Respiratory Acidosis and Alkalosis

Fundamentals of CO2 Homeostasis and Acid-Base Chemistry

Respiratory Acidosis – Mechanism of Hypoventilation

Respiratory Alkalosis – Mechanism of Hyperventilation

Effects on Electrolyte and Protein Ionization

Respiratory Compensation for Metabolic Disorders

Respiratory Acidosis – Causes of Hypoventilation

Central Nervous System Depression (Decreased Ventilatory Drive)

• Sedative medications (opioids, benzodiazepines, propofol, barbiturates): dose-dependent suppression of the respiratory centers in the medulla; particularly dangerous in combination with other CNS depressants or in elderly patients with reduced drug clearance
• Anesthesia (general, spinal, epidural): intentional or excessive CNS depression during procedures
• Neurological injuries: brainstem lesions, severe head trauma, or intracranial mass causing altered consciousness and loss of ventilatory drive
• Sleep apnea (obstructive or central): central sleep apnea involves reduced or absent ventilatory effort despite continued airway patency; obstructive sleep apnea involves airway collapse with preserved drive but inability to ventilate
• Hypothyroidism and myxedema coma: decreased metabolic rate and CNS depression
• Obesity hypoventilation syndrome (OHS): combination of severe obesity (BMI typically >30), daytime somnolence, and hypoventilation in the absence of primary lung or neuromuscular disease; thought to result from increased work of breathing, altered respiratory mechanics, and possible CNS CO2 insensitivity

Neuromuscular Weakness

• Guillain-Barré syndrome: acute demyelinating polyradiculoneuropathy causing ascending paralysis; respiratory failure typically occurs when vital capacity drops below 15 mL/kg
• Myasthenia gravis: autoimmune disorder of the neuromuscular junction causing weakness that preferentially affects proximal muscles; respiratory crisis occurs in 10-20% of patients
• Botulism: toxin blocks acetylcholine release at the neuromuscular junction, causing descending paralysis
• Spinal cord lesions (high cervical): injury above C4 interrupts phrenic nerve innervation, eliminating diaphragmatic function
• Muscular dystrophies and polymyositis: progressive muscle weakness eventually affecting respiratory muscles
• Medications: aminoglycosides (especially in patients with myasthenia gravis), neuromuscular blocking agents used in anesthesia

Airway Obstruction

• Severe asthma exacerbation ("status asthmaticus"): bronchial smooth muscle constriction, mucus plugging, and airway edema create flow limitation; "silent chest" (absent breath sounds) indicates severe obstruction and impending respiratory failure
• Anaphylaxis: laryngeal edema causing life-threatening airway obstruction
• Foreign body aspiration: complete or partial obstruction of trachea or mainstem bronchus
• Epiglottitis and retropharyngeal abscess: infectious causes of upper airway obstruction
• Vocal cord paralysis (bilateral): loss of abductor function prevents adequate airway opening during inspiration
• Tracheomalacia and bronchomalacia: structural collapse of airways during expiration

Parenchymal Lung Disease

• COPD exacerbation: acute increase in airway inflammation, mucus production, and bronchospasm superimposed on chronic baseline obstruction; represents the most common cause of acute respiratory acidosis in many populations
• Severe pneumonia or acute respiratory distress syndrome (ARDS): extensive consolidation or diffuse alveolar damage with ventilation-perfusion (V/Q) mismatch and impaired gas exchange
• Pulmonary edema (cardiogenic or non-cardiogenic): fluid-filled alveoli impair CO2 diffusion
• Interstitial lung disease: progressive fibrosis reduces functional alveolar surface area and increases work of breathing
• Massive pulmonary embolism: large thrombus can cause acute cor pulmonale and shock, potentially impairing ventilation despite often initially causing respiratory alkalosis
• Severe pneumothorax (tension or bilateral): mechanical restriction of lung expansion

Chronic Respiratory Acidosis Risk Factors

• COPD: emphysema and chronic bronchitis with progressive airway obstruction
• Cystic fibrosis: genetic disorder causing thick mucus and recurrent infections
• Morbid obesity: mechanical restriction of chest wall and diaphragmatic excursion
• Kyphoscoliosis: severe spinal deformity restricting chest wall movement
• Neuromuscular diseases: amyotrophic lateral sclerosis (ALS), Duchenne muscular dystrophy, post-polio syndrome

Respiratory Alkalosis – Causes of Hyperventilation

Hypoxemia-Driven Hyperventilation

• Pneumonia, ARDS, pulmonary embolism, acute asthma: V/Q mismatch or shunting activates peripheral chemoreceptors via hypoxemia
• High altitude: decreased atmospheric oxygen tension stimulates ventilation as a compensatory response
• Severe anemia: reduced oxygen-carrying capacity triggers increased ventilation
• Congenital heart disease with right-to-left shunt: venous blood bypasses pulmonary capillaries, causing arterial hypoxemia

Metabolic Acidosis Compensation

• Diabetic ketoacidosis (DKA): appropriately triggers Kussmaul respiration; respiratory alkalosis is common but inappropriate respiratory compensation suggests concurrent respiratory disease
• Salicylate toxicity: direct stimulation of respiratory centers in the medulla by salicylate, causing primary respiratory alkalosis with concurrent metabolic acidosis (a mixed disorder)
• Sepsis: endotoxin and inflammatory mediators directly stimulate ventilatory centers; often accompanied by metabolic acidosis from tissue hypoperfusion
• Other causes of metabolic acidosis: lactic acidosis, uremic acidosis, alcoholic ketoacidosis

Direct Respiratory Center Stimulation

• Anxiety and panic attacks: psychological stress triggers hyperventilation; often accompanied by perioral paresthesias, chest pain, and feelings of impending doom
• Pain: acute pain stimulates ventilation as part of stress response
• Pregnancy: progesterone has direct stimulatory effect on respiratory centers
• Fever and hyperthermia: elevated core temperature directly increases metabolic rate and CO2 production while also stimulating respiratory drive
• Thyroid storm: excess thyroid hormone increases metabolic rate and ventilatory drive

Hepatic and CNS Disease

• Hepatic encephalopathy: ammonia and other neurotoxins stimulate respiratory centers
• Intracranial mass or increased intracranial pressure: may cause Cheyne-Stokes respiration or central hyperventilation depending on location
• Meningitis and encephalitis: inflammation of meninges stimulates respiratory drive

Drug-Induced Hyperventilation

• Salicylates (aspirin): uncoupling of oxidative phosphorylation and direct medullary stimulation
• Sympathomimetics: beta-agonists and other catecholamine-releasing agents increase metabolic rate and ventilatory drive
• Methylxanthines (theophylline, caffeine): direct stimulation of respiratory centers

Mechanical Causes

• Pulmonary embolism: despite initial V/Q mismatch, the primary ventilatory stimulus is release of serotonin and other mediators from platelets coating the thrombus, causing reflex hyperventilation that often precedes significant hypoxemia
• Pneumothorax (small): may trigger reflex hyperventilation before substantial V/Q mismatch develops
• Atelectasis: stimulates peripheral chemoreceptors through hypoxemia

Respiratory Acidosis – Acute Presentation (PaCO2 rising acutely with pH <7.35)

• Headache: often described as diffuse, throbbing, and may be severe; mechanism involves cerebral vasodilation from elevated CO2 and associated increased intracranial pressure
• Altered mental status and confusion: elevated PaCO2 causes CNS depression; mild cases present with drowsiness or somnolence, while severe hypercapnia (PaCO2 >80 mmHg) may cause obtundation or frank coma ("CO2 narcosis")
• Tremor ("flapping tremor" or asterixis): fine, irregular tremor of outstretched hands seen in hypercapnic states
• Restlessness and anxiety: paradoxical initial presentation in some patients despite subsequent CNS depression

• Tachycardia: compensatory response to maintain cardiac output despite CNS depression
• Hypertension: catecholamine release from chemoreceptor stimulation
• Peripheral vasodilation and flushing: warmth and redness of skin from cutaneous vasodilation
• Arrhythmias: hyperkalemia-induced changes in myocardial conduction; acute hypercapnia predisposes to ventricular arrhythmias, particularly in patients with underlying cardiac disease
• Increased intracranial pressure (ICP): cerebral vasodilation from CO2 elevation can significantly increase ICP; particularly dangerous in patients with head trauma, mass lesions, or reduced compliance

• Use of accessory muscles: visible contraction of sternocleidomastoid, scalene, and intercostal muscles indicating increased work of breathing in conditions such as severe asthma or COPD exacerbation
• Pursed-lip breathing: COPD patients naturally adopt this breathing pattern to maintain positive end-expiratory pressure (PEEP) and prevent airway collapse
• Cyanosis: bluish discoloration of lips and nail beds indicates severe hypoxemia often accompanying respiratory acidosis from parenchymal disease

Electrolyte-related manifestations

• Hyperkalemia signs: peaked T waves on ECG, muscle weakness, cardiac irritability (see pathophysiology for mechanism)
• Hypocalcemia signs: perioral paresthesias, muscle cramps, tetany (though less prominent than in respiratory alkalosis)

Respiratory Acidosis – Chronic Presentation (compensated hypoventilation)

• Chronic headache: often overlooked or attributed to other causes; may be worse in morning due to hypoventilation during sleep
• Polycythemia: chronic hypoxemia from underlying lung disease stimulates erythropoietin production, leading to elevated hematocrit; helps compensate for impaired oxygen delivery
• Cor pulmonale: right ventricular hypertrophy and subsequent failure from chronic pulmonary hypertension secondary to hypoxemia and acidosis

Initial and confirmatory testing

• Arterial blood gas: the gold standard. Nothing else measures PaCO2 directly with the precision needed to classify the disorder. A venous blood gas plus serum bicarbonate is an acceptable screen (a normal venous PCO2 makes hypercapnia unlikely), but a suspected hypercapnic crisis warrants an arterial sample.
• Pulse oximetry and capnography: track oxygenation and ventilation trends, but SpO2 can be normal in early hypercapnia on supplemental oxygen — oximetry does not exclude respiratory acidosis.
• Basic metabolic panel: gives the HCO3− used for compensation math and detects the hyperkalemia of acidemia or hypokalemia/hypophosphatemia of alkalemia.

Stepwise interpretation

• Step 1 — pH: <7.35 acidemia, >7.45 alkalemia.
• Step 2 — is PaCO2 the culprit?: PaCO2 >45 mmHg with acidemia = respiratory acidosis; PaCO2 <35 mmHg with alkalemia = respiratory alkalosis.
• Step 3 — acute vs chronic: apply the compensation rules already given (roughly 1 mEq/L HCO3− rise per 10 mmHg acute PaCO2 rise vs 3–4 mEq/L when chronic). A markedly elevated HCO3− with a near-normal pH means the process is days old.
• Step 4 — look for a second disorder: calculate the anion gap on every gas, and use Winter's formula to confirm that any low PaCO2 is compensation rather than a primary respiratory alkalosis (classically in salicylate poisoning).

Localizing the lesion

• Alveolar–arterial oxygen gradient: normal in pure hypoventilation (opioids, brainstem injury, neuromuscular weakness); widened when parenchymal disease or shunt is present.
• Bedside respiratory mechanics: serial forced vital capacity and negative inspiratory force in Guillain-Barré or myasthenic crisis — falling values predict failure before the PaCO2 rises.
• Targeted studies: chest radiograph, CT pulmonary angiography, salicylate level, TSH, and polysomnography or nocturnal oximetry when obesity hypoventilation or sleep-disordered breathing is suspected, per American Academy of Sleep Medicine practice parameters.

Immediate stabilisation

• Airway, ventilation, reversible toxins first: hypercapnia is a ventilation problem, so the intervention must move air. Naloxone reverses opioid-induced hypoventilation; flumazenil is generally avoided because of seizure risk in mixed ingestion or chronic benzodiazepine use.
• Controlled oxygen: in COPD, GOLD advises titrating to an SpO2 of about 88–92%. Excess oxygen worsens hypercapnia through loss of hypoxic pulmonary vasoconstriction with V/Q mismatch and the Haldane effect — but never withhold oxygen from a hypoxemic patient.

First-line therapy for hypercapnic respiratory failure

• Noninvasive positive pressure ventilation (BiPAP): GOLD recommends NIV for COPD exacerbation with acidemia (pH ≤7.35) and PaCO2 >45 mmHg; it unloads respiratory muscles, offsets auto-PEEP, and reduces intubation and mortality.
• Bronchodilators: short-acting beta-2 agonist (albuterol) plus short-acting antimuscarinic (ipratropium), with systemic corticosteroids (prednisone) and antibiotics per GOLD for increased sputum purulence.
• Disease-specific therapy: plasma exchange or IVIG for Guillain-Barré and myasthenic crisis (American Academy of Neurology); nocturnal PAP and weight management for obesity hypoventilation (American Thoracic Society).

Escalation

• Intubation and mechanical ventilation: for NIV failure, depressed consciousness, inability to protect the airway, or hemodynamic instability. In severe obstruction use a low respiratory rate and prolonged expiratory time to limit breath-stacking, accepting permissive hypercapnia; ARDS is ventilated at low tidal volume (about 6 mL/kg predicted body weight, plateau pressure under 30 cmH2O) per the ATS/ESICM/SCCM guideline.

Respiratory alkalosis

• Treat the driver — sepsis, pulmonary embolism, pain, hypoxemia, fever. For anxiety-driven hyperventilation, reassurance and paced breathing; paper-bag rebreathing is not recommended because it can cause hypoxemia and masks lethal causes.
• Salicylate toxicity: sodium bicarbonate for serum and urine alkalinization, with hemodialysis for severe poisoning. Intubation is hazardous — losing the compensatory hyperventilation causes precipitous acidemia and CNS salicylate entry.

Contraindicated

• Bicarbonate for pure respiratory acidosis: it generates additional CO2 and can worsen intracellular acidosis.
• Sedatives and unnecessary respiratory depressants in any hypercapnic patient not on ventilatory support.

Complications of respiratory acidosis

• CO2 narcosis and coma: CSF pH falls faster than plasma pH because CO2 crosses the blood–brain barrier freely while HCO3− does not; signalled by progressive somnolence, asterixis, and obtundation. Emergency — impending respiratory arrest.
• Intracranial hypertension: hypercapnic cerebral vasodilation raises cerebral blood volume; in head trauma or a mass lesion this can precipitate herniation. Emergency.
• Arrhythmia: driven chiefly by the catecholamine surge of acute hypercapnia, coexisting hypoxemia, and direct myocardial effects of acidemia (depressed contractility, altered conduction). Transcellular K+ shift contributes only modestly in respiratory acidosis, so peaked T waves should prompt a hunt for an independent cause of hyperkalemia rather than being blamed on CO2 retention.
• Cor pulmonale: chronic hypoxemia and acidemia drive pulmonary vasoconstriction and remodeling; look for a loud P2, elevated JVP, and peripheral edema.

Complications of respiratory alkalosis

• Cerebral hypoperfusion: hypocapnic vasoconstriction produces lightheadedness, visual changes, syncope, and rarely seizure.
• Neuromuscular irritability: alkalemia increases albumin binding of calcium, lowering ionized calcium — perioral and acral paresthesias, carpopedal spasm, Chvostek and Trousseau signs.
• Impaired oxygen unloading: left shift of the oxyhemoglobin dissociation curve; matters most in anemia or shock.
• Hypokalemia and hypophosphatemia: predispose to arrhythmia and weakness.

Complications of treatment

• Oxygen-induced hypercapnia: uncontrolled high-FiO2 therapy in chronic hypercapnic COPD; rising PaCO2 with worsening somnolence. Emergency.
• Dynamic hyperinflation / auto-PEEP: inadequate expiratory time during ventilation of obstructive disease causes breath-stacking, hypotension, and pulseless electrical activity — disconnect the circuit and allow full exhalation. Barotrauma may present as sudden pneumothorax. Emergency.
• Post-hypercapnic metabolic alkalosis: rapid ventilator correction of chronic hypercapnia leaves the renally retained HCO3− unopposed; alkalemia, seizures, and blunted respiratory drive follow. Correct PaCO2 toward the patient's chronic baseline, not to 40 mmHg.
• NIV complications: mask intolerance, gastric insufflation, aspiration in the obtunded patient.
• Bicarbonate given for respiratory acidosis: paradoxical rise in PaCO2 and worsening intracellular acidosis.

• A "normal" PaCO2 in status asthmaticus is a red flag: the tachypneic asthmatic should be hypocapnic. Normalizing or rising PaCO2 with a silent chest means fatigue and impending arrest — the single best next step is intubation, not another albuterol nebulizer.
• Use the A–a gradient to localize hypercapnia: normal gradient points to hypoventilation with healthy lungs (opioids, brainstem lesion, myasthenia, obesity hypoventilation); a widened gradient points to parenchymal or vascular lung disease.
• The bicarbonate tells you how old the problem is: PaCO2 60 mmHg with HCO3− near 26 is acute; the same PaCO2 with HCO3− in the mid-30s and a near-normal pH is chronic. Ventilate a chronic retainer back to their baseline PaCO2, not to 40.
• Salicylate toxicity is the classic mixed disorder: primary respiratory alkalosis from direct medullary stimulation plus an anion-gap metabolic acidosis. Check that the measured PaCO2 matches Winter's formula — if it is lower than predicted, a primary respiratory alkalosis coexists.
• Target SpO2 88–92% in COPD (GOLD): the exam wants controlled oxygen and early BiPAP for pH ≤7.35 with PaCO2 >45 mmHg — but it also wants you to refuse to let a hypoxemic patient stay hypoxemic.
• Perioral tingling with carpopedal spasm after an argument is acute respiratory alkalosis with reduced ionized calcium; total calcium is normal. Reassurance and paced breathing — not a paper bag.
• Common distractor: sodium bicarbonate. It is wrong for pure respiratory acidosis (it makes more CO2). Reserve alkalinization for salicylate poisoning and selected metabolic acidoses.
• Do not over-read the potassium: acidemia shifts K+ out of cells, but the classically quoted ~0.6 mEq/L per 0.1 pH unit applies to mineral (inorganic) metabolic acidosis; the shift in respiratory acidosis is small. Peaked T waves in a hypercapnic patient should prompt a search for an independent cause of hyperkalemia — renal failure, rhabdomyolysis, ACE inhibitors/ARBs, potassium-sparing diuretics, or succinylcholine.