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Human Error in Medical Equipment

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/equipment-associated, directly or indirectly, human error facts and figures are as follows: [1]

  • • The Center for Devices and Radiological Health (CDRH) of the Food and Drug Administration reported that human errors account for around 60% of all medical device-associated deaths or injuries in the United States [29].
  • • A patient was seriously injured by over-infusion because the attending nurse wrongly read the number 7 as 1 [30].
  • • Human error, directly or indirectly, is responsible for up to 90% of accidents both generally and in medical devices [31,32].

Important Medical Equipment/Device Operator Errors

Past 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]:

  • • Incorrect selection of devices in regard to the clinical requirements and objectives.
  • • Mistakes in setting device parameters.
  • • Departure from following stated procedures and instructions.
  • • Wrong interpretation of or failure to recognize critical device outputs.
  • • Wrong decision-making and actions in critical moments.
  • • Untimely or inadvertent activation of controls.
  • • Over-reliance on automatic features of equipment/devices.
  • • Misssembly.

Medical Devices with High Incidence of Human Error

Over the years, many studies have been conducted to highlight medical devices with a high occurrence of human error. Consequently, the most error-prone medical devices were highlighted. These devices, in the order of least error-prone to most error-prone, are as follows [34]: [2]

  • • Permanent pacemaker electrode
  • • Administration kit for peritoneal dialysis
  • • Orthodontic bracket aligner
  • • Balloon catheter
  • • Glucose meter

Medical Equipment Maintainability and Maintenance

Medical 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 Maintainability

Past 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’ Application

There are many reasons for maintainability principles’ application. Some of the main reasons are as follows [37]:

  • • To determine the number of labor hours and related resources needed for carrying out the projected maintenance
  • • To lower projected maintenance time
  • • To determine the amount of downtime due to maintenance
  • • To lower projected maintenance cost through design modifications.

Maintainability Design Factors

There 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 Measures

There are various types of maintainability measures used in conducting maintainability analysis of engineering equipment/system. Two of these measures are as follows [37-39]:

• Mean Time to Repair (MTTR)

Frequently addressed maintainability design factors. It is defined by

FIGURE 9.2 Frequently addressed maintainability design factors. It is defined by


n is the number of units.

trJ 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:


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. [38-40].

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 Maintenance

For 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, X-ray machines, and ultrasound devices.

  • Classification II: Patient diagnostic equipment. Some examples of such equipment are spirometers, endoscopes, and physiologic monitors.
  • Classification Ш: Patient environmental and transport equipment Some examples of such equipment are patient beds, wheelchairs, and patient-room furniture.
  • Classification IV: Life support and therapeutic equipment. Some examples of such equipment are ventilators, lasers, and anesthesia machines.
  • Classification V: Laboratory apparatus. Some examples of such equipment are lab analyzers, lab refrigeration equipment, and centrifuges.
  • Classification VI: Miscellaneous equipment. This classification contains all other items that are not included in the previous five classifications, for example, sterilizers.


Similar to the case of the general maintenance activity, there are many indices that can be used for measuring the effectiveness of the medical equipment maintenance-related 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


ac is the number of repair requests completed per medical equipment. n is the total number of pieces of medical equipment.

Rrr 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


aal is the average turnaround time per repair.

Tlr 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


acr is the cost ratio.

Cm is the medical equipment service cost. It includes all parts, materials, and labor costs for unscheduled and scheduled service, including in-house, vendor, prepaid contracts, and maintenance insurance.

Cma s medical equipment acquisition cost.

For various classifications of medical equipment, a range of values for this index are available in Ref. [9].

Mathematical Models

Over 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.


This 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


Ca, is the average annual total cost of a piece of equipment. tje is the item/equipment life expressed in years.

C„j■ is the item/equipment operational cost for the first year. Cm( 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 tje and then equating it to zero, we get


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.

  • [1] Over 50% of all technical medical equipment problems are, directly or indirectly, due to operator errors [13]. • A fatal radiation overdose accident involving the Therac radiation therapydevice was the result of a human error [28].
  • [2] Contact lens cleaning and disinfecting solutions • Continuous ventilator (respirator) • External low-energy defibrillator • Trans-luminal coronary angioplasty catheter • Catheter guide wire • Catheter introducer • Peritoneal dialysate delivery system • Implantable pacemaker • Mechanical/hydraulic impotence device • Non-powered suction apparatus • Electrosurgical cutting and coagulation device • Urological catheter • Infusion pump • Intra-vascular catheter • Implantable spinal cord simulator
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