Definitions and epidemiology of the chronically critically ill.

Patients who are chronically critically ill constitute 5-10% of patients with acute respiratory failure but demand a disproportionate share of ICU resources. Epidemiologic studies and clinical trials require definitions for enrollment, and a uniform definition would be ideal to allow comparisons between studies. While a consensus definition exists (≥ 21 consecutive days of mechanical ventilation for ≥ 6 h/d), many study designs have required alternative definitions that include requirement for a tracheostomy, a different period of mechanical ventilation, or admission to a weaning facility. Regardless of definition, studies have indicated that the incidence of chronic critical illness has doubled in recent decades and may double again in the next decade. The overall 1-year survival for chronically critically ill patients is between 40% and 50%, depending on the cohort studied. New clinical prediction rules have been developed to better identify patients who are at high risk and low risk of death. These models could be enhanced by data on functional outcomes for survivors. The healthcare system has been adapting to the increase in chronic critical illness by increasing critical beds in short-term and long-term acute care hospitals, but continued monitoring of resources will be necessary, since the prevalence of chronic critical illness is expected to increase further.