Longevity medicine is back on the front pages. A mechanical heart is now the life-support technology for a third recipient, a 58-year-old retired auto worker whose cardiological diseases had brought him to the edge of death.

Physicians who successfully fight back mortality are a glamor industry, with a patient like William Schroeder, receiver number two, giving us Medicine Americana by drinking from a can of Coors beer to show that, fake ventricles and all, he's still all heart.

At the same time that the wonders of organ replacement were again a national fascination, or at least promoted that way by much of the media, the public was told that other unhealthy citizens hover near death. Their longevity concerns involve trying to survive childhood poverty.

According to the Children's Defense Fund, a Washington group whose sense of justice has not wavered in 15 years of advocacy, the nation's worst child-killer is poverty: "More American children die each year from poverty than from traffic fatalities and suicide combined," its report said. "Twice more children die from poverty than from cancer and heart disease combined."

It takes some thought adjustment to see poverty as a cause of death. Poor people, we know, die of cancer, heart disease and accidents like everyone else. Where, then, does poverty come in? For the poor, death becomes closer in proportion to health care being further removed. Mothers who can't get prenatal care have babies who die more easily in the first month of life. Poor nutrition weakens children's resistance to disease. Homeless children -- 22 percent of the people in shelters -- suffer from exposure. Two-thirds of poor children lack regular health insurance. Anemia, lead poisoning and death by fires from kerosene heaters are other health risks of growing up poor.

Federal statistics are kept on numerous health categories but not on childhood mortality among the nation's 13 million poor children. All that's available to date is one overlooked 1983 study by the Maine Department of Human Services. For poor children from 8 days to 17 years old, the death rate is three times that of other children. The Children's Defense Fund estimates that "over a five-year period more children die from poverty than the total number of American battle deaths in the Vietnam War."

The two branches of medicine that deal with primary health care for poor children and with the extraordinariness of mechanical-heart transplants offer stark choices about life-support systems.

Is the longevity of one middle-aged man so valuable a technological advance that it deserves greater respect than an inner-city health clinic that treats thousands of infants each year with low birth weight?

Why is medical heroism conferred on the transplant surgeons who save one life for a few months or a year and not on the general practitioner in Appalachia who adds combined thousands of years to his patients' lives by applying the simplest health techniques? Will the evening news programs ever lead off with the press conference of a South Bronx doctor, dressed in surgical greens, explaining how he just saved the life of a homeless man who nearly froze to death?

Occasionally the costs and benefits of American medicine are examined in a way that considers routine care against heroic care. Harvey Fineberg, the dean of the Harvard School of Public Health, writes in a recent issue of Technology Review that "the way we pose questions about medical expenditures is all-important. If asked 'what is it worth paying to save a life?' we are tempted to respond 'whatever it takes.' A more pertinent question is 'where will additional dollars produce the greatest health benefits?' . . . Policymakers cannot attend to more basic value judgments about whether the public can afford to pay for new procedures, whether current policies are fair to all patients, and whether other uses of the same resources might be more worthwhile." Shouldn't the finite health-care dollar be directed toward such groups as the nation's 13 million poor children rather than the latest transplant extravagancy?

For the nurses and doctors who have chosen to do whatever it takes, say, to eliminate low birth weight in poor children, federal funding continues to be withdrawn. The Children's Defense Fund estimates that the 1986 Reagan budget would take away $5.2 billion from poor children and their families. That is added to $10 billion in cuts since 1980. Money also is a life-support system.