President Reagan was able to reclaim his presidential authority with a clear head less than eight hours after signing it over to Vice President Bush because Reagan's post-operative pain was controlled through a relatively new technique.

Instead of injecting the usual dose of five to 10 milligrams of morphine into a muscle to control the pain caused by abdominal surgery, Reagan's surgeons injected one milligram of morphine directly into the tissue surrounding the president's spinal cord. This allowed the physicians to block the sensation of pain without flooding the body with mind-numbing narcotics.

"A milligram of morphine was placed in the spinal fluid at the conclusion of the operation," said White House press officer Mark Weinberg, who answered questions from the Bethesda Naval Hospital, where the president underwent surgery on Saturday. The president had received only one injection, and "he has not called for any pain medication since," Weinberg said on Monday.

"The president received a single injection of one milligram of morphine intrathecally through the membrane that surrounds the spinal cord at the end of the operation only, and it kept him very comfortable for the next 24 hours," Dr. Lee E. Smith, a colorectal cancer specialist at the George Washington University Medical Center and one of the doctors involved in President Regan's care, c + ecific region of the body without having to block the pain receptors in the brain itself.

"A narcotic injected into the spinal cord will interrupt the pain pathways or modify the pains that arise from the abdomen or lower extermities," Shin said. "It prevents the pain from reaching the brain's pain receptors."

This approach is similar to the so-called epidural anesthesia commonly used for women during childbirth, especially for Cesarean births, said Wall. Instead of injecting a narcotic such as morphine, however, the obstetrician injects a local anesthetic.

"They become numb totally and feel no sensation," said Wall, because the anesthetic blocks the function of all of the nerves that pass through that region, including the nerves that control movement, sensation and automatic activities like maintaining the semi-rigid condition of the blood vessels.

Injecting a narcotic, however, shuts down only the nerves that carry the pain signals. "That is a distinguishing charateristic between local anesthetic versus morphine or a narcotic," Wall said.

"Unlike the local anesthetic blocks, which impare motor, sensory and autonomic functions, the narcotics put into these spaces only provide analgesia," said Wall. "That makes it very nice for postoperative pain relief. The patient then has sensation, is able to feel the area, and is able to move it."

As with the epidural therapy for childbirth, doctors can insert a catheter into the spaces around the spinal cord to "give round-the-clock analgesia through the catheter," said Wall. "We use this type of therapy for patients in post-operative pain, or who are living in cancer pain or a variety of pain states."

Injecting morphine directly into the tissues around the spinal cord, however, does have some drawbacks, said Georgetown's Shin. "It has been associated with minor side effects, itching, nausea, vomiting and sometimes incontinence. It is not clear what causes these side effects. Itching can occur in up to 80 percent of the patients, vomiting 30 to 40 percent, and it has been reported that the rate of respiration can be can be suppressed."

"The most significant side effect is respiratory depression," said NIH's Wall. "The respiratory depression is the side effect which has scared people away the most from using this type of therapy." The narcotics move up the cerebrospinal fluid and eventually can be found in the brain, where it may alter the breathing pattern.

"You would decrease the volume of your breath and the number of breaths," Wall said. "You could become hypoxic have low amounts of oxygen in your blood and body tissues . Extreme respiratory depression can be fatal, but is very rare."

"The respiratory depression does not occur immediately, but hours down the line," Wall said. "It can occur 16 to 24 hours after you have given the injection. Therefore, it is recommended that there be close observation of these patients for at least a 24-hour period. In President Reagan's case, that is not going to be a problem."