Hyperthermia

When there is an imbalance and heat production > heat loss, hyperthermia results. Muscle cramps may be the first sign of hyperthermia. Next in the early spectrum of heat exhaustion is feeling hot, fatigued, and nauseated, but still sweating. And finally there is heat stroke when 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 and heat stress with exercise is associated with an increase sudden death as late as the next day. (In military recruits there is an increase in exercise related deaths the morning after intense exercise, especially in hot and humid conditions).

Heat gain 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 in order to keep your temperature in a normal range. Heat "unloading" occurs via:

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

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

Another potential risk associated with heat illness is hyponatremia. When you are losing sodium in sweat (15 - 50, avg 20 meq per liter, depending on sweat rate and acclimatization) and replacing 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 high concentrations of salt in their sweat (those with salt crusted on their jerseys after an event), who may need even more than the normal salt replacement. It has been speculated that their tendency to salt loss may be the result of these athletes carrying a single gene for cystic 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
  • but don't drink too much
  • thirst is a poor indicator of volume status, but current teaching is to use thirst as an indicator of your volume needs rather than programmed drinking while riding
  • top off the tank with a drink of 8 - 16 ounces 30 minutes before riding
  • and monitor you personal losses - weigh before and after a ride, monitor urine color. I'd suggest a maximum loss of 2 - 3 % of your body weight after a particularly hot ride.
• acclimatization 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

Here are a couple of riding tips if you are going to be in a competitive event.

• Pace yourself. Remember that 3 of every 4 Calories you are metabolizing (and the faster you go, the more Calories you will burn per minute) are converted to heat. Thus the more you are using per minute, the more potential for the body's heat balance to shift towards hyperthermia. Go slower and you will be able to maintain a pace longer in extremely hot conditions.
• Time your ride, if possible. It is cooler in the early morning (and late in the evenings). So get on the road early if you can.
• Fluids - don't overcompensate. Over the last few years the thinking has shifted from "drink before you are thirsty" towards allowing your thirst to dictate your fluid intake. But this means you have to listen to your body. Ignore your thirst and the risks of dehydration go up quickly.
• If you are going to be riding more than 4 hours, you should think about additional salt in your food and fluids. Sports drinks are not enough. Salty nuts, salty foods, and even salt tablets may be needed.

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 temperature 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.