A discrete event simulation model for coordinating inventory management and material handling in hospitals

Abstract

Inventory management of surgical instruments and material handling decisions of perioperative services are critical to hospitals’ and operating rooms’ (ORs) service levels and costs. However, efficiently coordinating these decisions is challenging due to their interdependence and the uncertainties faced by hospitals. These challenges motivated the development of this study to answer the following research questions: (R1) How does the inventory level of surgical instruments, including owned, borrowed and consigned, impact the service level provided by ORs? (R2): How do material handling activities impact the service level provided by ORs? (R3): How do integrating decisions about inventory and material handling impact the service level provided by ORs? Three discrete event simulation models are developed here to address these questions. Model 1, Current, assumes no coordination of material handling and daily inventory management operations. Model 2, Two Batch, assumes partial coordination, and Model 3, Just-In-Time (JIT), assumes full coordination. These models are verified and validated using real life-data from a partnering hospital. A thorough numerical analysis indicates that, in general, coordination of inventory management of surgical instruments and material handling decisions has the potential to improve the service level provided by ORs. More specifically, a JIT delivery of instruments used in short-duration surgeries leads to lower inventory levels without jeopardizing the service level provided.

Appendix

Material handling process The research team collaborated with the Perioperative Services Department (PSD) of Greenville Memorial Hospital (GMH). The PSD consists of three departments: the Materials Department (MD) and the Central Sterile Storage Department (CSSD), both of which are located on the mezzanine floor (see Fig. 2), and the Operating Room Department (ORD) located on the second floor. GMH has 46 AGVs that are used to complete tasks such as, the delivery of food, linen, trash, sterile surgical material (instruments and supplies), etc. Each task is assigned a priority level, which changes during the day. This assignment is an effort to balance the use of AGVs. For example, the movement of sterile surgical material from MD to ORD has the highest priority during 3-6pm. Thus, a total of 10 AGVs are dedicated to the delivery of sterile surgical material during this time period.

Every day, MD receives a list of instruments and soft goods that should be delivered to ORD between 3 and 6pm. This list is generated based on doctors’ preferences and will be used in surgeries scheduled the next day. Instruments and soft goods are loaded manually into clean case carts. A team of 5 employees is tasked with loading the AGVs. This team is assigned to other tasks during the second shift. Carts are then manually moved to one of the 4 detents available at MD. Detents are areas equipped with the rails necessary for loading and unloading an AGVs. Once a request for an AGV is submitted to AGV control system, an available AGV, closest to the MD, is assigned to the case cart. The case cart is loaded on an AGV. This movement of the AGV is depicted in Fig. 1 as “Path of AGV with Clean Cart.” To move the case cart to the 2nd floor, this AGV uses elevator J. The clean case cart is then dropped off at one of the 2 detents in the case cart storage area (CCSA). Since CCSA is located next to elevator J on the 2nd floor. The CCSA is not shown in Fig. 2. Since, delivery of food and linen take priority after 6pm, AGVs become increasingly unavailable for the movement of surgical case during those times. Thus, it is expected that the delivery of surgical case carts is completed before other services take priority. The case carts are stored at the CCSA overnight. The case cart is then moved manually to the OR. After the surgery, the soiled cart is moved manually to the detents on the second floor. Once a request for an AGV is submitted to AGV control system, the assigned AGV moves the soiled cart to the CSSD using the path “Path of AGV with Clean and Soiled Cart” depicted in Fig. 2. The soiled instruments are washed and sterilized at the CSSD, a process that takes up to 3 or 4 h. The sterilized instruments are loaded to a clean case cart and moved to MD for storage. The soiled case carts are washed at the cart washer. The washed case cart is moved to MD for the next cycle. The movement of AGVs with washed case carts is depicted in Fig. 2 as “Path of AGVs with Washed Cart”.

AGV scheduling and operations The scheduling of AGVs is completed in 2 steps: First, a fleet of AGVs is assigned to tasks during the day based on task priority. Tasks are the delivery of food, delivery of trash, delivery of linen, delivery of surgical carts, etc. Task priorities change during the day. Next, tasks are assigned to AGVs based on a version of the first-come-first-serve rule. For example, if case cart 1 Is ready for pick-up, the 1st available AGV which is located closest to the case cart, will be assigned to deliver the cart.

The operation of AGVs follows certain guidelines, such as, (1) AGVs are not allowed to pass each other; (2) if an AGV stops, then other AGVs following will also stop and maintain a safe distance; (3) at most 2 AGVs can use an elevator at the same time; (4) an AGV will not seize elevator J if every detent in the second floor is busy. These operational practices lead to congestion; (5) if no task are available, the AGV is moved to the parking area.