Renal/Urology · PANCE / PANRE

Respiratory Acidosis and Respiratory Alkalosis

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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Risk factors

Respiratory acidosis (alveolar hypoventilation): CNS depression (opioids, benzodiazepines, sedatives, anesthesia, brainstem stroke), neuromuscular disease (myasthenia gravis, ALS, Guillain-Barré, botulism, severe hypophosphatemia/hypoK), chest wall disease (kyphoscoliosis, flail chest, ankylosing spondylitis), obesity hypoventilation syndrome and OSA, airway disease (COPD exacerbation, severe asthma, upper airway obstruction), alveolar disease (severe ARDS, advanced pulmonary fibrosis), pneumothorax
Respiratory alkalosis (alveolar hyperventilation): pain, anxiety/panic, fever, sepsis, hypoxemia (pneumonia, PE, high altitude), CNS disease (CVA, tumor, infection), pregnancy (progesterone), liver failure, salicylate toxicity, mechanical overventilation, theophylline, catecholamine excess

Pathophysiology

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.

Clinical presentation

Symptoms

Respiratory acidosis: dyspnea, somnolence, confusion, headache (CO2-mediated cerebral vasodilation), asterixis, eventual coma ('CO2 narcosis')
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
Pulmonary embolism — Acute dyspnea, hypoxia, tachycardia; resp alkalosis with widened A-a gradient; CTPA, V/Q
Anxiety / panic attack — Resp alkalosis with paresthesias, carpopedal spasm, dizziness; diagnosis of exclusion after ruling out organic causes

Diagnostic workup

Diagnostic criteria

Acid-base diagnostic algorithm: (1) examine pH (acidemia or alkalemia), (2) determine primary disorder by direction of PaCO2 and HCO3 changes, (3) calculate expected compensation — deviation indicates additional primary disorder, (4) calculate anion gap and delta-delta in suspected mixed disorders. Acute resp acidosis: ΔHCO3 = 1 × (ΔPaCO2/10). Chronic resp acidosis: ΔHCO3 = 3.5-4 × (ΔPaCO2/10). Acute resp alkalosis: ΔHCO3 = -2 × (ΔPaCO2/10). Chronic resp alkalosis: ΔHCO3 = -4 to -5 × (ΔPaCO2/10).

Labs

ABG — pH, PaCO2, PaO2; calculate A-a gradient: A-a = (713 × FiO2 − PaCO2/0.8) − PaO2
BMP — bicarbonate, anion gap; assess compensation against expected values
Lactate, salicylate, glucose, ketones if mixed disorder suspected
CBC, troponin, BNP, D-dimer if dyspnea workup
Toxicology screen for opioid or salicylate ingestion

Imaging

Chest X-ray — pneumonia, pneumothorax, hyperinflation, edema
CT pulmonary angiogram if PE suspected
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

Acute respiratory acidosis (severe asthma, opioid overdose, neuromuscular crisis)

Immediate airway management; intubation if pH <7.20 or apnea
Reverse precipitant (naloxone, bronchodilators, IV magnesium for asthma)
Avoid excessive sedation in spontaneously breathing patients

Chronic respiratory acidosis (COPD, obesity hypoventilation, neuromuscular disease)

Target SpO2 88-92% to preserve hypoxic drive (in COPD)
Long-term non-invasive ventilation (BiPAP) at home for OHS and selected COPD/NMD
Pulmonary rehabilitation; smoking cessation
Avoid sedatives that further depress ventilation

Acute respiratory alkalosis (anxiety, pain, fever, sepsis, PE)

Identify and treat trigger
Supplemental oxygen if hypoxic
Anxiolysis (benzodiazepine) in pure anxiety after excluding organic causes
Heparin and CT-PA workup for suspected PE

Chronic respiratory alkalosis (pregnancy, chronic liver disease, high altitude)

Often no specific treatment needed
Address underlying disease
Acetazolamide for high-altitude prophylaxis (also accelerates acclimatization)

Second-line / adjunct

ECMO for refractory respiratory failure
Tracheostomy for chronic ventilator dependence
Stop hyperventilation-causing drugs (salicylates, theophylline, progesterone-containing agents)

Complications

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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