Case Study: Medical Device Human Factors

In this case study we discuss the human factors issues presented in the design of the NetGuard™ Patient Monitor. NetGuard is a low-cost wireless device intendted to bring cardiac arrhythmia monitoring to the numerous hospital patients who are unmonitored today. The NetGuard system consists of a patient-worn transmitter, disposable electrode patch, wireless access point, server and nurse’s station monitor. In this study we focus on the first three components.

The Needs of the Many

One of the challenges of medical device human factors is the diversity of individuals that interact with a device. We found that Caregivers (doctors/nurses/technicians), Patients and IT Administrators all had unique interests in how NetGuard human factors were addressed. These interests included:

Care Giver

Simplicity of Interface/Ease of Understanding
Designing a user interface for the patient worn transmitter that was clear; simple; and fit the power, size and cost profile of the device was a significant challenge for the design team. Extensive use of simulation was employed to rapidly iterate the design and gather feedback from caregivers and team members. This feedback along with the product requirements and associated risk analysis was used to bound the problem and converge on a design solution. In the end, the team selected a user interface consisting of two buttons, an audio transducer and two multicolor LEDs.

Ability to Locate a Down Patient
When a life-threatening arrhythmia is detected, the console at the nurse’s station indicates patient name and room number. Caregivers expressed concern that the patient might not be in their room. It is not unusual for hospital patients to be off-unit in various procedure areas, ambulating around the hospital or just down the hall visiting another patient. To address this scenario, both visual and audio local alarm indicators were added to the device. These alarms were optimized to provide maximum light and sound output within the limits of the device’s coin cell battery power supply.

Application on Patient
NetGuard is an ECG monitoring device requiring electrodes be applied to the chest of the patient. Proper placement is critical to obtaining the best electrical signal quality possible. Caregivers were particularly interested in correctly placing the device the first time as removal and reapplication of the adhesive backed electrode patch was viewed as being uncomfortable for the patient. The caregivers also wanted to minimize the amount of skin preparation and the pressure that was required to obtain good contact with the patient’s skin since application of pressure was viewed as uncomfortable as well.

The placement issue was addressed by designing a label that showed graphically the exact location of the electrodes on the body. The label is present on each device and while an additional cost, it serves as a constant reminder of the proper placement.

Finally, two of the three required electrodes were combined onto one carrier reducing by a third the possibility of mispositioning the electrodes. In order to support this design improvement, significant physiological study was required to ensure the integrity of the resulting ECG signal.

Patient preparation was reduced by adjusting the size of the electrode gel reservoirs and working with the manufacturing partner to closely control both the volume and the form of the gel pads. This work allowed good skin contact to be made with a minimum of patient preparation and lowered pressure required to apply the electrodes.

Finally, the caregiver was provided feedback, via a green LED turning from amber to green, indicating that a proper application had been completed and the leads were functional.

Patient

Comfort
Device size, weight, geometry and skin irritation were the primary design parameters driving patient comfort. Since product requirements stated that the device was to be worn by the patient for up to three days, every effort was made to reduce the size and weight. Lightweight materials, high density PCB assemblies, and battery technology and capacity were design choices driven by size and weight concerns.

Product geometry was very fluid during the design process. Key considerations included the need to optimize the acquired electrical signal via device placement on the body and the need to make the device comfortable as the patient moved around in their bed and hospital environment. Determining body placement was an iterative process. Once the location was chosen the overall external geometry was modeled and iterated to maximize comfort. Simple physical modeling was used extensively in this effort and proved invaluable in converging on the best device geometry.

Minimizing skin irritation was addressed in the design by reducing the actual skin surface area that contacted device adhesive, optimizing adhesive strength vs. three-day wear life and choosing biocompatible materials for all patient contact surfaces.

Intrusiveness
Underlying the requirement to ensure patient comfort was a desire on the part of the design team to make the device as unobtrusive to the patient as possible. Sick hospital patients are already barraged with invasive and non-invasive stimulus and the design team wanted to minimize Netguard’s intrusiveness as much as possible. Ideally the device would be applied and just “disappear” from the patient’s perspective. To this end, once the device was placed on the patient and properly set up, all local beepers and flashing lights are turned off.

IT Administration

Minimizing intrusiveness was also a consideration in the design of the wireless access point. The access points are typically located on the ceilings in patient rooms. For quick IT troubleshooting they incorporate a number of indicator LEDs which show the status of the wireless link. The designers recessed these lights so that the patient would not be bothered during the night by yet another set of flashing lights. The troubleshooting functionality was not impacted since the indicators were easily viewable from directly below the access point.

Role of Human Factors in Safety

The Netguard design team took a proactive approach to understanding how healthcare workers would use and misuse the product. This approach considered how the design could be optimized to avoid first-order use error as well as how the design could protect the patient if an actual use error occurred.

The system’s Failure Modes and Effects Analysis (FMEA) was the formal tool for this activity, identifying potential use errors, their potential effects on the patient and the mitigations implemented by the design team. Members from all the design disciplines participated in this activity. This was key for ensuring that the best risk mitigations were developed and implemented in the final NetGuard design.

For those interested in the complementary roles of risk analysis and human factors in medical device design, the following references are excellent places to start:

1. 1. AAMI/ANSI HE74:2001 Human Factors Design Process for Medical Devices

1. 2. ISO 14971-1:2007 Medical Devices – Application of Risk Management to Medical Devices

1. 3. IEC 60601-1-8, Ed. 1, Medical Electrical Equipment – Part 1-8: General Requirements for Safety – Collateral Standard: Alarm Systems – Requirements, Tests and Guidelines – General Requirements and Guidelines for Alarm Systems in Medical Equipment.