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PRESSURE MAPPING SURFACES FOR PRESSURE ULCER RISK REDUCTION

CONCEPTUAL FRAMEWORK

This study was based on a model derived from a classic study conducted by Kosiak and a conceptual schema for the study of the etiology of pressure sores formulated by Braden and Bergstom. Kosiak evaluated the interface pressure on skin over time. The model derived from Kosiak's study explained how external pressure in a specific area could lead to vasoocclusion, resulting in decreased tissue perfusion and possible ischemic injury to both deep and superficial tissues. Kosiak conducted further studies investigating pressure over time and found that the greater the external pressure, the less time needed for ischemic injury to occur.

Kosiak described a cutoff of 32 mm Hg as a guideline for measuring surface interface pressure. Although this number does not always predict actual perfusion, an interface pressure of 32 mm Hg or less is regarded as a useful guideline in determining the efficacy of a product in redistributing the interface pressure and thus lowering the risk of pressure ulcer development.

Within the Braden and Bergstrom schema, exposure of the skin to high interface pressures for prolonged periods will indeed lead to tissue damage. At the same time, exposure of skin to low interface pressures for prolonged periods may also lead to tissue damage if the tissue tolerance is compromised. Therefore, the tissue damage that occurs during prolonged OR procedures can be minimized by decreasing the interface pressure.

OPERATIONAL DEFINITIONS

-Interface pressure is the pressure load between the skin and the support surface.

- Peak interface pressure is the highest pressure load between the skin and the support surface.

- Average interface pressure is the average pressure load between the skin and the support surface of a full body or the specific area calculated by the XSENSOR pressure mapping device.

- Skin contact area is the total contact area between the skin and the support surface.

- Pressure redistribution is the pressure relief to a small concentrated area and the distribution of it over a larger area.

METHODS

This descriptive, comparative, quantitative study used a repeated-measures design in which the participants served as their own controls. A sample size calculation was conducted before recruiting volunteers and yielded a required sample size of 49. Sample size calculations were performed using the SAS software procedure POWER and were based on the paired t test as an approximation of the repeated-measures analysis of variance (ANOVA). We used preliminary data from five subjects to estimate both the expected differences between mattress types and the standard deviations of the differences. We powered based on the highest standard deviation of 6.5 to ensure adequate power under the worst case scenario. Because of the high number of comparisons, we used the Bonferroni correction for multiple comparisons to correct the global type I error at 0.05. We powered at the 90% level.

A convenience sample of 11 men and 40 women (N =51) participated in the study. Study participants were recruited from hospital staff with various body mass indexes. Eligibility criteria included volunteers who 1) work at the hospital, 2) agreed to self-report height and weight, 3) had 30 minutes to participate, and 4) agreed to have the pressure mapping performed on four different surfaces while lying flat for more than seven minutes per surface. The study was approved by the hospital institutional review board.

We tested the following OR table surfaces: 1) a standard three-layer viscoelastic memory foam surgical table surface, 2) an air-inflated static seat cushion that was used under the sacral area placed over a standard surgical table surface, 3) a two-layer OR surface consisting of a top layer of nonpowered self-contouring copolymer gel and a bottom layer of highdensity foam, and 4) a fluid immersion simulation surgical surface.

To evaluate the pressure redistribution properties of OR surfaces, we used full-body interface pressure testing. This method has been found to be valid and reliable in measuring interface pressure. The instrument used for measuring the interface pressure for this study was the XSENSOR X3 PX100 system. Pressure mapping systems are composed of a pressure sensing device that sends data to a computer program. The data are displayed as a color-coded map, a three-dimensional grid, and a numerical pressure value for each area. Numerical pressure values are typically expressed in millimeters of mercury (mm Hg) and reflect the pressure between the body and the surface used. The XSENSOR X3 PX100 system consists of a thin, 99.06 cmx 220.98 cm full body pressure mapping pad with 1664 sensing points. The sensors in the pad have 3.175 spatial resolutions. The pad was placed between the volunteer and the support surface and connected to the XSENSOR X3 PX100 display for real-time pressure mapping recording

All participants were instructed to lie flat on the surface for five minutes before the XSENSOR measurements were collected. During the pilot study, no difference was found in the interface pressure readings that were recorded between three minutes and 30 minutes for acclimation. However, five minutes was used as an acclimation period as recommended in a previous study. The data were displayed on the screen for a minimum of 1200 frames per participant and were recorded on each surface. The XSENSOR data collected were then downloaded to a computer using X3 medical v6.0 software. The peak pressures and surface contact areas recorded were then transcribed into Excel spreadsheets by two investigators, and all measurements were validated by two investigators.

A repeated-measures ANOVA was fitted to test for differences in average sacral pressure, peak sacral pressure, and sacral surface area between the four surfaces using the SAS software procedure GLM. Pairwise paired t tests were then performed to test for significant differences between each surface pair using the SAS software procedure TTEST with the paired option. We used the Bonferroni correction for multiple comparisons. A P value of 0.05 was considered statistically significant for the omnibus tests. For the pairwise comparisons, the Bonferroni correction indicated that a P value of 0.008 should be used for each comparison to control the global type I error at 0.05. Distributional assumptions were validated using residual diagnostic plots to assess normality.

RESULTS

Residual diagnostic plots revealed no skew or irregularity in the distribution of the residuals, indicating that the normal assumption was validated. All ANOVAs were significant at the 0.05 level (sacral average pressure P value, 0.0004; sacral peak pressure and sacral area P value, <0.0001; and sacral area skin contact area P value, <0.0001).

The average difference in sacral interface pressure between the surfaces was significant, with the air-inflated static seat cushion having the highest measured average pressure (23.9 mm Hg) and the fluid immersion simulation surgical surface having the lowest average sacral pressure (22.1 mm Hg). The sacral peak interface differences were also found to be significant. The sacral peak pressure was lowest in the air-inflated static seat cushion (35.8 mm Hg). When comparing the skin contact area, it was found that the air-inflated static seat cushion was significantly greater (250.2 cm2). These results suggest that the sacral interface pressure is better distributed with the use of the air-inflated static seat cushion than any of the other surface types used in this study.

The sacral average interface pressures were pairwise compared. The air-inflated static seat cushion and the fluid immersion simulation surgical surface were significantly different. Table 3 shows the results for the sacral peak interface pressure pairwise comparisons, where a P value of <0.08 is regarded as significant. When compared, all surfaces were significantly different except for the fluid immersion simulation surgical surface and the air-inflated static seat cushion, which were not significantly different from each other with the measured difference of0.09 mm Hg between the surfaces (P =0.9). The test showed that there was no difference in sacral peak pressures between the fluid immersion simulation surgical surface and the air inflated static seat cushion.

The results for the sacral skin contact area pairwise comparisons are summarized in Table 4. The sacral contact area pairwise comparisons showed that the difference between the surfaces was the greatest in the air-inflated static seat cushion and the fluid immersion simulation surgical surface with contact area at 36.6 (95% confidence interval: 25.9, 47.2).

The ANOVA results for the sacral average interface pressure indicate that the fluid immersion simulation surgical surface provided the lowest average pressure (22.1 mm Hg). Of the surfaces measured in this study, there was strong evidence to support that the air-inflated static seat cushion and the fluid immersion simulation surgical surface had the lowest sacral peak interface pressure (air-inflated static seat cushion, 35.8 mm Hg; fluid immersion simulation surgical surface, 35.9). The peak interface pressure pairwise comparison indicated no significance between the air-inflated static seat cushion and the fluid immersion simulation surgical surface measurements.

In relation to surface contact area, the ANOVA provided statistically significant results indicating the air-inflated static seat cushion as the surface that provides the largest skin contact area (250.21 in2) over the sacrum. Moreover, the pairwise comparisons test showed that there is a significant difference between the air-inflated static seat cushion and all surfaces measured in this study. The air-inflated static seat cushion outperformed all surfaces in increasing the measured skin contact area of the sacral region (difference ranging from 24.8to 36.6 in2) as measured by the XSENSOR device.