HYPERTHERMIA

What is normothermia?

Normal body temperature.

What is fever?

Regulated high temperature due to an increased hypothalamic set-point.

What is hyperthermia?

Uncontrolled increased temperature due to system failure.

What is hypothermia?

Abnormally low body temperature.

Key difference in control of fever vs hyperthermia?

Fever = regulated

Hyperthermia = unregulated

Key difference in mechanism of fever vs hyperthermia?

Fever = hypothalamic set-point increased

Hyperthermia = thermoregulatory failure

Do NSAIDs work in hyperthermia?

No

Ultimate mediator of fever?

PGE2.

What stimulates PGE2?

IL-1, TNF, LPS.

Effect of PGE2?

Increased hypothalamic set-point.

Can severe infection cause hypothermia?

Yes

What causes hyperthermia?

Heat gain > heat loss.

What happens to thermoregulation in hyperthermia?

Initially activated → eventually fails.

Final outcome of hyperthermia?

Dangerous sustained high temperature.

What is the key circulatory conflict in heat?

Blood must supply BOTH:

• Skin (cooling)

• Muscles (metabolism)

What causes dehydration in heat?

Sweating, exercise, high temperature.

What is the key effect of dehydration?

Decrease in plasma volume.

What happens to stroke volume in dehydration?

Decrease in stroke volume.

What increases during dehydration to compensate for reduced blood volume?

Heart rate increases.

What happens to skin blood flow in dehydration?

Decreased skin blood flow, which causes less heat loss.

What happens to sweating in dehydration?

Sweating decreases, because plasma volume is reduced, so the body conserves fluid instead of losing it via sweat.

What is the consequence of reduced sweating?

Impaired evaporative cooling → increased body temperature.

What happens to aldosterone during dehydration?

Increased release.

Why is aldosterone increased?

To increase Na⁺ reabsorption, which helps conserve water and restore blood volume.

What hormone increases water reabsorption?

ADH (vasopressin).

Most important factor affecting evaporation?

Relative humidity.

What triggers aldosterone release in dehydration?

1. ↓ plasma volume

2. ↓ blood pressure

3. Activation of RAAS (renin–angiotensin–aldosterone system)

What happens in high humidity?

Decreased evaporation which leads to decreased cooling.

What causes heat cramps?

Electrolyte imbalance (Na⁺ loss).

Key feature of heat stress?

Normal body temperature.

Main cause of heat exhaustion?

Dehydration + circulatory inefficiency.

What is the skin condition in heat exhaustion?

Moist (sweating present).

Body temperature in heat exhaustion?

Normal or mildly elevated (<38.8°C).

What is heat stroke?

Complete thermoregulatory failure.

Core body temperature in heat stroke?

>40.5°C.

Key symptom of heat stroke?

Anhidrosis (no sweating).

Why is anhidrosis dangerous?

No evaporative cooling thus leading to rapid overheating.

Symptoms of heat stroke?

• Hot, dry skin

• CNS dysfunction (confusion, seizures)

• ↑ Heart Rate, ↓ BP

Is sweating present in heat stroke?

No — anhidrosis occurs.

Step-by-step progression to heat stroke?

1. Heat stress

2. Sweating → fluid loss

3. ↓ plasma volume

4. ↓ cooling efficiency

5. ↑ body temperature

6. Thermoregulatory failure

7. Heat stroke

What happens to cardiac output in dehydration?

It decreases in efficiency.

What happens to heart rate in dehydration?

Increases due to compensation for the decreased cardiac output.

What happens at ~45°C?

Protein denaturation → cell death.

Hyperthermia vs heat exhaustion?

• Heat exhaustion = circulatory problem

• Heat stroke = thermoregulatory failure