The city's deputy chief medical examiner, who performed an autopsy on the body of 16-year-old Rita McDowell, testified in D.C. Superior Court yesterday that the high school student's death in March 1975 was caused by inflection and shock due to an abortion.

Dr. Brian D. Blackbourne, a government witness at the second-degree murder and perjury trial of Dr. Robert J. Sherman, rejected defense contentions that McDowell's death could have resulted from bacteria present in her body before she went to Sherman's clinic, followed by an acccumulation of fluid in her chest cavity after treatment at D.C. General Hospital.

Blackbourne also told the jury that in his opinion, air bubbles found in McDowell's heart during the autopsy "had little or no effect on her death".

McDowell died in the intensive-care unit of D.C. General four days after she went, with her mother, to Sherman for an abortion. Blackbourne testified that the severity of the infection suffered by McDowell and the brief length of time between her visit to Sherman's abortion clinic and her admission to the hospital led him to believe that the bacteria that caused her illness was introduced during a medical procedure.

The government contends that Sherman regularly performed incomplete abortions, used unsterile instruments and violated medical standards in order to cut costs at the clinic, once located at 1835 I St. NW.

During cross-examination by defense attorney Robert F. Muse, Blackbourne testified that he found 19 puncture marks during the autopsy, which he attributed to efforts by doctors at D.C. General to insert an intravenous line needed to monitor McDowell's blood circulation pressure. There were six punctures of the space in the chest that would be filled during expansion of the lungs, Blackbourne said.

During the trial the defense attempted to raise the possibility that those punctures allowed for an accumulation of fluid that could have resulted in McDowell's death.

Blackbourne testified that the six punctures were "unusual" but "not unique in my experience." When asked by Muse if the punctures were dangerous, Blackbourne responded, It has the risk of collpasing the lung or causing bleeding."

Earlier in his testimony, Blackbourne had characterized the doctors efforts to insert the line as "heroic resuscitation attempts on a critically ill patient." When later asked by Muse if improper administration of that procedure would keep vital treatment from a patient. Blackbourne said he could not tried to insert such an intravenous line himself.

Blackbourne also testified under cross-examination that he could "not definitely" say that the infection McDowell suffered was introduced at the time of her treatment by Sherman.

Blackbourne is expected to resume his testimony today.