RERC: Research R-4

R-4: Quantification and Development of a Pressure Index

Task Leader: Cliff Brubaker, PhD (rehabilitation scientist)

Co-investigators: David Brienza, PhD (rehabilitation scientist); John Horton, MD (physiatrist)

Duration/Staging of Task: This 36 month research task will be conducted in months 24-60 of the 60-month RERC cycle.

Rationale and Task Summary
The review of literature on seating provides substantial evidence that the formation of pressure ulcers is a complex issue that cannot be adequately addressed or explained solely in terms of seating interface pressures. Although true, it is also evident that contemporary decisions relevant to evaluation, prescription, and selection of seat cushions for pressure relief are based substantially more on the measurements and interpretations of interface pressure than on all other factors. The reason for this is rather obvious. Interface pressure is the only quantitative measurement that can be readily made in clinical environments. The concept of pressure and its importance has become the primary criterion in determining seating efficacy for many, perhaps most, contemporary clinicians. The coining of the term "pressure ulcer" in lieu of the now obsolescent term "decubitus ulcer" would attest to this focus and, perhaps, reflects a preoccupation with pressure in the determination of "good seating." Despite its apparent simplicity, pressure measurement is often poorly understood, and pressure measurements are frequently used inappropriately in decision processes.

We propose to improve the interpretation and utilization of pressure information by studying the association between pressure mapping data and pressure ulcer incidence. We further propose to develop a pressure mapping tool that will be optimized to evaluate only those aspects of the pressure information shown to be associated with pressure ulcer risk. The most important pressure-related factor is the peak pressure relative to the average pressure near the bony prominences of the buttocks. We, therefore, propose to develop a pressure index derived as the peak pressure divided by the average pressure.
The advantages of using the proposed pressure index over absolute pressure value maps are:

The scalar pressure index is simpler to interpret than a two-dimensional pressure map.
Relative pressure measurements are less demanding on the precision of the pressure measurement instrumentation, hence, resulting in lower cost measurement devices that are easier to maintain.

We believe the above formulation constitutes a simple and appropriate approach to the problem; however, we accept the possibility that a more complex formulation may be necessary to model pressure measurements to provide a discriminatory capability.

Research hypothesis and objectives

Objectives
1. To express seat interface pressure distributions as a function of the average interface pressure, i.e., a pressure index.
2. To determine the coincidence of pressure ulcers with ranges of values for the pressure index.
3. To formulate guidelines for pressure distribution based on the pressure index.
4. To develop an appropriate and affordable measurement instrument to obtain the pressure index.

Hypothesis: The pressure index will be higher among individuals who experience pressure ulcers than for those individuals who do not experience pressure ulcers.
The seating pressure-pressure ulceration relationship will be determined against the null hypothesis of no association between average and peak seating pressures, or the pressure index, and the presence of pressure ulcers after adjustment for other risk factors determined by previous studies to be significantly associated.

Expected Outcomes
This multifaceted task could produce a new method of interpreting pressure measurements (i.e., pressure index), new knowledge on the association between pressure measurements and pressure ulcers and a new and simpler pressure measurement tool for the assessment of seat cushion effectiveness.

Publications/Reports

Progress Report (12/31/99)

Task doesn't officially start until Year III of the RERC cycle

Progress Report (12/31/00)

Task doesn't officially start until Year III of the RERC cycle

Progress Report (12/31/01)

Protocol Development

Based on our view that commercially available scanning systems are most useful for determining relative pressure patterns rather than absolute pressure values we believe that it is necessary to determine the “seated weight” independently during data collection. To do this we have designed and are in the process of fabricating instrumentation to enable us to make this measurement. This instrumentation is described in the subsequent section. Our original intent was to develop (or purchase) a seating simulator with the capability to reliably reproduce seating environments for subjects and enable us to take various measurements including total and seated weight. This was ultimately rejected in favor of a simpler system and protocol for obtaining seated weight for each subject in his/her chair. We had also contemplated gathering data via home visits. This too has been superceded by a protocol that will be conducted exclusively at the Center for Assistive Technology (CAT). There are several reasons for these decisions. Perhaps the most compelling are enhanced efficiency, cost reduction and reliability of measurement under the closest circumstances for actual use (subject’s real seating environment). The weight measurement system will be placed in the subject’s chair with the cushion. The main wheels of the chair will be rolled up an inclined plane to achieve a level state of the seat surface. Seated weight will be recorded and the chair will then be returned to the original attitude where the weight will be recorded again and the angle of inclination of the seat surface will be recorded. Pressure data will be recorded using a pressure scanning system in both positions.

The collection of data in the CAT will provide a greater measure of safety, greater control and reliability, and more favorable access to the test population.

Instrumentation Development

We have fabricated a simple, yet effective, measurement system that enables us to determine the “seated weight” and calculate the location center of mass on the seat surface. This allows for more accurate calibration (and therefore) interpretation of the data obtained in the pressure scans. This system consists of rigid cross members that are placed on the seat frame members and a seat pan (to position the cushion) that rests on the cross members with 3 force sensors (2 on rear, 1 on front) interposed between the pan and cross members.

IRB Approval

We are now engaged in the arduous and rigorous process of obtaining IRB approval for our protocol. It is our belief that, in addition to enabling greater experimental control, the decision to conduct data collection in the CAT will favor the approval of our experimental protocol.

Analysis

We shall compute a variety of “Pressure Indices” to include our original index based on the ratio of peak to average pressures and all other indices that are under consideration in seating standards applications and published indices. These different pressure indices will be evaluated to determine their respective discrimination between individuals who experience pressure ulcers and those who do not over the course of this investigation.

Contact Person: Cliff Brubaker