HEAT STRESS

Hyperthermia

Heat gain/loss from your surroundings is dependent on the ambient temperature. As problems during exercise are usually related to shedding excess heat produced, let's focus on the ways your body can eliminate excess heat energy to maintain a temperature in a normal range. Heat "unloading" is dependent on

• radiation - an inefficient method
• convection - more efficient when the air is moving over your body as in cycling
• conduction - the way we lose heat while swimming
• sweating - by far the most effective way to shed heat

Sweating is the only effective way to cool your body if the ambient temperature is >92 degrees fahrenheit. And as it requires evaporation, it's effectiveness drops off quickly at >90% humidity. You have anywhere from 2 to 4 million sweat glands and can sweat up to 2 to 4 liters per hour. In fact Alberto Salazaar was estimated to lose 1 pint every 11 minutes in the 1984 Olympics. The heat energy loss from sweating is in the neighborhood of 580 Calories per hour.

When there is an imbalance of heat production > heat loss, hyperthermia occurs. Muscle cramps may be the first sign of hyperthermia. Next in the spectrum of heat exhaustion is the stage where you are hot, fatigued, nauseated, but still sweating. And finally there is heat stroke where you stop sweating, your core temperature begins to rise (often >104 F), and nervous system symptoms (altered state of consciousness or confusion) appear. It is potentially life threatening.

Another aspect of heat injury is hyponatremia. When you lose sodium in sweat (15 - 50, avg 20 meq per liter, depending on sweat rate and acclimitization) and replace it with pure water, your serum sodium concentration can drop below normal (hyponatremia) with serious consequences. Recent studies have shown that there are "salt losers", those with the salt crusted on their jerseys after an event, that may need even more than the normal salt replacement. It is thought that these athletes may carry the single gene for cyctic fibrosis (present in 5% of caucasians).

How do you protect yourself from hyperthermia? There are really just a few things you can do:

• Drink - and use fluids with some electrolyte replacement
  • don't drink too much
  • thirst is a poor indicator of volume status
  • use programmed drinking while riding - and drink 8 - 16 ounces 30 minutes before riding
  • monitor you personal losses - weigh before and after a ride, monitor urine color
• acclimitization helps - be properly trained for the duration/intensity of the event and train in the temperature and humidity where you will compete
• proper clothing - light color, wicks sweat, damp neck roll, layers you can shed as you heat up
• awareness of what fatigue may indicate if noted too early in an event

Hypothermia

Hypothermia is a decrease in your core body temperature. It is always a risk for the cyclist, especially in that combination of conditions that often occur with winter training.

What are the warning signs?

• Mild hypothermia (core body temprature above 95 degrees) - Uncontrollable shaking. Hands and feet turn white. Pulse may temporarily quicken. Decrease in manual dexterity - those hands just don't want to make the shifts or engage the brakes. This is a potentially life threatening situation, particularly if the conditions that led to it are ignored. The best thing to do is stop, find some warm shelter, and not even consider getting back on the bike until the shivering stops.
• Moderate hypothermia (core body temperature between 91 and 95) - Shivering actually slows. Heart rate begins to slow. Muscles get stiff.Ability to make good decisions is impaired.
• Severe hypothermia (core temperature below 91 degrees) - You begin to get that bluish hue. You can no longer move. At this point, you are unlikely to be able to save yourself.