A deputy city medical examiner, testifying as an expert witness for the government at the second degree murder trial of Dr. Robert J. Sherman, told a D.C. Superior Court jury yesterday that 16-year-old Rita McDowell died of shock and blood poisoning that resulted after an abortion.

Dr. William J. Brownlee also testified that in his opinion McDowell, who died on March 8, 1978, at D.C. General Hospital, got "the best care she could have received" when doctors treated her at D.C. General.

Sherman, 65, is charged with causing McDowell's death as a result of an incomplete abortion. He is also charged with 26 counts of perjury in connection with allegedly false statements made by or for him about conditions at an abortion clinic he once ran in Northwest Washington.

Sherman's defense attorneys have argued that they will show that McDowell's death resulted from grossly improper treatment she received at D.C. General and not as a result of Sherman's conduct.

During cross-examination of government medical witnesses, the defense has focused on numerous attempts by doctors to reinsert a needle into McDowell's chest cavity as part of a medical procedure to monitor her blood circulation. The defense contends that those repeated punctures allowed blood to collect in McDowell's chest cavity, causing her death.

Brownlee testified, however, that fluid accumulation found during an autopsy was insufficient to cause McDowell's death. Brownlee, who witnessed the autopsy and reviewed McDowell's medical record, also testified that air found in McDowell's heart was unrelated to her death. Brownlee is expected to be cross-examined today by defense attorney Robert F. Muse.

Earlier yesterday, Dr. Carl W. Bourne, who treated McDowell in the intensive care unit, testified that the weakened condition of McDowell's veins - a result of shock-necessitated the repeated attempts to reinsert the needle for monitoring blood circulation. When asked by defense attorney Constance O'Bryant about the possible risks of numerous punctures, Bourne said, "We had a dying patient on our hands, so we had to get a line in."

Bourne also testified that McDowell told him she had had an abortion about three days before she was addmitted to the hospital. Earlier in the trial, another doctor testified that McDowell said she had not tried to induce the abortion herself.