Effects of Exercise in Hot Weather

Effects of Exercise in Hot Weather

Heat exposure and dehydration impair performance and pose major problems at competitions in higher temperatures. This can place severe stress on athletes.

The thermoregulatory centre is located in the anterior hypothalamus. When blood temperature reaches 37C (98.6F), the Benzinger reflex results in dilation of blood skin vessels and sweating occurs. Humans can only survive for short periods if core temperatures are above 41C (106F)

Physiological responses to heat:
Heat Stress Can Speed The Pulse By Skin:
• Heat – heart rate increases
• Stress – stroke volume increases
• Can – cardiac output increases
• Speed – skin blood vessels dilate
• The – total peripheral resistance decreases
• Pulse – pulse pressure widens
• By – blood is shunted from visceral organs to the skin
• Skin – sweat glands are stimulated

Evaporation is the most important of the mechanisms for defence against increased heat production. As sweat evaporates, heat is transferred by conduction from the skin to the environment, which cools the skin. The rate of this evaporation is dependent on a number of factors such as the amount of area of skin exposed to the environment, air movement/wind velocity, temperature of air, amount of humidity. Humidity is important as evaporation decreases in higher humidity

Acclimatisation is best achieved by exercising in heat – the major physiological adjustments take about 7-14 days. This can be done in an area with a hotter climate or in an artificially controlled environmental chamber – but may depend on humidity of environment that is being trained for (principle of specificity). Training intensities should initially be reduced on first exposure to a warmer climate and the athlete monitored (body weight, urine output and colour, symptoms) as intensity increases. Acclimatisation is lost within a few weeks.

Adaptations that occur to exercising in heat:
• sweat production and skin vasodilation begins at a lower core temperature
• sodium is preserved as sweat is more dilute
• increase in heat loss via radiation and convection (from increased skin blood flow)
• more sweat is produced
• there is a reduction in the rise of core temperature
• blood volume increase
• increased oxygen consumption
• improved exercise economy
• heart rate decreases
• ‘comfort’ exercising in heat improves

Heat illnesses/injury:
Disorders of thermoregulation are on a continuum: Heat stress  heat cramps  heat exhaustion  heat stroke  death. The pathophysiological mechanism is based on heat gain exceeding heat loss.
As dehydration occurs  blood volume is decreased  decrease in cardiac filling pressure and stroke volume  increase in heart rate  decreased circulatory efficiency, decreased performance (reduction in oxygen delivery) and decreased blood flow to skin  decrease in heat loss.

Risk factors for heat injury:
Higher temperature; higher humidity; no wind; no cloud cover; individual risk factors (eg chronic illness, very young or older, dehydration, drugs (eg tricyclic antidepressants), cardiac disease, sweating disorder, fever, eating disorder; hyperthyroidism; alcohol consumption, activity/exercise levels (especially prolonged activity).

Heat oedema:
• mildest form of heat illness; transitory swelling of hands and feet
• dependent swelling develops in the un-acclimatised – due to transient peripheral vasodilation from the heat; usually more a nuisance, often resolving in a few days.
• improves with periodic exercise and leg elevation
• do need to remove rings and pay attention to footwear fitting due to swelling.

Heat cramps:
• unheralded short term painful cramps from muscle spasm after prolonged, usually intense, exercise  weakness, fatigue.
• occurs in muscle groups that are worked the most – usually only affects part of muscle; more common in those who are less acclimatised.
• may be warning sin of impending heat exhaustion
• treat with fluid & sodium replacement; gentle stretching, maybe massage; ice for muscle.

Heat syncope:
fainting after exercise – dyspnoea; weakness; piloerection; profuse sweating; cutaneous flushing;
due to pooling of venous blood due to sudden loss of foot and calf muscle pumps, the shunting of blood through cutaneous vessels, reduction of cardiac output, and cerebral ischaemia.
treat with fluid replacement, lie down and leg elevation in cool place, then start walking around

Heat exhaustion:
excessive rise in core body temperature (0.6-1.70C) syncope; dyspnoea; weakness; piloerection; headache; hypotension; profuse sweating; cutaneous flushing; nausea and vomiting; irritability
distinction between heat exhaustion and heat stroke sometimes difficult (on a continuum) – generally heat exhaustion has no mental state changes and heat stroke does – also sweating absent in heat stroke  division between two conditions may be somewhat arbitrary.
treat with rest, cooling and rehydration – recovery should be rapid and feel better in 2-3 hours – if not  IV fluid rehydration and more intensive cooling

Heat stroke:
reaction to heat  get a ‘paralysis’ of the body’s thermo-regulatory mechanism  tissue damage to many organ systems
medical emergency.
Can be divided into classical heat stroke (usually in susceptible individuals eg obese, chronically ill etc in summer heat waves) and exertional heat stroke (usually in athletes, military personal)
more common in the less fit and less experienced athlete
clinical features – dizziness, nausea, vomiting, seizures, headache, hypertension, tachycardia, lack of co-ordination, pale and dry skin – progresses to loss of consciousness
risk of progression to organ failure, rhabdomyolysis, disseminated intravascular coagulation (DIC) and death
Immediate treatment – move to shaded area, elevate legs, rehydration (weak electrolyte solution or water) and cooling (poor fluid over body, wet towels; ice bath for 5-10 minutes).

Careful not to induce hypothermia (indicated by shivering)
Most will recover and be ambulatory in 30-60 minutes if cooling started promptly.

Transfer to hospital if severe and issues with consciousness or cardiac abnormalities; IV fluids; oxygen and respiratory assistance

Complications of heat injury:
• thermoregulation is disordered for a leats several days
• Rhabdomyolysis. Uncommon complication of heat stroke. Muscle membrane injury  intracellular contents leak  toxic to kidney – sometimes called the “muscular meltdown”. Can progress to renal failure.
• cardiac conduction abnormalities, congestive heart failure
• acute respiratory distress syndrome (ARDS)
• disseminated intravascular coagulation
• acute renal failure
• liver abnormalities
• haemostatic abnormalities

Prevention of heat illnesses:
acclimatisation to cope with heat – takes 10-14 days  body works more efficiently in heat
reduced workload during acclimatisation
knowledge of heat stress charts  cancellation/postponement of sports event
adequate hydration before, throughout and during, exercise (thirst is poor guide to hydration)
appropriate clothing
availability of trained personal during competition
warm down after exercise
control of individual risk factors and identification of ‘at risk’ athletes