The temperature in the Northwest apartment never fell below 64 degrees. Not particularly uncomfortable for most people. But for the 84-year-old woman it was lethal. She was found lying uncovered in her bed, her body so numbed by cold that she couldn't speak.

Within hours of being rushed to the hospital, she was dead. Cause: accidental hypothermia, a sudden drop in body temperature.

Hypothermia kills half its victims, causing tens of thousands of deaths among the nation's elderly each year. The National Institute on Aging estimates that more than 2.5 million older Americans could become victims, especially the chronically ill, the poor unable to afford enough heat and those older than 75 for whom risks increase five-fold.

"It slips up on you like a thief in the night," says Rep. Claude Pepper (D-Fla.), chairman of the House Aging subcommittee on health and long-term care.

Most elderly victims succumb while alone at home, in relatively mild weather with temperatures between 50 and 65 degrees. Developing over hours, or even days, the hypothermia dulls the mind to such an extent that the victim becomes too confused to recognize the danger and seek help. HH ypothermia, which literally means low temperature, occurs when the H body loses heat faster than it makes it. The body's internal or "core" temperature of vital organs such as the brain, heart and lungs drops.

Normally, the human body maintains its internal temperature around 98.6 degrees Fahrenheit. A temperature drop to 95 degrees or lower signals danger. As the temperature falls, blood pressure drops and pulse rate slows dramatically. Without proper first aid, cardiac arrest occurs, bringing death.

For most people under 65, mild cold poses no problem because good circulation, proper nutrition and physical activity keep heat production up. One expert notes that barring unusual circumstances, a healthy person exposed to temperatures as low as 55 degrees can maintain a constant body temperature indefinitely.

When cooled, the body takes several actions to protect its core temperature. Blood vessels near the skin constrict, reducing the flow of warm blood near the surface, decreasing heat loss. If the body continues to lose heat too rapidly, muscles involuntarily contract or tighten, causing shivering. This burns sugar stored in the muscle and turns it into heat.

Age weakens such lines of defense.

"About 10 percent of those over 65 have a physiological defect related to body-temperature regulation," says Dr. Richard Besdine of Harvard University's School of Medicine and an expert on the effects of the cold on the elderly. "The heat-conserving mechanisms of the body don't work as well.

Sensitivity to temperature changes diminishes with age. Blood vessels near the skin don't constrict, and the ability to shiver may be completely gone. The layer of fat just under the skin, which once served as insulation, is often gone, too. Circulation is poor.

In some cases, chronic diseases such as arthritis and Parkinson's disease immobilize a person, limiting heat-producing activity. In others, diseases such as diabetes and hypothyroidism directly interfere with the body's ability to regulate its internal temperature. One 80-year-old man suffered from hypothermia in 70-degree temperatures because he had an underactive thyroid.

Drugs can contribute to hypothermia. Alcohol expands blood vessels; barbiturates, sedatives, tranquilizers, blood pressure medications, vasodilators and antidepressants impair the body 's ability to adjust to temperature shifts. Poor nutrition, insufficient sleep and inactivity also increase the chances of hypothermia.

Hypothermia is difficult to detect because its signs often mimic other diseases such as stroke, diabetic coma and heart disease. Victims complain of cold only during the early stages, when body temperature is between 95 and 97 degrees. For unknown reasons, victims stop complaining when the body temperature goes below that, and a special low-reading thermometer, calibrated below 94 degrees, is needed to prove hypothermia.

The condition should be suspected during cold weather if the older person exhibits any of these symptoms: confusion, apathy, irritability, hostility or aggressiveness, difficulty speaking, slowed breathing and heart rate, stiff muscles, puffy face, trembling on one side of the body or in one arm or leg, cool or cold skin, pale skin with large, irregular blue or pink spots, poor coordination and jerky movements, excessive sleepiness. Rewarming And Prevention

If hypothermia is suspected, call a doctor, ambulance, rescue squad or local emergency room immediately. While waiting, handle the person very gently -- the heart is extremely weak when the body is cold. Conserve heat by covering the person with blankets, quilts or extra clothing. Make sure that the head and neck are covered.

The rewarming process is hazardous; the chances of sudden shock and heart failure are enormous.

Under no circumstances should the victim be given hot drinks or food. Do not raise the feet -- cold blood from the legs will flow into the body "core" and further lower body temperature.

While rapid rewarming in a warm bath is good for the young, it has proved lethal for the elderly. Hospitals warm the older hypothermic patient slowly with blankets and closely monitor the victim to keep the temperature from rising more than 1 degree an hour.

Prevention is far better than treatment. Those older than 65 should get plenty of rest, eat a diet high in carbohydrates, drink at least 1 1/2 quarts of fluid a day and avoid excessive use of alcohol. They should check with a doctor to see if prescription drugs they are taking increase the risk of hypothermia. Certain drugs used to treat anxiety, depression and nausea can have this effect.

Experts recommend keeping household temperatures, especially in key living and sleeping areas, 65 degrees or higher. Sick persons and those over 75 may need higher temperatures.

For information on financial assistance for home heating and insulation:

* D.C. Energy Office, 420 Seventh St. NW, Room 500, 727-1800.

For general information on hypothermia:

* Center for Environmental Physiology, 1511 K St. NW, Suite 1100, 737-3795.

* American Association of Retired Persons, Program Development Section, 1909 K St. NW, 872-4700.

* National Institute on Aging, Building 31, Room 5C35, Bethesda, 496-1752.