DONALD G.HARKCOM never saw the station wagon that speared him as he pulled off I-95 on his way home last October. His seat belt was not fastened. Before he knew it, his legs had been crushed under the dashboard, the rest of his body thrown wildly into the passenger seat amid jagged chrome, torn padding and shredded metal. His pelvis was shattered, his lungs punctured, his stomach, spleen and intestines jammed up through his diaphragm, his leg gashed open. As his chest filled with blood and he gasped for breath, the Dumfries Triangle Rescue Squad fought to cut him from the wreckage.

Twenty years ago, the rescue might have seemed pointless. Harkcom would have died, probably from shock, before a surgeon even got near him. Today, accident victims like Harkcom almost routinely survive the mayhem automobile accidents can wreak, thanks to a revolution in American emergency medicine brought home from the Vietnam war.

Although 52,000 Americans were killed in Southeast Asia between 1961 and 1975, at least that many Americans have been saved each year since 1980 because of shock-trauma units, helicopters and battlefield surgery techniques learned in the Vietnam war.

HARI HARKCOM called Dynatech Data Systems Inc. at 7:00 that night last October to ask whether her husband had left on time. He had, and she began to suspect what her son blurted out: "Dad's probably been in an accident."

"Don't say that!" she snapped.

The Harkcoms had been childhood sweethearts and had been married 20 years. Their recent move to Dumfries from a job-poor town in southern Pennsylvania had drawn them even closer. How had he once described it? "When I get a pain, she does, too."

When the call came from Potomac Hospital, Shari Harkcom collapsed on the floor and bawled. Her husband was being helicoptered to the Washington Hospital Center's Medical Shock Trauma Acute Resuscitation unit -- MedSTAR.

She swallowed two Valiums and called her brother for a ride to the hospital. "I was scared Don was going to die before I saw him."

Meanwhile, a shock-trauma nurse and paramedic on board the helicopter inserted a tube into Harkcom's mouth to help him breathe and attached monitors to track his vital signs. He was wearing MAST -- military anti-shock trousers -- inflatable, plastic pants that reduce internal bleeding. It was his first helicopter ride.

He was conscious and in terrible pain: "They told me that when we landed, there would be a bunch of people working on me and I shouldn't get upset."

The shock-trauma team that whisked Harkcom into an emergency area at one point numbered 12 physicians, nurses and technicians. Assuming the worst, they swarmed over him, taking X-rays, running lab tests, inserting intravenous lines to stabilize his falling blood pressure, a major cause of shock. Each body part was checked. An eight-foot-high life-support column of gauges, suction tubes and monitors dangled over Harkcom's body like a robot from Star Wars. Within minutes, Dr. Mark L. Buchly, the team leader, had Harkcom's X-rays and a sense of what the MedSTAR teamed faced: "He was in really bad shape."

Shari Harkcom arrived at the hospital just as her husband was being wheeled into an operating room. Buchly stopped the cart as she rushed forward and grabbed her husband's hand. "I can't breathe," he told her. She began sobbing.

Only minutes later Buchly had his hands inside Harkcom's chest, searching for the aorta, afraid it might be torn. He saw that Harkcom's diaphragm, the muscle that separates the chest and abdominal organs, had been ripped and that air was escaping from the lungs. As tubes sucked blood from Harkcom's chest and inflated his lungs, Buchly found the aorta. It was okay. He mended the torn organs quickly and moved them back where they belonged.

When he finally emerged from the operating room hours later, Buchly smiled at Shari Harkcom. "I knew Don was okay then," she said. "That doctor, he's a saint."

Within a few days another physician began operating on Harkcom. Dr. Stephen F. Gunther, chairman of orthopedic surgery at the hospital, had to rebuild Harkcom's hip, which had been broken in six places, with pins and stainless-steel plates. After three operations, long hours of physical therapy, and weeks in bed, Harkcom finally left Washington Hospital Cen.

"I was lucky," he said. " . . . If it hadn't been for that helicopter and Dr. Buchly, I'd be dead." He was right.

EMERGENCY MEDICINE was little more than first aid in the 1960s. Only 5 percent of ambulance drivers had any medical training; many were undertakers driving ambulances for extra income. Competing ambulance companies in large cities sometimes fought over victims like cabbies seeking fares. Patients were taken to the nearest hospital or one that they requested, regardless of injuries or the hospital's capabilities.

Heart attacks killed 700,000 people in 1972; half of them died trying to get to a hospital. Studies showed 70 percent of motor vehicle deaths in the 1960s occurred in rural areas with no emergency medical facilities. Even in most larger cities, emergency care was pedestrian. Only 25 hospitals in the entire nation in 1972 had a doctor on duty in emergency rooms 24 hours a day.

At the same time that an estimated 100,000 persons were dying each year in this country because of inadequate emergency medical care, thousands of soldiers suffering from much worse wounds were being saved in Vietnam.

The war, says Dr. Donald L. Custis, retired surgeon general of the Navy, Vietnam veteran, and former chief medical director of the Veterans Administration, made the medicl establishment admit that emergency medicine was a specialty, a necessity all its own.

It is not unusual for a war to teach doctors about medicine. The Civil War taught how to evacuate mass casualties, says Dr. R. T. Joy, a Vietnam veteran and chairman of the medical history department at the Uniformed Services University of the Health Sciences in Bethesda. World War I taught about shock. Treatment of burns and the use of antibiotics were advanced in World War II. The Korean war helped develop surgery to repair arteries.

Vietnam produced the modern helicopter air ambulance and advances in team surgery, anesthesia, burn treatment, shock and orthopedics. It also gave thousands of doctors an education that they would never have received from textbooks.

"Fights between citizens and their enemies are rare, but are frequent and almost daily between soldiers," noted Hippocrates, the father of medicine. "He who would become a surgeon, therefore, should join an army and follow it."

Those doctors who volunteered for Vietnam, or who were compelled to go there, were thrust into medicine they had never known, massive injuries that were so overwhelming that they forced surgeons to invent extraordinary procedures.

"There were times when you simply had to make up things as you went along during surgery," said Dr. Larry Carey, chairman of the department of surgery at Ohio State University and a former combat surgeon in Vietnam. "You would try new procedures, take desperate steps. Not because you wanted to experiment, but because you had no other choice."

Sometimes the risks led to miracles such as the discovery that warming a unit of blood in a microwave oven before a transfusion could prevent a patient whose body temperature has fallen from going into shock. A physician who tried that back home where a hospital review board was looking for abnormal behavior in the operating room might have lost his license.

The number of casualties and severity of wounds also gave surgeons in Vietnam a chance to learn from unusual situations. "At night when I'm lying in bed, I think about the war sometimes and I remember this one kid . . . ," says Dr. Pete Parker, who was a surgeon near the demilitarized zone during the Tet offensive in 1968.

"This kid had been hit by a sniper . . . with an AK47. Just one bullet hole, a small one. He was awake and alert and doing reasonably well, so we got him into the operating room and I opened him up. He had a tremendous amount of blood in there and we got it all out and there was a small hole in his liver that was just slowly oozing blood. I sewed up some of the liver and put in several large sutures . . . Everything was going along nicely and I was about to finish when . . . " Parker paused. "To this day I wish I had done something differently, but I could feel that big bullet in the back of the liver and I thought it probably would give him some trouble, so I made this incision, it was just a tiny little incision that I put in the liver to get that bullet out, and the liver just decompressed and he bled like I'd never seen a person bleed before and by the time that I got the hemorrhage under control, he had cardiac arrest and died.

"That has stayed in my memory forever," Parker said. "It has helped me, because now I know that you are much better off just keeping your fingers crossed when you get the bleeding in a liver stopped . . .

"I remember that right before we put that guy to sleep, he said: 'Please don't let me die, doc! I don't want to die in Vietnam!' And I remember saying, 'We won't let you die here, son,' and then he went and he died."

WAR PUSHED SURgeons farther than they thought they could go, Parker said. "Most of our sleep during Tet would be in the hallway. I mean, you would go finish 12 to 15 hours in surgery and lie down in the hall and in an hour and a half another helicopter would come down and a corpsman would come in and say, 'We got another one, doc!' and you would get up and go right to it."

The challenge was between skill and demons: fatigue and death.

Some doctors cracked. Parker remembers an orthopedist who curled himself into a fetal position during a rocket attack and refused to move for 24 hours.

"You were on your own," Carey said. He had been in Vietnam only three days when he was told to decide who among the wounded would be treated first at a Naval hospital in Danang. "There was a rocket attack and we got 75 wounded young men in a 35-minute period. Your first reaction was to throw up your hands and say 'Jesus, I can't handle this,' but you do, and you become good at it because you must.

" . . . I used to watch surgeons who had been in combat," said Carey. "They had a certain confidence. They did not hesitate or doubt." He searches for a word. "I guess Vietnam gives you what might best be called surgical chutzpah."

And some doctors got hooked on the high.

"I was ready to go home when my 13 months were up," said Parker. "I couldn't have taken it anymore. But when I came back to the States in September 1968, I found that I was spellbound by Vietnam.

"Everything seemed unimportant here. It just seemed that I left where I was really needed. I was a very important cog in a wheel that had to keep turning and all of a sudden, I came back here and everything seemed anticlimactic, everything seemed small in comparison. It was hard for me to get worked up about taking out some old lady's gall bladder.

"It took me a long time to get Vietnam out of my mind, my system. I missed it so much that I was going to go back and would have except my wife was pregnant," Parker said. "It sounds strange, but Vietnam was one of the greatest experiences that I ever had."

The seduction of emergency-room medicine is not altogether different from battlefield surgery. "This is really the last bastion for the old-time general surgeon," said Buchly, who is not a veteran. "You never know from one moment to the next what you are going to be doing. You are always on the brink. It demands a lot."

DR. R ADAMS COWLEY, a Baltimore surgeon, had discovered in the early 1960s that severely injured people needed to get to a hospital within 60 minutes. After what he called the ''golden hour," death was almost certain from shock, which occurs when there is not enough blood to supply oxygen and other nutrients to body tissues. In 1961, Cowley opened the nation's first shock-trauma center at the University of Maryland campus in Baltimore, but many patients couldn't get there in time.

The success of helicopters in Vietnam impressed Cowley, and he decided to convince state authorities to help. In 1970, the state bought the first of seven state police helicopter. Last year, they airlifted 1,600 critically injured patients to Cowley's shock-trauma center.

In 1973 Sen. Alan Cranston (D- Calif.) sponsored the Emergency Medical Systems Act, which offered federal funds to states to upgrade emergency care based on procedures used in Vietnam.

By 1981, nearly 200 emergency regions had been formed. "The program was saving lives," Cranston said. Federal studies found that there were 42 percent fewer deaths from automobile accidents in regions that had adopted the full program.

After President Reagan was elected, administration officials lumped money for the emergency programs with other funds given to states as block grants. Cranston warned that the move could destroy the emergency program because states might spend the money other ways.

Reagan himself is alive today because of emergency care he received at the shock-trauma unit at George Washington University Medical Center after he was shot by John W. Hinckley Jr. in 1981. Dr. Benjamin Aaron, director of chest and cardiovascular surgery at the hospital, said, "Only because the president got prompt and highly skilled modern shock-trauma care was he in no danger of dying."