By Kenneth G. Lavelle, MD, FACEP, FF/NREMT-P
Anytime the human body participates in exercise, it generates heat as a byproduct of metabolism.
This is normal and the body has developed four ways to dissipate this heat to keep its internal temperature within the narrow range required for efficient function: radiation, evaporation, convection and conduction.
Radiation is heat the body gives off to the atmosphere. Evaporation is the loss of heat that occurs when liquid on the body changes to vapor. This liquid is often sweat released by the body during exercise.
Convection is the loss of heat to air or water that is flowing by the body. Conductive heat loss occurs when the body is in direct contact with an object and the heat is transferred to that object.
For a firefighter who wearing full PPE, heat loss is minimized. This is a concern for those of us providing them with medical care, especially on hot days.
Mechanism of heat loss
Sweating is a mechanism of heat loss with unintended consequences that are often improperly managed. Most of us know that the body can become dehydrated during exercise, so we need to maintain hydration by drinking before, during and after the exercise.
But what are the other complications of sweating and what are the best options to prevent them?
Before exercise it is important to maintain proper hydration throughout the day. Once we are thirsty we already have some degree of dehydration and our abilities can suffer.
We need to maintain adequate water intake on any routine day, with additional fluids before exercise. Athletes should have 18 to 20 ounces of water or a sports drink two to three hours before exercise and then 7 to 10 ounces 10 to 20 minutes before the exertional activity.
This sounds great, but most of the time we cannot anticipate when our day of training and station chores will be interrupted by a working structure fire in the summer. Although cover assignments may permit some pre-incident planning, it is reasonable to try to pre-hydrate and pre-fuel while en route to or at a cover assignment, as the chances of being called to the “saddle” are more likely.
The beverage selected to maintain hydration and performance depends on the anticipated duration of activity, again a challenge for firefighters. If the duration of the activity is anticipated to be less than 1 hour, then water alone before and during the event may be acceptable.
However for longer incidents, it is important to have carbohydrates present as well to make sure there is adequate storage of them for use in the upcoming activity. Electrolytes that are lost in sweat (sodium and potassium in particular) are more difficult to load up on before the exercise because the body does not store extra electrolytes — it will release them in the urine.
During the exercise or activity, 7 to 10 ounces of fluid is needed every 10 to 20 minutes to approximate the sweat and urine losses. Even if we do not urinate, urine accumulates in the bladder and may as well have already left the body.
Again, if the duration of the firefighting activity and post-fire exertion is less than 1 hour, water alone may be acceptable. But longer incidents where only water is ingested, have a unique risk — hyponatremia.
Managing water intoxication
Hyponatremia, or low sodium, is also known as water intoxication, although this term is not very descriptive. The concentration of sodium in the body is fairly narrow, between 135 and 145 mEq/L (Milli-Equivalents per liter).
Sodium and, to a lesser extent, potassium are present to a significant amount in our sweat. So it is critical to make sure these losses are replaced during rehydration.
If they are not, continuing to ingestion only water will cause the body’s concentration of sodium to decrease. As these levels fall below 125 mEq/L, an individual can suffer headaches, disorientation, fatigue and vomiting. At levels below 120 mEq/L, seizures, brain damage, coma and death can occur.
Sodium and other electrolytes are present in sport drinks, salty snacks and other fluids. Hyponatremia is unlikely to occur as long as a person does not ingest only plain water for long periods of time. Limit plain water to 12 quarts per day.
The next substance to be concerned about during exercise is the carbohydrate, usually obtained from sugars. Carbohydrates provide us with the energy to do work.
These should definitely be added to any rehydration/rehab regimen when the activity is longer than 1 hour, because glycogen stores in the liver (where extra carbohydrates are initially stored) may be exhausted in this time. The body needs between 30 and 60 grams per hour during exercise.
The carbohydrate in fluids is usually a form of sugar, but the type and concentration is important. Fructose in particular should be limited because it can contribute to gastrointestinal distress, like vomiting and diarrhea.
Foods that can cause this “dumping syndrome” include fruits and sodas. Sodas also have the adverse effect of making one feel more full due to the carbonation, so they really have no place on the fireground. Other sugars such as sucrose and glucose are less likely to cause this, but can if ingested in large amounts.
Regardless of the type, their concentration should be limited. If there is greater than 8 percent carbohydrate (>8 grams per 100mL of fluid) then the absorption of fluid in the stomach and primarily small intestine can be affected. Popular sports drinks approach this threshold, while having lower concentrations of electrolytes.
Some agencies diluting sports drinks with water to reduce their thickness. This may help with the high-sugar concentration, but can dilute the electrolytes that are already somewhat low.
For short- to moderate-length activities, this should not be a problem. But longer events such as an all-day evolution, the risk of hyponatremia can rise.
A new carbohydrate has recent been showing up in the military literature — rice-based rehydration solutions. These have carbohydrates with a variety of complexity, and in one small study showed that they lead to less dehydration than other types of replacement fluids.
Further research is needed before these can definitely be recommended. But, they do offer an interesting alternative and are available in powder packets that can be added to water. They may also cause less gastrointestinal upset than sugar-based fluids.
Once the activity is complete, it continues to be important to replenish water, electrolytes and carbohydrates to return the body to optimum performance. This is even more critical for emergency services, who are unable to anticipate when the next call to duty will occur.
Our bodies are our tools. And the public we serve and our fellow firefighters depend on us to have our tools operating at peak levels.
Proper hydration and nutrition is an important part of this. Take a moment to consider what you are putting in your body at the peak time of vulnerability.