Recommendations on Standards for the Design of Medical Diagnostic Equipment for Adults with Disabilities, Advisory Committee Final Report

19. Medical table install base derived from U.S. medical distribution sales data, as provided by Global Healthcare Exchange (GHX).

20. Some tables may require installation of a modified top to meet the 19” standard but would not require changing out the installed base.

AVAILABLE DATA DO NOT SUPPORT DEPARTING FROM THE CURRENTLY ACCEPTED STANDARD OF 19-INCH TRANSFER SURFACE HEIGHT

The Medical Diagnostic Equipment Technical Advisory Committee appropriately tried to determine what the optimal accessible transfer surface height is based on available data. In particular, the Advisory Committee spent a great deal of time discussing the Anthropometry of Wheeled Mobility (AWM) Project.²¹ In that study, which the U.S. Access Board commissioned, Dr. Steinfeld served as the lead investigator. This study measured the physical characteristics of people who use wheeled mobility devices and some of the characteristics of those devices. The resting position of a person before attempting an independent transfer is important, and the Committee members valued the study’s useful information in evaluating potential recommendations. However, because the Wheeled Mobility Anthropometry project only studied static positioning of users in their devices, it did not identify optimal transfer surface heights, and did not assess the ability of wheeled mobility device users to transfer independently from their mobility device onto an examination table or chair.

In evaluating data about seat height, the Committee took into account the relevant transfer height from the wheelchair. The AWM Project measured the rear compressed seat height of the wheeled mobility device with the user seated in it. For transfer purposes, however, the most relevant height is the wheelchair front edge. The height of the front edge of the seat or cushion is important because the user will shift to the front of the seat (to avoid having to lift over the side wheel) before moving to the side to complete the transfer. Unfortunately, the AWM Project did not measure the height of those same users' front wheelchair edges.

There is a recognized international standard defining various measurements of wheelchairs: ISO 7176-7:1998 Wheelchairs — Part 7: Measurement of seating and wheel dimensions. Figures 6 and 7 below are selected screenshots from this international standard. These illustrations identify several key measurements. The most important illustrations for present purposes are "seat plane angle," "effective seat depth," and "seat surface height at the front edge." The standard focuses on measurement procedure so it does not prove any actual measurements. However, as the images below make clear, the seat reference plane and effective seat depth will dictate the difference between the seat surface height at the front edge and the height of the seat at the rear of the wheelchair. Note further that wheelchair height measurements generally do not take into account the height of the cushion, which the consumer will need to clear at the front edge to enable a successful transfer.

In addition, section 4 of the Paralyzed Veterans of America’s “Guide to Wheelchair Selection”²² also illustrates the distinction between the following heights:

…the seat surface height at the front edge (which excludes the effect of a seat cushion, typically measuring 2-4" in depth), the seat height at the rear of the seated surface, and the relevant transfer surface height for clearing the seat cushion.²³

Furthermore, the PVA’s “Clinical Application Guide to Standardized Wheelchair Seating Measures of The Body And Seating Support Surfaces” defines the seat surface height as “the distance from the floor to the top of the seat at front edge, in area intended for thigh loading.” It goes on to state that:

This measure is clinically relevant because it impacts the user’s overall sitting height, clearance under tables, clearance of foot supports above casters or ground, and functional activities such as transfers.²⁵ 

Therefore, it cannot be inferred that the seat surface height at the front edge is the same as the seat height measured in the AWM Project. Similarly, these illustrations used in measuring individuals for manual wheelchairs indicate that the AWM Project’s data cannot be used to make a direct assessment of the table height needed to accommodate wheelchair users effectively. Based on figures 5-7 below, a 17-inch rear compressed height measured in the AWM Project could easily correspond with a 20- or 21-inch uncompressed seat surface height at the front edge. This could in turn mean that individual users participating in the AWM Project measured below 19 inches would be able to transfer comfortably to a table surface for which the highest uncompressed surface is 19 inches.

Note further in figure 5 that the rear seat height is considerably lower than the height of the wheelchair wheels, which typically measure 22-26 inches. Any transfer from the rear of the chair would require the user to transfer up and over the wheelchair wheel, again, well above a minimum low-range height of 19 inches. Based on this difficulty of transferring over the wheelchair wheel, Committee members noted transferring over the wheelchair wheel is an extremely unlikely scenario. This emphasizes the fact that the front edge of the seat is the most relevant for transfer.

In addition, a second study commissioned by the U.S. Access Board and evaluated by the Advisory Committee (the Pittsburgh study²⁶) determined that manual wheelchair-users, who are generally the users most likely to have the lowest seated heights, are generally able to accommodate a 2-inch difference in height between one’s wheelchair and a transfer surface. Consequently, even if one posited that the AWM Project finding of 17 inches at the rear of the seat compressed corresponded to an uncompressed height of 17 inches at the front edge of the wheelchair, the maximum height of 19 inches at the highest point of a table or chair transfer surface would still fall within the 2-inch differential identified by the Pittsburgh study as accessible to most manual wheelchair users. Of note, Committee members pointed out that the Pittsburgh study utilized a sample of younger and more active subjects than the AWM Project. As such, broad conclusions for a broader population of persons with disabilities, and direct correlations with the AWM Project, should be avoided.

Figure 5: Adapted diagram from the “Paralyzed Veterans of America’s Guide to Wheelchair Selection” showing difference in measured height between the AWM Project, wheelchair manufacturer’s height per ISO 7176-7, and the uncompressed upholstery measurement height.

Figure 6: Wheelchair manufacturer’s height and seat construction types (showing variation in compression when person’s weight is applied) per ISO 7176-7

Figure 7: Wheelchair manufacture standardized measurements per ISO 7176-7

NOTES

^21. See Analysis of Seat Heights for Wheeled Mobility Devices at: http://udeworld.com/analysis-of-seat-height-for-wheeled-mobility-devices. The seat heights ranged from 16.3 inches to 23.9 inches for manual wheelchair users; 16.2 inches to 28.9 inches for power wheelchair users; and 18.8 inches to 25.3 inches for scooter users. Seat heights for males were typically higher than for females. Thirty (30) percent of male manual wheelchair users and 6 percent of male power wheelchair users had seat heights equal to or less than 19 inches. All the male manual wheelchair users and 92 percent of the male power wheelchair users had seat heights equal to or less than 25 inches. Thus, transfer surfaces that are adjustable from 17 inches minimum to 25 inches maximum during patient transfer accommodate significantly more patients who use mobility devices.

^22. Available at http://www.wheelchairnet.org/WCN_ProdServ/Docs/PDF/AXbook_Sec4a.pdf.

^23. Ibid.

^25. Ibid.

^26. Human Engineering Research Laboratories, University of Pittsburgh, The Impact of Transfer Set-Up on the Performance of Independent Transfers: Final Report. Available at: http://herl.pitt.edu/ab/transfer_assessment_report.pdf (visited May 22, 2013).