A Prospective Assessment of Sacral Pressures in Healthy Volunteers Seated Upright and Reclined With Legs Elevated in a Recliner

Prolonged, unrelieved pressure is a major risk factor for pressure ulceration, and interface tissue pressures have been shown to be very high when a person is sitting. Using convenience sampling methods, 23 healthy participants (four men, 19 women, mean age 45 years, body mass index [BMI] range 20–45) participated in a prospective pilot study to evaluate the effect of BMI and two commonly used seating positions in standard hospital reclining chairs on tissue (especially sacral) interface pressures.   Measurements were obtained when volunteers were seated upright with plantar surfaces of feet touching the floor (Position 1) and reclining with legs resting on an elevated surface (Position 2). Measurements were obtained for 6 minutes using a thin, flexible force sensing array 430-mm x 430-mm seat mat. Average pressure, maximum pressure, and the number of sensors reading >60 mm Hg and >80 mm Hg were used for analysis across the total surface and at the sacrum. Participants were categorized by BMI (category 1: 20–22, category 2: 23–28, category 3: 29+; no participants had a BMI <20). Leg elevation reduced average pressure across the total surface (from 42.9 mm Hg to 40.0 mm Hg, P = 0.015) and the number of sensors reading >60 mm Hg at the sacrum (from 31.4 to 27.1, P = 0.047). BMI and position were significantly correlated with the number of sensors reading >80 mm Hg (P = 0.008) and average pressure (P = 0.031). Pairwise comparisons showed significant differences existed between BMI categories 1 (average delta: -3.63, indicating down position is better) and 3 (average delta: 4.67, indicating up position is better) for the difference in number of sensors above 80 mm Hg (P = 0.030). Research is needed to further explore the relationship between BMI and tissue pressure, but the results of this study suggest that for patients with a BMI >29, elevating the heels/reclining the chair significantly reduces sacral tissue interface pressure. Further research specific to pressure relief in the sitting position is needed, as neither position examined in this study was found to reduce interface pressures to generally considered safe levels for reduced-mobility patients.

Pressure ulcers are a significant cause of morbidity and mortality among patients with limited mobility.¹ These multifactorial wounds can lead to intense pain and serious infectious processes, including sepsis and osteomyelitis.^2-4 Furthermore, hospital-acquired pressure ulcers contribute to 60,000 deaths annually, making them one of the leading iatrogenic causes of death in the United States.³ Due to their serious and complex nature, pressure ulcers are associated with prolonged hospitalizations, necessitate additional medical care, and ultimately increase related costs.¹ The yearly incidence of pressure ulcers has increased by nearly 80% since 1993, with an associated economic burden estimated at $11 billion.^1,2

Major risk factors for pressure ulcer formation include shear, friction, moisture, and temperature, as well as confounding factors such as nutritional deficiencies, advanced age, and hypotension.⁵ One of the most important extrinsic factors is prolonged, unrelieved pressure to a bony prominence.⁶ A recent pilot study by Lachenbruch et al⁷ used laser Doppler flowmetry to measure postload reactive hyperemia on the sacrums of healthy volunteers to determine the role of pressure, stress, and temperature on tissue ischemia. The authors found interface pressures and temperature to be the most significant factors leading to ischemia, which in turn could translate into pressure ulcer risk.

Akron General Medical Center (Akron, OH) has instituted numerous pressure ulcer prevention strategies, including purchase of new mattresses to better redistribute pressures experienced by patients immobilized during surgery or in intensive care units. However, concern for trauma and postsurgical patients who are often moved into a chair while still experiencing periods of reduced mobility (common practice is for postsurgical patients to be out of bed within 12 to 24 hours), justifies examination of seated pressures and positions.

Many studies have used pressure mapping technology to evaluate interface pressure — ie, pressure between an object and the surface on which it rests. For example, Brienza et al⁶ analyzed data from a randomized clinical trial using two types of seat cushions in a sample of 32 elderly skilled nursing facility patients at high risk for developing a pressure ulcer. The study aimed to assess the effectiveness of pressure-redistributing seat cushions in wheelchair users and to elucidate the relationship between interface pressures and pressure ulcer development. Results showed average and peak interface pressure measurements on wheelchair seat cushions were higher for patients who developed sitting-acquired pressure ulcers than those who did not.⁶ This study ultimately provides evidence that pressure mapping technology is a helpful method for evaluating surfaces for their pressure redistribution capabilities and potential for preventing pressure ulcers.

A study by Shechtman et al⁸ prospectively examined the interface pressures of wheelchair cushions in a sample of 40 wheelchair users recruited from a rehabilitation hospital. The authors found cushion pressure-redistributing abilities were largely dependent on the individual’s body mass index (BMI).⁸ Using a nonrandomized, cross-over study design, Williams et al⁹ compared the seat interface pressures of two standard high-back hospital chairs and a similar chair with a gel overlay in 18 intensive care unit patients. Of the three chairs tested, no seating surface reduced peak pressure sufficiently or addressed all requirements for patients sitting out of bed; requirements include but are not limited to practical utility and adjustments to support the patient (eg, slide boards, footplate adjustments). Additionally, the study determined many patients sustained high interface pressures for the duration of the sitting period.

Few studies have focused on pressures relative to position. Defloor and Grypdonck¹⁰ conducted a prospective, nonrandomized study on the effects of body posture and cushion type on seat interface pressures in 56 healthy volunteers. Their results demonstrated reclining the seat backward resulted in lower sacral pressures. In contrast, while in the upright sitting position, elevation of the legs increased sacral pressures, but in the reclined position, elevation of the legs decreased sacral pressures. Additionally, reclining with legs elevated produced a lower average maximum pressure than sitting upright in an armless hospital chair.

A prospective, nonrandomized group study of 63 healthy volunteers conducted by Stinson et al¹¹ found that elevating the legs while in an upright sitting position did not impact maximum pressure, but approached statistical significance in terms of average pressure. Stinson also examined reclines of 10˚, 20˚, and 30˚ and found reclining the back of the chair from the neutral position did not significantly alter average or maximum pressure at either 10˚ or 20˚; however, a 30˚ recline significantly reduced the average pressure on the sacrum.

The purpose of the current pilot study was to investigate whether a difference in sacral pressure exists between persons seated upright with the plantar surfaces of their feet touching the floor and persons reclining with their legs resting on an elevated surface and whether this difference is affected by participant BMI. Because the sacrum is one the most common anatomical locations for pressure ulcer development⁵ and because data on sacral pressure measurements relative to positioning are limited, the results of this study may provide caregivers evidence for position recommendations and justify further testing.

This study was approved by the hospital (Akron General Medical Center, Akron, OH) Internal Research Review Board for human subjects research. The volunteer study sample was recruited from hospital employees and volunteers via email and posted signs. Demographic information (age, weight, height) was collected as self-reported, and no financial incentive was given for participation. BMI was calculated as weight in lb times 703 divided by height in inches squared (weight in kg divided by height in meters squared¹² with 703 conversion factor from US units to metric).

Instrumentation. The Force Sensing Array (FSA) pressure mapping system (Vista Medical, Inc, Winnipeg, Canada) FSA 4D Modular Seating System with 21-inch x 21-inch Soft Flex Sensing Mat was used to measure interface pressures at the buttock-seat interface. The system comprises a pressure-sensing stretch mat that contains 256 sensors in a 430-mm x 430-mm sensing area. Each sensor measures 23.8 mm x 28.8 mm with a gap of 3.1 mm between sensors. The mat was connected to the FSA computer software via one interface module. Data were recorded as color-coded maps of pressure distribution, three-dimensional grids, and numerical output parameters including summary statistics. The FSA system was calibrated previously by its manufacturer according to factory standards. The same standard hospital room recliner was used for all measurements. The FSA pressure mapping system has been demonstrated to be an appropriate tool for comparative evaluation of pressure differences between multiple positions and multiple surfaces.^6,13,14

Procedure. The thin, flexible FSA sensor array seat mat was placed on the standard hospital room recliner chair. Volunteers wore only a single layer of plain clothing during the study. Each participant was asked to sit directly on the FSA mat in two specific positions (see Figure 1):
    • Position 1 (Down): The person was seated with his/her feet firmly resting on the floor with the seat back in its most upright position, which was considered the 0˚ position (see Figure 1a).
    • Position 2 (Up): The person was seated with his/her legs elevated on the extended foot rest of the recliner chair, and the seat back in the reclining position, which was 20˚ from the upright position (see Figure 1b).

After 6 minutes in each designated position, data with a corresponding pressure image were collected by the FSA software. A settling period of 6 minutes before data capture was chosen for this study because previous research¹⁵ with the FSA pressure mapping system found this to be an optimal sitting time for stable pressure measurement.

In order to uniformly collect pressure data from a specified location across participants, the pressure maps were pixelated to reveal individual sensors using the “Show color blocks” feature available with the FSA software. The subjects with the greatest and least BMI were used to determine the size of the rectangle necessary to indicate the sacral area of every participant.

Summary statistics also were generated by the FSA software, including average pressure and maximum pressure associated with the image.

Data collection and analysis. Measurements of average pressure, maximum pressure, and sensors registering 60 mm Hg or 80 mm Hg and above in the sacral region were used for data analyses. The number of sensors registering >60 mm Hg was used because many studies consider 60 mm Hg an acceptable limit for patients who can self-relieve pressure.^16-19 An alternative measure of 80 mm Hg was used as an indication of high pressures (based on unpublished observations). Statistical analyses were performed using the IBM Statistical Package for Social Sciences for Windows Version 19.0 (SPSS, Chicago, IL). Data were analyzed across the total surface of the array, as well as with a 7-sensor x 9-sensor rectangular area centered on the sacrum. Pressure measurements were analyzed after all sensor values <10 mm Hg were eliminated to rid the analysis of “noise,” considered to be random fluctuations of sensors not indicative of true pressure in the area of interest. Values used for data analyses included average pressure, maximum pressure, the number of sensors reading >60 mm Hg in the sacral region, and the number of sensors reading >80 mm Hg. Definitions of these terms are listed in Table 1.

Average pressure and maximum pressure were compared across Positions 1 (Down) and 2 (Up) using paired t-tests. A paired t-test between positions also was conducted on the number of sensors reading at least 80 mm Hg across the total surface, which was always equal to the number of sensors at the sacrum. In contrast, the number of sensors reading 60 mm Hg and above differed between the total surface and the sacrum. Because the sacrum is the area of interest in the current study, only the number of sensors at the sacrum was compared across Positions 1 and 2.

Data were divided into three BMI categories for further analysis: Category 1 = 20–22, Category 2 = 23–28, and Category 3 = 29+. Although the standard categories as defined by the World Health Organization¹² are underweight (BMI <18.5), healthy weight (18.5 – 25), overweight (25 – 30), and obese (30+), the alternative categories were chosen because they best stratified this limited participant pool for analysis. The two positions (Up and Down) were compared across BMI categories using a one-way analysis of variance (ANOVA) and subsequent pairwise comparisons with Bonferroni correction. The difference (delta) in number of sensors (60 mm Hg and 80 mm Hg) between positions, maximum pressure, and average pressure were analyzed. The delta was used in order to remove the impact of BMI on interface pressure (ie, high BMI correlates to more sensors).

Finally, visual ranking of color-coded pressure maps was used as an evaluation tool to compliment the numerical analyses. Pixelated pressure maps for each participant were evaluated by one of the authors. The two positions were compared and visually coded according to which position appeared to better reduce pressure. The following numbers were assigned: 1 (Up), 2 (no discernible difference between maps), or 3 (Down). Correlation between the visual ranking of maps for pressure distribution and the numerical values between the two positions was examined using Spearman’s rho. Strength of correlations was determined at the  = 0.01 level. For the BMI correlation, an important point to emphasize is that the current study did not focus on the impact of BMI on interface pressure measurements; rather, the analysis was designed to focus on the impact of BMI on position of least pressure. Therefore, visual ranking was compared to the difference in the measurements taken between the two positions.

The study sample included 23 participants, four men and 19 women, 18 to 64 years (average 45 years, SD 16). All were in good health. The sample also reflected diversity in body types; average BMI was 26 (SD 6.2, range 20 to 45); 17 participants fell into the healthy and overweight categories.

Ultimately, a 7-sensor x 9-sensor rectangle (see Figure 2) was determined to be most indicative of the sacral pressure region. This method removes confounding pressure measurements generated by the participant’s legs. The maximum pressure and sensors reading >80 mm Hg were found only in the sacral area; thus, their analysis was unaffected by this methodology. However, average pressure changed considerably through use of the 7-sensor x 9-sensor rectangle. Pressure values centered at the sacrum were much higher than pressure values at the legs, which creates a lower average pressure value across the entire surface than is observed with respect to the sacrum alone. Table 2 summarizes the effects of using a 7-sensor x 9-sensor area versus the total surface area.

Statistical analysis results comparing the two positions are summarized in Table 3. Position 2 was found to reduce the average pressure of the total surface examined (from 42.9 mm Hg to 40.0 mm Hg; P = 0.015) and to reduce the number of sensors reading >60 mm Hg in the sacral region (from 31.4 to 27.1 sensors; P = 0.047). In Table 3, the Up and Down positions are compared across BMI categories using the delta value between the positions. A significant interaction between BMI categories and position was found when the difference in the number of sensors >80 mm Hg (P = 0.008) and average sacral pressure (P = 0.031) were examined. Further pairwise comparisons showed significant differences between BMI categories 1 (average delta of -3.63 indicating the down position is better) and 3 (average delta of 4.67 indicating the up position is better) for the difference in number of sensors >80 mm Hg (P = 0.030). The difference between BMI categories 1 (average delta of -3.63) and 2 (average delta of 2.67) for the difference in number of sensors >80 mm Hg also approached statistical significance (P = 0.056).

For comparison of numerical values to visual examination of color-coded pressure maps, all variables, including average sacral pressure, showed a significant correlation with visual ranking (see Table 4). Quantifying the apparent relationship between position and BMI by visual ranking showed individuals with a BMI <29 were equally likely to be ranked as Up, Same, or Down for the position of greater pressure redistribution (see Table 5), while individuals with a BMI >29 were increasingly likely to have Position 2 (Up) ranked as the position of greater pressure redistribution.

This study found sensors registering >80 mm Hg occurred only in the sacral region, the primary area of interest, making this a favorable method of measuring high pressure. Recording the number of sensors reading >60 mm Hg in the sacral region also assisted in quantifying the approximate area of the sacrum affected by high pressure and provided more insight into how the two positions differ across the surface.

Interventions including the use of turning schedules and specialized support surfaces are commonly implemented to address the risk of impaired mobility. The National Pressure Ulcer Advisory Panel²⁰ (NPUAP) guidelines support regular repositioning to relieve or redistribute pressure and avoid shear and frictional forces. In addition, the NPUAP advises postures that increase pressure should be avoided. In the current study, Position 2 (elevation of the legs and reclining the chair) significantly reduced average pressure across the total surface as well as the number of sensors registering >60 mm Hg in the sacral region when outcomes of all participants were analyzed together. These results indicate in this pilot study elevation of the legs with a 20˚ back recline better redistributed pressure than sitting upright with the plantar surface of the feet resting on the floor. Similarly, Stinson et al¹¹ found leg elevation approached statistical significance for reducing average pressure; additionally, the authors determined that a 30˚ angle of recline significantly reduced average pressure at the seat interface. However, because no analyses specific to the sacrum were reported, a comparison to the current study is limited to measurements across the total seating surface. Stinson et al¹¹ also measured angle of recline and elevation separately; in the current study, the angle of recline and leg elevation may not be acting independently.

Defloor and Grypdonck¹⁰ also assessed chair recline and elevation simultaneously, finding a 20˚ recline with legs elevated best reduced maximum pressure. Although the current study confirms elevating the legs with a 20˚ chair recline enhances pressure redistribution, the difference in terms of maximum pressure between the two positions was not statistically significant.

The present study also demonstrates pressure distributions in the two positions are related to participant BMI. A significant correlation was found between BMI category and position when the difference in number of sensors >80 mm Hg and average sacral pressure were examined. Further analysis through pairwise comparisons elucidated significant differences between BMI categories 1 (20–22) and 3 (29+) for the difference in number of sensors 80 mm Hg and approached statistical significance for the difference in average sacral pressures. A pairwise comparison between BMI categories 1 (20–22) and 2 (23–28) also approached statistical significance for the difference in number of sensors >80 mm Hg. These results indicate significant difference between BMI categories 1 and 3 exists and a difference between BMI categories 1 and 2 may exist. Therefore, distinct seating position recommendations may be made, at a minimum, for individuals with a BMI of 29 or above. Sitting position for individuals with an intermediate BMI may not be as crucial.

For individuals within BMI category 1, measurements were similar with no discernible difference between positions. For persons in BMI category 2, Position 2 (Up) conferred a slight advantage, because these measurements were typically lower than those taken during Position 1 (Down). Among individuals in BMI category 3 (29+), measurements taken in Position 2 (Up) were, on average, much lower than those taken in Position 1 (Down). These results also were supported by the visual rank comparison of each participant’s pressure mapping image. In previous studies involving healthy participants,^21,22 maximum pressure has been shown to be highly correlated and average pressure less correlated with visual ranking.

Many studies have focused on the impact of BMI on interface pressure readings. Stinson et al¹¹ found a significant positive correlation between BMI and average pressure in the sitting position but found no significant correlation between BMI and maximum pressure. In contrast, Kernozek at al²³ found a significant negative correlation between BMI and maximum pressure in the sitting position. The current study examined the interaction between BMI category and position as opposed to the impact of BMI on interface pressure reading. The results suggest different positions are better suited to different BMI categories. When reviewing the results of the current and previously published studies, it can be concluded the relationship between BMI and interface pressure readings is more complicated than a positive or negative linear relationship and further research is needed to establish the full impact of BMI and sitting position on interface pressures.

Ultimately, the purpose of this study was to begin to provide caregivers with evidence-based recommendations for patients with limited mobility sitting out of bed. Small pilot studies using healthy volunteers can provide guidelines for the design of more rigorous studies using at-risk patients. Results indicate for individuals in BMI categories 1 and 2 (20–28), neither position offers a significant reduction in pressure. Therefore, caregivers should recommend a seating position based on other factors, such as comfort. However, for individuals in BMI category 3 (29+), a significant difference was noted between the two positions. These individuals should be encouraged to spend more time with the legs elevated and with the seat back reclined. Because neither position reduced maximum pressure to <60 mm Hg, caregivers must realize positioning may not be enough to prevent pressure ulcers in the sitting position. Finally, if a patient experiences exacerbated mobility limitations due to particular positions (eg, leg elevation), those positions should be discouraged.

Another consideration is the use of the standard hospital recliner. This product can be recommended for the supervised setting such as acute hospital or skilled care, but use in the unsupervised home environment requires further investigation.

Limitations to this study include sample size (N = 23), which was considerably smaller when the data were subdivided into BMI categories. Another challenge regarding the BMI subdivision was no participants were in the underweight category. Future research with more participants representing all BMI categories equally would be beneficial; some studies^23-25 suggest individuals in the underweight category are most at risk for pressure ulcer development. A comparison²³ of peak seat-interface pressures in a population of 75 institutionalized elderly showed highest pressures in the thin group (BMI <20). Examination of data²⁴ from the 2006 and 2007 International Pressure Ulcer Prevalence Surveys revealed the highest prevalence of pressure ulcers was in the underweight category (BMI <18.5). A study²⁵ analyzing 3,214 elderly (65) patients admitted to two large inner-city hospitals over 4 years found underweight (BMI <18.5) individuals had the highest prevalence of pressure ulcers.

All participants in the current study were healthy volunteers with an average age of 45 years. Previous studies^19,26 have found skin tissues differ depending on the health and age of individuals, making the results of this study difficult to generalize to a disabled or elderly population. Higher maximum pressures were observed in a group of 12 spinal cord injury patients compared to 10 nondisabled individuals in a study¹⁹ examining typical wheelchair sitting positions; a study²⁶ found young individuals experienced lower maximum interface pressures than elderly participants while lying on pressure-redistribution mattresses.

Interface pressures are a proxy measure for pressure ulcer development and should be considered in that context. For example, a small, prospective, repeated measures study²⁷ of wheelchair seat cushions found spinal cord injury patients may experience higher interface pressures compared with healthy elderly individuals. A small, prospective study²⁸ of interface pressures over the ischial tuberosities of individuals who had paraplegia compared to individuals who were healthy found differences in pressure distribution in the sitting position across the seating surface, with higher pressures observed in the group with paraplegia.

Pressure mapping does not measure shear or frictional forces,⁵ nor does it measure the pressure extant on deeper tissues,²⁹ all of which may contribute to ulceration and deep tissue injury. However, this method does provide insight into one known risk factor for the development of pressure ulcers, which makes it a valuable contribution to understanding pressure and positioning. Additionally, associations between high peak-interface pressures during sitting and the subsequent development of pressure ulcers have been demonstrated,⁶ which adds support to the use of pressure mapping as one of many tools to ascertain risk. However, thoughts on the best way to quantify and report interface pressures differ considerably, which makes comparisons between studies problematic. The current study utilized the commonly reported peak and average pressure values. Numbers of sensors above a pressure threshold also were utilized in order to provide additional insight. Various discussions of how best to quantify and interpret interface pressure measurements have been published elsewhere and exemplify various viewpoints^30-32; however, this methodological issue remains outside the scope of the current discussion.

For patients with a BMI higher than 29, an out-of-bed reclined sitting position with heels elevated is recommended over an upright out-of-bed sitting position to reduce interface pressure and may help prevent pressure ulcers. No recommendation for seating position can be made for individuals with a BMI of 20 to 28 or for underweight (untested) individuals. Further research specific to pressure redistribution in the sitting position is needed, because neither position examined in this study was found to reduce interface pressures to the extent that may be optimal to help prevent pressure ulcer development. The current study design could be replicated in future research with a defined patient population susceptible to pressure ulcer formation, such as surgical patients. Also, future studies of this nature should include larger samples in order to draw more definitive conclusions and to further investigate the relationship between BMI and positioning and interface pressures. Finally, caregivers should be cognizant of other preventive measures, such as frequent posture changes as tolerated by the patient and use of pressure-redistribution cushions. Other factors such as comfort, skin integrity, positioning needs, activity level, cognition, continence, and nutrition, among others, also must be taken into consideration when evaluating pressure ulcer prevention strategies.³³

Judith Fulton, PhD, is a sub-project PI on a grant from the Ohio Department of Development issued to the Clinical Tissue Engineering Center (Cleveland Clinic, Cleveland, OH, USA) (tissue engineering), PI on contracts with Silvergreen Ltd, Vantaa, Finland (silver wound dressing); and Sterionics Inc, Cleveland, OH, USA (nonthermal plasma) for animal trials, SubI on sponsored clinical trial with Healthpoint, Smith & Nephew Biotherapeutics, Fort Worth, TX, USA (spray on cells), CoI on NIH R15 grant in collaboration with The University of Akron (oxygen-releasing polymer); all funds paid to Akron General Medical Center. Dr. Fulton also is an advisory board member, Sterionics, Inc.

Ms. Miller is a research assistant, Akron General Medical Center, Wound Center, Akron, OH. Ms. Aberegg is a summer fellowship student, Akron General Medical Center; and a student, Cornell University, Ithaca, NY. Ms. Blasiole is a research nurse coordinator; Dr. Parker is a plastic surgeon; and Dr. Fulton is Director, wound research, Akron General Medical Center, Wound Center.

Please address correspondence to: Stephannie K. Miller, MPH, Akron General Medical Center, Wound Center, 1 Akron General Avenue, Akron, OH 44307; email: smiller8705@gmail.com.