Heat Intolerence

                          Heat Intolerance.

                        PKGhatak, MD

Heat intolerance is not a disease, it is a symptom. Some people would say “it is too hot in here” and others might look at him in surprise because they felt comfortably cold in the same room.

The question is who is right or is there a medical explanation for this temperature perception difference between people.

Yes, there is a physiological reason for heat intolerance.

The human body's Heat Center in the brain is connected with the skin temperature receptors and internal organ receptors. The Heat Center (HC) is located in the Hypothalamus of the brain which is connected with the pituitary endocrine gland and the center for the sympathetic and parasympathetic nervous system which is located in the hindbrain.

By varying the output, the heat center the body is able to maintain a constant core temperature all through the winter and summer, whether the body is fully clothed or not, that property is known as Euthermic.

Source of body heat.

The human body derives heat from 5 separate sources:1. Basal metabolism, 2. The specific dynamic action of food, 3. Physical activities, 4. Shivering and  5. Non-shivering thermogenesis.

Heat Center regulates temperature settings but the metabolic process generates heat. It is like a computer; multiple programs are running in the background. The metabolic process continues at all times during the lifetime of the individual. The organ functions continue at a basal level and that generates heat. This is called Basal metabolism. In humans, the Basal Metabolic Rate (BMI) is 76 kilocalories/Kg body weight /Hour. This is the main source of body heat.

2. Specific dynamic action of food (SDA). The energy is required for digestion, absorption, transport and synthesis and breakdown of food. This portion of the energy requirement is in addition to the Basal metabolism and this is called SDA or the Thermal effect of food. Protein requires the most energy, about 20 % of the calories contained in protein food. This is the reason we feel comfortably warm after eating a steak.

3. Physical activities. It does not require any explanation. We all get up and move around when watching a long football game to shake off the cold.

4. Shivering. Everyone can recall having been exposed to cold and wind, and the body begins to shiver. Muscle contractions generate body heat.

5. Non-shivering thermogenesis takes place in Brown Fat and muscles. Adrenaline is released at the sympathetic nerve terminal of brown fat tissue and muscle. Adrenaline accelerates the conversion of fatty acid and glucose to heat, ketones, H2O and CO2.

Excessive body heat and heat intolerance.

Menopause is a sign of decreasing the secretion of reproductive hormones in women which begins at age 45 to 55. Hot flashes, emotional changes, sleep disturbance, irritability, and excess sweating are some of the common symptoms. In rare cases, men also have menopausal symptoms.

Deceased gonadal hormone level in the brain produces relative hypoglycemia. That triggers an increase in hypothalamic, and endocrine secretion and sympathetic discharge. Increased metabolism leads to increased heat production.

Hyperactive thyroid gland.

Excessive secretion of Thyroxine (hormone of the thyroid gland) increases BMR, generates fine muscle tremors, and increases heart rate and blood pressure. Catabolism of fatty tissues and muscles produces excessive heat.

Drug abuse. Amphetamines and related compounds used in the treatment of attention deficit disorder and obesity are often abused as recreational pills. Palpitations, high BP and muscle tremors are usual symptoms. Increased metabolic rate and muscle tremors produce extra body heat. Cocaine, antipsychotic and anti-depressed drugs also produce excessive body heat by increasing metabolism and interfering with heat dissipation from the body.

Anticholinergic drugs. Anticholinergic drugs decrease sweat production and prevent cooling from evaporation and loss of heat. The core body temperature rises.

Caffeine. It is a cardiovascular and neuronal stimulant. It produces small vessel constriction and decreases heat dissipation. A mild increase in temperature is usual in therapeutic doses. In poising, caffeine raises the core temperature. 

Heat stroke. When exposed to high heat in a humid environment, the body fails to dissipate heat by radiation and evaporation. The core temperature rises and derails the functioning of metabolic and enzyme systems. It turns into an emergency situation and without decreasing the core temperature by therapeutics and fluid administration, permanent neurological and cardiac damage leads to fatalities.

Malignant Hyperthermia. This is another life threatening medical condition. It is an inherited disease inherited as an autosomal dominant trait. In susceptible patients, exposed to volatile anesthetic agents during general anesthesia, large amounts of calcium leave the calcium stores and enter the skeletal muscle fibers. Increased intramuscular calcium produces sustained muscle contractions. An excessive amount of heat is generated. The contracted state of muscles obstructs blood flow in the muscles. When the blood supply is cut off for a prolonged time muscles undergo necrosis which is called Rhabomyolysis. The released myoglobin from the necrotic muscle can block the kidney filters, producing metabolic acidosis and hyperkalemia (high serum potassium). Followed by cardiovascular collapse and death even death. 

Multiple sclerosis.  In Multiple sclerosis, the demyelation may directly affect the hypothalamus and autoionic control center. Excessive heat generation may also be accompanied by derailed heat dissipation because of spinal cord lesions in multiple sclerosis.

Diabetes mellitus can damage the autonomic nervous system and excessive heat and fluctuation of body heat may result.

Multisystem atrophy of the nervous system. The autonomic center damage is the result of increased heat.

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