Tales about patients saved by a last minute advance in medicine are legion. True stories, like the case of Allison Shank, are not.

When she was born with a defective heart Dec. 27 at Alexandria Hospital, Allison had a one in 20 chance of surviving. Through the use of a still experimental drug - used only eight times in this country - and a surgical technique developed only 18 months ago, a cardiologist and a surgeon at Children's Hospital National Medical Center have already boosted to 50-50 the infant's chances for a normal life.

Allison's skin was bluish when she was born because one of the chambers of her heart was one tenth the normal size.There was also a blockage where there should have been a valve permitting blood to flow from the heart to the lungs so that the blood would pick up oxygen and turn bright red.

In the past 10 years, Children's Hospital has cared for 42 children suffering from the same condition, called pulmonary atresia. None of those children lived past age 2, and only one of the 42 lived that long.

In addition to having the blockage, Allison, like the other 42 infants, suffered because she did not have a second defect - a hole between the right nd left ventricles, or lower chambers of the heart - which often occurs in cases like Allison's. Those infants born with such a defeat at least have a way for the blocked chamber to be relieved of the pressure caused by the blood flow's having no valve to escape through.

Further complicating Allison's condition, a natural back-up system, which usually helps infants with her particular problem, failed.

Prior to birth, a tube called the patent ductus provides some flow of blood between the aorta and pulmonary artery, insuring oxygenation of the blood. If all is well, the duct automatically constricts and shuts down shortly after birth.

When the heart malfunctions, however, the duct often remains open, providing a vital backup system.

By the time Allison arrived at Children's Hospital, however, about six hours after her birth, her ductus was shutting down.

About 12 hours after she was born, tests were completed and Drs. Stephen Shapiro, a cardiologist and Frank M. Midgley, a cardiovascular surgeon, discussed using Protaglandin El, a still experimental drug, to keep Allison's ductus open long enough to get her ready for heart surgery.

According to Shapiro, "There are less than 20 reports in the literature" of the drug's being used on humans and it has been only eight times in this country.

The drug, manufactured by the Upjohn company and cleared by the Food and Drug Administration for clinical experimental use, "has been shown . . . to keep the ductus open. The idea is to keep (it) open long enough to get the child in a stable enough condition to operate," said Shapiro.

The experimental nature of the procedure was explained to Allison's father, Thomas Shank, of Springfield, who said, "I didn't have time to be nervous . . . I was sort of worried about it, but I figured they knew what was best for her."

Shapiro said it was explained to Shank that the drug, which cost $1,800 a dose when first develped - but now costs next to nothing - "would not cure anything, but would enable surgery.

"The risks in the dose we use," which is a minute, "are unknown," said Shapiro. "There are all kinds of riks in higher doses. (But) it has a much more marked effect on the ductus than anywhere else in the body." Allison, he said, was the first patient at Children's to receive Prostaglandin El.

Less than 24 hours after she was born. Allison was in surgery. And once again she benefited from the use of a new medical procedure.

The Prostaglandin dilates, or opens, the ductus much the way of a pencil might dilate a soft, hollow noodle. But just as the noodle collapses when the pencil is withdrawn, so the ductus closes up when the patient is not given Prostaglandin.

To prevent this, Dr. Midgley injected the ductus with Formalin, a preservative that holds the tube in an open position. The technique has been in use for only about 18 months, said Midgley.

Midgley then made a small hole in the right ventricular chamber of the heart, enabling him to "punch holes" in the blocked valve.

Asked the size of the child's heart, Midgley reached into his desk and, with a flourish, held up a golf ball. "That's about it," he said. One quarter of the ball had been painted red, indicating the size of the chamber in which the surgeon worked.

In about six months, Allison will return to Children's Hospital for further tests."In about six months, these things (the ductus) just close off like an old iron pipe," said Midgley. "It doesn't actually shut off, but it just slowly carries less blood."

At that point the doctors may find that the valve itself is allowing enough blood to flow through the chamber. If it is not, an artifical ductus will have to be constructed to buy Allison a few years until she can undergo open heart-surgery and the construction of a new valve.

Yesterday, a month and a half after the surgery, Midgley said he and Shapiro are "enthusiastically encouraged" about Allison's chances now. "She's through a very high-risk period," he said.

"The experience is that most (such patients) die shortly after birth," added Shapiro. The mortality rate in the first month of life is about 60 per cent, he said.

"The sign that I felt we made it" with Allison's parents, said Midgely, "was that on the third day they named her. Up till then she was 'baby girl.'"

"We wanted to give her a name, one way or another," said Thomas Shank, "but in the confusion, it just took that long to get it on paper."

Reviewing the initial success with Allison, Frank Midgley commented: "My mentors lived through the years when this wasn't possible. The base-line of seemingly inoperable cases gets whittled away at."