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In South Dakota, up to 1,500 head of cattle died across the state from the heat. And in eastern Iowa, the scorching sun caused a portion of Interstate 380 to buckle. The weather also sent dozens of people to hospitals, canceled outdoor sporting events and caused sporadic power outages.

In such conditions, dehydration can lead to heat exhaustion and potentially deadly heat stroke. During a heat wave, that can happen in a matter of hours in older people if they over-exert themselves, don’t drink enough water or are frail and don’t get out of uncooled homes, said Dr. Chris Carpenter, an emergency medicine physician at Washington University School of Medicine in St. Louis.

Heat exhaustion can cause muscle cramps, low blood pressure, rapid pulse and nausea. It can be treated at home, by drinking water, getting into an air-conditioned room or sitting in front of a fan and misting the body with cool water.

But affected people should be monitored for mental changes and to make sure their temperature does not rise above 102 because the condition can quickly lead to heat stroke. A medical emergency, heat stroke involves temperatures of 104 or higher and can cause seizures, loss of consciousness and death.

Medicines many older people take also may make them more vulnerable to the heat. These include diuretics for high blood pressure, which increase urination _ and make it more important to drink plenty of water, Dale said.

Some types of drugs can interfere with sweating and raise body temperature, including some medicines for insomnia, nausea, prostate conditions, Parkinson’s disease and even Benadryl. Many list “dry mouth” as a side effect _ a tip-off to drink more water, Zich said.

There aren’t specific guidelines on how much water older people should drink in a heat wave.

Dale said he generally tells his older patients to drink a quart of water throughout the day, and to drink even if they don’t feel thirsty.

Doctors also advise older patients to avoid alcohol and coffee during extreme heat because they can cause the body to lose fluid and contribute to dehydration.

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Online:

Centers for Disease Control and Prevention: http://emergency.cdc.gov/disasters/extremeheat/elderlyheat.asp

American Geriatrics Society: http://www.healthinaging.org/public_education/hot_weather_tips.php

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AP Medical Writer Lindsey Tanner can be reached at http://www.twitter.com/LindseyTanner.