The case mix of patients admitted to the ICU annuallyvaries very little as it depends on the clinical activities in thehospital. Therefore the resources required to meet the demands of itsclinical management are predictable. An important consequence of thisis that the use of ICU resources (particularly staff) is almostconstant over time, so that it is possible to define staffing levelsthat will satisfy demand (as measured by severity of illness or riskof death). Because the nursing staff is the most important (about 90per cent of the total) and the most constantly present in the ICU, itis often used as a measure of level of ICU staffing.
Fourlevels of care (LOCs) based upon P/N ratios (the number ofpatients assisted by one nurse per shift) were defined in 1983 at theBethesda Consensus Conference. Staffing levels were proposed for onlythree of these, as the ICUs in the fourth level, where the nursingrequirements were difficult to distinguish from those in the generalward, were not considered to be true ICUs. The LOCs were defined onthe basis of the average nursing workload in the ICU measured atpatient level by means of the Therapeutic Intervention Scoring System(TISS) (Cullen etal 1974). The work of oneexperienced ICU nurse in each nursing shift is represented by 40 to45 TISS points daily. Therefore average daily patient requirementsare at least 40 TISS points ( P/N=1/1) in the ICU, between 39and 20 TISS points (P/N between 1.5 and 2) in high-dependencycare, between 19 and 10 TISS points (P/N between 2 and 4) inthe medium- to high-level care, and less than 10 TISS points inmedium- to low-level care. The division of ICUs into three levels ofcare corresponds to a generally experienced need to match theconcentration of resources to the intensive care requirements ofpatients. Therefore medium-care units are generally located next toICUs in the majority of hospitals. However, the traditionalclassifications of intensive care, high-dependency care, and mediumcare should be abandoned, because they are extremely difficult todefine, and should be replaced by the calculation of P/Nratios as the quantitative definition of LOCs.
The relation between workload as measured by TISS and the daily activities of the nursing staff has recently beenre-examined ( ReisMiranda,,,etal 1995). After simplifying, updating, and validating a new version of TISS, it wa sdetermined that one TISS point corresponded to 10.8 min of nursingwork and that one nurse could develop the equivalent of 46 TISS points per shift. Therefore it is possible to calculate the P/N ratio of an ICU by analyzing the nursing workload in agiven period of time. Each computed P/N ratio is itself anLOC. However, previous research has shown that it is possible toaggregate the ICUs into three LOCs, to each of which one-third of thetotal intensive care population will be admitted when stratifying thepatients by severity of illness ( ReisMI^ ICUs with patients requiring a P/Nratio below 1.6 are in LOC III, ICUs with patients requiring anaverage P/N ratio of 2.5 (range between 1.7 and 2.9) are inLOC II, and ICUs with patients requiring a P/N ratio above 3.0are in LOC I. Although the number of patients is similar in each LOC, there are more beds in the higher LOCs because the average stay islonger.
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