Principles of Pressure Management

Douglas A. Hobson, Ph.D.

RERC on Wheeled Mobility*
Department of Rehabilitation Science and Technology
University of Pittsburgh

*support by: the National Institute on Disability and Rehabilitation Research



Pressure Sore Incidence and Costs

  • 40% occur in hospital- (lying ulcers-sacrum, heels, elbows)
  • 30% occur post discharge (sitting ulcers -sacrum, ischium)
  • Cause of mortality in about 4% of SCI people
  • Repair costs-$15-60k/ incident
  • Social/psychological costs immeasurable!!


Populations at Risk

  • Spinal Cord Injured
  • Immobile Elderly
  • Diabetics
  • ALS, MS, MD
  • Immobility is greatest risk factor

Risk assessment scales have been developed (e.g., Braden scale, Norton scale)




The Etiology of Pressure Ulcers (prime factors)

  • If repeated assault occurs, the breakdown process will begin.
  • Deep tissue distortion due to shear forces also major contributing factor.
  • Surface sores can result from repeated surface friction/abrasion.





Pressure Tolerance Guidelines


The Etiology of Pressure Ulcers (prime factors)

  • If repeated assault occurs, the breakdown process will begin.
  • Deep tissue distortion due to shear forces also major contributing factor.
  • Surface sores can result from repeated surface friction/abrasion.



Other Contributing Factors

  • Immobility- close correlation between onset and frequency of movement.
  • Loss of sensation-evidence that denervated tissue has reduced nutrition and oxygen perfusion.
  • Body Type- does not seem to be any relationship between body-type and deep ulcers. Surface friction may increase with heavier people.
  • Nutrition-inadequate dietary intake can result in muscle atrophy, anemia, low protein levels and vitamin C deficiency reducing skin integrity.
  • Infection-increased metabolic rate, higher oxygen demand endangers ischemic tissue.



Typical Superficial Ulcer Development




Development of Typical Deep Ulcer






Five Clinical Stages of Pressure Development

  • Stage I - blanching of reddened area experiencing reactive hyperemia
  • circulation intact, redness disappears in reasonable time (1-2 hrs)
  • Stage II - blanching does not occur, redness remains
  • Stage III - ulceration progresses beyond the dermis to subcutaneous tissue
  • redness remains around edges, hardening of tissue occurs
  • Stage IV - ulceration progresses to fat layer and muscle becomes swollen.
  • Stage V - necrosis (dying tissue) penetrates deep fascia, muscle and sometimes bone. Surgical repair necessary, requires 2-6 months of inactivity.




Pressure Ulcer Risk Factors



Principles of Clinical Pressure Management

  • Maximize the surface area: decrease the pressure on any one location (peak pressures)
  • Redistribute body weight: use support surface shape and materials with required properties.
  • Minimize asymmetries: i.e., unequal loading of pelvic structures and tissues.

Training for pressure relief: i.e., regular weight relieving movements

  • Dietary instruction: critical to both good health and skin integrity
  • Instruction for lifting/ transferring: whenever possible person should instruct care-givers
  • Personal hygiene and skin care: very important!




Skin Viability Measurement Techniques

  • Skin blanching
  • Measurement of interface pressure (single site and mapping)
  • Thermography
  • Oxygenation of blood flow
  • Measurement of tissue deformation - Brienza

Measurement tissue stiffness - Brienza



Pressure Measurement Technologies


  • Electropneumatic bladder


  • Pneumatic bladder

  • Fluid filled bladder

  • Thick film resist. Element


  • Capacitive element



Single Cell vs. Mapping Devices


  • Single bladder type devices are:
    • more accurate and repeatable
    • more difficult to use
    • provide limited information

allow single or continuous measurements

  • Mapping devices:
    • provide posture and relative pressure information
    • allow graphical displays
    • are quicker to use and provide much more information
    • are more expensive

allow single or continuous measurements




Clinical Applications (Lipka, 1997)

  • Objective representation of peak pressure
  • Differential comparison of support surfaces
  • Effectiveness of weight shifting interventions
  • Wheelchair configuration set-up
  • Clinical validation



Pressure Mapping Systems



Pressure Mapping as Biofeedback



Shifting to One Side



Shifting Forward


Comparative Study Ferguson-Pell & Cardi (1992)

  • Three computer-based devices.
  • Four different cushion types.

Results, comparisons of readings between different cushions, same subject, may produce errors as much as 10mmHg.




Interpretation of Pressure Data:
A Few Precautions

  • Comparison of absolute pressure values can be misleading.
  • Measurement device may be altering the distribution of surface forces (e.g, hammocking).

Relative pressure comparisons are probably most useful.



Cushion Types



Key Properties of Cushion Materials
(Sprigle, 1992)

  • Density - weight/volume ratio.
  • Stiffness - measure of softness.
  • Resilience - ability to recover shape.
  • Dampening - absorb impact loads.

Envelopment - surface area covered.



Positioning for Pressure and Postural Management

  • planer vs. sling seat surface
  • provide appropriate pressure relieving cushion
  • incline seat (5-10°)
  • firm contoured back, reclined 10-20°

match backrest height to user needs.

  • add lumbar pad (optional).
  • adjust arm and foot rests for optimal weight distribution.
  • provide weight relief accessories to wheelchair (recline/tilt) as necessary.

provide training on weight relief and use of seating and/or wheelchair system.




  • Ferguson-Pell, M. & Cardi. (1992). Pressure Mapping, XXX, Oct., 30-35
  • Ferguson-Pell, M. & Cardi. (1993). Development and comparative evaluation of W/C pressure mapping. Assistive Technology. 5:78-91
  • Sprigle, S. (1993). Using Seat Contour during Seating Evaluations of Individuals with SCI, Assistive Technology. 5:24-35.

Brienza, D.; Karg, P. & Brubaker, C. (1996). Seat cushion design for elderly wheelchair users based on minimization of soft tissue deformation using stiffness and pressure measurements. IEEE Transactions on Rehabilitation Engineering, 4(4)320-328.