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Human Error in Medical EquipmentTable of Contents:
Human errors are universal and are committed each day around the globe. Past experiences over the years clearly indicate that although most are trivial, some can be quite serious or fatal. In the area of health care, one study reported that in a typical year approximately 100,000 Americans die due to human errors [17]. Nonetheless, some of the medical device/equipmentassociated, directly or indirectly, human error facts and figures are as follows: ^{[1]}
Important Medical Equipment/Device Operator ErrorsPast experiences over the years indicate that there are many types of operator errors which occur during medical equipment/device operation or maintenance. Some of these are as follows [33]:
Medical Devices with High Incidence of Human ErrorOver the years, many studies have been conducted to highlight medical devices with a high occurrence of human error. Consequently, the most errorprone medical devices were highlighted. These devices, in the order of least errorprone to most errorprone, are as follows [34]: ^{[2]}
Medical Equipment Maintainability and MaintenanceMedical equipment maintainability may simply be described as the probability that a failed piece of medical equipment will be restored to its acceptable operating state. Similarly, medical equipment maintenance is all actions necessary for retaining medical equipment in, or restoring to, a specified condition. Both medical equipment maintainability and maintenance are discussed below, separately [35,36]. Medical Equipment MaintainabilityPast experiences over the years clearly indicate that the application of maintainability principles during designing the engineering equipment has helped to produce effectively maintainable end products. Their application in the medical equipment’s design can also be quite helpful for producing effectively maintainable end medical items. This section presents three aspects of maintainability considered useful for producing effectively maintainable medical equipment. Reasons for Maintainability Principles’ ApplicationThere are many reasons for maintainability principles’ application. Some of the main reasons are as follows [37]:
Maintainability Design FactorsThere are many maintainability design factors and some of the most frequently addressed factors are shown in Figure 9.2 [38]. Additional information on these factors is available in Refs. [9,38]. Maintainability MeasuresThere are various types of maintainability measures used in conducting maintainability analysis of engineering equipment/system. Two of these measures are as follows [3739]: • Mean Time to Repair (MTTR) FIGURE 9.2 Frequently addressed maintainability design factors. It is defined by where n is the number of units. t_{rJ} is the repair time required to repair unit j; for j = 1,2,3, ..., n. Xj is the constant failure rate of unit j for j = 1, 2, 3,..., n. • Maintainability Function This measure is used for predicting the probability that the repair will be accomplished in a time t, when it starts on an item/equipment at time t = 0. Thus, the maintainability function, m(t), is expressed as follows: where t is the time. /(f) is the probability density function of the repair time. Equation (9.3) is used for obtaining maintainability functions for various probability distributions (e.g., exponential, Weibull, and normal) representing failed item/ system/equipment repair times. Maintainability functions for various probability distributions are available in Refs. [3840]. Example 9.1 Assume that the repair times of a medical equipment/system are exponentially distributed with a mean value (i.e., mean time to repair (MTTR)) of 5 hours. Calculate the probability that a repair will be accomplished in 15 hours. Thus, in this case, the probability density function of repair times is defined by By inserting Equation (9.4) and the specified data value into Equation (9.3), we obtain Thus, the probability of accomplishing a repair within 15 hours is 0.9502. Medical Equipment MaintenanceFor the purpose of maintenance and repair, medical equipment may be classified under six classifications [41]: ^{•} Classification I: Imaging and radiation therapy equipment. Some examples of such equipment are linear accelerators, Xray machines, and ultrasound devices.
IndicesSimilar to the case of the general maintenance activity, there are many indices that can be used for measuring the effectiveness of the medical equipment maintenancerelated activity. Three of these indices are as follows [41]: • Index I This index measures how often the customer has to request for service per medical equipment and is defined by
where a_{c} is the number of repair requests completed per medical equipment. n is the total number of pieces of medical equipment. R_{rr} is the total number of repair requests. As per one study, the value of this index ranged from 0.3 to 2 [9]. • Index II This index measures how much time elapses from a customer request until the failed medical equipment is fully repaired and put back in service. The index is defined by
where a_{al} is the average turnaround time per repair. T_{lr} is the total turnaround time. m is the total number of work orders or repairs. As per one study, the turnaround time per medical equipment repair ranged from 135 to 35.4 hours [9]. • Index III This index is a cost ratio and is defined by where a_{cr} is the cost ratio. C_{m} is the medical equipment service cost. It includes all parts, materials, and labor costs for unscheduled and scheduled service, including inhouse, vendor, prepaid contracts, and maintenance insurance. C_{ma} ^{s} medical equipment acquisition cost. For various classifications of medical equipment, a range of values for this index are available in Ref. [9]. Mathematical ModelsOver the years, a large number of mathematical models have been developed for performing engineering equipment maintenance analysis. Some of these models can equally be used for performing medical equipment maintenance analysis. One of these models is presented below. ModelThis mathematical model can be used for determining the optimum time interval between item replacements. The model is based on the assumption that the item/ equipment average annual cost is composed of average investment, operating, and maintenance costs. Thus, the average annual total cost of a piece of equipment is defined by where C_{a}, is the average annual total cost of a piece of equipment. t_{je} is the item/equipment life expressed in years. C„j■ is the item/equipment operational cost for the first year. C_{m(} is the item/equipment maintenance cost for the first year. C,„ is the investment cost. i is the amount by which maintenance cost increases annually. j is the amount by which operational cost increases annually. By differentiating Equation (9.8) with respect to t_{je} and then equating it to zero, we get where t‘_{e} is the optimum time between item/equipment replacements. Example 9.2 Assume that for a medical equipment, we have the following data values: C,„= $200,000 j = $2,000 / = $500 Determine the optimum replacement period for the medical equipment under consideration. By substituting the above specified data values into Equation (9.9), we obtain
Thus, the optimum replacement period for the medical equipment under consideration is 40 years.

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