Antoinette C. Purdy, 39, and invalid who was trapped briefly in her burning Lexington Park, Md., home Sept. 30, died Thursday at St. Mary's Hospital in Leonardtown of smoke inhalation and double pneumonia.

Mrs. Purdy, who in recent years had been incapacitated by amyotrophic lateral sclerosis, the paralyzing disease that killed baseball star Lou Gehrig, was carried from blazing house by a carpenter who rushed from across the street, a neighbour said.

Unable to speak or use her right arm or legs, Mrs. Purdy could not flee unaided. The only other person in the house at the time was her 3-year-old son, Bob.

The boy poured a glass of water on the living room sofa in which the fire broke out, but then ran from the home with his hair badly singed, according to relatives and St. Mary's County fire officials.

Frances Garnes, Mrs. Purdy's mother, said the first at the Purdy's home, at 7 Hartman Dr., began after the 3-year-old had knocked a lighted lamp onto the sofa. Sometime later, Mrs. Purdy's 9-year-old daughter , Karen, found the lamp lying on the couch, its bare bulb scorching a cushion.

The 9-year-old then called her 17-year-old sister, Valerie, at work and asked her what to do. The older sister told the girl to check for sparks and to smother them, according to Mrs. Garner. The 9-year-old found only what appeared to be harmless scorch marks and left home for school, Mrs. Garner said.

However, fire officials said, the cushion apparently smoldered for about 20 minutes and then burst into flame. Unable to get help or instructions from his paralyzed mother, the 3-year-old boy ran to the kitchen where he got a glass of water and threw it at the sofa in an effort to douse the fire.

Failing in the attempt, and frightened by the blaze, the boy ran screaming from the home through a patio door, Mrs. Garner said. Then Mrs. Purdy, who had been in a family room adjacent to the living room, realized there was a fire and manage to drag herself within arm's reachof the patio door and drew it shut. The woman's mother said Mrs. Purdy later indicated to her that she did so to prevent the 3-year-old from returning to the house.

Moments later a carpenter working at a house across the street heard the child screaming and saw smoke pouring from the Purdy home. He ran to the house, broke the glass patio door and carried Mrs. Purdy outside.

St. Mary's County fire fighters and brought the blaze under control. Fire officials said the fire left the home extensively damaged but habitable.

Mrs. Purdy, a 1955 graduate of Anacostia High School, was a housewife most of her adult life. Before her marriage she had worked for a year as a secretary with the Acacia Mutual life Insurance Co. in Washington.

She had been a member of Citizens for Community Schools Inc. and a leader of a Camp Fire Girls group.

Other survivors include her husband, Robert H., of the home; two other daughters, Barbara Mayor and Elaine Dean, both of Ridge, Md., and a grandmother, Margaret Hossmann, of Marlow Heights.