The defense case in the second-degree murder and perjury trial of former Washington abortionist Dr. Robert J. Sherman moved toward a close yesterday with additional testimony from a medical expert in connection with the treatment of 16-year-old Rita McDowell at D.C. General Hospital.

McDowell died in the hospital's intensive care unit in March 1975, four days after she went to Sherman's clinic for an abortion.Sherman's defense lawyers have argued during the trial in D.C. Superior Court that it was her treatment at D.C. General rather than Sherman's conduct, that resulted in McDowell's death.

Testimony yesterday from Dr. Robert B. Wagner, a surgeon who specializes in the chest, heart and blood vessels, supported the defense theory that doctors at D.C. General made an excessive number of attempts to reinsert a needle into McDowell's chest as part of a medical procedure.

According to other medical experts for the defense, the repeated punctures allowed fluid to accumulate in McDowell's lungs, interfering with respiration. Those experts have also testified that air entered McDowell's veins during the procedure, caused a blockage that interrupted the heart's pumping action and eventually resulted in death.

The government contends McDowell died as a result of blood poisoning and shock due to an abortion.

Wagner explained to the jury yesterday that the procedure performed at D.C. General involves insertion into the chest or arm of a needle through which a thin tube is threaded into a vein and then pushed toward the heart. The procedure is used to monitor blood circulation in treatment of patients in shock, Wagner said.

Wagner testified that in his opinion, after a maximum of six attempts to place this line into the chest, the high risk of complications requires the doctor to look for another entry site -- like the arm -- through which to insert the line. Testimony earlier in the trial disclosed that 19 attempts to insert the line through McDowell's chest had been made by doctors at D.C. General.

The defense is expected to rest its case Monday morning, and the government is expected to present additional evidence to counter defense testimony. Sherman is not expected to testify at the trial.