Measurement of the Influences of Support Surfaces on Physical Factors

Eric Flam, PhD
NTL Associates, Inc.
East Brunswick, New Jersey

The ultimate measurements of the effectiveness of support systems in the prevention and treatment of pressure ulcers are clinical outcome studies on patients.¹ This is a difficult, time-consuming, and expensive process. In addition, due to the complexities of management of these patients and their usually poor health conditions, many patients must be recruited for studies to obtain sufficient data for meaningful comparison of effectiveness. Therefore, alternative, less time-consuming, and lower cost-measurement procedures have been and are being developed to estimate this effectiveness. Some of these are presented below.

The underlying physical causes of pressure ulcers are pressure, shear, and friction.² The common denominator for these physical factors is pressure. Shear and friction are significantly increased when the skin is moist. Therefore, the support system must sufficiently reduce the pressure, lower the covering surface drag, and keep excess moisture from the skin to prevent and successfully treat pressure ulcers. This article will discuss the measurement of the influences of support surfaces on these physical factors.

Pressure

Pressure is currently measured at the interface between the patient and the support surface. The probes that sense the pressure generally include the following.

Pneumatic and electropneumatic probes. These are air-inflatable bladders and may be single units or in an array. In use, the probes are placed under the body sites to be measured. They are then inflated until the pressure within the bladder is either equal or related to the interface pressure.

Electroresistive probes. These probes are made from materials that change their electrical resistance as pressure is applied to their surface. In use, the probe mat is placed under the body site to be measured. The change in resistance of the sensors is measured and is interpreted as the interface pressure.

Major concerns that exist with these current measurement procedures include:

* Measurement inconsistencies because human volunteers or patients are required to determine the pressure distribution of the support structures.

* Interface probes and probe arrays may create a "hammock effect" because they add an additional layer between the person and the support surface. This can lead to a distorted result.

* The initial site of pressure ulcer formation is often not at the interface between the patient and the support structure.

Pressure measurement systems that can overcome these concerns and are consistent with clinical outcome studies are needed. Such systems should usher in a new paradigm that will give the pressure ulcer community a more definitive means of developing and evaluating support products.

Initial evaluations on a new system that appears to overcome these concerns have yielded results consistent with the outcomes of clinical trials involving hundreds of patients using various support structures.

Shear

Shear may be measured through use of a weighted sled moving against the skin.³ The surface of the sled is made from a bed sheet or the covering surface of the support system that will be in direct contact with the patient. The maximum shear is produced just before the sled begins to slide across the skin.

Friction

Friction is measured through use of the same weighted sled and procedure used for shear measurement. Friction occurs when the sled moves across the skin.

Moisture Vapor Permeability

Moisture vapor permeability of the surfaces of the support structure and covering is measured by determining the loss of weight of the water in a special cup sealed with the material(s) of interest.⁴

Skin Moisture

Skin moisture is measured directly from the moisture vapor rising from the skin with an instrument that analyzes the amount of water vapor passing between an internal pair of humidity indicators. It is also measured indirectly with a probe placed on the skin that detects the level of electrical resistance of the skin. The resistance decreases as the skin moisture level increases.

Summary

Standard, well-accepted methods exist for the measurement of the shear, friction, moisture vapor permeability, and skin moisture management capability of support surfaces. However, significant problems exist with the current methods of determining pressure management capability. Beta tests with a new pressure measuring system, which appears to overcome these problems, have indicated its validity with clinical outcome studies. ***

References

1. Panel for the Prediction and Prevention of Pressure Ulcers in Adults. Clinical Practice Guideline Number 3: Pressure Ulcers in Adults: Prediction and Prevention. Rockville, MD: US Department of Health and Human Services. Agency for Health Care Policy and Research; 1992. AHCPR Publication 92-0047.

2. Allman RM. Pressure ulcers among the elderly. New England Journal of Medicine 1989;320:850–3.

3 Flam E. Skin maintenance in the bed-ridden patient. Ost/Wound Manag 1990;28:48–54.

4. E 96-80, Procedure BW: Standard test methods for water vapor transmission of materials. 1988 Annual Book of ASTM Standards. Philadelphia, PA: American Society for Testing and Materials, 1988;04.06: 605–612.