Technology update Defining ‘active’ pressure redistribution


a


100 150 200 250


50 0 Mattress A Mattress B Mattress C


existing mattress or presented as a single- layer overlay to be placed on top of the existing foam mattress.


Cycle duration and frequency Cycle duration is the time taken to complete one inflation-deflation cycle and will depend on both the speed of air transfer and the number of cells in each sequence. Typically, but not exclusively, cycle duration ranges from 7.5 to 20 minutes, with 10 minutes perhaps the most common. Cycle frequency is generally sequential in that one cycle follows another, but some devices have a ‘periodic’ active phase where one or more cycles are followed by a reactive or ‘static’ interval, hence, it is important to be able to differentiate between frequency and duration when selecting a device.


Cycle amplitude Amplitude, or the range between the highest and lowest pressure applied to the skin during the inflation and deflation cycle [Fig 2], is a primary design consideration and varies widely. In order to achieve the lowest pressure on the skin during cell deflation, the cells adjacent to the deflating cell need to contain sufficient air pressure to provide support to the patient. This will depend on the construction of the cells themselves, how the quickly the air flows between them and, critically, whether the pump has sufficient power to support the patient during the inflation phase. At the same time, the deflating cell needs


sufficient clearance to reduce contact with the skin. It is important to note that tightly fitted covers and sheets can create an artificial ‘hammock’ across the deflating cell, reducing the potential benefit derived from off-loading[11]


.


Rate of change One important characteristic of the inflation-


b P=<0.0001 P=<0.0001


2000 4000 6000 8000 10000


0


P=<0.03


P=<0.01


Mattress A


Mattress B Figures 3a nd 3b. Comparing interface pressure (IP) and heel perfusion in three different active mattresses[17] .


deflation process is the ‘rate of change’, or speed of the air transfer during the fill-empty-fill cycle, as this directly influences the duration of the off-loading phase. If air is moved slowly out of a cell, it takes longer to achieve the minimum pressure (off-loading) point, but slower air transfer does have the advantage that the cell movement is less noticeable for the patient. As with all surfaces, there has to be a balance between therapeutic performance and patient acceptability and comfort.


CLINICAL SIGNIFICANCE As each patient presents with a unique and changing risk profile, it is not possible to determine a universally ‘safe’ pressure-duration threshold for each individual[9][12]


. A principal


design goal is, therefore, to mimic the protective effect of repositioning by periodically reducing contact with the support surface to a level that is as low as practically achievable for as long as possible. Off-loading cycles ideally occur several times each hour to reduce the risk of ischaemia- reperfusion injury; a condition associated with vessel occlusion (closure or blockage) for as little as one to two hours[13]


. While the optimal cycle duration has yet to be


determined, volunteer studies suggest shorter cycle times (five minutes) to be marginally favoured over longer cycles[14]


. There will be a


physiological cut-off point where the cycle is either too fast for the tissue to reperfuse or too slow to prevent ischaemia. The cycle has to be long enough for full


reperfusion, which is particularly important for patients with vascular pathology and a longer oxygen recovery index[13][15]


. Similarly, patients


with spinal injuries have been shown to require off-loading for up to five minutes to fully restore cellular function[16]


, making a minimum 10-minute www.woundsinternational.com www.woundsinternational.com 55


Mattress C


References


10.Bansal C, Scott R, Stewart D, Cockerell CJ. Decubitus ulcers: A review of the literature. Int J Derm, 2005; 44(10): 805–10.


11. Swain I, Bader D. The measurement of interface pressure and its role in soft tissue breakdown. J Tissue Viability, 2002; 12(4): 132–46.


12. Loerakker S, Manders E, Strijkers GJ et al. The effects of deformation, ischemia, and reperfusion on the development of muscle damage during prolonged loading. J Appl Phys, 2011; 111(4): 1168–77.


13. Jiang L, Qian T, Wang Y et al. Ischemia-Reperfusion Injury- Induced Histological Changes Affecting Early Stage Pressure Ulcer Development in a Rat Model. Ostomy Wound Management, 2011; 57(2): 55–60.


14.Mayrovitz H, Sims N. Effects of different cyclic pressurization and relief patterns on heel skin blood perfusion. Adv Skin Wound Care, 2002; 15(4):158–64.


15.Masaki N, Sugama J, Okuwa M et al. Heel Blood Flow During Loading and Off- Loading in Bedridden Older Adults With Low and Normal Ankle-Brachial Pressure Index A Quasi-Experimental Study. Biol Res Nurs, 2012 Apr 23. [Epub ahead of print]


Technology and product reviews


mmHg


Perfusion (Arbitrary units)


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