of surgical and systemic therapy that are now available for treating diseases that were formerly untreatable, and the multitude of antimicrobial drugs currently available and capable of selecting a more resistant microbial flora.

CHANGING FEATURES OF THE POPULATION OF HOSPITALIZED PATIENTS

The increase in mean life expectancy in the population as a whole is reflected in the increasing age of hospitalized patients. In an acute-care community general hospital in Madison, WI, for example, the percentage of patients 65 years of age or older rose from 13% in 1970–1973 to 24% by 1987 (2). Similarly, at the Massachusetts General Hospital, the mean age of admissions increased steadily over a 7-year period (1986–1992) from 54 to 57 years. Increasing age as a risk factor for nosocomial infections is seen in the analysis of nosocomial infections in elderly patients reported by the National Nosocomial Infections Surveillance (NNIS) system between 1986 and 1990 (3). Whereas elderly patients (>65 years of age) represented only 31% of total discharges from hospitals, 54% of nosocomial infections occurred in this group.

CHANGES IN TYPES OF PROCEDURES PERFORMED FOR COMPLEX ILLNESSES

Extensive surgical procedures are now carried out almost routinely, correcting a variety of incapacitating or debilitating illnesses and restoring patients to productive lives. Many of these procedures have been introduced and performed in large numbers only in the past several decades. Innovations in cardiac surgery or orthopedic surgery serve as examples (4, 5). In 1960 the modern era of heart valve replacement began with the first successful placement of a caged-ball valve in the subcoronary aortic valve position. Currently, an estimated 46,000-58,000 prosthetic heart valve replacement procedures are performed annually in the United States (6). The first saphenous vein aortocoronary artery bypass graft for coronary artery disease was carried out in 1967. Over the ensuing 25 years this procedure has become one of the most commonly performed operations in the United States. About 390,000 such procedures are performed each year (6).

Paralleling development of innovative and extensive operative procedures for treatment of cardiac disease have been similar advances in reconstructive orthopedic surgical procedures. The first total hip replacements in this country began in 1966, and current estimates are that ≈125,000 such procedures are performed annually in North America



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