The exiled shah of Iran should be able to leave the United States once doctors have removed a gallstone blocking his bile duct, but the doctor who designed the technique says the operation should not be performed before next Wednesday.

Dr. H. Joachim Burhenne, of Vancouver, B.C., interviewed by telephone, siad it would risk failure of his "basket" technique for removing the remaining gallstone if performed before a full five weeks after the Shah's surgery on his gallbladder. The five-week interval will be up next Wednesday.

He said he had been consulted on the case by the shah's doctors at New York Hospital-Cornell Medical Center here, and he estimates an 80 percent chance of success for the operation. He added that he had personnaly taught two doctors at the New York institution how to do it.

For the patient, the operation is simple and painless. No anesthesia is needed, and it can take as little as half an hour. The shah would be able to travel immediately if the operation is successful, the New York Hopital doctors said.

The doctors said a week ago there was a 50-50 chance of success, but Dr. Burhenne said that was unduly pessimistic. "They didn't ask me what the chances were," said the Vancover rediologist who developed in 1973 the technique that has become standard in gallstone surgery, "They should know it's 80 percent from the medical literature."

He confirmed reports that the last gallstone is lodged in the bile duct inside the liver -- a spot generally inaccessible to the surgeon's knife. The gallstones that surgeons recovered from the shah were in the section of duct outside the liver.

The Berhenne technique was developed to avoid having to reoperate for gallstones that cant't be found when they migrate from the gallbladder into the bile duct system. With the success of the Burhenne technique, surgeons now often deliberately leave stones in parts of the duct difficult or dangerous for them to reach because it is easier and safer for the Berhenne basket to do it later.This, says Burhenne, was apparently the case with the shah.

The Burhenne technique involves leaving a tube in place through the skin into the bile duct, both as a surgical drain and as a mold for a channel. It takes the full five weeks for this channel for form firmly, and trying to search for the stones sooner could cause the channel to collapse, making it hard or impossible to slide in the stone retrieval basket.

After five weeks, the drainage tube is withdrawn, and a flexible tube is inserted through the channel. Then the little wire basket is threaded through the new tube. Using flouroscopy, the tube with the basket at its tip is moved through the duct to the stones. When the wire basket is pushed out through the tip of the tube, it opens slightly, grasps the stone, and it is pulled out. t

If it fails, doctors at New York Hospital would have to resort to surgery, an "exceedingly grave" operation that they said could not take place for at least a month, maybe two.