A portable power system that eventually could allow artificial-heart recipients to lead more normal lives underwent its first human test today, hooked up to William J. Schroeder's mechanical pump on two occasions for a total of more than an hour.

"It worked just fine," said Dr. Peter Heimes, the West German inventor of the 11-pound device, after this afternoon's first test, which lasted about 22 minutes. "I'm very happy it worked as expected." After the initial test, Heimes said Schroeder "reached his hand out to me and said thank you and then asked for some ice cream."

A second test tonight lasted about an hour. The results were fine, according to a hospital statement.

Reporting on the first test, Heimes said Schroeder's artificial heart skipped a few beats as the research team switched him off the external 323-pound machine that has kept him alive since last Sunday's surgery and on to the portable unit, a process that took less than three seconds.

The heart also stopped briefly when it was returned to the main power supply.

But the patient, who was described as "quite tired" today and suffering from post-surgery "blahs" after a restless night, "didn't recognize any differences," said Heimes.

Readings of the heart's activity levels did not vary significantly during the "very important" experiment, said Heimes. The only difference was a "little bit softer beating of the heart," he added. This results from a different and gentler pumping action that could ultimately produce less wear and tear on the valves in the mechanical heart.

"It's the beginning of the end of the view of the artificial heart as a cumbersome device that doesn't give people the quality of life they want," said Dr. Robert Jarvik, head of Symbion Inc. and inventor of the mechanical heart. His company also has a license on the Heimes power pack, and Heimes works for a new German subsidiary of Symbion.

Two years ago, during his 112 days on the Jarvik-7 heart, dentist Barney B. Clark was permanently attached by two plastic air lines to a 375-pound heart drive console. Given this restriction, critics questioned what kind of life he would lead even if he recovered.

The portable power pack, which can be worn over the shoulder and looks much like a camera bag, has been under development at the Heimholtz Institute in West Germany since 1977 but was only recently approved for experimental use in this country.

The federal Food and Drug Administration gave the Humana Heart Institute International permission to use the portable unit for up to three hours a day as part of this second artificial-heart implant.

Heimes said that the unit potentially could be used around the clock and allow its wearer to do "very normal" activities, or up to "75 percent of the maximum a human can do."

This would include moderate running and bicycle riding, but not something as stressful as running a marathon, he said.

The unit has its own microprocessor that automatically adjusts the heartbeat to the needs of the body, beating more slowly at rest and speeding up when exercising.

The mechanical heart costs about $15,000 and the large power unit more than $40,000, Jarvik said. The portable pack costs about $10,000.

Until today the portable unit had been tested only on 54 animals, with the longest use for 17 days, said Heimes.

Both the large and small power systems drive the mechanical heart with air pumped through two lines in Schroeder's upper abdomen. The large console is plugged into electrical outlets, with a backup system in case of power failure. The Heimes unit is run by a rechargeable nickel-cadmium battery pack. The batteries run for about five hours but are designed to be changed without interrupting the functioning.

Heimes said he believes the unit is "100 percent safe," citing duplication of both the air compressors and the battery packs. An alarm sounds if one unit fails and it automatically switches to the backup systems, he said.

Heimes said he thought today's test was "significant" because the patient's condition remained stable before, during and after the switch from the large console. In days to come, Schroeder's time on the portable unit will be gradually increased, provided he is up to it, until the approved maximum is reached.

Today Schroeder, who was out of bed earlier this morning, was in bed resting when the portable unit was tested.

In today's morning medical update, Humana's Dr. Allan M. Lansing said that the 52-year-old Jasper, Ind., man had not slept well last night because of coughing episodes to clear his lungs, which have accumulated mucus since the surgery. Schroeder also had an eventful day yesterday, when he got out of bed for the first time. As a consequence, Lansing said that he hoped today to have him "rest as much as possible."

He described Schroeder as saying, "I'm just sort of very weary." Lansing said that this "syndrome I call the blahs" is a normal consequence of major surgery, one he often encounters in other open-heart patients several days after their operations. "It does not need to be treated. It goes away," he said.

Lansing acknowledged that doctors might have given their star patient too much activity yesterday. "We did it because he felt great . . . . We felt it would be good for him," he said.

Lansing said it was important for Schroeder to get more sleep, because exhausted patients in intensive care units can sometimes become confused and disoriented. He said there were no signs of mental changes in Schroeder that would be associated with "any brain problem."