Heat Tetany

Heat tetany is carpopedal spasm and perioral and fingertip paraesthesia caused by hyperventilation induced respiratory alkalosis in people under heat stress. It is brief and usually benign, but it can be an early sign of progressing heat illness.

Epidemiology. It affects unacclimatised workers in intense heat, particularly with anxiety or strenuous work. It is distinct from heat cramps, which are painful muscle spasms from sodium and fluid loss.

Mechanism of symptoms. Alkalosis increases albumin binding of calcium, lowering ionised calcium and increasing neuromuscular excitability.

Importance. Hyperventilation can also reflect early heat exhaustion, heat stroke, sepsis or metabolic acidosis, so these need exclusion.

Pathophysiology.

Heat and anxiety increase respiratory drive. Falling PaCO2 raises blood pH, shifts calcium onto albumin and lowers ionised calcium without a change in total calcium.

Neuromuscular irritability then causes paraesthesia, carpopedal spasm and sometimes laryngeal spasm.

• Tingling around the mouth and in the fingers and toes, with carpal and pedal spasm (flexed wrists, adducted thumbs).
• Rapid deep breathing, light headedness, chest tightness and anxiety.
• Positive Chvostek and Trousseau signs in some.
• Normal temperature or mild elevation, with clear mental state.

Red flags.

• Confusion, core temperature over 40 degrees Celsius or persistent vomiting (heat illness).
• Chest pain, hypoxaemia or unilateral leg swelling (pulmonary embolism).
• Laryngospasm, seizures or QT prolongation (true hypocalcaemia).

• Core temperature, oxygen saturation, glucose and respiratory rate.
• Arterial or venous gas: raised pH with low PaCO2 (under 4.7 kPa) and normal oxygenation.
• Ionised calcium, corrected calcium, magnesium, potassium, phosphate and ECG (QT interval).
• Consider salicylate level, ketones and chest radiograph when the picture is atypical.

Differential diagnosis. Hypocalcaemia, hypomagnesaemia, heat cramps, early heat exhaustion or heat stroke, panic disorder, pulmonary embolism, diabetic ketoacidosis, salicylate toxicity, asthma, pneumothorax.

• Move to a cool place, reassure and coach slow diaphragmatic breathing at about 6 to 8 breaths/minute. Paper bag rebreathing is not advised because of hypoxia risk.
• Oral rehydration with fluids containing sodium; exclude and treat heat illness.
• Calcium is not needed for alkalosis related tetany. Give calcium gluconate 10% 10 mL (2.25 mmol) intravenously over 10 minutes only for proven hypocalcaemia (ionised calcium under 1.0 mmol/L or corrected calcium under 1.9 mmol/L) or QT prolongation. May repeat once after 15 minutes; further dosing by infusion under specialist direction. Monitor ECG and calcium; avoid extravasation and mixing with bicarbonate or phosphate; caution with digoxin; no renal adjustment but monitor in renal failure.
• Correct hypomagnesaemia: magnesium sulphate 2 g (8 mmol) intravenously over 20 minutes for documented deficiency with symptoms. Maximum 2 g each dose before repeat levels. Monitor reflexes, respiratory rate and blood pressure; reduce dose when eGFR is under 30.

Monitoring. Symptoms and respiratory rate until resolution; repeat blood gas if symptoms persist.

Referral and escalation. Emergency assessment for any red flag; occupational physician for acclimatisation and heat management at work.