Establishing predictive indicators for the status of loaded soft tissues

The complex interplay between mechanical forces, tissue response and individual susceptibility to pressure ulcers

OBJECTIVE

The most recent edition of the International Clinical Practice Guideline for the Prevention and Treatment of Pressure Ulcers/Injuries was released in 2019. Shortly after, in 2020, the first edition of the SECURE Prevention expert panel report, focusing on device-related pressure ulcers/injuries, was published as a special issue in the Journal of Wound Care. A second edition followed in 2022. This article presents a comprehensive summary of the current understanding of the causes of pressure ulcers/injuries (PU/Is) as detailed in these globally recognised consensus documents.

METHOD

The literature reviewed in this summary specifically addresses the impact of prolonged soft tissue deformations on the viability of cells and tissues in the context of PU/Is related to bodyweight or medical devices.

RESULTS

Prolonged soft tissue deformations initially result in cell death and tissue damage on a microscopic scale, potentially leading to development of clinical PU/Is over time. That is, localised high tissue deformations or mechanical stress concentrations can cause microscopic damage within minutes, but it may take several hours of continued mechanical loading for this initial cell and tissue damage to become visible and clinically noticeable. Superficial tissue damage primarily stems from excessive shear loading on fragile or vulnerable skin. In contrast, deeper PU/Is, known as deep tissue injuries, typically arise from stress concentrations in soft tissues at body regions over sharp or curved bony prominences, or under stiff medical devices in prolonged contact with the skin.

CONCLUSION

This review promotes deeper understanding of the pathophysiology of PU/Is, indicating that their primary prevention should focus on alleviating the exposure of cells and tissues to stress concentrations. This goal can be achieved either by reducing the intensity of stress concentrations in soft tissues, or by decreasing the exposure time of soft tissues to such stress concentrations.

Development of ultra-high-performance supercritical fluid chromatography-mass spectrometry assays to analyze potential biomarkers in sweat

Liquid chromatography-mass spectrometry methods were required to afford the rapid separation and detection of purines and small organic acids. These compounds are found in sweat and sebum and are potential biomarkers for the early detection of pressures sores. Two ultra-high-performance supercritical fluid chromatography-mass spectrometry assays have been successfully developed for both classes of compounds. Separation for purines was achieved using a gradient of supercritical carbon dioxide and methanol with a 1-aminoanthracene sub 2 μm particle size column followed by positive ion electrospray ionization. Separation for organic acids was achieved using a gradient of supercritical carbon dioxide and methanol (50 mM ammonium acetate 2% water) with a Diol sub 2 μm particle size column followed by negative ion electrospray ionization. Calibration curves were created in the absence of internal standards and R² values > 0.96 were achieved using single ion monitoring methods for the protonated purines and the deprotonated acids. The two new assays afford rapid analytical methods for the separation and detection of potential biomarkers in human sweat leading to the early detection and prevention of pressure sores.

Our contemporary understanding of the aetiology of pressure ulcers/pressure injuries

In 2019, the third and updated edition of the Clinical Practice Guideline (CPG) on Prevention and Treatment of Pressure Ulcers/Injuries has been published. In addition to this most up‐to‐date evidence‐based guidance for clinicians, related topics such as pressure ulcers (PUs)/pressure injuries (PIs) aetiology, classification, and future research needs were considered by the teams of experts. To elaborate on these topics, this is the third paper of a series of the CPG articles, which summarises the latest understanding of the aetiology of PUs/PIs with a special focus on the effects of soft tissue deformation. Sustained deformations of soft tissues cause initial cell death and tissue damage that ultimately may result in the formation of PUs/PIs. High tissue deformations result in cell damage on a microscopic level within just a few minutes, although it may take hours of sustained loading for the damage to become clinically visible. Superficial skin damage seems to be primarily caused by excessive shear strain/stress exposures, deeper PUs/PIs predominantly result from high pressures in combination with shear at the surface over bony prominences, or under stiff medical devices. Therefore, primary PU/PI prevention should aim for minimising deformations by either reducing the peak strain/stress values in tissues or decreasing the exposure time.

Positron emission tomography study of effects of two pressure-relieving support surfaces on pressure ulcer development

OBJECTIVE

To study the pathophysiological cascade of pressure ulcer (PU) development consisting of tissue deformation, inflammation and hypoxia.

METHOD

In this crossover study, deformation was measured with computerised tomography (CT) linked with contact area reflecting immersion and envelopment. Inflammation and hypoxia were measured using subepidermal moisture (SEM), skin temperature and tissue perfusion with positron emission tomography. These variables were investigated under 90 minutes of pressure exposure caused by two functionally different support surfaces-a regular foam mattress and a minimum pressure air (MPA) mattress.

RESULTS

A total of eight healthy volunteers took part in the study. There was major tissue deformation when the participants lay on a foam mattress while the tissues retained their original shape on the MPA mattress (p<0.0001). During the pressure exposure, the skin temperature increased significantly on both support surfaces but the final temperature on the foam mattress was about 1oC higher than on the MPA mattress (p<0.0001). SEM increased on both support surfaces compared with an unexposed reference site, but the cause may be different between the two support surfaces. Tissue perfusion was lowest in the skin followed by subcutaneous tissues and highest in the muscles. The pressure exposure did not cause any substantial changes in perfusion. The results showed that tissue deformation was more pronounced, the support surface contact area (envelopment), was smaller and the skin temperature higher on the foam mattress than on the MPA mattress, without significant differences in tissue perfusion.

CONCLUSION

In this study, the MPA mattress support surface had mechanobiological properties that counteracted tissue deformation and thereby may prevent PUs.

Terminal Ulcers, SCALE, Skin Failure, and Unavoidable Pressure Injuries: Results of the 2019 Terminology Survey

GENERAL PURPOSE

To present the results of the 2019 study of healthcare professionals' consensus and opinions regarding terminology for terminal ulcers, Skin Changes At Life's End, skin failure, and unavoidable pressure injuries to improve clinical care and to foster research into current criteria for unavoidable skin changes at the end of life.

TARGET AUDIENCE

This continuing education activity is intended for physicians, physician assistants, NPs, and nurses with an interest in skin and wound care.

LEARNING OBJECTIVES/OUTCOMES

After completing this continuing education activity, the participant should be better able to:1. Explain the survey methodology and identify the consensus statements.2. Synthesize the open-ended questions and respondent comments and their implications for clinical care and research.

ABSTRACT

This article reports the results of a global wound care community survey on Kennedy terminal ulcers, Skin Changes At Life's End, Trombley-Brennan terminal tissue injuries, skin failure, and unavoidable pressure injury terminology. The survey consisted of 10 respondent-ranked statements to determine their level of agreement. There were 505 respondents documented. Each statement required 80% of respondents to agree (either "strongly agree" or "somewhat agree") for the statement to reach consensus. Nine of the 10 statements reached consensus. Comments from two additional open-ended questions were grouped by theme. Conclusions and suggested recommendations for next steps are discussed. This summary is designed to improve clinical care and foster research into current criteria for unavoidable skin changes at the end of life.

Establishing a measurement array to assess tissue tolerance during loading representative of prosthetic use

BACKGROUND

In the early stages of rehabilitation after primary amputation, residual limb soft tissues have not been mechanically conditioned to support load and are vulnerable to damage from prosthetic use. There is limited quantitative knowledge of skin and soft tissue response to prosthetic loading.

METHODS

An in-vivo protocol was developed to establish suitable measures to assess tissue tolerance during loading representative of early prosthesis use. Ten participants without amputation one participant with trans-tibial amputation were recruited, and pressure applied to their calf in increments from 20 to 60 mmHg. Measurements were recorded at relevant skin sites including interface pressures, transcutaneous oxygen (T_CPO₂) and carbon dioxide (T_CPCO₂) tensions and inflammatory biomarkers.

FINDINGS

At the maximum cuff pressure, mean interface pressures were between 66 and 74 mmHg, associated with decreased T_CPO₂ values. On the release of pressure, the ischaemic response was reversed. Significant upregulation (p < 0.05) in inflammatory biomarker IL-1α and its antagonist IL-1RA were observed at all sites immediately following loading.

INTERPRETATION

The protocol was successful in applying representative prosthetic loads to lower limb tissues and monitoring the physiological response, both in terms of tissue ischemia and skin inflammation. Results indicated that the measurement approaches were sensitive to changes in interface conditions, offering a promising approach to monitor tissue status for people with amputation.

The relationship between pressure offloading and ischial tissue health in individuals with spinal cord injury: An exploratory study

Objectives: To compare thickness and texture measures of tissue overlying the ischial region in able-bodied (AB) individuals vs. individuals with spinal cord injury (SCI) and to determine if there is a relationship between pressure offloading of the ischial tuberosities (IT) and tissue health in individuals with SCI. Design: Exploratory cross-sectional study. Setting: University setting and rehabilitation hospital. Outcome Measures: Thickness and texture measurements from ultrasound images of tissues overlying the IT were obtained from AB individuals (n = 10) and individuals with complete or incomplete traumatic and non-traumatic SCI American Spinal Injury Association Impairment Scale (AIS) classification A-D (n = 15). Pressure offloading was measured in individuals with SCI and correlated with tissue health measurements. Results: The area overlying the IT occupied by the muscle was significantly greater in the SCI when compared with AB cohort. The area occupied by the muscle in individuals with SCI appeared to lose the striated appearance and was more echogenic than nearby skin and subcutaneous tissue (ST). There was no correlation between offloading times and thickness, echogenicity and contrast measurements of skin, ST and muscle in individuals with SCI. Conclusion: Changes in soft tissues overlying the ischial tuberosity occur following SCI corresponding to the loss of striated appearance of muscle and increased thickness of the area occupied by the muscle. Further studies using a larger sample size are recommended to establish if thickness and tissue texture differ between individuals with SCI who sustain pressure injuries vs. those who do not.

Bioengineering considerations in the prevention of medical device-related pressure ulcers

BACKGROUND

In recent years, it has become increasingly apparent that medical device-related pressure ulcers represent a significant burden to both patients and healthcare providers. Medical devices can cause damage in a variety of patients from neonates to community based adults. To date, devices have typically incorporated generic designs with stiff polymer materials, which impinge on vulnerable soft tissues. As a result, medical devices that interact with the skin and underlying soft tissues can cause significant deformations due to high interface pressures caused by strapping or body weight.

METHODS

This review provides a detailed analysis of the latest bioengineering tools to assess device related skin and soft tissue damage and future perspectives on the prevention of these chronic wounds. This includes measurement at the device-skin interface, imaging deformed tissues, and the early detection of damage through biochemical and biophysical marker detection. In addition, we assess the potential of computational modelling to provide a means for device design optimisation and material selection.

INTERPRETATION

Future collaboration between academics, industrialists and clinicians should provide the basis to improve medical device design and prevent the formation of these potentially life altering wounds. Ensuring clinicians report devices that cause pressure ulcers to regulatory agencies will provide the opportunity to identify and improve devices, which are not fit for purpose.

Fluid-permeable enzymatic lactate sensors for micro-volume specimen

Sensing of lactate in perspiration provides a way to monitor health and control exercise. The volume of perspiration is miniscule, and the efficient collection of perspiration is desired for its effective sensing. We developed mesh-type enzymatic electrodes fabricated on textile meshes and integrated the meshes into an enzymatic biofuel cell. We tested them as self-powered lactate sensors for a small volume of lactate solution. A fluid-permeable enzymatic anode was fabricated based on an insulating textile mesh that was coated with carbon nanotubes (CNTs) and lactate oxidase. The anode was further coated with polyurethane to increase the linear range by limiting the diffusion of lactate while maintaining the advantages of the original textile mesh, such as flexibility, stretchability, and permeability. Permeability of the mesh-type lactate-oxidizing anode allowed a vertically stacked structure of the anode and a previously developed air-breathing cathode. This resulted in a small overall device size (1 cm2). The mesh-type sensor was tested using a small flow rate of lactate solution, and a moderate linearity of amperometric response for a wide concentration range (5 to ≥20 mM) was confirmed. The fluid-permeable anode and enzymatic biofuel cell show the potential of the sensor for continuous monitoring of lactate in perspiration on skin.

Reducing the Risk for Pressure Injury During Combat Evacuation

BACKGROUND

Combat casualties undergoing aeromedical evacuation are at increased risk for pressure injuries. The risk factors pressure and shear are potentially modifiable via solutions appropriate for en route care.

OBJECTIVES

To compare transcutaneous oxygen levels and skin temperatures in healthy participants under offloaded (side lying) and loaded (supine or supine with 30° backrest elevation) under 4 conditions: control (no intervention), Mepilex sacral and heel dressings, LiquiCell pad, and Mepilex plus LiquiCell.

METHODS

Participants were randomly assigned to 4 groups according to ideal body weight. Backrest positions were randomized. Transcutaneous oxygen level and temperature were measured on the sacrum and the heel; skin interface pressure was measured with an XSensor pressure imaging system. Measurements were obtained for 5 minutes at baseline (offloaded), 40 minutes with participants supine, and 15 minutes offloaded.

RESULTS

In the 40 healthy participants, interface pressure, transcutaneous oxygen level, and skin temperature did not differ between the 4 groups. Peak interface pressures were approximately 43 mm Hg for the sacrum and 50 mm Hg for the heel. Sacral transcutaneous oxygen level differed significantly between unloaded (mean, 79 mm Hg; SD, 16.5) and loaded (mean, 57 mm Hg; SD, 25.2) conditions (P < .001) in a flat position (mean, 85.2 mm Hg; SD, 13.6) and with 30° backrest elevation (mean, 66.7 mm Hg; SD, 24.2) conditions (P < .001). Results for the heels and the sacrum were similar. Sacral skin temperature increased significantly across time (approximately 1.0°C).

CONCLUSIONS

The intervention strategies did not differ in prevention of pressure injuries.

Demonstrating the Potential of Using Transcutaneous Oxygen and Carbon Dioxide Tensions to Assess the Risk of Pressure Injuries

Pressure injuries have a high incidence in elderly and critically ill patients, and can endanger lives in severe cases. The key to reducing the incidence of pressure injuries is to find an objective, noninvasive, automatic and consistent scientific method for assessing pressure injuries. To serve this need, we conducted a clinical study to investigate the potential of using transcutaneous oxygen tension (TcPO2) and transcutaneous carbon dioxide tension (TcPCO2) for assessing pressure injuries. From the results of the study we found that first, the values of TcPO2 and TcPCO2 are sensitive to the change of pressure imposed on the measured region and to the risk status of a pressure injury when a pressure is imposed. Second, the magnitude of change in TcPO2 and TcPCO2 is higher in patients with a high risk of a pressure injury compared with those who have a low risk. Third, TcPO2 and TcPCO2 are both significantly correlated with the Braden score, the widely used score for assessing the risk of a pressure injury. Therefore, TcPO2 and TcPCO2 have a potential to be an effective and convenient scientific tool for assessing the risk of pressure injuries.

An advanced magnetic resonance imaging perspective on the etiology of deep tissue injury

Early diagnosis of deep tissue injury remains problematic due to the complicated and multifactorial nature of damage induction and the many processes involved in damage development and recovery. In this paper, we present a comprehensive assessment of deep tissue injury development and remodeling in a rat model by multiparametric magnetic resonance imaging (MRI) and histopathology. The tibialis anterior muscle of rats was subjected to mechanical deformation for 2 h. Multiparametric in vivo MRI, consisting of T2, T2*, mean diffusivity (MD), and angiography measurements, was applied before, during, and directly after indentation as well as at several time points during a 14-day follow-up. MRI readouts were linked to histological analyses of the damaged tissue. The results showed dynamic change in various MRI parameters, reflecting the histopathological status of the tissue during damage induction and repair. Increased T2 corresponded with edema, muscle cell damage, and inflammation. T2* was related to tissue perfusion, hemorrhage, and inflammation. MD increase and decrease was reported on the tissue’s microstructural integrity and reflected muscle degeneration and edema as well as fibrosis. Angiography provided information on blockage of blood flow during deformation. Our results indicate that the effects of a single damage-causing event of only 2 h of deformation were present up to 14 days. The initial tissue response to deformation, as observed by MRI, starts at the edge of the indentation. The quantitative MRI readouts provided distinct and complementary information on the extent, temporal evolution, and microstructural basis of deep tissue injury-related muscle damage.

NEW & NOTEWORTHY We have applied a multiparametric MRI approach linked to histopathology to characterize damage development and remodeling in a rat model of deep tissue injury. Our approach provided several relevant insights in deep tissue injury. Response to damage, as observed by MRI, started at some distance from the deformation. Damage after a single indentation period persisted up to 14 days. The MRI parameters provided distinct and complementary information on the microstructural basis of the damage.

Technologies to monitor the health of loaded skin tissues

There are many situations where the skin and underlying soft tissues are compromised by mechanical loading in the form or pressure, or pressure in combination with shear. If sustained, this can lead to damage in the tissues particularly adjacent to bony prominences, resulting in chronic wounds. An array of bioengineering technologies have been adopted to assess the integrity of loaded soft tissues. This paper aims to review these approaches for the quantification, simulation and early detection of mechanically-induced skin damage. The review considers different measurements at the interface between the skin and support surface/medical device, involving pressure, shear, friction and the local microclimate. The potential of the techniques to monitor the physiological response of the skin to these external stimuli including biophysical measurement devices and sampling of biofluids are critically analysed. In addition, it includes an analysis of medical imaging technologies and computational modelling to provide a means by which tissue deformation can be quantified and thresholds for tissue damage defined. Bioengineering measurement and imaging technologies have provided an insight into the temporal status of loaded skin. Despite the advances in technology, to date, the translation to clinical tools which are robust and cost effective has been limited. There is a need to adapt existing technologies and simulation platforms to enable patients, carers and clinicians to employ appropriate intervention strategies to minimise soft tissue damage.

The effect of pressure and shear on tissue viability of human skin in relation to the development of pressure ulcers: a systematic review

Pressure ulcers are a significant problem in health care, due to high costs and large impact on patients' life. In general, pressure ulcers develop as tissue viability decreases due to prolonged mechanical loading. The relation between load and tissue viability is highly influenced by individual characteristics. It is proposed that measurements of skin blood flow regulation could provide good assessment of the risk for pressure ulcer development, as skin blood flow is essential for tissue viability. . Therefore, the aim of this systematic review is to gain insight in the relation between mechanical load and the response of the skin and underlying tissue to this loading measured in-vivo with non-invasive techniques. A systematic literature search was performed to identify articles analysing the relation between mechanical load (pressure and/or shear) and tissue viability measured in-vivo. Two independent reviewers scored the methodological quality of the 22 included studies. Methodological information as well as tissue viability parameters during load application and after load removal were extracted from the included articles and used in a meta-analysis. Pressure results in a decrease in skin blood flow parameters, compared to baseline; showing a larger decrease with higher magnitudes of load. The steepness of the decrease is mostly dependent on the anatomical location. After load removal the magnitude of the post-reactive hyperaemic peak is related to the magnitude of pressure. Lastly, shear in addition to pressure, shows an additional negative effect, but the effect is less apparent than pressure on skin viability.

The physiological response of soft tissue to periodic repositioning as a strategy for pressure ulcer prevention

BACKGROUND

Individuals who have reduced mobility are at risk of developing pressure ulcers if they are subjected to sustained static postures. To reduce this risk, clinical guidelines advocate healthcare professionals reposition patients regularly. Automated tilting mechanisms have recently been introduced to provide periodic repositioning. This study compared the performance of such a prototype mattress to conventional manual repositioning.

METHODS

Ten healthy participants (7 male and 3 female, aged 23-66 years) were recruited to compare the effects of an automated tilting mattress to standard manual repositioning, using the 30° tilt. Measures during the tilting protocols (supine, right and left tilt) included comfort and safety scores, interface pressures, inclinometer angles and transcutaneous gas tensions (sacrum and shoulder). Data from these outcomes were compared between each protocol.

FINDINGS

Results indicated no significant differences for either interface pressures or transcutaneous gas responses between the two protocols (P>0.05 in both cases). Indeed a small proportion of participants (~30%) exhibited changes in transcutaneous oxygen and carbon dioxide values in the shoulder during a right tilt for both protocols. The tilt angles at the sternum and the pelvis were significantly less in the automated tilt compared to the manual tilt (mean difference=9.4-11.5°, P<0.001). Participants reported similar comfort scores for both protocols, although perceived safety was reduced on the prototype mattress.

INTERPRETATION

Although further studies are required to assess its performance in maintaining tissue viability, an automated tilting mattress offers the ability to periodically reposition vulnerable individuals, with potential economic savings to health services.

Bone geometry on the contact stress in the shoulder for evaluation of pressure ulcers: finite element modeling and experimental validation

This research presents the finite element modeling (FEM) of human-specific computed tomography (CT) data to study the effect of bone prominences on contact stress in the shoulder for prevention of pressure ulcers. The 3D geometry of scapula, skin, and surrounding soft tissues in the shoulder was reconstructed based on the anonymous CT data of a human subject in a prone posture (without loading on the shoulder) for FEM analysis of the contact stress. FEM analysis results show that the maximum stress is located at the prominence of the scapula with sharp bone geometry. This demonstrates that stress concentration at the bone prominence is a significant factor to cause the high contact stress, which is a source for pressure ulcers. For experimental validation, a physical shoulder model manufactured by 3D printing of the bone geometry and the mold for molding of tissue-mimicking silicone was developed. Compression tests of the mattress foam and silicone were conducted to find the nonlinear stress-strain relations as inputs for FEM. Experiments of compressing the shoulder model against the foam were carried out. Three flexible force sensors were embedded inside the model to measure the contact forces and compared to the FEM predictions. Results show that the FEM predicted forces match well with the experimental measurements and demonstrate that FEM can accurately predict the stress distributions in the shoulder to study the effect of bone geometry on the inception of pressure ulcers.

Lactate in human sweat: a critical review of research to the present day

This review provides a critical overview of the literature published in the area of lactate in human sweat between 1934 and the present. The first section summarizes the relevant pieces of literature, the second evaluates the literature across a range of topics and the third presents potential applications for sweat lactate measurements. Key factors that may affect sweat lactate are discussed in detail in the second section of this review. Both acetylcholine and catecholamine hormonal signals can trigger sweating independently of one another; differences in sweat output and lactate concentrations are compared. The primary triggers for sweating, exercise and heat are also compared, with exercise-induced sweating being further examined with respect to the exercise intensity. This review examines the significant variations in sweat lactate between bodily sites as well the relationship to physiological parameters such as gender, age and physical fitness, along with the effect of climate adaptation on sweat lactate concentrations.

Effects of electrical stimulation-induced gluteal versus gluteal and hamstring muscles activation on sitting pressure distribution in persons with a spinal cord injury

STUDY DESIGN

Ten participants underwent two electrical stimulation (ES) protocols applied using a custom-made electrode garment with built-in electrodes. Interface pressure was measured using a force-sensitive area. In one protocol, both the gluteal and hamstring (g+h) muscles were activated, in the other gluteal (g) muscles only.

OBJECTIVES

To study and compare the effects of electrically induced activation of g+h muscles versus g muscles only on sitting pressure distribution in individuals with a spinal cord injury (SCI).

SETTING

Ischial tuberosities interface pressure (ITs pressure) and pressure gradient.

RESULTS

In all participants, both protocols of g and g+h ES-induced activation caused a significant decrease in IT pressure. IT pressure after g+h muscles activation was reduced significantly by 34.5% compared with rest pressure, whereas a significant reduction of 10.2% after activation of g muscles only was found. Pressure gradient reduced significantly only after stimulation of g+h muscles (49.3%). g+h muscles activation showed a decrease in pressure relief (Δ IT) over time compared with g muscles only.

CONCLUSION

Both protocols of surface ES-induced of g and g+h activation gave pressure relief from the ITs. Activation of both g+h muscles in SCI resulted in better IT pressure reduction in sitting individuals with a SCI than activation of g muscles only. ES might be a promising method in preventing pressure ulcers (PUs) on the ITs in people with SCI. Further research needs to show which pressure reduction is sufficient in preventing PUs.

Physiological measurements of tissue health; implications for clinical practice

Pressure mapping alone insufficiently describes tissue health. Comprehensive, quantitative non invasive assessment is crucial. Interface pressures (IPs) and transcutaneous blood gas levels [transcutaneous tissue oxygen (T(c) PO(2) )] were simultaneously assessed over both ischia and the sacrum to investigate the hypotheses: (i) tissue oxygenation decreases with sustained applied pressure; (ii) tissue oxygen and IP are inversely correlated in loaded soft tissues; (iii) multisite assessments are unnecessary because healthy individuals are symmetrical. Measurements were taken at 5-minute intervals for 20 minutes in both sitting and supine lying for a cohort of 20 able-bodied adults. There were no statistically significant changes over time for either variable in 96% of timepoint comparisons. Specifically, no significant differences were seen between 10 and 20 minutes in either position. These findings imply that a 10-minute assessment can reliably indicate tissue health and that tissue may adapt to applied load over time. No statistically significant correlations between T(c) PO(2) and IP were observed. However, the left and right ischia were significantly different for both variables in supine lying (P < 0.001) and for sitting IP (P < 0.010). Thus, even in this healthy cohort, postural symmetry was not observed and should not be assumed for other populations with restricted mobility. If a multisite technique cannot be used, repeated tissue health assessments must use the same anatomic location.

Skin blood flow response to 2-hour repositioning in long-term care residents: a pilot study

PURPOSE

The purpose of this noninvasive pilot study was to examine the changes in transcutaneous oxygen (tcO2), skin temperature, and hyperemic response in the heels, sacrum, and trochanters in a 2-hour loading-unloading condition in nursing home residents who are positioned in supine and lateral positions.

DESIGN

A 1-group, prospective, repeated-measures design was used.

SUBJECTS AND SETTING

Nine subjects (5 males, 4 females) with a mean age of 85.3 ± 10.86 years (mean ± SD) who required help in turning and positioning at a skilled nursing facility participated in the study.

METHODS

Oxygen and temperature sensors were placed on the heels, trochanters, and sacrum. The subject was (1) positioned lateral for 30 minutes (preload); (2) turned to the supine position with head of the bed at 30° for 2 hours (both sacrum and heels were on the bed surface) (loading); and (3) positioned lateral for 2 hours (unloading). Subjects were turned to either the right or the left side.

RESULTS

Friedman test showed no statistical differences in tcO₂ or skin temperature on the sacrum, heels, or trochanters during preload, supine, and lateral positioning (P > .5). Individual data revealed that hyperemic response was seen in 6 of the 9 subjects when the position was changed from supine to lateral. Only one-third of the subjects attained a sacral tcO₂ of 40 mm Hg or more at the end of the 2-hour lateral positioning. tcO₂ on both heels decreased within the first 30 minutes of loading.

CONCLUSION

Two hours of staying in the supine position lowered sacral oxygenation to less than 40 mm Hg, in some subjects, regardless of whether there was adequate tcO₂ at preload. Repositioning to a lateral position after 2 hours of placement in a supine position did not cause the tcO₂ to return to preload level. The efficacy of a 2-hour repositioning schedule requires further investigation. Since heel tcO₂ was reduced after 30 minutes of loading, further work is needed to determine whether the heels should be offloaded with more frequent repositioning.

New concepts in the prevention of pressure sores

Pressure sores are a serious, and costly, complication for many patients with reduced mobility and sensation. Some populations, such as those with spinal cord injury (SCI), remain at high risk throughout their lifetime. Prevention is highly preferable and while the concept is readily definable, it is much more challenging to develop valid preventative measures. Subjective and objective approaches to risk factor assessment before pressure sores develop are reviewed, including risk status scales and emerging techniques to assess deep tissue injury. Devices to prevent pressure sores have traditionally focused on pressure-relieving cushions and mattresses. Technological advances being applied in the development of new pressure sore prevention devices are presented. Clinical evidence-based practice is integral to pressure sore prevention. Comprehensive assessment must include evaluation of systemic diseases, anatomical and physiological factors, together with environmental and psychosocial factors, which can all contribute to pressure sore development. Extrinsic factors need to be considered in conjunction with intrinsic tissue health factors and are reviewed together with an evaluation of currently available clinical practice guidelines. This chapter presents the broad diversity of factors associated with pressure sore development and highlights the need for an interdisciplinary team approach in order to maximize successful prevention of pressure sores.

Traditional Japanese formula kigikenchuto accelerates healing of pressure-loading skin ulcer in rats

We evaluated the effect of kigikenchuto (KKT), a traditional Japanese formula, in a modified rat pressure-loading skin ulcer model. Rats were divided into three groups, KKT extract orally administered (250 or 500 mg/kg/day for 35 days) and control. KKT shortened the duration until healing. Immunohistochemically, KKT increased CD-31-positive vessels in early phase and increased α-smooth muscle actin-(α-SMA-) positive fibroblastic cells in early phase and decreased them in late phase of wound healing. By Western blotting, KKT showed the potential to decrease inflammatory cytokines (MCP-1, IL-1β, and TNF-α) in early phase, decrease vascular endothelial growth factor in early phase and increase it in late phase, and modulate the expression of extracellular protein matrix (α-SMA, TGF-β1, bFGF, collagen III, and collagen I). These results suggested the possibility that KKT accelerates pressure ulcer healing through decreases of inflammatory cytokines, increase of angiogenesis, and induction of extracellular matrix remodeling.

Reproducibility of transcutaneous oxygen pressure measurements in persons with spinal cord injury

OBJECTIVES

To assess the reproducibility and the effects of the subjects' characteristics on the reproducibility of transcutaneous oxygen pressure (TcPO2) measurements in the sacral area in persons with spinal cord injury during loading in the supine position.

DESIGN

Test-retest study.

SETTING

Physical medicine and rehabilitation center.

PARTICIPANTS

Thirty spinal cord-injured American Spinal Injury Association grade A subjects.

MAIN OUTCOME MEASURES

Two TcPO2 monitoring sessions in the sacral area during loading in the supine position were performed at 24-hour intervals, including the measurement of absolute resting sacral and chest TcPO2 values and the calculation of regional perfusion index (RPI) and delta from rest oxygen pressure, taking into account systemic TcPO2 changes.

RESULTS

The intraclass coefficient of the sacral TcPO2 absolute resting value, RPI, and delta from rest oxygen pressure was .787 and .798, .704 and .635, .760 and .465, respectively, at 20 and 40 minutes. The only characteristic with an influence on RPI reproducibility was the subject's smoking status, whereas age, weight, time since injury, lesion level, and presence of pressure ulcer showed no influence.

CONCLUSIONS

TcPO2 measurement is a reproducible method for assessing cutaneous microcirculation during loading over 20-minute monitoring sessions, with RPI exhibiting better reproducibility than delta from rest oxygen pressure at 40 minutes.

The transport profile of cytokines in epidermal equivalents subjected to mechanical loading

Pressure ulcer risk assessment might be optimized by incorporating the soft tissue reaction to mechanical loading in the currently used risk assessment scales. Cytokines, like IL-1alpha, IL-1RA, IL-8, and TNF-alpha, might be used to determine this tissue reaction, since they are released after 24 h of mechanical loading of epidermal equivalents. In the current study, the release and transport of these cytokines with time was evaluated. Epidermal equivalents were subjected to 20 kPa for different time periods (1, 2, 4, 6, 8, 16, and 24 h). Compared to the unloaded control group, a significant increase was found for IL-1alpha (4.7-fold), IL-1RA (4.8-fold), and IL-8 (3.6-fold) release after 1 h loading. For TNF-alpha, the release was significantly increased after 4 h of loading (5.1-fold compared to the unloaded situation), coinciding with the first signs of gross structural tissue damage. These cytokine values were determined in the surrounding medium and a transport model was developed to evaluate the distribution of cytokines inside the culture. These simulations revealed that all IL-8 and TNF-alpha was released from the keratinocytes, whereas most of the IL-1alpha and IL-1RA remained inside the keratinocytes during the 24 h loading period. In conclusion, IL-1alpha, IL-1RA, and IL-8 appear promising biochemical markers for pressure ulcer risk assessment, since their release is increased after 1 h of epidermal loading and before the onset of structural tissue damage.

The effects of pressure and shear on capillary closure in the microstructure of skeletal muscles

Deep tissue injury (DTI) is a severe pressure ulcer, which initiates in muscle tissue under a bony prominence, and progresses outwards. It is associated with mechanical pressure and shear that may cause capillaries to collapse and thus, induce ischemic conditions. Recently, some investigators stipulated that ischemia alone cannot explain the etiology of DTI, and other mechanisms, particularly excessive cellular deformations may be involved. The goal of this study was to evaluate the functioning of capillaries in loaded muscle tissue, using animal and finite element (FE) models. Pressures of 12, 37, and 78 kPa were applied directly to one gracilis muscle of 11 rats for 2 h. Temperatures of the loaded and contralateral muscles were recorded with time using infrared thermography (IRT) as a measure of the ischemic level. In addition, a non-linear large deformation muscle-fascicle-level FE model was developed and subjected to pressures of 12-120 kPa without and with simultaneous shear strain of up to 8%. For each simulation case, the accumulative percentage of open capillary cross-sectional area and the number of completely closed capillaries were determined. After 2 h, temperature of the loaded muscles was 2.4 +/- 0.3 degrees C (mean +/- standard deviation) lower than that of the unloaded contralateral limbs (mean of plateau temperature values across all pressure groups). Temperature of the loaded muscles dropped within 10 min but then remained stable and significantly higher than room temperature for at least 30 additional minutes in all pressure groups, indicating that limbs were not completely ischemic within the first 40 min of the trials. Our FE model showed that in response to pressures of 12-120 kPa and no shear, the accumulative percentage of open capillary cross-sectional area decreased by up to 71%. When shear strains were added, the open capillary cross-sectional area decreased more rapidly, but even for maximal loading, only 46% of the capillaries were completely closed. Taken together, the animal and FE model results suggest that acute ischemia does not develop in skeletal muscles under physiological load levels within a timeframe of 40 min. Since there is evidence that DTI develops within a shorter time, ischemia is unlikely to be the only factor causing DTI.

Periodically relieving ischial sitting load to decrease the risk of pressure ulcers

OBJECTIVE

To investigate the relieving effect on interface pressure of an alternate sitting protocol involving a sitting posture that reduces ischial support.

DESIGN

Repeated measures in 2 protocols on 3 groups of subjects.

SETTING

Laboratory.

PARTICIPANTS

Twenty able-bodied persons, 20 persons with paraplegia, and 20 persons with tetraplegia.

INTERVENTIONS

Two 1-hour protocols were used: alternate and normal plus pushup. In the alternate protocol, sitting posture was alternated every 10 minutes between normal (sitting upright with ischial support) and with partially removed ischial support (WO-BPS) postures; in the normal plus pushup protocol, sitting was in normal posture with pushups (lifting the subject off the seat) performed every 20 minutes.

MAIN OUTCOME MEASURE

Interface pressure on seat and backrest.

RESULTS

In WO-BPS posture, the concentrated interface pressure observed around the ischia in normal posture was significantly repositioned to the thighs. By cyclically repositioning the interface pressure, the alternate protocol was superior to the normal plus pushup protocol in terms of a significantly lower average interface pressure over the buttocks.

CONCLUSIONS

A sitting protocol periodically reducing the ischial support helps lower the sitting load on the buttocks, especially the area close to ischial tuberosities.

The Prevention and Treatment of Pressure Ulcers

Pressure ulcers remain a significant secondary complication for many individuals with spinal cord injury (SCI). Technological advances have the potential to affect both the prevention and treatment of pressure ulcers. The focus of this article is hi-tech devices and methodologies. The current state-of-the-art methods are discussed and conceptual approaches are presented.

Viscoelastic Properties of Ovine Adipose Tissue Covering the Gluteus Muscles

Pressure-related deep tissue injury (DTI) is a life-risking form of pressure ulcers threatening immobilized and neurologically impaired patients. In DTI, necrosis of muscle and enveloping adipose tissues occurs under intact skin, owing to prolonged compression by bony prominences. Modeling the process of DTI in the buttocks requires knowledge on viscoelastic mechanical properties of the white adipose tissue covering the gluteus muscles. However, this information is missing in the literature. Our major objectives in this study were therefore to (i) measure short-term (HS) and long-term (HL) aggregate moduli of adipose tissue covering the glutei of sheep, (ii) determine the effects of preconditioning on HS and HL, and (iii) determine the time course of stress relaxation in terms of the transient aggregate modulus H(t) in nonpreconditioned (NPC) and preconditioned (PC) tissues. We tested 20 fresh tissue specimens (from 20 mature animals) in vitro: 10 specimens in confined compression for obtaining the complete H(t) response to a ramp-and-hold protocol (ramp rate of 300mm∕s), and 10 other specimens in swift indentations for obtaining comparable short-term elastic moduli at higher ramp rates (2000mm∕s). We found that HS in confined compression were 28.9±14.9kPa and 18.1±6.9kPa for the NPC and PC specimens, respectively. The HL property, 10.3±4.2kPa, was not affected by preconditioning. The transient aggregate modulus H(t) always reached the plateau phase (less than 10% difference between H(t) and HL) within 2min, which is substantially shorter than the times for DTI onset reported in previous animal studies. The short-term elastic moduli at high indentation rates were 22.6±10kPa and 15.8±9.4kPa for the NPC and PC test conditions, respectively. Given a Poisson’s ratio of 0.495, comparison of short-term elastic moduli between the high and slow rate tests indicated a strong deformation-rate dependency. The most relevant property for modeling adipose tissue as related to DTI is found to be HL, which is conveniently unaffected by preconditioning. The mechanical characteristics of white adipose tissue provided herein are useful for analytical as well as numerical models of DTI, which are essential for understanding this serious malady.

Measuring tissue perfusion during pressure relief maneuvers: insights into preventing pressure ulcers

BACKGROUND/OBJECTIVE

To study the effect on tissue perfusion of relieving interface pressure using standard wheelchair pushups compared with a mechanical automated dynamic pressure relief system.

DESIGN

Repeated measures in 2 protocols on 3 groups of subjects.

PARTICIPANTS

Twenty individuals with motor-complete paraplegia below T4, 20 with motor-complete tetraplegia, and 20 able-bodied subjects.

METHODS

Two 1-hour sitting protocols: dynamic protocol, sitting configuration alternated every 10 minutes between a normal sitting configuration and an off-loading configuration; wheelchair pushup protocol, normal sitting configuration with standard wheelchair pushup once every 20 minutes.

MAIN OUTCOME MEASURES

Transcutaneous partial pressures of oxygen and carbon dioxide measured from buttock overlying the ischial tuberosity and interface pressure measured at the seat back and buttocks. Perfusion deterioration and recovery times were calculated during changes in interface pressures.

RESULTS

In the off-loading configuration, concentrated interface pressure during the normal sitting configuration was significantly diminished, and tissue perfusion was significantly improved. Wheelchair pushups showed complete relief of interface pressure but incomplete recovery of tissue perfusion.

CONCLUSIONS

Interface pressure analysis does not provide complete information about the effectiveness of pressure relief maneuvers. Measures of tissue perfusion may help establish more effective strategies. Relief achieved by standard wheelchair pushups may not be sufficient to recover tissue perfusion compromised during sitting; alternate maneuvers may be necessary. The dynamic seating system provided effective pressure relief with sustained reduction in interface pressure adequate for complete recovery of tissue perfusion. Differences in perfusion recovery times between subjects with spinal cord injury (SCI) and controls raise questions about the importance of changes in vascular responses to pressure after SCI.

Assessment of mechanical conditions in sub-dermal tissues during sitting: A combined experimental-MRI and finite element approach

A common but potentially severe malady afflicting permanent wheelchair users is pressure sores caused by elevated soft tissue strains and stresses over a critical prolonged period of time. Presently, there is paucity of information regarding deep soft tissue strains and stresses in the buttocks of humans during sitting. Strain and stress distributions in deep muscle and fat tissues were therefore calculated in six healthy subjects during sitting, in a double-donut Open-MR system, using a "reverse engineering" approach. Specifically, finite element (FE) models of the undeformed buttock were built for each subject using MR images taken at the coronal plane in a non-weight-bearing sitting posture. Using a second MR image taken from each subject during weight-bearing sitting we characterized the ischial tuberosity sagging toward the sitting surface in weight-bearing, and used these data as displacement boundary conditions for the FE models. These subject-specific FE analyses showed that maximal tissue strains and stresses occur in the gluteal muscles, not in fat or at the skin near the body-seat interface. Peak principal compressive strain and stress in the gluteus muscle were 74+/-7% and 32+/-9 kPa (mean+/-standard deviation), respectively. Peak principal compressive strain and stress in enveloping fat tissue were 46+/-7% and 18+/-4 kPa, respectively. Models were validated by comparing measured peak interface pressures under the ischial tuberosities (17+/-4 kPa) with those calculated by means of FE (18+/-3 kPa), for each subject. This is the first study to quantify sub-dermal tissue strain and stress distributions in sitting humans, in vivo. These data are essential for understanding the aetiology of pressure sores, particularly those that were recently termed "deep tissue injury" at the US National Pressure Ulcer Advisory Panel (NPUAP) 2005 Consensus Conference.

Wavelet analysis of skin blood oscillations in persons with spinal cord injury and able-bodied subjects

OBJECTIVE

To assess the blood oscillations in the skin over the ischial tuberosity (high-risk area for pressure ulcer) using spectral analysis of laser Doppler flowmetry signals based on wavelet transform.

DESIGN

Wavelet analysis of skin blood oscillations in persons with spinal cord injury (SCI) and able-bodied subjects.

SETTING

Seating and body support interface laboratory.

PARTICIPANTS

Ten men were recruited for this study, of whom 5 were able-bodied subjects (age, 31.2+/-3.3 y) and 5 were persons with SCI (age, 37.2+/-7.3 y).

INTERVENTIONS

External pressure of 16.0 kPa (120 mmHg) was applied to the ischial tuberosity via 1 specifically designed pneumatic indentor. The loading duration was 30 minutes.

MAIN OUTCOME MEASURES

Skin blood flow was monitored for 10 minutes prior to loading and 20 minutes after the prescribed loading period. With spectral analysis based on wavelet transform, 5 frequency intervals were identified (.01-.02, .02-.06, .06-.15, .15-.40, .40-2.0 Hz) corresponding to endothelial related metabolic, neurogenic, myogenic, respiratory, and cardiac activities, respectively.

RESULTS

The relative amplitude of the metabolic component for persons with SCI was significantly lower (F=5.26, P=.032) during the resting conditions as compared with able-bodied subjects. During the postloading period, the response of oscillatory activities was evidently lower in the skin over the ischial tuberosity for persons with SCI when compared with able-bodied subjects. In addition, the relative amplitude of the neurogenic component (.02-.06 Hz) during postloading was significantly lower for persons with SCI (F=5.44, P=.029).

CONCLUSIONS

These findings suggest that the contributions of endothelial related metabolic and neurogenic activities to the blood perfusion regulation become relatively less for persons with SCI during the resting and postloading periods, respectively.

Effects of prolonged surface pressure on the skin blood flowmotions in anaesthetized rats—an assessment by spectral analysis of laser Doppler flowmetry signals

The objective of this study is to assess the effect of prolonged surface compression on the skin blood flowmotion in rats using spectral analysis based on wavelets transform of the periodic oscillations of the cutaneous laser Doppler flowmetry (LDF) signal. An external pressure of 13.3 kPa (100 mmHg) was applied to the trochanter area and the distal lateral tibia of Sprague-Dawley rats via two specifically designed pneumatic indentors. The loading duration was 6 hours/day for 4 consecutive days. Five frequency intervals were identified (0.01-0.04 Hz, 0.04-0.15 Hz, 0.15-0.4 Hz, 0.4-2 Hz and 2-5 Hz) corresponding to endothelial related metabolic, neurogenic, myogenic, respiratory and cardiac origins. The absolute amplitude of oscillations of each particular frequency interval and the normalized amplitude were calculated for quantitative assessments. The results showed that (1) tissue compression following the above schedule induced significant decrease in the normalized amplitude in the frequency interval of 0.01-0.04 Hz both in the trochanter area (p < 0.001) and tibialis area (p = 0.023), (2) prolonged compression induced significant increase in the absolute amplitude (p = 0.004 for the trochanter area and p = 0.017 for the tibialis area) but significant decrease in the normalized amplitude (p = 0.023 for the trochanter area and p = 0.026 for the tibialis area) in the frequency interval of 0.15-0.4 Hz, and (3) at the tibialis area, the flowmotion amplitude (frequency interval 0.15-0.4 Hz) measured prior to the daily tissue compression schedule was found to be significantly higher on day 4 than the measurements obtained on day 1. However, this finding was not observed at the trochanter area. Our results suggested that prolonged compression might induce endothelial damage and affect the endothelial related metabolic activities.

Can near infrared spectroscopy measure the effect of pressure on oxygenation of sacral soft tissue?

OBJECTIVE

To test whether near infrared spectroscopy (NIRS) is applicable for the examination of the influence of external pressure on oxygenation of the soft tissues in the sacral area.

METHOD

Tissue oxygenation was measured in 33 healthy volunteers in the prone position. A NIRS probe was positioned over the sacrum and external pressure was applied in 10 mmHg increments, from 20 mmHg to 200 mmHg and then decreased. At each level, tissue oxygen saturation (StO2) was measured. To test reproducibility, the protocol was repeated in six volunteers, in whom the thickness of the soft-tissue envelope at different levels of external pressure was assessed using ultrasound.

RESULTS

There was wide variability in StO2 courses between the 33 subjects, with a non-linear relationship between pressure and StO2. The only consistent finding was that the StO2 was significantly higher after decreasing pressure than at the initial pressure of 20 mmHg, which is indicative of reactive hyperaemia. Despite the application of high external pressures, reasonable tissue oxygenation was maintained in 19 of 33 subjects. Reproducibility of the measurements was poor. Comparison of soft-tissue thickness with corresponding StO2 values showed that, with increasing pressure, the percentage decrease in tissue thickness was higher than the decrease in tissue oxygenation.

CONCLUSION

This study confirms that NIRS is not useful for assessing tissue oxygenation in pressure ulcer research due to unacceptable inter-individual variability and poor reproducibility of measurements.

Compression-induced damage in a muscle cell model in vitro

Soft tissue breakdown can be initiated at the muscle layer associated with bony prominences, leading to the development of pressure ulcers. Both the magnitude and duration of pressure are important factors in this breakdown process. The present study utilizes a physical model, incorporating C2C12 mouse myoblasts in a homogeneous agarose gel, to examine the damaging effects of prolonged applied pressure. Identical cylindrical cores cut from the agarose/cell suspension were subjected to two separate compressive strains, of 10 and 20 per cent. The strain was applied for time periods ranging from 0.5 to 12 hours, using a specially designed loading apparatus. After each compression period, sections taken from the central horizontal plane of the individual constructs were stained using either haematoxylin and eosin or with the fluorescent probes, Calcein AM and ethidium homodimer-1, and assessed for cell damage. It was found that constructs subjected to the higher strain values demonstrated significantly higher values of non-viable cells for equivalent time points compared to the unstrained constructs. Further analysis on sections using the DNA nick-translation method suggested that this increase was primarily due to apoptosis. These findings imply a relationship between the duration of applied compression and damage to muscle cells seeded in the gel, which was particularly apparent at the strain level of 20 per cent, equivalent to a clinically relevant pressure of 32 mmHg (4.3 kPa). Such an approach might be useful in establishing damage threshold levels at a cellular level.

In Vivo Muscle Stiffening Under Bone Compression Promotes Deep Pressure Sores

Pressure sores (PS) in deep muscles are potentially fatal and are considered one of the most costly complications in spinal cord injury patients. We hypothesize that continuous compression of the longissimus and gluteus muscles by the sacral and ischial bones during wheelchair sitting increases muscle stiffness around the bone-muscle interface over time, thereby causing muscles to bear intensified stresses in relentlessly widening regions, in a positive-feedback injury spiral. In this study, we measured long-term shear moduli of muscle tissue in vivo in rats after applying compression (35 KPa or 70 KPa for 1∕4–2 h, N=32), and evaluated tissue viability in matched groups (using phosphotungstic acid hematoxylin histology, N=10). We found significant (1.8-fold to 3.3-fold, p<0.05) stiffening of muscle tissue in vivo in muscles subjected to 35 KPa for 30 min or over, and in muscles subjected to 70 KPa for 15 min or over. By incorporating this effect into a finite element (FE) model of the buttocks of a wheelchair user we identified a mechanical stress wave which spreads from the bone-muscle interface outward through longissimus muscle tissue. After 4 h of FE simulated motionlessness, 50%–60% of the cross section of the longissimus was exposed to compressive stresses of 35 KPa or over (shown to induce cell death in rat muscle within 15 min). During these 4 h, the mean compressive stress across the transverse cross section of the longissimus increased by 30%–40%. The identification of the stiffening-stress-cell-death injury spiral developing during the initial 30 min of motionless sitting provides new mechanistic insight into deep PS formation and calls for reevaluation of the 1 h repositioning cycle recommended by the U.S. Department of Health.