L
DSerum bicarbonate of 34 mEq/L
Reveal answer & full explanation
Correct answer: D — Serum bicarbonate of 34 mEq/L
AArterial blood pH value of 7.36
BArterial PaCO2 value of 62 mmHg
CSerum anion gap of 10 mEq/L
DSerum bicarbonate of 34 mEq/L✓
Why Serum bicarbonate of 34 mEq/L is correct
In acute respiratory acidosis the kidney has not yet adapted, so HCO3 rises only ~1 mEq/L per 10 mmHg rise in PaCO2. With PaCO2 62 (a ~22 mmHg rise above 40), acute compensation would predict an HCO3 of only ~26.
An HCO3 of 34 reflects the robust renal adaptation of chronic respiratory acidosis (~3.5-4 mEq/L per 10 mmHg over 2-5 days). The classic teaching point is that HCO3 >30 in a hypercapnic patient implies chronic disease (or a coexisting metabolic alkalosis), making the elevated bicarbonate the single finding that distinguishes chronic from acute.
Why the others are wrong
Arterial blood pH value of 7.36: a near-normal pH is consistent with compensation but does not by itself separate chronic from acute, since an acute disorder can also show a borderline pH early, and pH alone does not quantify the renal response.
Arterial PaCO2 value of 62 mmHg: hypercapnia confirms a respiratory acidosis but says nothing about chronicity, because both acute and chronic respiratory acidosis are defined by an elevated PaCO2.
Serum anion gap of 10 mEq/L: a normal gap simply argues against a superimposed anion-gap metabolic acidosis and is a non-discriminating finding for distinguishing acute from chronic respiratory acidosis.
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Ventilation matches CO2 production to PaCO2. Hypoventilation raises PaCO2 and lowers pH; hyperventilation lowers PaCO2 and raises pH. CO2 readily diffuses through cell membranes, so changes in PaCO2 promptly affect intracellular and extracellular pH. Renal compensation occurs through changes in proximal tubular bicarbonate reabsorption and distal H+ secretion/ammoniagenesis: acute disorders see minimal HCO3 shift, while chronic disorders show robust HCO3 adaptation over 2-5 days. Expected compensation rules: acute resp acidosis raises HCO3 ~1 per 10 mmHg ΔPaCO2; chronic raises HCO3 ~3.5-4 per 10. Acute resp alkalosis lowers HCO3 ~2 per 10; chronic lowers HCO3 ~4-5 per 10.
Respiratory alkalosis: dyspnea, anxiety, light-headedness, paresthesias (especially perioral and fingers), carpopedal spasm from acute reduction in ionized calcium, syncope, chest tightness
Signs / physical exam
Resp acidosis: hypoventilation, cyanosis, papilledema, asterixis, depressed level of consciousness; tachypnea with shallow breathing in muscle weakness
Resp alkalosis: tachypnea, Trousseau and Chvostek signs (functional hypocalcemia), tetany; underlying cause clues (fever, pain, focal neuro deficits)
Classic findings
COPD patient with chronic hypercapnia (chronic resp acidosis): elevated HCO3 with mild pH derangement, sleepy on oxygen — preserve hypoxic drive at SpO2 88-92%. Hyperventilating young patient with carpopedal spasm: acute resp alkalosis from anxiety, sepsis, or PE — search for organic cause.
Differential diagnosis
Metabolic acidosis with respiratory compensation — Primary low HCO3 with appropriate hyperventilation; pH low or normal; calculate Winter formula
Metabolic alkalosis with respiratory compensation — Primary high HCO3 with appropriate hypoventilation; pH high or normal
Mixed acid-base disorder — Compensation outside expected range — suspect a second primary disorder; common in critically ill (e.g., sepsis with resp alkalosis + lactic acidosis + AKI)
Salicylate toxicity — Mixed resp alkalosis (early, direct stimulation of medulla) + anion-gap metabolic acidosis (later); tinnitus, hyperthermia, AMS
CT or MRI brain if CNS cause suspected (stroke, ICH, brainstem lesion)
Echocardiogram if right heart strain or shunt suspected
Diagnostic algorithm
Disorder
pH
PaCO2
HCO3 (compensation)
Classic causes
Acute resp acidosis
↓ <7.35
↑ >45
↑ 1 per 10 ΔPaCO2
Opioid overdose, severe asthma, neuromuscular crisis
Chronic resp acidosis
↓ or near normal
↑ >45
↑ 3.5-4 per 10 ΔPaCO2
COPD, OHS, kyphoscoliosis, OSA
Acute resp alkalosis
↑ >7.45
↓ <35
↓ 2 per 10 ΔPaCO2
Anxiety, pain, sepsis, PE, fever, early salicylate
Chronic resp alkalosis
↑ or near normal
↓ <35
↓ 4-5 per 10 ΔPaCO2
Pregnancy, chronic liver disease, high altitude
Respiratory acid-base disorders — expected compensation determines acute vs chronic and detects mixed disorders.
Treatment
First-line
Treat the underlying cause — reverse opioid with naloxone, treat asthma/COPD exacerbation with bronchodilators (albuterol, ipratropium, levalbuterol) and systemic corticosteroids (prednisone, methylprednisolone, dexamethasone), antibiotics for pneumonia, anticoagulation for PE
Restore alveolar ventilation — non-invasive ventilation (BiPAP) is first-line for COPD exacerbation hypercapnia and obesity hypoventilation; mechanical ventilation if NIV failure or severe acidemia (pH <7.20)
Resp acidosis with CO2 narcosis from oversedation: airway support, naloxone, flumazenil (if benzodiazepine and no chronic use)
Resp alkalosis: identify and treat trigger (pain, sepsis, hypoxemia, PE, salicylate); reassurance and breath control in pure psychogenic hyperventilation
Avoid bag rebreathing in hyperventilation — risks dangerous hypoxemia and overshoot hypercapnia; treat the cause instead
Resp acidosis: CO2 narcosis with coma, cardiac arrhythmias (especially with hypoxemia), pulmonary hypertension and cor pulmonale (chronic), increased intracranial pressure, hyperkalemia from H+/K+ exchange
Resp alkalosis: cerebral vasoconstriction → confusion, syncope, seizures; carpopedal spasm and tetany from functional hypocalcemia; arrhythmias; coronary vasospasm (especially with cocaine), worsened tissue oxygenation (leftward shift of oxyhemoglobin curve)
PANCE pearls
In COPD, give oxygen to maintain SpO2 88-92% — overcorrection can worsen hypercapnia via V/Q derangement and loss of hypoxic drive.
Salicylate toxicity classically produces mixed respiratory alkalosis + anion-gap metabolic acidosis — check level in any unexplained mixed disorder, especially with tinnitus and hyperthermia.
Acute resp acidosis raises HCO3 only ~1 per 10 mmHg PaCO2 rise; HCO3 >30 in hypercapnia implies chronic disease or coexisting metabolic alkalosis.
Pregnancy produces chronic respiratory alkalosis (progesterone-driven) — a 'normal' PaCO2 of 40 in a pregnant patient is abnormal.
Carpopedal spasm during hyperventilation reflects acute drop in ionized calcium, not total hypocalcemia.
References
Berend 2014 — Berend K et al. Physiological approach to assessment of acid-base disturbances (NEJM 2014;371:1434-1445)
GOLD 2024 — Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2024 Report: Strategy for the Diagnosis, Management, and Prevention of COPD
Wedro & Olshaker — Wedro BC, Olshaker JS. Approach to acid-base disorders (Emerg Med Clin North Am 2014;32:459-483)
Pal 2020 — Pal A et al. Respiratory acidosis (StatPearls 2023)
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