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A Prospective, In vivo Evaluation of Two Pressure-redistribution Surfaces in Healthy Volunteers Using Pressure Mapping as a Quality Control Instrument

Empirical Studies

A Prospective, In vivo Evaluation of Two Pressure-redistribution Surfaces in Healthy Volunteers Using Pressure Mapping as a Quality Control Instrument

Index: Ostomy Wound Manage. 2013;59(2):44‚48.


  Deep tissue injury (DTI) can rapidly evolve into a higher stage pressure ulcer. Use of pressure-redistribution surfaces is a widely accepted practice for the prevention of pressure ulcers in acute care patients, particularly in departments where care processes limit mobility.

A 15-year-old patient developed a sacral DTI 24 hours after completion of a lengthy (12-hour) electrophysiology (EP) study and catheter ablation. A root cause analysis (RCA) conducted to investigate the origin of the hospital-acquired suspected DTI prompted a small investigation to evaluate the pressure-distribution properties of the EP lab surface and an OR table pad. Five healthy adult employee volunteers were evaluated in the supine position by placing a sensing mat between the volunteer and the test surface. Interface pressures (on a scale of 0 mm Hg to 100 mm Hg) were captured after a “settling in” time of 4 minutes, and the number of sensors registering very high pressures (above 90 mm Hg) across the surface were recorded. On the OR table pad, zero to six sensors registered >90 mm Hg compared to two to 20 sensors on the EP lab surface. These data, combined with the acquired DTI, initiated a change in EP lab surfaces. Although interface pressure measurements only provide information about one potential support surface characteristic, it can be helpful during an RCA. Studies to compare the effect of support surfaces in all hospital units on patient outcomes are needed. 

Potential Conflicts of Interest: none disclosed


  According to the National Pressure Ulcer Advisory Panel1 (NPUAP), suspected deep tissue injuries (DTIs) are complex wounds that present as intact areas of purple or maroon skin discoloration as a result of underlying tissue damage. The category suspected deep tissue injury was incorporated into the NPUAP pressure ulcer staging system in 2007. DTIs are of particular concern to clinicians because they are often difficult to identify and can evolve rapidly to reveal damage and necrosis of deeper tissues.1 In 2009, 36.8% of the pressure ulcers recorded in a survey of 86,932 acute care patients were facility-acquired, with 1% of these wounds documented as DTI.2 A survey of Medicare statistics reveals that healthcare-associated pressure ulcers account for approximately $2.2 to $3.6 billion in healthcare costs each year.3-5 A review6 of clinical evidence, animal studies, and in vitro models indicates that procedures lasting >3 hours are associated with increased rates of pressure ulcer formation in surgical patients; incidence continues to increase with extended duration thereafter.6

  Hospitals are continually evaluating ways to decrease hospital-acquired pressure ulcers (HAPUs), such as by utilizing pressure-redistributing mattresses. These surfaces are meant to either redistribute or reduce interface pressure, ideally to levels below 32 mm Hg, the average capillary closure pressure in human skin.7 Studies comparing the efficacy of various surfaces indicate that differences exist among them. For example, a randomized controlled trial8 evaluating 446 surgical patients showed the use of a dry viscoelastic polymer pad in the operating room (OR) resulted in significantly fewer postoperative pressure ulcers when compared to use of a standard surgical mattress (P = 0.010). Hoshowsky and Schramm9 randomly assigned 505 surgery patients to six experimental mattress groups and determined a viscoelastic dry polymer mattress overlay on a standard OR mattress was most effective in preventing pressure ulcers, followed by a foam and gel OR table mattress when compared to a standard, 2-inch foam OR table mattress.9 A randomized clinical trial10 comparing standard hospital surfaces to a viscoelastic polymer foam mattress in a population of 1,168 elderly acute care, orthopedic, and rehabilitation patients found a viscoelastic polymer foam mattress significantly (P = 0.004) reduced the incidence of blanching erythema at 7 days after admission to the acute elderly care, elderly rehabilitation, or orthopedic wards at three hospital sites. A Cochrane review11 examining the effectiveness of pressure-relieving surfaces in reducing pressure ulcer incidence found 53 randomized controlled or quasi-randomized studies that reported clinical outcomes. The authors concluded high-specification foam mattresses (surfaces with varying foam densities) were more effective for preventing pressure ulcer formation than standard ordinary hospital foam mattresses.

  In addition to prospective trials for evaluation of pressure ulcer development, pressure-mapping equipment can be utilized to assess the effectiveness at redistributing pressure of various patient support surfaces. Pressure mapping technology is used to evaluate the interface pressures between a patient and a test surface. A thin, flexible mat containing an array of sensors is placed between the patient and the test surface. The sensors capture pressure values and transmit them to a computer containing specialized software, which generates a map that then can be interpreted both visually and numerically. High and low pressures are generally color-coded with a corresponding numerical pressure scale (mm Hg).

  Practical clinical experience suggests measures such as turn schedules, offloading heels, moisture management, and pressure-redistributing surfaces can be readily implemented in departments that care for high-risk, immobile patients, such as persons in intensive care units and the OR. In addition to preventative measures, root cause analysis (RCA) techniques should be employed when a HAPU is discovered. RCA is a tool often utilized in healthcare to investigate and categorize the primary cause of an incident that occurs with unacceptable consequences. Understanding how and why an event occurred is critical in identifying effective recommendations to prevent similar occurrences.12

  A review of the literature13 suggests more studies are warranted to examine patient risk of pressure ulcer development and preventative measures during diagnostic or interventional procedures. Messer13 conducted a comprehensive literature review and identified 43 relevant studies to answer the research question: Which intrinsic and extrinsic risk factors for pressure ulcer development identified in the literature are most likely to place the ancillary procedure patient population at high risk for pressure injury? The specific factors identified include high interface pressures on procedure tables, shear with patient movement/positioning, advanced age, anesthesia or sedation, fever, sepsis, and hypotension. The literature review13 also identifies several animal trials that demonstrate pressure-related tissue injuries at higher pressures for shorter periods of time can produce as much tissue damage as lower pressures for longer periods of time. Prospective studies using healthy volunteers13-15 show evidence of high interface pressures (from 97.7 mm Hg14 to more than 170 mm Hg15) present on exam/procedure table surfaces. Also, clinical experience suggests tissue interface pressures above the capillary closure of 32 mm Hg and shear forces from movement and positioning are common in the ancillary setting.13 This evidence underscores the importance of identifying all clinical areas where implementation of pressure ulcer preventive measures should be considered. A recent hospital case illustrates the importance of instituting pressure ulcer preventive measures in ancillary procedure areas such as the electrophysiology (EP) lab.

Case Report

  Mr. K, a 15-year old male, was referred to the wound center for evaluation and treatment of a sacral DTI that developed 24 hours after completion of a difficult 12-hour EP study and catheter ablation. Mr. K presented to the EP lab for treatment of Wolff-Parkinson-White Syndrome (WPW), commonly seen in children and young adults. Patients with WPW have pre-excitation that presents as supraventricular tachycardia (SVT). WPW initially may be managed through antiarrhythmic therapy by causing a decrease in the sinus node rate and blocking AV node conduction, decreasing episodes of SVT. Radiofrequency catheter ablation is recommended for patients who do not respond to drug therapy.16 Mr. K experienced episodes of palpitation associated with rapid heartbeats for approximately 1 year, sometimes occurring after he participated in a sporting event. He also complained of becoming light-headed but did not lose consciousness. He reported not being able to shorten episodes with coughing, deep breathing, and other Val salver methods. Diagnosis of WPW was made by EKG. Mr. K was evaluated via stress testing and experienced the WPW pattern throughout exercise. For this reason and symptoms suggestive of tachycardia, an EP study with a possible curative catheter ablation was recommended. He was not on any medications before the EP study and ablation except for fish oil, a multivitamin, and cough syrup as needed.

  A RCA was instituted to identify factors contributing to the development of Mr. K’s HAPU. With the understanding that pressure and duration of pressure are important causative factors in the development of DTIs, the wound center recommended testing the table surface used during the procedure (2.5-inch Tempur-Pedic® EP lab surface constructed of a proprietary viscoelastic material, Tempur-Pedic North America, Inc, Lexington, KY) to evaluate interface pressures. A comparison to another pressure-redistribution surface (a 4-inch viscoelastic pressure redistribution OR table pad, Medline Industries, Inc, Mundelein, IL) might generate results that could be beneficial in identifying alternatives.

  A force-sensing array (FSA) high-resolution pressure map system (Vista Medical, Inc, Winnnipeg, Canada) was utilized to measure areas of high pressure typically experienced over bony prominences of the body. Wound center employees compared the pressure-distribution qualities of the EP lab surface to the OR table pad, which had previously been found to exhibit good pressure distribution qualities (unpublished data).


  Participants. This support-surface comparison was conducted among five healthy, able-bodied employee volunteers (four women, one man) electing to participate. Institutional Review Board approval was not required.

  Materials/products. Two surfaces were used for comparison in this project: the existing EP lab surface and the OR table pad. Both surfaces are radiolucent. The FSA Pressure Mapping System and accompanying software were used to map interface pressures and perform subsequent data analysis. The sensing mat consists of 1,024 sensors within a 1,920 mm x 762 mm sensing area. The FSA system has been previously noted to have 95% accuracy, in addition to high reliability and repeatability of interface pressure measurements.17

  Procedure. The sensing mat was placed directly on each test surface. Volunteers were asked to lay in the supine position with their arms across their chests to create a uniform posture. A thin pillow was used to support the head. All volunteers were tested in the same way, on the same day, and on the same surfaces to reduce the effect of extraneous environmental variables such as temperature and humidity. Pressure maps with corresponding pressure values in mm Hg (on a scale of 0 mm Hg to 100 mm Hg) were generated at the 4-minute mark (time was allowed for “settling in” to obtain consistent readings). Four minutes has been demonstrated to be a minimum effective waiting time before data capture in a prospective study18 of 12 healthy volunteers comparing pressure maps on a variety of cushions over time using the FSA system.

  Data collection and analysis. One data frame was captured for each participant over the entirety of each surface, yielding a total of two data frames per participant (one for each surface). All data were stored on a research-dedicated laptop computer.

  Data analysis involved counting the total number of sensors reading 90 mm Hg or above for each generated map. The threshold of 90 mm Hg was chosen because it is indicative of high pressure. A prospective comparative study conducted by Justham et al14 found that interface pressures of 16 healthy adults lying on a standard x-ray table averaged 97.7 mm Hg at the sacrum (the bony prominence that typically exhibits the most severe pressures). Sensors in contact with the pillow supporting the occipital region were not included in data analysis. Raw data (number of sensors out of 1,024 that exhibited pressures >90 mm Hg) were entered into Microsoft® Excel 2007 (Microsoft Corporation, Redmond, WA) for statistical comparison using a two-tailed paired t-test.


  Volunteer weight ranged from 132 lb to 215 lb, height ranged from 62 inches to 72 inches, and body mass index (BMI) ranged from 22 to 32 (see Table 1). Visual examination of pressure maps shows noticeably larger areas of red (pressures of 90 mm Hg or above) when volunteers were on the EP lab surface than on the OR table pad (see Figure 1), corresponding to more sensors registering 90 mm Hg or higher — 9.6 average number of sensors out of 1,024 on the EP surface versus 2.6 average number of sensors on the OR table pad (see Table 2) — but the difference was not statistically significant (P = 0.090).   The EP lab surface with two to 20 sensors registering 90 mm Hg or above would appear to support suspicion that it would be less effective at reducing pressure than the OR table pad with fewer (zero to six) sensors registering 90 mm Hg or above; however, a high degree of variability was observed between volunteers. When data points are arranged in order of BMI, a trend can be seen, with fewer areas of high pressure on the OR table pad than on the EP lab surface pad in persons with a higher BMI.


  HAPUs are considered to be preventable, and DTIs often progress quickly to Stage II ulcers or deeper.1 Although many factors contribute to the formation of a DTI, a prolonged period of high pressure is perhaps the most obvious cause. Any hospital unit in which patients may remain immobile for extended periods of time is especially likely to deal with DTI formation.

  RCA is beneficial in cases such as the one presented in order to ascertain where in the progression of care a pressure ulcer could have developed and which interventions might help improve patient outcomes.

  The results of this small study suggest the EP lab surface used during Mr. K’s lengthy procedure may have been inadequate in reducing very high tissue-interface pressures that are normally experienced over bony prominences. The results also suggest that differences between pressure-redistribution surfaces of similar composition exist as far as the number of areas of high tissue-interface pressures. According to an extensive review of randomized controlled trials assessing clinical outcomes of various pressure ulcer interventions published by Reddy et al,19 only nine of the eligible 59 reviewed studies compared two or more specialized static patient support surfaces to one another. Of those nine studies, three showed a reduction in the incidence of pressure ulcers, indicating that some static support surfaces may have advantages over others.19 Although five randomized or quasi-randomized controlled trials of support surfaces in the operating room were identified by a Cochrane Review,11 few studies have examined pressure ulcer risks in other ancillary settings such as radiology or procedure labs.13 When choosing a support surface for a specialized healthcare area, literature reviews11,19 can provide a starting point for identifying specific mattress types such as high-specification foam mattresses, overlays, or constant low-pressure devices. Research is needed to address the gaps in knowledge surrounding currently available pressure-redistribution surfaces. Although the comparison presented here sheds some light on the difference between two static surfaces, a large, defined sample is required to test these differences. Moreover, future studies should investigate the effect of different surface types currently available to an EP lab on the incidence of HAPUs.

  In this facility’s case, the study results prompted a change in EP lab pressure-redistribution surfaces. The study also increased awareness of pressures experienced by immobile patients and has resulted in the onset of systematic testing of surfaces for pressure-distribution capabilities in departments throughout the hospital. Although patient immobility is essential in the EP lab, routine turning and skin examination has been promoted hospital-wide.

  This study also raised new questions worth investigating: What are the differences, if any, in support surface effectiveness for patients with a high or low BMI? The concept that a single surface may not be ideal for all body types has been presented by others in the literature; body morphology and postures should be considered when testing new mattress options, particularly for very thin or very obese patients.20,21 In a prospective study of 38 healthy volunteers conducted by Moysidis et al,20 interface pressures present in the supine position on three different mattress types were analyzed in terms of BMI; the authors concluded that body shape has as much bearing on mattress effectiveness as BMI. Tall, thin patients benefited more from viscoelastic foam mattresses, while small, obese patients benefited from constant low pressure devices.20 Another research study21 used 100 bedridden elderly patients with extreme bony prominences to develop a buttock model for testing sacral pressures in a supine position at 30° elevation on a variety of pressure-relieving mattresses. Researchers found that using lower-stiffness foam mattresses resulted in lower maximum interface pressures for this patient population.21


  Limitations of the study include small sample size, use of healthy volunteers not under anesthesia, environment not identical to EP lab, and relatively short time periods on each surface. Interface pressures were recorded, but deep tissue pressures and the condition of the tissues that experienced high pressures were not examined. Moreover, tissue interface pressure is only one variable in the determination of support surface characteristics and is not considered a patient-outcome variable. Other factors that were not tested but can contribute to pressure ulcer formation include sheer, moisture, and temperature.


  A comparison of support surface interface pressures was conducted with healthy volunteers after an RCA traced the likely source of a hospital-acquired DTI to the EP lab. Pressure-mapping technology revealed differences in the number of very high-pressure areas (90 mm Hg or higher) between the standard EP lab mattress and another radiolucent surface. Overall, the number of high pressure readings averaged 9.6 for the EP lab surface and 2.6 for the OR table pad evaluated.

  The use and appropriateness of pressure-redistribution surfaces should be considered for all departments caring for hospitalized patients. Pressure mapping can be used in creative ways to obtain information about one variable that may affect the incidence of HAPUs. The RCA process is a beneficial tool in identifying causative factors with HAPU incidences. Analysis of findings and ongoing pressure ulcer prevalence studies are critical in implementing corrective measures that may help prevent future pressure ulcer development.

 Ms. Miller is a research assistant; Dr. Parker is a plastic surgeon; Ms. Blasiole is a research nurse coordinator; Ms. Beinlich is Director of the wound center; and Dr. Fulton is Director of Wound Research, Akron General Medical Center, Akron, OH. Please address correspondence to: Stephannie Miller, BA, Wound Healing and Limb Preservation Center, Akron General Medical Center, 400 Wabash Avenue, Akron, OH 44307; email: