Surge capacity: analysis of census fluctuations to estimate the number of intensive care unit beds needed.

OBJECTIVE: To compare methods of characterizing intensive care unit (ICU) bed use and estimate the number of beds needed. STUDY SETTING: Three geographic regions in the Canadian province of Manitoba. STUDY DESIGN: Retrospective analysis of population-based data from April 1, 2000, to March 31, 2007. METHODS: We compared three methods to estimate ICU bed requirements. Method 1 analyzed yearly patient-days. Methods 2 and 3 analyzed day-to-day fluctuations in patient census; these differed by whether each hospital needed to independently fulfill its own demand or this resource was shared across hospitals. PRINCIPAL FINDINGS: Three main findings were as follows: (1) estimates based on yearly average usage generally underestimated the number of beds needed compared to analysis of fluctuations in census, especially in the smaller regions where underestimation ranged 25-58 percent; (2) 4-29 percent fewer beds were needed if it was acceptable for demand to exceed supply 18 days/year, versus 4 days/year; and (3) 13-36 percent fewer beds were needed if hospitals within a region could effectively share ICU beds. CONCLUSIONS: Compared to using yearly averages, analyzing day-to-day fluctuations in patient census gives a more accurate picture of ICU bed use. Failing to provide adequate "surge capacity" can lead to demand that frequently and severely exceeds supply.