Table 1.
Key definitions and model assumptions for a medical intensive care unit/step-down unit
| Term | Definition | Assumptions |
|---|---|---|
| Bed types | ||
| ICU bed | ICU bed that can treat all acuity classes | Beds are identical and always staffed |
| SDU bed | SDU bed that can treat lower acuity patients | Beds are identical and always staffed |
| Patient characteristics | ||
| Acute | Those with a critical care–level diagnosis (27) or who require a critical care–level intervention (28) | Patients originating from non-ED locations are higher priority |
| Subacute | Those admitted directly to SDU or admitted to the ICU who do not either have a critical care–level diagnosis or receive a critical car–-level intervention | Patients originating from non-ED locations are higher priority |
| Short-stay | Those who stay less than 2 wk (acute patients) or 24 h (subacute patients) | Classification used to estimate service time distribution from past data |
| Long-stay | Those who stay longer than 2 wk (acute patients) or 24 h (subacute patients) | Classification used to estimate service time distribution from past data |
| Model inputs | ||
| Arrival rate | Number of patients per hour who require an ICU or SDU bed | Estimated from past data on patient throughput; calculated as the reciprocal of the average time between consecutive patient arrivals |
| Service time | Duration of time spent receiving care in the ICU or SDU | Estimated by fitting probability distributions to past data; can vary by acuity level and short or long stay |
| Time to transfer | Duration of time between request for transfer out of ICU or SDU and physical transfer to wards | Estimated using multivariate regression; can depend on concurrent patient census levels in the ICU/SDU and medical wards |
| Model outcomes | ||
| Wait time | Duration of time between ICU or SDU bed request and physical transfer out of unit | Shorter wait times are preferred; can be validated against past data |
| Bed occupancy | Number of bed-hours that are occupied divided by total available bed-hours | Higher occupancy is more profitable but results in lower bed availability |
Definition of abbreviations: ICU = intensive care unit; SDU = step-down unit.