Emergency Medicine Emergency Medicine · Physiology

Respiratory Alkalosis Causes

Respiratory alkalosis (low PaCO2 + high pH) results from hyperventilation. Identifying the cause prevents unnecessary treatment of the compensatory response.

✦ The Mnemonic

"PANICS Causes Respiratory Alkalosis"

Pain · Anxiety · Neurological · Infection/fever · CNS · Salicylates

P Pain / Psychological Most common in ED — anxiety, panic attacks, hyperventilation syndrome

A Altitude Low FiO2 → hypoxia → hyperventilation

N Neurological Brain injury, stroke, raised ICP stimulating respiratory centre

I Infection / Fever Sepsis stimulates respiratory centre via cytokines; early sepsis sign

C CNS stimulants Salicylate toxicity (early) — direct respiratory centre stimulation; also causes metabolic acidosis

S Severe anaemia / Liver failure Hepatic encephalopathy; anaemia — hypoxia drives hyperventilation

📚 Clinical Breakdown

Hyperventilation syndrome: anxiety → hyperventilation → ↓PaCO2 → cerebral vasoconstriction + perioral/fingertip paraesthesia + carpo-pedal spasm (hypocalcaemia of alkalosis). Treatment: reassurance, breathing into paper bag (raises CO2), treat underlying anxiety.

Respiratory alkalosis in sepsis: early sepsis characteristically causes a respiratory alkalosis before any metabolic acidosis develops. A patient with respiratory alkalosis and no obvious cause (anxiety, pain) should be evaluated for early sepsis — check lactate, blood cultures, and organ function.

Compensation: in chronic respiratory alkalosis, the kidneys excrete bicarbonate (takes 3–5 days) — pH normalises. This is why chronic altitude dwellers have lower bicarbonate than expected. Expected compensation: for every 1 kPa fall in PaCO2, bicarbonate falls 4–5 mEq/L (chronic) or 1–2 mEq/L (acute).

Most common cause Anxiety/panic — hyperventilation syndrome

Early sepsis marker Respiratory alkalosis before metabolic acidosis

Salicylate toxicity ABG Respiratory alkalosis + metabolic acidosis

Treatment of HVS Reassurance + controlled breathing

⭐ Clinical Pearl

Electrolyte effects of alkalosis: respiratory alkalosis drives K+ and PO4 intracellularly (H+ leaves cells to buffer extracellular pH). Severe hyperventilation can cause symptomatic hypophosphataemia and hypokalaemia — relevant in patients on ventilators or with prolonged panic attacks.

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