The average daily charge for a bed in the intensive-care unit at American hospitals shot up more than 18 percent to $356 in 1981, a record, according to a survey released yesterday.

In major cities, the cost was much higher than the average, with the highest mark recorded in Los Angeles, where an intensive-care bed for one day costs $687 on the average.

The District of Columbia was not far behind, with intensive-care beds costing an average of $617 a day. Statewide in Maryland, the average was $395; in Virginia it was $332.

In 1980, the national average was $300.50 and then contributed to a booming annual national health care bill that for the first time rocketed past $250 billion. But this year, the costs jumped still higher, according to the new survey published by Equitable Life Assurance Society of the United States and based on reports from 2,658 of the nation's 6,000 nonfederal hospitals.

Michael D. Bromberg, executive director of the Federation of American Hospitals, an organization representing private hospitals in America, gave the three following reasons for the rise:

"For more than a year there has been a shortage of nurses all across the country. The short-term answer to the shortage has been wage increases, and that has led to a price war. Paying more for nurses has had a rippling effect on other hospitals' workers as well. But for intensive care, where the ratio of nurse to patient is about 1-to-1, wages would be one important reason for the rise in costs."

"Another reason is that there has been an increase in admissions of patients over 65. For patients under 65 there has been no percentage increase in admissions, but for those over 65, there has been a 5 percent increase over the last year. It costs more than twice as much for elderly patients to be treated, because they are less mobile, and their average stay is longer." The elderly also use intensive care more frequently than younger people.

"Still, as much as three-quarters of the rise is just general inflation."

Intensive-care unit beds are a hospital's most expensive because patients require more attention from doctors and nurses as well as more tests and sophisticated equipment.

The lowest average intensive care cost was reported in Cleveland, Miss.: $91.

The "1981 National Survey of Hospital Room and Board Charges" also reported on average charges in less expensive hospital beds--private and semiprivate.

The 1981 average room and board bills for patients in private rooms were $152.50--up 18 percent from a year ago. Average charges in semiprivate rooms also hit a record--$151, up 15 percent from the 1980 figure, $131.

Equitable sent questionnaires to 3,076 hospitals, requesting that they exclude from their reports geriatric and orthopedic beds and beds reserved for maternity, nursery and psychiatric care.

The survey, done in the third quarter of 1981, drew an 86 percent return. The report said the room and board charges reported are exclusive of ancillary or special charges.

In the survey of average semiprivate room charges in the states, Virginia and Maryland fell below the average, at $103-$123 and $123.50-$143.50, respectively. The District of Columbia's were higher, at $185-$219.