Technology and product reviews Increasing contact area Key Points


1. Active (alternating) surfaces have a unique ‘signature’ described by cycle


frequency, duration, amplitude and rate of change


2. Physiological outcome can differ significantly in response to unique performance characteristics


3. Evidence arising from one active surface cannot be automatically attributed to another surface


4. Standardised performance tests and field studies are required to enable informed selection


5. There is no single clinically safe pressure- time threshold


6. Design goals aim for time-sensitive, complete or near-complete off-loading


7. Active surfaces are recommended for both prevention and treatment


8. Active therapy is the modality of choice for patients who cannot be regularly repositioned.


Unloading (Deflation)


1 in 2 Low amplitude cycle Duration of lowest pressure


Figure 2. Key performance characteristics: cycle frequency, duration, amplitude and rate of change. Unloading


pressure several times each hour, even during sleep[5]


. This periodic off-loading is followed by a period


of vessel dilatation, which serves to increase blood flow beyond that normally seen at baseline (reactive hyperaemia), reverse the hypoxic state and restore cellular equilibrium[6]


. However, when References


6. Lorenzo S, Minson CT. Human cutaneous reactive hyperaemia:


role of BKCa channels and sensory nerves. J Physiol. 2007; 585(1): 295–303


7. Reswick JB, Rogers JE. Experience as Rancho de los Amigos Hospital with devices and techniques to


prevent pressure sores. In: Kenedi RM, Cowden JM and Scales JT


Eds. Bedsore Biomechanics. 1976; Macmillan Press, London.


8. Exton-Smith AN, Sherwin MA. Prevention of pressure sores significance of spontaneous


body movements. Lancet. 1961; 278(7212): 1124–26.


9. Sprigle S. Sonenblum S. Assessing evidence support pressure redistribution of pressure for


pressure ulcer prevention: A review. J Rehab Dev, 2011; 48(3): 203–14.


motor, sensory or cognitive pathology results in partial or total immobility, the patient is exposed to increasing risk[7,8]


. The most accepted hypothesis is that


pressure injury develops when tissue located usually, but not always, over a bony prominence is exposed to forces of sufficient magnitude, direction or duration to result in tissue ischaemia, cell disruption and cell death[9]


. Severe pressure damage can occur


in less than two hours in the most vulnerable patient[10]


preventative strategy. As immobility is clearly a dominant risk


factor, the foundation of preventative care has traditionally been focused on pressure redistribution through assisted repositioning — a relatively effective, if labour intensive, activity when carried out diligently. Whether assisted or spontaneous, this repeated application and removal of pressure is, in part, simulated by active support surfaces, given


54 Wounds International Vol 3 | Issue 2 | ©W ol 3 | Issue 3 | ©W ol 3 | Issue 3 | ©Wounds International 2012 , hence the importance of time in any


of performance criteria for all therapeutic support surfaces[3]


time


that both duration and magnitude of pressure are prioritised. In 2007, NPUAP (USA) published a list


, which considered that,


alongside basic cell configuration, active surfaces have four clinically important, inter- dependent and measurable performance characteristics — cycle frequency, duration, amplitude and ‘rate of change’, ie the speed at which the cells shift between the inflated and deflated state [Fig 2].


Cell configuration Active surfaces typically operate on a one-in-two cycle giving a matched interval between the duration of loading (50%) and off-loading (50%). Less commonly, some devices operate a one-in-three or one-in- four sequence providing a larger supporting area either side of the single deflating cell. Other surfaces have different cell sequencing over different parts of the body, such as the sacrum and heel. Individual air cells can be of different


shape, depth and overall dimension and may be stacked in layers with variable functionality in each layer. Cells can be configured to such a depth as to replace the


Pressure redistribution Active Figure 1. Principal modalities for pressure redistribution.


Loading (Inflation)


Amplitude time Point of lowest pressure time 1 in 2 High amplitude cycle Reactive


e.g. cut, layered or formed foam, static air, gel, fibre, low air-loss, air fluidised


Alternate inflation deflation of support surfaces


e.g. alternating pressure air mattress


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