61 Reliability in Medical Device Industry

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Although reliability engineering tools are well established, their application to the medical industry is ..... Prediction Procedure for Electronic Equipment) standard ...

61 Reliability in Medical Device Industry Vaishali Hegde Respironics Inc. Sleep and Home Respiratory Central Facility Monroeville, Pennsylvania, 15146, USA

Abstract: The medical industry is one of the fastest growing segments of US economy. Increasingly, medical equipment is being used outside a controlled hospital environment. The complexity and increased use of medical equipment in non-hospital environments has made the need for safe, reliable products imperative. Although reliability engineering tools are well established, their application to the medical industry is fairly new. This chapter discusses the reliability tools and standards applicable to the medical equipment industry.



The medical industry is one of the fastest growing segments of US economy. In 2004, health care spending in the US reached $1.9 trillion, and was projected to reach $2.9 trillion in 2009 [1]. In 2004, the US spent 16 percent of its gross domestic product (GDP) on healthcare. It is projected to rise to 20% in the next decade. United States is not the only country in the world spending large sums of money on health care. Countries all over the world are spending more and more on health care every year. According to the Organization for Economic Cooperation and Development, health care spending accounted for 10.9 percent of the GDP in Switzerland, 10.7 percent in Germany, 9.7 percent in Canada and 9.5 percent in France [2]. Health care spending is increasing all over the world due to increased life expectancy and consequently the increased aging population. Reduction in mortality during the 20th century led to large increases in life expectancy. By 2000, life expectancy in the US was approximately 76.9

years. According to US Census Bureau’s projections, by 2030, the older population in US is projected to be double that of 2000, growing from 35 million to 72 million. By 2050, they project the older population to be around 86.7 million. In other words, nearly 1 in 5 Americans will be age 65 and over in 2030 [3]. In 2000, about 92 percent of people aged 65 and over had made at least one health care visit to a doctor’s office, an emergency room, or a hospital during the past year (NCHS, 2003a). Among people 65 and older, the number of health care visits increased with age. Chronic diseases have caused most elderly deaths throughout the last 50 years. Diseases of heart, malignant neoplasms (cancer), cerebrovascular diseases (stroke), chronic obstructive pulmonary diseases/chronic lower respiratory diseases and pneumonia and influenza were the top 5 causes of death for people aged 65 and over, in year 2000 [3]. Treatment of these chronic diseases requires several different types of medical devices. All data mentioned above, points


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to an explosive growth in the medical device and medical services market in the world. When it comes to medical equipment, failure is not an option. From critical devices such as oxygen concentrators, lasers, ventilators, MRI scanners, insulin pumps, implantable pacemakers to instruments as straightforward as stethoscopes, injections, and thermometers; medical equipment must be reliable. The medical industry must have higher reliability standards than other fields. However, a high reliability standard is hard to maintain in today’s environment of intense global competition, pressure for shorter product-cycle times, stringent cost constraints, higher customer expectations for quality and reliability and complex global and heterogeneous markets. Medical electronic products are increasingly being used in non-hospital environments by nonclinical personnel. This is because sophisticated medical products are becoming more compact and portable. For example, a few years ago, an oxygen concentrator weighed 75 pounds, had to be plugged into the wall for power, and had to be used in a controlled clinical environment. Today, a portable oxygen concentrator weighs only 8 pounds, can operate on batteries and can be used in a home environment. Developing and producing a medical electronic product reliable enough to continuously repeat certain key functions over and over again like clockwork and alarming appropriately in case of a fault condition, is now more important than ever before. Today, reliability takes on even greater significance since medical devices designed for specific applications are becoming more expensive. It is not uncommon for cancer treatment or surgery equipment to cost a few million dollars. While hospitals may be relatively profitable, they cannot afford to have too many of these expensive products. If one such medical system incurs downtime due to failure, it can adversely affect the hospital’s bottom line, its reputation and the wellbeing of its patients [4]. In some cases, medical electronics products in use for several years require updating to increase their reliability as greater healthcare demands are placed on them. The financial impact of having an unreliable or unsafe medical product in the market can be

devastating for a medical device manufacturer. Since December 13, 1984, the FDA Medical Device Reporting (MDR) regulations have required firms who have received complaints of device malfunctions, serious injuries or deaths associated with medical devices to notify FDA of the incident. The MDR regulation provides a mechanism for FDA, manufacturers, importers and user facilities to identify and monitor significant adverse events involving medical devices. The goals of the regulation are to detect and correct problems in a timely manner. MedWatch, the FDA’s safety information and adverse event reporting program is set up for health professionals and consumers to report serious adverse reactions and problems related to drugs, medical devices, cosmetics etc. MedWatch plays a critical role in FDA’s post marketing surveillance – the process of following the safety profile of medical products after they’ve begun to be used by consumers. The FDA publishes a weekly FDA Enforcement Report that contains all enforcement actions including recalls, field corrections, seizures and injunctions. This report is published on the internet at http://www.fda.gov/opacom/Enforce.html. It also maintains MAUDE (Manufacturer and User Facility Device Experience database) which contains reports of adverse events involving medical devices. Depending on the severity of MDR or MedWatch report filed, a defective medical device may have to be recalled. A recall is a method of removing or correcting products that are in violation of laws administered by the Food and Drug Administration (FDA). Filing an MDR or recalling a medical device takes a significant amount of time, effort and resources on the part of the manufacturer. It can have a negative impact on a device manufacturers’ reputation in the marketplace. It may lead to product liability lawsuits, particularly in the US. All this could consequently lead to an adverse impact on sales and profits. Low reliability can have a significant impact on service, repair, and warranty costs as well. In fact, warranty cost is inversely proportional to the reliability of a medical device. Hence, more and more manufacturers are willing to invest in

Reliability in Medical Device Industry

reliability related tasks to try and reap the benefits in terms of warranty costs. A 5% increase in reliability focused development costs will return a 10% reduction in warranty costs. A 20% increase in reliability focused development costs will typically reduce warranty costs by half and a 50% increase in reliability focused development costs will reduce warranty cost by a factor of 5 [5].


Government (FDA) Control

As the use of medical devices for critical life supporting functions increases, governments around the world are regulating their design, manufacture, quality, reliability, proper use and disposal. The Food and Drug Administration (FDA) is the U.S. Government agency that oversees most medical products, foods and cosmetics. Within FDA, the Center for Devices and Radiological Health (CDRH) oversees the safety and effectiveness of medical devices and radiationemitting products. The FDA is responsible for protecting the public health by assuring the safety, efficacy, and security of human and veterinary drugs, biological products, medical devices, the nation’s food supply, cosmetics, and products that emit radiation. The FDA is also responsible for advancing the public health by helping to speed innovations that make medicines and foods safer, and more affordable, and helping the public get the accurate, science based information they need to use medicines and foods to improve their health. Products regulated by FDA include all foods except meat and poultry, prescription and nonprescription drugs, blood products, vaccines and tissue transplantation, medical devices and radiological products, including cellular phones, animal drug and feed, and cosmetics. In addition to setting product standards, FDA regulates the labeling of products under its jurisdiction. This information must be valid, well documented, and not misleading. FDA plays a major role in protecting consumers and the public health. The Federal Food, Drug, and Cosmetic Act of 1938 authorized the FDA to take formal or informal regulatory measures against the misbranding or adulteration of medical devices.


Amendments to this act in 1976 empowered the FDA to regulate medical devices during their design and development phases. The Safe Medical Device Act passed in 1990, authorized the FDA to implement the Preproduction Quality Assurance Program. This program requires medical device manufacturers to address deficiencies that lead to failure during design. Nowadays, similar types of regulations concerning medical devices are being followed in other countries. For example, the Medical Device Directive (MDD) of the European Union (EU) outlines the requirements for medical devices in the EU countries.


Medical Device Classification

The FDA has established three regulatory classes for medical equipment based on the level of control necessary to assure the safety and effectiveness of the device. The class to which a device is assigned determines, among other things, the type of premarketing submission / application required for FDA clearance to market. Device classification is risk based, that is, the risk the device poses to the patient and/or the user is a major factor in the class it is assigned. All classes of devices are subject to General Controls which are the baseline requirements of the Food, Drug and Cosmetic (FD&C) Act. The three classes and the requirements which apply to them are: • Class I - General Controls Class I devices are subject to the least regulatory control. They present minimal potential for harm to the user and are often simpler in design than Class II or Class III devices. Class I devices are subject to "General Controls" as are Class II and Class III devices. Examples of Class I devices include elastic bandages, examination gloves, and handheld surgical instruments. • Class II - General Controls and Special Controls Class II devices are those for which general controls alone are insufficient to assure safety and effectiveness, and existing methods are available to provide such assurances. In addition to complying with general controls, Class II devices are also


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subject to special controls. Special controls may include special labeling requirements, mandatory performance standards and postmarket surveillance. Examples of Class II devices include powered wheelchairs, infusion pumps, and surgical drapes. • Class III - General Controls and Premarket Approval Class III is the most stringent regulatory category for devices. Class III devices are those for which insufficient information exists to assure safety and effectiveness solely through general or special controls. Premarket approval by FDA, is required for this class of devices. Premarket approval is the required process of scientific review to ensure the safety and effectiveness of Class III devices. Class III devices are usually those that support or sustain human life, are of substantial importance in preventing impairment of human health, or which present a potential, unreasonable risk of illness or injury. The reliability activities that need to be performed during the life cycle of a product, depends to some degree on the classification of the product. Class III devices need maximum analyses and testing because they support or sustain human life.


Reliability Program

Reliability engineering tools are well established however their application to the medical industry is fairly new. The first step in launching a safe and reliable medical product is a good reliability program. What are the hallmarks of a good reliability program? Developing realistic reliability goals early, planning an implementation strategy and then executing the strategy are all key features of a good reliability program. The four phases of a reliability program are [6] • Concept Phase • Design Phase • Prototype Phase • Manufacturing Phase Several different reliability tools are available for use in the four phases of a reliability program. In the concept phase, one can use benchmarking and

gap analyses to develop a reliability program and integration plan. In the design phase, one can use reliability modeling and predictions, derating analysis/component selection, worst case circuit analysis, thermal analysis, electromagnetic analysis, design of experiments, risk management / FMECAs, fault tree analysis, human factors analysis and software reliability analysis to increase reliability of a product. In the prototype phase, one can perform highly accelerated life testing (HALT), design verification testing (DVT), and reliability demonstration testing (RDT). In the manufacturing phase, the reliability tools that can be used are highly accelerated stress screening (HASS), on-going reliability testing, FRACAS/CAPA system setup, and end-of-life assessment. 61.4.1

Concept Phase Benchmarking

Benchmarking is the process of comparing the current project, methods, or processes with the best practices in the industry. Benchmarking is crucial to both a startup company as well as an established company that is coming out with a new product to assure the new product is competitive based on reliability and cost. For example, company A, a new entrant into the medical manufacturing market, wants to start selling a new type of dialysis pump. Company A should check the warranty information, annual service plan and recommended maintenance schedule of leading competitor products to try and benchmark their dialysis pump with respect to MTBF, failure rate, service interval etc. against competitor pumps. Gap Analysis Gap Analysis compares your current capabilities with what is expected of your product in the industry. Before performing a gap analysis you have to set a reliability goal and perform a review of your current capabilities. Gap = Goals - Current Capabilities. For example, a ventilator manufacturer currently has a ventilator on the market that has a field reliability of 0.988. It is now

Reliability in Medical Device Industry

planning to add oxygen blending feature to this ventilator. They would like the new ventilator to have a reliability goal of 0.9999 to meet customer expectation. The process of determining the current capability (0.988), the goal (0.9999), and the gap (0.9999-0.988 = 0.0119) is called gap analysis. The output of this phase is the reliability program. The program generally quantifies out of box failure rate, reliability within warranty period, and reliability throughout life of the product. It also defines a schedule of the different activities that will be performed to achieve your reliability goal. 61.4.2

Design Phase Reliability Modeling and Predictions A reliability prediction is a method of calculating the reliability of a product by assigning a failure rate to each individual component and then summing all of the failure rates. Standards such as MIL-HDBK-217, Bellcore, PRISM, CNET, HRD5 etc can be used for reliability predictions. A reliability model presents a clear picture of functional interdependencies and provides the framework for developing quantitative product level reliability estimates to guide design trade-off process. Models are helpful for identification of single points of failure, making numerical allocations, evaluating complex redundant configurations, and showing all series-parallel relationships. Derating Analysis / Component Selection In MIL-STD-721 derating is defined as using an item in such a way that applied stresses are below rated values. Limitation of electrical, mechanical and environmental stresses is critical to high reliability. One common rule of thumb used in electronics is that 50% derating can decrease the failure rate by 30%. Telcordia SR332 (Reliability Prediction Procedure for Electronic Equipment) standard gives derating information. Component selection must be based on the basis of derating analysis. Design engineers should select appropriate component ratings to avoid overstressing them. In case of implantable medical devices, biocompatibility and reliability of the


material and components should be considered as well, while making component selections. Worst Case Circuit Analysis Worst case circuit analysis (WCCA) is an analysis technique which, by accounting for component variability, determines circuit performance under a worst case scenario, i.e., under extreme environmental or operating conditions. The output of a WCCA allows an assessment of actual applied part stresses against rated part parameters, which can help ensure the application of sufficient part stress derating to meet design requirements. One of the most critical steps involved in completing a meaningful WCCA is the development of a part characteristic database. This database should contain a composite of information necessary for quantifying sources of component parameter variation. Once these sources have been identified, the database can be used to calculate worst case component drift for critical parameters. Thermal Analysis Temperature is one of the important variables that impacts system reliability. Therefore, the thermal design of a system must be planned and evaluated carefully. Thermal Analysis techniques can be used to evaluate the circuit board, casing, and junction temperatures and check for components that exceed their temperature limits. The analysis can also be used to identify the hot spots and components on the board that designers may modify by various means to eliminate thermal problems. Typical solutions are adding heat sinks, thermal pads, and thermal vias, using thicker ground or power planes, local conduction pads, thermal screws, changing of radiation emissivity by coatings, or relocating components. Electromagnetic Analysis Electromagnetic compatibility, or EMC means that a device is compatible with (i.e., no interference is caused by) its electromagnetic (EM) environment and it does not emit levels of EM energy that cause electromagnetic interference (EMI) in other devices in the vicinity. A medical device can be vulnerable to EMI if the levels of EM energy in its environment exceed the EM immunity (resistance)


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to which the device was designed and tested. The different forms of EM energy that can cause EMI are conducted, radiated, and electrostatic discharge (ESD). EMI problems with medical devices can be very complex, not only from the technical standpoint but also from the view of public health issues and solutions. The Center for Devices and Radiological Health (CDRH) encourages manufacturers of electro-medical equipment to use the IEC 60601-1-2 standard, a widely recognized standard issued by the International Electrotechnical Commission, Geneva, Switzerland. The standard provides various limits on emissions and immunity. FMECA Failure modes, effects and criticality analysis (FMECA) is a reliability evaluation and design review technique that examines the potential

level. There are several different types of FMEAs. For example, design FMEAs, process FMEAs, functional FMEAs, system FMEAs etc. The basic approach for all the FMEAs is the same. A sample section of a FMEA worksheet is given in Figure 61.1. Fault Tree Analysis A fault tree analysis is a systematic, deductive methodology for defining a single specific undesirable event and determining all possible reasons or failures that could cause that undesirable event to occur. The undesired event is the top event in the fault tree and is generally a catastrophic or complete failure of the product. Fault trees use concepts of logic gates to determine the overall reliability. In medical devices, patient harm is the most common undesired top event. The results of

Fig. 61.1: Sample Section of a FMEA Worksheet

failure modes within a system or lower indenture level, in order to determine the effects of failures on equipment or system performance. FMECA uses a “bottom up” approach. This approach begins at the lowest level of the system hierarchy and traces up through the system hierarchy to determine the end effect on system performance. The criticality portion of this method allows us to place a numerical value or rating on the criticality of the failure effect on the entire system or user. After the initial analysis, one can prioritize the failure modes, provide mitigations, reanalyze, rescore and confirm that risks are at an acceptable

FTA can be used as a troubleshooting tool during service calls, after the product is available in the market. A sample section of a fault tree is given in Figure 61.2.

Reliability in Medical Device Industry


rates [7]. The Mills model was developed by H.D. Mills [8] by reasoning that an estimation of faults remaining in software could be made through a seeding process that assumes a homogeneous distribution of a representative class of faults. Before starting the seeding process, fault analysis is performed to determine the expected types of faults in the code and their relative frequency of occurrence [7]. 61.4.3

Fig. 61.2: Sample Section of a Fault Tree Human Factors Analysis Human factors analysis should be performed with safety, manufacturing and maintainability in mind. Human beings are famous for not reading and following instructions. Surrounding distractions is a major concern in the hospital environment. Medical device manufacturers have to make sure that hospital staff is able to use the medical equipment correctly and efficiently in spite of all the other surrounding hospital distractions. They also have to make sure that symbols used on their medical devices meet standards approved by the FDA. AAMI (Association for the advancement of medical instrumentation) has published a technical information report on graphical symbols for electrical equipment in medical practice. Software Reliability Analysis Software is an important component of medical devices. A software life cycle can be divided into design, coding, and testing. If good practices are followed in all stages of the software life cycle a reliable software product will be ensured. The USAF Model and the Mills Model are two software reliability models that are generally considered useful for medical devices. The USAF model was developed by the U.S. Air Force Laboratory in Rome, NY. to predict software reliability during the initial phases of the software life cycle. The model begins by developing the predictions of fault density and then transforming them into other reliability measures such as failure

Prototype Phase

Testing is an important part of any product development cycle. It is performed to verify performance, understand failure modes and weaknesses of the product, benchmark your product against competitor products, determine reliability and safety of the product, and study the impact of stress on performance of the product. Reliability and safety testing is crucial in medical devices. Testing can be performed in two different modes: standard or accelerated. In standard mode, tests are performed in ambient temperature, at typical operating stress conditions. In accelerated mode, parameters such as temperature, voltage, current, cycling, are increased well above their normal levels to reduce test times. In the prototype phase, one can perform highly accelerated life testing (HALT), design verification testing (DVT), and reliability demonstration testing (RDT). HALT Highly Accelerated Life Testing is a quick and cost effective tool used for discovering design issues and improving design margins. Thermal stress and vibration stress are the two main stresses applied during HALT. Generally a product is subjected to a cold thermal cycle, a hot thermal cycle, a fast thermal transition cycle, a vibration cycle, and a combined thermal-vibration cycle. A root cause analysis is performed at the end of the test to determine the cause of failures. Design and processing changes can be made based on HALT findings. A verification HALT can be performed to ensure that problems are fixed and new problems have not been introduced. For example, during a


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HALT test, a ventilator with an operating range of 5C to 40C, provided incorrect therapy above 35C and shut down at 65C. The root cause of failure was determined to be the temperature rating of a proximal pressure flow sensor. The standard sensor was changed to an extended temperature range sensor to solve the problem. On the same ventilator, the internal battery was disconnecting at very low vibration levels. The battery connector was changed to a positive latch, mini lock type connector to prevent disengagement of the battery. Both the sensor issue and the battery connector issue would not have been discovered before product release if HALT had not been performed. DVT Design Verification Testing is performed to verify product functionality. Generally, this testing is performed under normal operating conditions; no external stresses are applied. The results of design verification testing are generally required during pre-market notification 510(k) submittal to the FDA. RDT Reliability Demonstration Testing is used to validate reliability prediction analyses and gather a measure of confidence that the released product will achieve a certain reliability target. In this test, a sample of units is tested at slightly accelerated stresses for several months. The stresses are higher than normal operating stress conditions but lower than HALT stress conditions. The stresses are held constant thus enabling you to calculate the acceleration factor for the test. MTBF (mean time between failures) can be obtained from test results. This test is generally performed once before product release. 61.4.4

Manufacturing Phase

In the manufacturing phase, the reliability tools that can be used are highly accelerated stress screening (HASS), on-going reliability testing, FRACAS/CAPA system setup, and end-of-life assessment. HASS Highly Accelerated Stress Screening is used to reduce out of box quality returns, decrease field service and warranty costs, and detect and correct process defects. Typical failures that HASS will find are soldering defects, bent IC leads, socket failures, incorrect components or component placement and programming errors. It is recommended that a HALT be performed prior to performing a HASS. HALT will help to identify the operating and destructive limits of a product. This information is essential while developing a HASS process. ORT On going reliability testing is performed to get an indication of the reliability of the product before it is shipped to customers. In this test, a sample of product is taken off a production line and tested for a period of time, adding the cumulative test time to ensure that the reliability target will be met. The samples are rotated on a periodic basis to get an on-going reliability. One must take care not to wear out any components because these units are shippable units and you cannot risk taking significant life out. FRACAS/CAPA system setup Failure reporting and corrective action systems (FRACAS) or Corrective and Preventive Action systems (CAPA) provide a framework for controlling corrective action processes. At its core, FRACAS is a closed loop corrective action system which enables you to collect failure data, analyze data and determine root cause, document the corrective action and implement controls to prevent the reoccurrence of the failure. An integral part of CAPA is a Failure Review Board (FRB). The Board is typically a cross-functional group representing Quality, Reliability, Engineering, Manufacturing, Regulatory and other departments depending on the nature of the business. The Board is responsible for reviewing and approving recommended corrective actions and evaluating effectiveness of corrective actions after implementation. A FRACAS system is essential for medical device manufacturers because of the

Reliability in Medical Device Industry

FDA Medical regulations.



(MDR) EOL Assessment End of Life assessment is performed to determine when a product is starting to wear out, whether the preventive maintenance strategy is effective and whether the predictions performed during design phase were accurate. EOL assessment uses the Weibull plotting technique to figure out where the product is on the bathtub curve. Field failure data is used to determine the number of days before failure.


Reliability Testing

There are many different reasons for performing reliability testing. The main reasons are to induce failure modes as well as detect unanticipated failure modes so corrective actions can be implemented, to determine if items or system meet reliability requirements, to compare estimated failure rates to actual failure rates, to monitor reliability growth over time, to determine the safety margin in a design, to estimate MTBF or MTTF values, and to identify weaknesses in the design or parts [9]. O’Connor states that reliability testing is a part of an integrated test program involving statistical testing, to optimize the design of the product and the production processes; functional testing, to confirm the design; environmental testing, to ensure that the product can operate under the projected environments; reliability testing, to ensure that the product will operate for its expected life; and safety testing, to ensure safety of the product for use by humans, animals or property [10]. Developing a test plan is the first step of an integrated test program. The Reliability Toolkit provides a test plan outline which includes the following steps: define purpose and scope, list reference documents, define test item facilities, test requirements, test schedule, test conditions, test monitoring, test participation, failure definitions, test ground rules, and test documentation [11].


Test procedures are required for the proper execution of the test plan. Test procedures must address calibration and proofing the test equipment. They must have a detailed description of the tools, parts, adjustments, hook ups, datasheets, tools and materials required for the test. The Reliability Toolkit provides a reliability test procedure checklist that includes equipment operation, on/off cycles, operation modes, exercising methods, performance verification procedure, failure event procedure, and adjustments and preventive maintenance schedule [11]. There are several different types of reliability tests that can be performed over the life of the product cycle. Reliability Development/Growth Tests (RD/GT), Reliability Qualification Tests (RQT), Performance Tests, Screening Tests and Probability Ratio Sequential Tests (PRST). 61.5.1

Development / Growth Test

This type of test is run to determine if there is a need to change the design to achieve reliability specification, or to verify improvements occurring in design reliability after changes have been made. These tests are generally run in the prototype or development phase. Estimates of MTTF or MTBF can be obtained from these tests. 61.5.2

Qualification Test

The objective of this type of test is to determine if a design is acceptable for the intended function. Qualification tests in many cases include aspects of vibration, shock, temperature cycling and other environmental considerations the product will see in use. 61.5.3

Acceptance Test

Acceptance testing is statistically derived to determine if an item is to be accepted or rejected for use, either individually or on a lot basis. This method of testing does not provide MTBF, MTTF or any other quantifiable measure.



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Performance Test

Performance testing is conducted on completed designs and normally manufactured items to verify the reliability predictions and test results in the preproduction phase. This testing provides a benchmark for comparison of previous activities to see if they are effective in delivering a design that meets reliability requirements.


MTBF Calculation Methods in Reliability Testing

Screening tests are performed with the intent of eliminating the infant mortality period. By eliminating infant mortality period, the yield of finished product is improved, and a higher reliability is achieved out in the field.

MTBF (mean time between failures) is used frequently to assess reliability of a medical product. There are five methods to estimate MTBF in reliability testing [7]. • Time terminated, failed items replaced • Time terminated, failed items not replaced • Failure terminated, failed items replaced • Failure terminated, failed items not replaced • No failures All the methods assume exponential distribution (constant failure rate). Each method is described below.





Sequential Testing

Sequential testing for products designed to operate for some period of time is outlined in MIL-STD781. These tests called Probability Ratio Sequential Tests (PRST) are based on the ratio of an acceptable MTBF which should have a high probability of acceptance to an unacceptable MTBF which should have a low probability of acceptance. Items are placed on test, and failures that occur are plotted against test time. A decision is made based on the plotted point. The decision is one of three, accept the items as meeting the required MTBF, reject the items as not meeting acceptable MTBF, or continue testing. Sequential test plans provide the least amount of time to make a decision when product is either very good or very bad as compared to reliability requirements. When the product is borderline, testing can be continued for an indeterminate period of time, which is an unacceptable situation. To prevent the tests from continuing indefinitely there are specific truncation points designed in these tests. The test will terminate at either a given amount of failures or a predetermined test time, if a decision has not been made.

Time Terminated, Failed Items Replaced

In this method, testing is terminated at a preassigned time, and all failed items are replaced immediately after their individual failures. MTBF can be calculated using

MTBF = MTd n


where Td is the test duration time, M is the number of items placed on test and n is the total number of failures. One does not need to record the time to failure of each individual unit on test. For example, a medical device manufacturer places fifteen identical handheld oximeters on test, for 2400 hours. Six of the fifteen oximeters failed in the 2400 hour time period. All failed oximeters are replaced as soon as they fail. An estimate of the MTBF of the tested oximeters can be given by:

MTBF = (15)(2400) 6 ≡ 6000hours Therefore, the MTBF of the oximeters is 6000 hours.

Reliability in Medical Device Industry


Time Terminated, Failed Items not Replaced

In this method, testing is terminated at a preassigned time, and the failed items are not replaced. MTBF can be calculated using

 n  (61.2) MTBF =  Ti + ( M − n)Td } n  i =1  where Ti is the time to failure, M is the number of items placed on test; Td is the test duration time and n is the total number of failures. One needs to record the time to failure of each individual unit on test. For example, a manufacturer started a test with five identical defibrillators. Two of the five defibrillators failed after 75 and 110 hours. The failed defibrillators were not replaced. The test was terminated at a predetermined time of 500 hours. What is the MTBF of the tested defibrillators?


Failure Terminated, Failed Items not Replaced

In this method, testing is terminated at a predetermined number of failures, and the failed items are not replaced. MTBF can be calculated using

MTBF = {(75 + 110 ) + (5 − 2 )(500 )} 2 ≡ 842.5hours


 n  MTBF =  Ti + ( M − n)Td } n  i =1 


where Ti is the time to failure, M is the number of items placed on test; Td is the test duration time and n is the total number of failures. 61.6.5

No Failures Observed

In this method, MTBF cannot be calculated because no failures are observed. However, a lower one-sided confidence limit can be calculated to state the minimum limit of the MTBF for a specified confidence level.

LOCL = (2Tt ) χ α2 , m


Therefore, the MTBF of the defibrillators is 842.5 hours. 61.6.3

Failure Terminated, Failed Items Replaced

In this method, testing is stopped at a predetermined number of failures, and each failed item is replaced. MTBF can be calculated using (61.3) MTBF = MTd n where Td is the test duration time, M is the number of items placed on test and n is the total number of failures. The equations for time terminated and failure terminated are equivalent when failed items are replaced.


Reliability related Standards and Good Practices for Medical Devices

Nowadays, compliance to certain safety, quality, and usability standards is a pre-requisite to gaining approval from FDA to market a medical device in the USA. There are over 700 medical standards available on different topics for different types of devices. Some standards directly or indirectly related to medical device quality, reliability, safety, and usability are given in Table 61.1.


Vaishali Hegde Table 61.1. Reliability Standards

Standard Reference No.

Standard Title / Description

ANSI/AAMI/ISO 14971:2000 and 14971:2000/A1:2003

Medical devices – Application of risk management to medical devices


Safe current limits for electro medical apparatus

ASTM F1100 – 90

Standard Specification for Ventilators Intended for Use in Critical Care

ASTM F1246 – 91

Standard Specifications for Electrically Powered Home Care Ventilators, Part 1 – Positive Pressure Ventilators and Ventilator Circuits

ASTM F1463

Specification for alarm signals in medical equipment used in anesthesia and respiratory care

ASTM F0792

Guide for computer automation in the clinical laboratory


Performance evaluation of ethylene oxide sterilizers – EO test packs, good hospital practice

AAMI TIR 12:2004

Designing, testing, and labeling reusable medical devices for reprocessing in health care facilities: A guide for medical device manufacturers

AAMI TIR 32:2004

Medical device software risk management

AAMI HE-1988

Human factors engineering guidelines and preferred practices for the design of medical devices Guidance for the content of Pre-market Submissions for Software Contained in Medical Devices Issued May 11, 2005 by CDRH

ISO 10651-6

Lung ventilators for medical use – Particular requirements for basic safety and essential performance

ISO 10993-1

Biological evaluation of medical devices – Part 1: Evaluation and testing

ISO 15001

Anesthetic and respiratory equipment – Compatibility with oxygen

ISO 8185:1997

Humidifiers for medical humidification systems

ISO 9000 series

ISO 9000 series of quality standards

ISO 1348

Quality systems – medical devices – particular requirements for the application of ISO9002

IEC 60601-1-1

Medical Electrical Equipment Part 1: General requirements for safety – Collateral Standard: safety requirements for medical electrical systems

IEC 60601-1-2

Medical Electrical Equipment Part 1: General requirements for safety – Collateral Standard: electromagnetic compatibility.

IEC 60601-1-6

General requirements for safety – Collateral Standard: Usability





Reliability in Medical Device Industry

Standard Reference No.


Standard Title / Description

IEC 60601-1-8

General requirements for safety – Collateral Standard: General requirements, tests and guidance for alarm systems in medical electrical equipment and medical electrical systems

IEC 68-2-64

Environmental Testing Part 2:Test methods Test Fh: Vibration, broad-band random (digital control) and guidance

IEC 1123

Reliability testing compliance test plans for success ratio

IEC 605

Equipment reliability testing


Standard for safety of medical and dental equipment

UL 2601-1

Medical Electrical Equipment, Part1: General Requirements for Safety


Electronic reliability design handbook


Reliability prediction of electronic equipment


Procedures for performing a failure mode, effects and criticality analysis


Reliability test methods, plans, and environments for engineering development, qualification and production


Failure reporting, analysis and corrective action system (FRACAS)

References [1]




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Rand Corporation. The cost and benefits of reliability in military equipment, 1988. [6] Schenkelberg F. Reliability integration 3-day training course. Ops A La Carte LLC, Oct, 2004. [7] Dhillon BS. Medical device reliability and associated areas, CRC Press, 2000. [8] Mills HD. On the statistical validation of computer programs. IBM Federal Systems Division, Gaithersburg, MD, Report No. 72-6015, 1972. [9] CRE Primer 1998, Quality Council of Indiana. [10] O’Connor PDT. Practical reliability engineering. (3rd edition). John Wiley and Sons New York, 1996. [11] Systems reliability division. Rome Laboratory Reliability Engineer’s Toolkit: New York. 1993; April.