50-58 owm1007 reger · dr. reger is a biomedical engineer and director for rehabilitation...

50 OstomyWound Management

FEATURE

Soft tissue breakdown disables an estimated 1.3 to

3 million patients per year in the US.1,2 The total

cost of associated healthcare is $8 billion per

year.3 Miller and Delozier4 estimated that in 1992,

treating patients with pressure ulcers cost $1.335 bil-

lion (average charge was $21,675). It is universally

accepted that a reduction of pressure between the

body and the support surface interface will reduce or

prevent the occurrence of pressure ulcers. Landis’5

observation (using a microinjection method) that 32

mm Hg capillary pressure is a threshold above which

pressure ulcer occurs is an often-used industry guide-

line for testing the effectiveness of a support surface.

The healthcare industry and healthcare providers have

been active in developing new products and treatment

guidelines to reduce the occurrence of pressure ulcers.

Currently, more than 200 support surfaces6 are

available that aim to either redistribute or reduce

interface pressures to below the 32 mm Hg threshold.

These pressure-relieving and pressure-reducing

Support Surface InterfacePressure, Microenvironment, andthe Prevalence of Pressure Ulcers:An Analysis of the LiteratureSteven I. Reger, PhD, CP; Vinoth K. Ranganathan, MS, MBA; and Vinod Sahgal, MD

External pressure is the most frequently considered stress factor in the formation of ulcers. A review and analysis of existing litera-ture addressing the relationship between pressure ulcer prevalence and interface pressures at various anatomic sites was conduct-ed. Results suggest a nearly non-existent or slightly negative correlation between interface pressure and ulcer prevalence in generaland spinal cord injured populations, respectively. Despite limitations of the analysis methods used, the observed lack of a direct rela-tionship confirms the results of other studies and suggests that ulcer formation also may involve factors secondary to pressure andmechanical factors (eg, temperature, moisture, duration of the applied load, atrophy, and posture). Based on currently available infor-mation, clinicians should include these considerations when selecting a support surface. Studies directly relating primary stress fac-tors and tissue viability with prevalence and incidence of pressure ulcers are needed to better understand the benefits of pressure-relieving support surfaces and to improve the effectiveness of prevention and treatment.

KEYWORDS: ischemic necrosis, interface pressure, shear, prevalence, tissue breakdown

Ostomy Wound Management 2007;53(10):50–58

This article is adapted from a presentation at the Case Western Reserve University School of Medicine conference,Evidence-based Practice in Wound Care, held in Cleveland, Ohio, September 2006.

Dr. Reger is a biomedical engineer and Director for Rehabilitation Technologies; Mr. Ranganathan is Program Manager for Research;and Dr. Sahgal is Department Chairman, Department of Physical Medicine and Rehabilitation, Cleveland Clinic, Cleveland, Ohio. Pleaseaddress correspondence to: Steven I. Reger, PhD, Department of Physical Medicine and Rehabilitation, Cleveland Clinic, 9500 EuclidAvenue /C21, Cleveland, Ohio 44195; email: regers@ccf.org.

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products have been widely applied with the hope of

reducing pressure ulcer frequency. The effectiveness

of support surfaces is usually tested by their ability to

reduce interface pressures in healthy populations.

Many studies7-17 have been published by researchers,

clinicians, and the support surface industry to aid

healthcare staff in choosing the right support surface

for their patients. These studies mostly evaluate or

compare the different support products using inter-

face pressure measurements alone, mainly because

this method is readily available, easy to use, non-

invasive, and reasonably accurate.9,10,11,13

Despite such efforts, data from a 2003 nationwide

inpatient sample18 indicate that the rate of hospital

stays related to pressure ulcers has increased 63% from

280,000 cases in 1993 to 455,000 cases in 2003; of these

patients, 72% with pressure ulcers were 65 years of age

and older and 19% were between 45 and 64 years of

age.18 The five most common conditions for pressure

ulcer-related hospital stays were septicemia, pneumo-

nia, urinary tract infection, aspiration pneumonitis,

and congestive heart failure. Common concomitant

conditions for patients admitted primarily for pres-

sure ulcers were paralysis, spinal cord injury, substance

abuse, malnutrition, multiple sclerosis, stroke, and

senility.18 The average charge for treating pressure

ulcers was nearly $37,800 per person for a total cost of

approximately $17.2 billion in the year 2003. The

prevalence (the number of patients with ulcers divid-

ed by the number of patients at risk for ulcers) and

incidence (the number of patients who develop a pres-

sure ulcer after admission to a hospital) of pressure

ulcers in acute care settings has remained

steady at approximately 15.3% and 7.6%,

respectively, for the period 1999 to

2004.19 This has led many researchers to

revisit the role played by pressure in the

development of pressure ulcers and the

effectiveness of pressure-relieving sup-

port surfaces.

Due to the paucity of other relevant

data, healthcare providers continue to

rely on research studies that primarily

address interface pressure to select pres-

sure-relief support devices. They fail to

adequately consider all the other factors

contributing to the formation of ischemic necrosis —

ie, temperature, moisture, duration of the applied

load, atrophy, and posture. As a result, support sur-

faces provide mixed, or in some cases, no benefits for

patients. Correlating the interface pressure measured

for various pressure-relieving support surfaces with

relevant prevalence or incidence information will be

useful in understanding the role of pressure in reduc-

ing pressure ulcers and the effectiveness of pressure-

relieving systems.

The objective of this overview is to critically review

the literature in order to analyze the relationship

between interface pressure and pressure ulcer occur-

rence at various anatomic sites and to briefly review

the role of other factors in the development of pres-

sure ulcers.

MethodsA MEDLINE® search of English-language literature

published from 1975 through December 2006 was

conducted to identify publications that addressed

either interface pressures on pressure-relieving sup-

port surfaces or pressure ulcer prevalence in the gen-

eral population and spinal cord injured (SCI) patients.

Criteria for article selection included: reports of inter-

face pressure or prevalence values for at least three

anatomic locations on the body, identification of sup-

port surfaces tested, and presentation of data in an

easily analyzable format (tables or graphs). The search

identified eleven publications7-17 (see Table 1) that

reported interface pressures and six19-24 (see Table 2)

that reported pressure ulcer prevalence information at

October 2007 Vol. 53 Issue 10 51

KEY POINTS• Making appropriate support surface decisions remains a complicated

task, in part because support surface characteristics and methods usedto test them vary widely.

• This review suggests that the most commonly reported effect of thesesurfaces, a reduction in tissue interface pressure, may not translate intoreduced pressure ulcer incidence and prevalence rates.

• While awaiting the results of support surface effectiveness studies, clini-cians should consider the effects of these surfaces on all pressure ulcerrisk factors.

Ostomy Wound Management 2007;53(10):50–58

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52 OstomyWound Management

TABLE 1STUDIES REPORTING TISSUE INTERFACE PRESSURE USING

VARIOUS SUPPORT SURFACES

First author/reference

Lilla et al (1975)7

Boorman et al(1981)8

Berijan et al(1983)9

Bar (1991)10

Allen et al(1993)11

Hedrick-Thompsonet al (1993)12

Allen et al(1994)13

Collier (1996)14

Sachse (1998)15

Fontaine et al(1998)16

Hardin et al(2000)17

Subjects/position

10 SCI patients, lyingdown supine position

Nine patients, lying supine

28 cancer patients, lyingdown supine position

25 SCI patients, sittingposition

Six healthy subjects, lyingdown supine position

15 healthy volunteers, lyingdown supine and lateralposition

10 healthy volunteers, lyingdown supine and sittingposition

90 patients from a generalward, lying down supineposition

10 healthy volunteers lyingdown supine and lateralposition

11 healthy subjects, lyingdown supine and lateralposition

Six healthy subjects,supine and lateral positions

Support surface

Standard mattresses andfour types of water beds

Air-fluidized mattress, stan-dard mattress

Eight different surfaces (twodynamic, six static)

Foam, gel, and air seatingcushion

Segmented foam mattress

Standard mattress and sixtypes of pressure-reducingmattress

Two continuous airflow mat-tresses and two alternat-ing pressure air mattresses

Standard mattress andseven types of foam mat-tresses

Standard mattress and fivetypes of pressure reliefmattresses

Alternating pressure overlay,air filled mattress and fluidoverlay

Dynamic low-air-loss andstatic fluid mattress

Average InterfacePressure (mm Hg)

Occiput (44)Sacrum (41)Knee (67)Heel (65)Occiput (30)Scapula (11)Sacrum (15)Buttock (24)Heel (70)Sacrum (29)Dorsal spine (25)Trochanter (94) Heel (65)Ischium (71)

Occiput (51)Scapula (24)Elbow (28)Sacrum (20)Buttock (29)Heel (68)Sacrum (20)Trochanter (35)Heel (68)Occiput (53)Scapula (19)Elbow (26)Sacrum (18)Buttock (31)Heel (77)Scapula (14)Sacrum (29)Trochanter (71)Heel (61)Sacrum (43)Trochanter (73)

Scapula (12)Sacrum (15)Trochanter (30)Heel (14)Sacrum (28)Trochanter (54)Heel (44)

Comments

Comparison study. Pressure measuredusing a manometer with 2-cm diam-eter bladder

Evaluation study. Pressure measuredusing a pressure sensor (28-mmdiameter)

Support surface evaluation study.Pressure measured using amanometer with a 1 sq inch inflat-able bladder

Evaluation study. Pressure measuredusing two 28-mm diameter electrop-neumatic-type sensors

Reliability study.Pressure measured using a Talley

SA500 Pressure Evaluator with 28-mm diameter sensor pad

Comparison study. Pressure measuredusing Oxford pressure Monitor (20-mm diameter sensors)

Evaluation study. Pressure measuredusing a Talley SA500 PressureEvaluator with 28-mm diameter sen-sor pad

Evaluation study.Pressure measured using Talley

Pressure Monitor 3 (96 sensors)

Evaluation study. Multimodality (pres-sure, TcPO2, and blood flow).Pressure measured using a 15 x 15(60 cm x 60 cm) sensor array

Comparison study — lab setting.Measured pressure (Talley SD-500)and shear (RIK shear sensor)

Evaluation study. Pressure measuredusing X-sensor pressure mappingsystem

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various anatomic sites for either the general popula-

tion or SCI patients.

To gauge the relationship between reported inter-

face pressure and ulcer prevalence, the pressure ulcer

prevalence information and tissue inter-

face pressure reported (summarized in

Table 1 and Table 2 and shown in the

Figures) was weighted based on the

reported sample size and then averaged

for each anatomic location. This weight-

ed average was used to perform correla-

tion analysis between interface pressure

and prevalence of pressure ulcers at var-

ious anatomic locations. The sample

sizes for studies reporting interface

pressures were small compared to the

prevalence studies.

Many inconsistencies and a lack of

standardization were noted among the

studies analyzed. For studies reporting

interface pressure, the interface pres-

sures for each location over all surfaces

tested were averaged because not

enough information about the tested

support surfaces or methodology was

available to more reliably compare stud-

ies. Also, the intention was not to evalu-

ate or compare support surfaces, but

rather to assess the relationship between

various pressure-relieving support

devices and the prevalence of pressure

ulcers in the acute care setting. To fur-

ther investigate the role of pressure on

pressure ulcer prevalence, anatomic

locations were selected for which both

interface pressure and prevalence infor-

mation was available for the general as

well as the SCI patient population.

ResultsThe occurrence of pressure ulcers and

the corresponding measurements of

body interface pressures at different

anatomic regions are highly variable

depending on tissue health, thickness,

support characteristics, and method of

measurement. Few papers reported interface pressure

at more than three locations; the locations most

reported are the sacrum, trochanter, and heels. Only

one paper7 reported data on more than two locations

October 2007 Vol. 53 Issue 10 53

TABLE 2STUDIES AND LITERATURE REVIEWS REPORTINGPRESSURE ULCER PREVALENCE AND ANATOMIC

LOCATION DISTRIBUTION

Author(s)/reference

El-Toraei andChung (1977)20

Noble (1981)21

Yarkony andHeinemann(1995)22

Cuddigan et alNPUAP (2001)23

Garber andRintala (2003)24

Whittington andBriones (2004)19

Subjects

2,500 SCI patients

2,626 SCI patients

14,704 SCI patients

42,817 patients in acutecare facilities

553 SCI patients

31,969 patients in acutecare (2004)

22,455 patients in acutecare (2000)

Prevalence (%)

n/a

11%

n/a

14.8%

39%

16%

16%

Anatomic locationdistribution

Sacrum (15%)Ischium (21%)Trochanter (19%)Scapula (2%)Elbow (2%)Sacrum (16%)Ischium (31%)Trochanter (10%)Malleolus (10%)Heel (11%)Occiput (3%)Scapula (4%)Elbow (2%)Sacrum (37%)Ischium (9%)Trochanter (4%)Knee (2%)Malleolus (8%)Heel (16%)Occiput (1%)Scapula (2%)Elbow (7%)Sacrum (37%)Ischium (8%)Trochanter (5%)Knee (3%)Malleolus (6%)Heel (30%)Sacrum (14%)Ischium (35%)Trochanter (14%)Feet/Ankle (26%)Elbow (4%)Sacrum (29%)Ischium (16%)Heel (25%)

Elbow (5%)Sacrum (26%)Ischium (13%)Heel (25%)

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in the SCI population. Figure 1 shows the distribution

of pressure ulcers at various anatomic locations for the

general population and SCI patients. Except for

occiput, elbow, and heels, the distribution of pressure

ulcers was similar between the two populations.

The weighted average interface pressures at various

anatomic locations as reported in the publications list-

ed in Table 1 are shown in Figure 2. As can be expect-

ed, the interface pressure for healthy volunteers and

the general population is much lower than in the SCI

population25 except for the occiput. A correlation

analysis was performed between the interface pres-

sures and the prevalence for four locations (occiput,

sacrum, ischium, heel) where data were available for

both the general population and SCI patients (see

Table 3). A slightly negative correlation was found for

both the general population and SCI patients. Both

groups had 31% of occurrence of distribution for

pressure ulcers at the sacrum but SCI patients had a

70% higher interface pressure

compared to the general popula-

tion. Regarding heel ulcers, the

general population had a much

higher occurrence (27% versus

16%) than SCI patients, while the

reported interface pressure was

actually lower (61 mm Hg versus

65 mm Hg).

DiscussionThe data presented have to be

interpreted with caution due to the

limitations and shortcomings of

using data from multiple sources,

years, surfaces, patient groups, investigators, and meas-

urement technologies. Missing data, small sample sizes,

and lack of consistency in methods used further limit

data interpretation. However, until data from a ran-

domized controlled study with continuous or frequent

measurements of interface pressure at various anatom-

ic sites and collection of confounding factors and ulcer

incidence data are available, the information presented

here may help develop new research theories and

increase understanding of the role of pressure redistri-

bution in the development of these ulcers.

Despite the limitations discussed, the analysis pre-

sented suggests that no direct or positive relationship

exists between interface pressure and the distribution

of pressure ulcers at various anatomic locations. This

observation suggests that other factors, such as sup-

port surface microenvironment, influence ulcer for-

mation at these anatomic sites.

More than 100 biomechanical and pathophysiolog-

ic risk factors for ischemic skin and soft tissue necro-

sis have been identified.1 Of these, external pressure

has been the most frequently discussed risk factor in

the formation of ulcers. Other primary stress factors

associated with ulcer formation are shear,26 friction,

and the resulting deformation of soft tissues.

Secondary or environmental factors important in bed

immobility are temperature, moisture, duration of the

applied load, atrophy, and posture. These factors influ-

ence tissue quality by reducing the strength and the

rigidity of soft tissues and increase the friction coeffi-

cient of the skin.

54 OstomyWound Management

TABLE 3CORRELATION ANALYSIS BETWEEN REPORTED INTERFACE

PRESSURE AND PRESSURE ULCER OCCURRENCE DISTRIBU-TION (WEIGHTED AVERAGES) FOR THE GENERAL POPULATION

AND SCI PATIENTS AT SELECTED ANATOMIC LOCATIONS

Location

OcciputSacrumIschiumHeel

Correlation Coeff

Occurrence in% (range)

131 (26-37)12 (8-17)27 (23-30)

Interface Pressurein mm Hg (range)

49 (23-51)24 (12-43)26 (22-29)61 (14-102)

Occurrence in% (range)

3 31 (14-37)14 (9-35)16 (11-26)

Interface Pressurein mm Hg (range)

44417165

General Population Spinal Cord Injury Patients

-0.118 -0.191

TABLE 4PRESSURE INDUCED TISSUE INJURY

ACCELERATES WITH INCREASING BODYTEMPERATURE

Body Temperature25°C35°C45°C

OutcomeNo break downPartial-thickness injuryFull-thickness breakdown

Experimental pig model: 100 mm Hg pressure applied for 5 hours28

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Shear. Shear stress in soft tissues is gen-

erated by the tangential force component

of body weight on the contact area exter-

nally and by the parallel and opposite tan-

gential force on the bony prominence

internally. Tangential forces acting on the

skin develop shear stress in the tissues

through friction and cause the tissue layers

to slide against each other. The amount of

sliding depends on the looseness of the

connective fibers between tissue layers. If

the fibers are tight, the skin and subcuta-

neous tissue will be subjected to higher

shear stress; if the fibers are loose, more

sliding than shear stress will occur. Shear

stress is reduced by decreasing tangential

force and increasing contact area. Loose

covers and increased immersion in the

support medium also increase contact area

and further reduce shear stresses. When

shear-induced tissue sliding occurs, blood

vessels approaching the skin surface per-

pendicularly will bend and occlude at the

connective layers between the tissue

planes. Thus, shear will increase the effect

of pressure in reducing flow through the

blood vessels.27 Conversely, if shear stress is

reduced, tissues can tolerate higher pres-

sures without blood flow occlusion.

Friction. Friction describes the ability

of a surface to prevent motion due to

forces tangential to the contact area. The

tangential or frictional force depends on

the perpendicular force and the friction

coefficient independent of the contact area. When the

cover of the support surface is designed to allow

movement over its foundation, slippage occurs

between the cover and the bed and not within the tis-

sue layers; thus, tension (stretching) in the skin and

blood vessel occlusion are decreased due to a lower

friction coefficient (see Figure 3). The tangential force

is reduced most effectively by decreasing the friction

coefficient on the support surface. The effect of a high

friction coefficient is shown in Figure 4. The effective-

ness of properly inflated air, water, and viscous fluid or

gel supports is based on these principles.

Combinations of these biomechanical principles are

commonly used in modern support surfaces to create

a better physical environment for tissue survival.

Temperature. Elevated body temperature raises the

metabolic activity of tissues by 10% for every one

degree Celsius of temperature increase, concurrently

increasing the need for oxygen and an energy source at

the cellular level.28 If the patient has impaired circula-

tion from local pressure and shear, tissues will starve

and release lysozymes, inducing autodigestion of cyto-

plasma and reducing skin integrity. Metabolic activity

may cease from lack of energy and the accumulation of

October 2007 Vol. 53 Issue 10 55

Figure 1. Pressure ulcer occurrence distribution (weighted average) at variousanatomic locations for the general and SCI population. Standard deviations arenot shown if data were from a single study. Courtesy of the Cleveland Clinic.

Figure 2. Tissue interface pressure (weighted average) at various anatomic loca-tions for the general and SCI population. Standard deviations are not shown ifdata were from a single study. Courtesy of the Cleveland Clinic.

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waste products, compromising skin integrity.29 It has

been shown in animal studies30 that pressure-induced

tissue injury accelerates with increasing body tempera-

ture (see Table 4). Increases in skin temperature also

induce the sweat response, increasing the potential for

moisture to accumulate at the skin-support interface.

Moisture. Moisture from sweating or

incontinence will hydrate the skin, dissolve

the molecular collagen crosslinks of the

dermis, and soften the stratum corneum

(maceration). Skin maceration reduces

stiffness, threatening the near complete

loss of connective tissue strength and the

erosion of the dermis under the action of

shear forces. Another result of skin hydra-

tion is the rapid increase of the epidermal

friction coefficient, which promotes adhe-

sion of the skin to the support surface and

increases shear, easy sloughing, and ulcer-

ation. Compounding the destructive effect

of stress, excess moisture dilutes the skin’s

natural acidity, reducing antibacterial

properties of the epidermis and increasing

the threat of sepsis.29,30

Two excellent technologies for control-

ling the microclimate at the skin-support

surface interface are available. Low- and

high-air-loss31 and air-fluidized support

systems are designed to reduce stress and

temperature, facilitate moisture evapora-

tion, and prevent heat accumulation at the

skin-support surface interface. The evapo-

ration of 1 Kg of water from the skin at the

support surface will remove 580 Kcal of

heat from the body through the “latent

heat of vaporization.”28 Thus, for an aver-

age person with 1.8 m2 skin surface and

water loss of 26.7g/m2/hour, the cooling

power is 27.9Kcal/hour. With proper

design and nursing care aimed at main-

taining a physiologic water balance,

dynamic air loss support surfaces are able

to control interface pressure, shear, and

skin temperature and moisture.

Pressure. Pressure stress in soft tissues

arises from the force component perpen-

dicular to the external contact area and from body

weight acting through the nearest bony prominence.

In the design of support surfaces, the objective is to

increase contact area by greater “immersion,” allowing

the body to sink deeper into the surface, distributing

the force and reducing the pressure. Cyclic transfer of

56 OstomyWound Management

Figure 3. The lack of skin stretch in the case of low friction coefficient between thebed sheet and the mattress cover and the bed. Courtesy of the Cleveland Clinic.

Figure 4. Skin stretch on one side and folding of tissues on the side opposite to thedirection of movement (see inset) in the case of high friction coefficient between thebed sheet and the mattress cover and the bed. Courtesy of the Cleveland Clinic.

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weight from high-pressure areas is the main principle

underlying alternating pressure support systems but

resultant alternating pressure gradients, which are

related to shear stress, may damage adipose tissues and

capillaries that lack tensile strength.26,27

Pressure and shear stress have a similar effect on tis-

sues; both reduce blood flow and, subsequently, tissue

perfusion.31 A simple reduction of tissue stretching

(shear) can nearly double the ability of tissues to with-

stand pressure without the development of ischemia.27

With this fact in mind, many support surfaces are

designed to reduce shear, improving the weight-bear-

ing tolerance of soft tissues.32

Patients most affected. Pressure ulcers are most

prevalent in two groups of patients — those with neu-

rological disorders (SCI, stroke, head trauma) and the

elderly.2,21,23,24 The similarity is not surprising because

the loss of muscle strength, skin and muscle protein,

and muscle mass as a result of aging is similar to the

losses observed in patients with neurological disor-

ders. Skin changes in mechanical strength and suscep-

tibility to external loads and alterations in subcuta-

neous tissues that accompany aging and neurological

disorders indicate a significant reduction of tissue via-

bility from normally innervated tissues.

ConclusionThe analysis methods employed in this assessment

were limited because few studies33,34 have reported

both pressure and corresponding pressure ulcer inci-

dence or prevalence at various anatomic sites.

Available control data lack standardization (no stan-

dard “standard” mattress exists) and studies tend to

compare mattresses with different features from dif-

ferent manufacturers. As a result, a direct comparison

between most studies is not possible or accurate.

Unfortunately, healthcare providers rarely have data

other than interface pressure to evaluate and select

support surfaces for their patients. However, the

National Pressure Ulcer Advisory Panel (NPUAP) is

currently developing standardization methodology

for support surfaces through its Support Surface

Standardization Initiative (S3I).35 This analysis of pub-

lished data, despite its limitations, failed to demon-

strate a direct or positive relationship between meas-

ured interface pressures and the occurrence of pres-

sure ulcers at various anatomic locations. In the

absence of more definitive data, these observations

suggest that clinicians need to consider the ability of a

pressure-relieving support surface to not only redis-

tribute pressure but also to control microenviron-

ment. - OWM

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