Heat Syncope

Heat syncope is a brief loss of consciousness from heat induced peripheral vasodilation, venous pooling and volume depletion, usually after prolonged standing, rising suddenly or finishing exertion in a hot setting. It is self-limited and has normal or mildly raised core temperature.

Epidemiology. Common in unacclimatised workers and in older adults. Workers on blood pressure lowering drugs or diuretics are at higher risk.

Setting. Hot indoor or outdoor work, ceremonies and parades with prolonged standing, and the early days of a heat wave.

Importance. It is a benign diagnosis made only after excluding cardiac and other causes. It can also be the first sign of progressing heat illness.

Pathophysiology.

Heat dilates cutaneous vessels and sweating reduces plasma volume, so venous return falls. Standing adds gravitational pooling, and cerebral perfusion drops transiently.

• Light headedness, dizziness, visual dimming, nausea and sweating followed by brief collapse.
• Rapid recovery after lying flat, with no postictal state.
• Warm or flushed skin; heart rate may be high; core temperature under 40 degrees Celsius.
• Blood pressure low when standing and recovering on lying.

Red flags.

• Syncope during exertion, with palpitations, chest pain or breathlessness.
• Abnormal ECG, family history of sudden death, or known structural heart disease.
• Injury from the fall, persistent confusion or core temperature over 40 degrees Celsius.

• Detailed history of events, preceding symptoms, exertion and drugs.
• Lying and standing blood pressure and heart rate; core temperature; glucose.
• ECG in all; electrolytes, creatinine and haemoglobin in older adults or when volume depletion is suspected.
• Echocardiography and rhythm monitoring when exertional, recurrent or ECG-abnormal.

Differential diagnosis. Arrhythmia, structural heart disease (aortic stenosis, hypertrophic cardiomyopathy), vasovagal syncope, hypoglycaemia, drug induced hypotension, haemorrhage, seizure, heat exhaustion and early heat stroke.

• Lay flat with legs raised, move to a cool place, loosen clothing and monitor.
• Oral rehydration with fluids containing sodium, 500 mL over 30 to 60 minutes, then drinking to thirst.
• Intravenous 0.9% sodium chloride 500 to 1000 mL over 1 to 2 hours if unable to drink, hypotensive or taking diuretics. Maximum set by blood pressure and heart failure risk. Monitor blood pressure and heart rate; use caution in heart failure and renal impairment.
• Return to work after full recovery, normal ECG and no other cause, with rest in the shade for the remainder of the shift and no work at heights or with machinery for 24 hours.
• Prevention: acclimatisation, breaks with seated rest, avoiding prolonged static standing, leg muscle movement, hydration, and review of antihypertensives and diuretics with the treating physician.

Monitoring. Repeat lying and standing blood pressure after treatment; review within 1 week.

Referral and escalation. Cardiology for any red flag, exertional syncope or abnormal ECG; occupational physician for fitness-for-duty assessment after recurrent events.

Special groups. Older adults and workers on diuretics, ACE inhibitors, angiotensin receptor blockers or alpha blockers need medicines review.