Defining ‘active’ pressure redistribution
01/09/12 | Complex wounds, Infection | Lyn Phillips, Richard Goossens, Makoto Takahashi, Michael Clark
This paper will explore the design principles of ‘active’ (alternating) support surfaces, discuss how specific characteristics might influence physiology, pathophysiology and the prevention of pressure ulcers, and introduce the rationale for a standardised performance measurement.
Pressure ulcer prevention guidelines routinely include the prescription of regular patient repositioning, and a pressure-redistributing surface for beds and chairs. However, selecting a support surface from the rapidly expanding list of available options is difficult. Reliable information from high quality clinical trials is scant, the terminology for reporting support-surface performance is confusing and the measurements used to describe product performance are not, as yet, standardised.
Nevertheless, an informed prescription requires a basic understanding of support surface functionality and an appreciation that surfaces are not generic with respect to clinical performance.
What is ‘active’ therapy?
Since 2007, the National Pressure Ulcer Advisory Panel (NPUAP), and, more recently, the European Pressure Ulcer Advisory Panel (EPUAP), has classified support surfaces into one of two functional categories, as determined by the primary method of pressure redistribution [Fig 1].
Included here for clarity, these range from simple foam, gel and non-powered, air-filled surfaces, through to powered low-air-loss and air-fluidised beds. Measurable performance characteristics include immersion into, and envelopment by, the supporting materials mentioned above. By increasing the surface area that supports the body, the applied pressure is lowered, however, unless the patient is repositioned, the pressure remains constant and may still be sufficient to occlude the circulation to the tissue. This modality has been covered in a previous issue of this publication and will, therefore, not be discussed further
These are powered devices designed to periodically redistribute pressure by repeatedly loading and unloading the pressure beneath the body. Unloading, or pressure removal, is typically achieved through the alternate inflation and deflation of a series of air-filled cells, giving rise to the more traditional description of ‘alternating therapy’ or ‘alternating pressure air mattress’. Unlike reactive surfaces, cyclical pressure redistribution continues even in the absence of patient movement, although the degree of off-loading varies by device.
The purpose, form and function of active pressure redistribution can best be described by first revisiting standard physiology. Essentially, as terrestrial mammals, human beings are naturally exposed to periods of relatively high, non-uniform, pressure. Even so, most do not develop tissue injury thanks to complex and highly successful, protective physiological mechanisms, including spontaneous movement; a subconscious behavioural response, which redistributes pressure several times each hour, even during sleep.
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. However, when 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. Severe pressure damage can occur in less than two hours in the most vulnerable patient, hence the importance of time in any 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 that both duration and magnitude of pressure are prioritised.
In 2007, NPUAP (USA) published a list of performance criteria for all therapeutic support surfaces, 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].
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 existing mattress or presented as a single-layer overlay to be placed on top of the existing foam mattress.
- Active (alternating) surfaces have a unique ‘signature’ described by cycle frequency, duration, amplitude and rate of change
- Physiological outcome can differ significantly in response to unique performance characteristics
- Evidence arising from one active surface cannot be automatically attributed to another surface
- Standardised performance tests and field studies are required to enable informed selection
- There is no single clinically safe pressure-time threshold
- Design goals aim for time-sensitive, complete or near-complete off-loading
- Active surfaces are recommended for both prevention and treatment
- Active therapy is the modality of choice for patients who cannot be regularly repositioned.