Description of the condition
Pressure ulcers (also known as pressure sores, bed sores or decubitus ulcers) are defined as "a localized injury to the skin and/or underlying tissue usually over a bony prominence, as a result of pressure, or pressure in combination with shear" (EPUAP 2009; NPUAP 2009). Pressure ulcer classification systems provide a method by which the depth and severity of the injury can be described and documented (Appendix 1). Pressure ulcers have traditionally been categorised according to severity, from a category I pressure ulcer (non-blanching erythema (redness) of intact skin) up to category IV (full thickness tissue loss to bone or muscle). However, recently two additional categories have been defined - 'unstageable/unclassified' and 'suspected deep tissue injury' (EPUAP 2009; NPUAP 2009). As pressure ulcers are categorised according to depth of the wound, these categories are used when the depth of the wound is unknown. The 'unstageable' category describes wounds in which slough or necrosis (loose or dead tissue) obscures the wound bed. 'Suspected deep tissue injury' describes wounds in which it is suspected that there is deeper damage, such as when bruising or a blood blister is present. Although these additional categories were initially described for use in the USA, they have started to be used and investigated worldwide (EPUAP 2009; Gefan 2008).
It has been estimated that the mean cost of treating a pressure ulcer varies from GBP 1, 214 (Category 1) to GBP 14,108 (Category 4) (Dealey 2012), and the total cost in the UK is GBP 1.4 to 2.1 billion annually (4% of total NHS expenditure) (Bennett 2004). A more recent estimate of the annual costs in the United States is USD 9.1 to 11.6 billion (AHRQ 2011). The majority of the costs are due to nursing time, and more severe pressure ulcers have higher costs that relate to higher complication rates (e.g. infections or longer hospital stay). As well as having a financial cost, pressure ulcers have a massive impact upon health-related quality of life; their presence and treatment have been found to affect people's lives emotionally, mentally, physically and socially (Gorecki 2009; Spilsbury 2007).
It is theorised that when pressure ulcers develop, they start internally at the bone and progress outwards; this is because experimental and theoretical models have indicated that internal pressures near a bony prominence are three to five times higher than those experienced in the skin when under pressure (Gefan 2008; Le 1984). The heel and the sacrum are frequently reported as being the most common sites for pressure ulcers to develop (Amlung 2001; Barczak 1997; Whittington 2000), which is probably because these are areas where there is little subcutaneous tissue over the bones to provide padding to offset the forces of pressure or shear, or both.
Why is the heel such a high risk area?
The heel has a thickened dermis and a large fatty pad to the plantar aspect, which is well adapted to absorb shock from the calcaneum when walking and running. However, the posterior heel has a smaller surface area, little subcutaneous (tissue) volume, and no muscle to distribute pressure and provide cushioning over the bone. This leads to higher pressures being exerted directly over the bone when a person is in a supine, or seated position, with the heels on a foot stool. Therefore people who have reduced mobility, or who spend extended periods supine (e.g. patients in acute and long-term care facilities), have an increased risk for heel pressure ulcer development.
The skin over the posterior heel is supplied with oxygen by small branches from the calcaneal and peroneal arteries; the small size of the blood vessels in this area makes the skin more susceptible to ischaemia (lack of oxygen) when under prolonged pressure (Cichowitz 2009). Conditions that affect the circulation of the lower limb, such as peripheral vascular disease, also make the heel more susceptible to ischaemia as they reduce blood flow to the lower limb. Although circulatory conditions are commonly associated with older age, they can be present in younger people such as those with diabetes or hypertension, or smokers (Vogt 1992). Other circulatory problems, such as chronic venous disease and heart failure, can increase the risk of developing a heel pressure ulcer, as they lead to an increase in pedal oedema (swelling of the feet due to fluid accumulation), which impairs the delivery of oxygen and nutrients to the tissues, and also the disposal of metabolic waste products (Ryan 1969). Oedema also increases the weight of the limb, which, in turn, increases normal resting pressures.
When there is an acute reduction in the circulating volume of blood, subcutaneous tissue is one of the first tissues in which vasoconstriction (muscular narrowing of blood vessels) occurs, and the last to regain normal perfusion once the circulating volume has been restored (Gottrup 1987). This makes the feet and heels very susceptible to ischaemia during an acute illness, as the sympathetic nervous system and some medications preserve the body's organs through increasing the central circulating volume available to them, while decreasing the peripheral circulating volume.
Shear forces are a common problem in the acute and long-term care population due to poor positioning in beds or chairs, which can lead to the patient sliding downwards. Shear forces are also exerted when patients use their heel as a pivot point to reposition themselves. Friction can also cause an increased risk in heel pressure ulceration during poorly-conducted moving and handling of patients, or when patients are agitated or have tremors that can lead to their heels rubbing against bed sheets.
Peripheral neuropathy (reduced or altered ability to sense, in this case in the feet) is one of many complications of diabetes, resulting in significant morbidity and mortality (Callaghan 2012). Although neuropathy is most common amongst diabetics, it is also associated with other conditions such as alcoholism, stroke, demyelinating diseases such as multiple sclerosis, and conditions such as Guillain-Barré syndrome, which can have quite a rapid onset (White 2004). Peripheral neuropathy can lead to an increased risk of heel pressure ulcers, as people with neuropathy are unaware of pain and pressure, and so do not respond to them.
Description of the intervention
There are a number of different ways through which the extent and duration of pressure can be reduced, or removed, at the heel. This can be done through changing a person's position, using equipment that reduces pressure at the heel - or completely removes the pressure at the heel - or a combination of these methods. The equipment available includes whole body devices (e.g. mattresses) and devices specific to the foot (e.g. heel cups, booties or splints).
Mattresses and mattress overlays tend to come in two types - alternating pressure (AP) and constant low pressure (CLP). CLP mattresses include foam mattresses and overlays, low air-loss mattresses, air-fluidised bead beds and air overlays.
Heel-specific off-loading can include simple methods such as use of pillows, wedges or other aids to lift the heels off the bed, as well as specific splints, heel troughs or other medical devices that completely remove pressure from the heel.
Heel-specific CLP devices include foam or gel foot protectors or heel cups, air-filled foot protectors and sheepskin products.
Heel-specific low friction devices include dressings or booties designed to reduce friction and shear at the heel.
How the intervention might work
Risk of ulceration is thought to be related to both the amount of pressure on the skin, and the duration for which it is applied. High pressures for a short time do not cause harm, similarly low pressures for a long time are considered safe. The two approaches to reducing risk tend to work by either reducing the amount of pressure on the body, or the duration of the applied pressure. Whole body devices (mattresses or mattress overlays) generally fall into one of two categories - CLP or AP. CLP devices are thought to work by reducing the magnitude of the applied pressure by distributing the body weight over a larger surface area (as pressure is related both to the force applied, and to the area over which it is spread; mathematically, Pressure = Force/Area). AP devices reduce the duration of pressure by alternately inflating and deflating air-filled cells in a mattress over a set cyclical period.
Heel-specific devices also tend to fall into one of three categories - CLP devices, off-loading devices or low friction devices. CLP heel devices (e.g. a gel or foam heel cup or bootie) are designed either to reduce the magnitude of the applied pressure by spreading it over a larger area, or reduce the effects of the forces of friction or shear, or both. Off-loading devices are designed to remove the pressure of the 'at-risk' body site completely. This could be through using a pillow or wedge under the calf to leave the foot suspended above the mattress, or through supporting the foot or calf in a splint or trough, thereby leaving no pressure on the heel. Low friction devices consist of dressings or booties that do not reduce the magnitude of pressure at the heel, but are used to reduce risk of pressure ulcer development through reducing the forces of friction and shear. There are no heel-specific AP devices, although the heel section of some mattresses may alternate and work in this manner.
Why it is important to do this review
There are a number of different devices available that aim to prevent heel pressure ulcers. However, there appears to be a lack of evidence-based guidance in this area to assist practitioners in deciding which device or pressure-relieving method should be used specifically to reduce pressure ulcer incidence at the heel. This review will help to identify whether any device helps reduce the incidence, or prevents deterioration, of pressure ulcers that develop on the heel; and particularly, whether there is a device that could be considered to be the most effective in prevention of heel pressure ulcers in terms of incidence, health-related quality of life and cost.