Table 1 Examples of explanations that start each chapter, as described in Section 6, with the reference numbers in brackets changed to this article's reference numbers

a. Explanation at start of chapter 5: Definitions - OR Efficiency (Table 2). This chapter contains definitions used throughout the article and for the other scenarios.

   Case duration is defined as the time from when a patient enters an OR until he or she leaves the OR.

   Turnover time is the time from when one patient exits an OR until the next patient on that day's OR schedule enters the same OR [2, 18]. Separating turnover time from case duration permits the two to be studied statistically as separate processes. Turnover times include cleanup times and setup times, but not scheduled delays between cases.

   An example of a scheduled delay would be when the first patient of the day is scheduled for surgery that is anticipated to be completed by 11 AM, and the second case of the day in the OR will be performed by a different surgeon who is not scheduled to be available until 1 PM.

   Elective OR workload of a surgical service is its total hours of elective cases including turnover times throughout the workday, not just during scheduled OR hours [1].

   Under-utilized OR time is the positive difference between allocated OR time and the OR workload [1].

   Over-utilized OR time is the positive difference between OR workload and allocated OR time. When allocated OR time and scheduled OR hours are the same (e.g., a service is allocated an OR for 10 hr from 7 AM to 5 PM) and allocated OR time has not been released, then over-utilized OR time is the same as the hours that ORs run past the end of scheduled OR hours [1]. Hourly employees often receive overtime when working during over-utilized hours.

   Inefficiency of use of OR time equals the sum of two products: hours of under-utilized OR time multiplied by the cost per hour of under-utilized OR time and hours of over-utilized OR time multiplied by the cost per hour of over-utilized OR time [1]. The cost per hour of over-utilized OR time is invariably more expensive than the cost per hour of under-utilized OR time, in that staff want to get home on time.

   OR efficiency is the value that is maximized when the inefficiency of use of OR time has been minimized [1].

b. Explanation at start of chapter 14: Releasing OR Time Based on OR Efficiency (Table 2). The scenario in Table 4 is an example of application of the explanation.

   Occasionally, a service will have filled its allocated OR time and have another case to schedule. Then, OR efficiency is enhanced by scheduling the case into the OR time of the service expected to have the most under-utilized OR time, assuming availability of the surgeon, equipment, etc [1, 19, 20]. The service "releasing" its allocated OR time is not losing access to OR time, because its surgeons can continue to book cases.

   Services fill their allocated OR time at different rates. For example, at one hospital, the median times between when a patient was scheduled for surgery and the actual day of surgery ranged from 2 to 27 days. Whereas outpatient ophthalmology scheduled cases weeks before the day of surgery, cardiac surgery scheduled cases a few days before the day of surgery. Consequently, to maximize OR efficiency, allocated OR time cannot be released for all services the same number of pre-specified days before surgery [1, 19].

   Predicting which service will have the most under-utilized OR time on the day of surgery is not the same as determining which service has the largest difference between allocated and scheduled OR time when the new case is scheduled. However, the difference in OR efficiency between the two methods is very small. Thus, the service that should have its OR time released is the one with the most unscheduled but allocated OR time [1, 20].

   Whenever possible, the OR manager should indeed put the case into the OR time of the service with the most allocated but unscheduled OR time. This is particularly important for long (> 3 hr) cases scheduled at medium (and small) surgical suites [20]. OR managers can reasonably compromise on releasing the OR time of the service with the largest difference between allocated and scheduled OR time when the new case is short (1 hr).

c. Explanation at start of chapter 18: Day of Surgery Decisions (Table 2). The scenario in Table 5 is an example of application of the principle.

   On the day of surgery, OR efficiency is maximized by minimizing the hours of over-utilized OR time [1, 11, 19].