A pathologist testifying as a defense witness in the second-degree murder trial of former Washington abortionist Dr. Robert J. Sherman told a D.C. Superior Court jury yesterday that 16-year-old Rita McDowell's death was caused by an air blockage in her heart that interfered with its pumping action.

Dr. Stephen C. Woodward, director of the surgical pathology division at Doctors Hospital here, testified that the air accumulated during repeated attempts by doctors at D.C. General Hospital to insert a needle into McDowell's chest cavity as part of a medical procedure to monitor her blood circulation.

Woodward, certified by the court as an expert in pathology, said the repeated attemps to put the intravenous line in place also allowed blood to collect in the lining of McDowell's chest cavity, resulting in respiratory failure.

Woodward's testimony was key to the defense theory that McDowell's death was caused by grossly negligent treatment at D.C. General, where she died on March 8, 1975, four days after she went to Sherman's clinic at 1835 I St. NW for an abortion.

The city's deputy chief medical examiner testified for the government, however, that McDowell's death resulted from blood poisoning and shock due to an abortion. The medical examiner testified that in his opinion, air bubbles found in McDowell's heart during an autopsy "had little or no effect on her death."

Sherman, 65, also is charged with 25 counts of perjury in connection with what the government contends was an attempt to cover up the circumstances of McDowell's death. The case is now in its fifth week of trial before Judge Fred B. Ugast.

Ugast is expected to rule today on a defense motion that the charges against Sherman be dismissed or a mistrial declared because of government interference with defense witnesses. Prosecutors in the case have vigorously denied allegations that they harrassed and intimidated witnesses.

Woodward testified that McDowell had exhibited symptoms of an air blockage when doctors attempted to reinsert the needle, according to hospital records. Contrary to testimony from government witnesses, he told the jury he did not think the procedure was essential to McDowell's treatment.

Woodward also testified that McDowell's condition was complicated by a blood-clotting diesease that he believed resulted from an incomplete abortion.