With the spring high school tennis season in full swing, 16-year-old Nick Mantzouris has it in his mind to become the most dominating junior in the area since Harold Solomon. His body, however, may not go along.

Mantzouris, a 6-foot-3 sophomore at Sherwood High School, played No. 1 on his team's varsity as a ninth-grader and finished the season with an 18-0 record.

But every time he steps on the court, Mantzouris must also prepare to confront a far different kind of opponent. He has a rare, nonhereditary heart condition called paroxysmal atrial tachycardia (PAT), which can occur in the young as well as in the old. If uncontrolled, the attacks can lead to serious complications and would therefore limit any sports participation.

"There is an extremely small chance of my having heart failure on the tennis court. Whatever risks there are, I feel they are worth taking in order to continue playing," said Mantzouris. "Tennis means everything to me."

Dr. Frank Galioto, a pediatric cardiologist at Children's Hospital and coordinator of the hospital's innovative sports medicine program, is Mantzouris' physician.

He said there is "a group of cells in the upper chamber of the heart which serves in all of us as a pacemaker and sends electrical signals to the rest of the heart to make it beat in an orderly fashion."

Mantzouris, who had PAT diagnosed at the age of 13, has another group of cells that can begin beating rapidly and can complete at times with his normal pacemaker. When the abnormal pacemaker dominates, his heartbeat speeds up and brings on an attack.

Normally, after he runs for 20 minutes, his heart beats at 160 counts per minute. But during his spells of PAT, his heart rate is well over 220 beats per minute. At those times, the amount of blood his heart pumps can be reduced by as much as 30 percent.

Although the attacks can come during any part of the day or night, they are more common during stress situations. Mantzouris says they occur only when he is "running or moving quickly, especially in a match, because I'm more nervous and tense. I get very dizzy, drowsy and weak and I can actually see my heart pounding through my shirt."

The attacks, which generally strike once every two tournaments he enters, occur more frequently when he is overtired. If he is in good shape and well rested, he says, the PAT generally will not happen.

"If the PAT goes on unrelieved for hours, he could pass out," explained Galioto. "But the medications he takes have reduced the number and severity of the attacks. If, despite the medicine, an attack occurs, various body maneuvers (such as deep knee bends or breath holding) may control the PAT and return the heart's rhythm to its normal beat."

During a match, when Mantzouris gets a PAT, he says he will "finish the game and when we switch sides, I'll sit down on a bench and relax all my muscles and breathe in easily until I can't inhale any more. Then I let the air out slowly and repeat the routine." Generally, that stops the attack for the moment.

"There is always a danger in anybody doing anything," said Galioto. "Nick and his family know that he can get these attacks and become ill, but for Nick it is worth the small hazard.

"We all know of athletes who have had surgery on their knees, overcame their handicap and resumed normal activities. Here's a boy with a cardiac handicap which he has to live with. It's a nuisance which interferes with his level of competition. He's determined to perform well despite the handicap, so our goal is to allow him to participate in activities as fully and as safely as possible."

Galiota monitors Mantzouris's condition through frequent conversations with the player's mother, Kay, and by occasionally recording Mantzouris's heart under playing conditions, with the aid of sophisticated devices that will not hamper play.

Mantzouris, who lives in Ashton, has received special permission from his principal at Sherwood to leave school early every day. Instead of taking woodwork, physical education and language classes, he is driven by his mother to the Northern Virginia Racquet Club, 45 minutes away, where he gets three credits for school by working out and playing against other top juniors for three hours in a clinic run by the club's pro, Jack Schore.

"I got permission to miss those classes because I'm an athlete and I want to make tennis my career. It's sort of an on-the-job training program," Montzouris said.

Schore, who has coached many promising juniors and has 110 youngsters of varying abilities in his program, says Mantzouris has "the greatest potential for future stardom of anybody I've worked with. For being that tall, he has a rare combination of touch and power games."

Mantzouris is treated the same as the rest of Schore's charges and is put through a demanding training routine. If a PAT attack occurs, he stops immediately and rests.

Manzouris' weakness, says Schore, "is that he puts out against top players, but when he's up against opponents with less ability, he tends to relax and to not take the match seriously. Some days he's great but at other times he's ordinary."

In February, Mantzouris upset four seeded players, including one with a high national ranking, on his way to winning the 16-and-under division of the prestigious Port Washington Tennis Classic in New York.

Then, one week later, he was obliterated, 6-2, 6-0, by Danny Goldie in the finals of the Middle Atlantic Lawn Tennis Association 16-and Under Grand Prix Playoffs.

Mantzouris dreams about joining the professional circuit. To do so, he must become more consistent in his ground strokes, improve his strength and quickness and learn how to keep his game at a peak against all competition.

But for now, at least, Mantzouris already has his toughest opponent-PAT-under control. So one day, that dream may be reality. CAPTION: Picture, Nick Mantzouris: budding tennis star. By Richard Darcey-The Washington Post