A comparative assessment of interface pressures generated by four surgical theatre heel pressure ulcer prophylactics

Abstract

Current heel protection devices used in the operating room do not comply with the consensus document of the European and National (North American) Pressure Ulcer Advisory Panels. A complying prototype has been tested. Prospective cohort study comparing interface pressures. While using the prototype device, the heel interface pressure is significantly [mean 0·0 mmHg, standard deviation (SD) 0·0] less than the viscose elastic gel (VEG) mat (mean 174·8 mmHg, SD 64·5), the Action^® heel block (mean 182·3 mmHg, SD 70·8) and the theatre table (mean 193·2 mmHg, SD 57·1). At the Achilles tendon, the prototype device (mean 16·2 mmHg, SD 19·0) is significantly superior to the Oasis (mean 183·7 mmHg, SD 67·4) and Action^® heel blocks (mean 112·3 mmHg, SD 64·7). At the lateral malleolus, the prototype device (mean 0·0, SD 0·0) is better than the Action^® (mean 24·3 mmHg, SD 53·4) and Oasis heel blocks (mean 20·9 mmHg, SD 49·2). At the calf, the prototype (mean 53·7 mmHg, SD 23·0) imposed more pressure than all other devices tested but was not statistically significant compared with the theatre table or the VEG mat. It is possible to design a device that protects the heel, lateral malleolus and Achilles tendon without causing hyperextension of the knee and consequent popliteal vein compression, thereby complying with the above guidelines.

INTRODUCTION

All patients who are anaesthetised and supine are prone to developing heel and ankle pressure ulcers. In October 2009, the National Pressure Ulcer Advisory Panel (NPUAP) and the European Pressure Ulcer Advisory Panel (EPUAP) (the peak pressure ulcer panels in North America and Europe, respectively) issued a consensus document, with a section dedicated to prevention of pressure ulcers which originate in the operating theatre. Within that section, there is a paragraph dedicated to care of the foot. It states:

Elevate the heels completely (offload them) in such a way as to distribute the weight of the leg along the calf without putting all the pressure on the Achilles tendon. The knee should be in slight flexion. Hyperextension of the knee may cause obstruction of the popliteal vein and this could predispose the individual to deep vein thrombosis (1).

The question we ask in this study is whether a device that complies with the requirements of the EPUAP/NPUAP consensus document generates lower interface pressures compared with those devices currently in use.

A pressure ulcer is an area of localised damage to the skin and/or the underlying tissue usually over a bony prominence, as a result of pressure, or pressure in combination with shear (1). Although it is commonly thought to be a slow process, in actual fact it is an acute injury. Capillary filling pressure is approximately 32 mmHg (2), and it is assumed that interface pressures above this may lead to ischaemia and eventual necrosis of the skin even in the absence of vascular disease. However, Le et al. (3) showed that the pressure within the tissue close to bone is three to five times the interface pressure. Thus, devices used to measure interface pressures are likely to underestimate pressures at the point where the deep tissue injury occurs. This helps to explain the deep tissue injury, which is usually the precursor to the pressure ulcer itself. It may also help to explain why, in areas where there is no bone (e.g. the calf), pressures above this level do not generally lead to ulceration. Despite these limitations with interface pressure readings, Gefen points out that the well‐studied interface pressure curves (4), Reswick and Rogers and the more accurate sigmoid curve, differ at the extremes only. It turns out that the differences between the various devices that we tested were so large that this limitation was not considered a problem.

Pressure ulcers are common and are costly to the health budget. The prevalence of pressure ulcers in inpatients in the UK is 4·4% and their short‐ and long‐term management costs the NHS about £1·4–£2·1 billion annually (5). Studies from the USA suggest that the cost of treatment of hospital acquired pressure ulcers is $US9·1–$US11·6 billion (6). This figure includes inpatient medical management of the condition as well as long‐term rehabilitation with the largest proportion of the cost in nursing care. Approximately, 25% of these ulcers begin in the operating theatre (7) and heel and ankle pressure ulcers constitute 34% of all pressure ulcers (8). This means that heel and ankle pressure ulcers which begin in the operating theatre cost the US health system between $US760 and $US960 million each year.

Devices that are currently in use for the prevention of heel and ankle pressure ulcers either offload (such as the Oasis) or cradle the heel (Action^® heel block) or try to distribute the pressure on a pad such as a viscose elastic gel (VEG) mat. Heel offloading devices allow the knee to drop back and this causes popliteal vein compression in up to 64% of supine and anaesthetised patients (9). A prototype elevation device has been designed specifically for use in the operating theatre and to satisfy the requirements of the EPUAP/NPUAP consensus statement (1). It has been designed to offload the heel, distribute the weight of the leg along the calf, protect the skin over the Achilles tendon, protect the ankle and prevent popliteal vein compression by flexing the knee slightly.

METHODS

The Ethics Committee of the South East Sydney and Illawarra Area Health Service and the Wollongong University approved this study. One hundred and sixteen consecutive subjects (61 male and 55 female) were recruited at an outpatient vascular laboratory. Each subject was tested on each device. All subjects consented and completed this study. Age and body mass index (BMI) were recorded. Three commonly used pressure ulcer prophylactic devices and a prototype were evaluated. These were the Action^® Heel Support (Figure 1), Oasis Elite VEG heel block (Figure 2), Action^® Overlay VEG mat (Figure 3) and the prototype leg elevation device (Viater^® Medical, Sydney, Australia –Figure 4). A standard theatre table mattress was also evaluated (Denyers International, Melbourne, Australia). Interface pressure was measured for the three devices, the VEG pad and the theatre table. Measurements were taken at the heel, Achilles tendon, lateral malleolus and calf.

Figure 1.

Action^® Heel Support.

Figure 2.

Oasis Elite Viscose Elastic Gel heel block.

Figure 3.

Action^® Overlay Viscose Elastic Gel mat.

Figure 4.

Prototype elevation device.

The XSensor X3 (XSensorTechnology Corporation, Alberta, Canada) pressure mapping system was used to measure peak interface pressures at each of the four anatomical sites. It was calibrated as per the specification sheet. The maximum recordable pressure was 256 mmHg. Readings were recorded manually from the output screen and pictorial representations of the data were also collected. Subjects lay on an operating theatre mattress. Each device was placed in turn. Measurements were taken 2 minutes after the device was put in place and the subject was comfortable. This was to allow the pressure to stabilise. Three measurements were taken for each device and the mean used for evaluation.

STATISTICS

On the basis of a previous pilot study, a basic power analysis of 95% confidence interval was used to calculate the minimum sample size required to be likely to detect the effects being investigated here. In some cases, the effect was large enough that power was large with small samples, such as the effect size when comparing the pressure on the heel when using prototype as compared with a theatre mattress. Conversely, larger samples were required (after power analysis was complete) for smaller effect sizes, for example, when comparing the pressure on the Achilles when using prototype as compared with the Action heel block. Interestingly, in the final study, the effect size was larger than expected in favour of the prototype. The results were analysed using Excel 2003 with add‐in Analyse‐it. A normal distribution was assumed because, according to the Mean Value Theorem, our sample size is sufficient to make that assumption. Student's t tests were used to compare results.

RESULTS

There were 61 males and 55 females with an average height, weight and BMI of 169 cm, 78 kg and 27·3 (Table 1). Average age was 56 years. Typical pressure maps are shown in 5, 6. The mean pressures and standard deviations for each of the devices at each of the anatomical locations are shown in Table 2, and the corresponding Pvalues are shown in Table 3.

Table 1.

Demographic data with mean and standard deviations

	Height (cm)	Weight (kg)	Age	Body mass index
Mean	169	78·1	55·7	27·3
Standard deviation	8	14·5	18·3	4·7

Figure 5.

Typical pressure maps for four devices (‘Block 1’ is the Action^® Heel Support, ‘Block 2’ is the Oasis Elite Viscose Elastic Gel heel block).

Figure 6.

Typical pressure map for the prototype.

Table 2.

Mean pressures (mmHg) and standard deviation (SD) recorded for each of the devices and the respective anatomical positions

	Heel		Achilles tendon		Lateral malleolus		Calf
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
Viscose elastic gel	174·8	64·5	0·2	2·6	0·0	0·0	42·9	19·7
Theatre table	193·2	57·1	0·4	3·8	0·4	2·2	48·4	32·9
Oasis	0·2	2·0	183·7	67·4	20·9	49·2	17·6	24·5
Action	182·3	70·8	112·3	64·7	24·3	53·4	27·4	14·7
Prototype	0·0	0·0	16·2	19·0	0·0	0·0	53·7	23·0

Table 3.

Levels of significance for each device versus the prototype elevation device

	Heel	Achilles	Lateral malleolus	Calf
Viscose elastic gel mat	P < 0·0001		Could not be calculated	ns
Theatre table	P < 0·0001		ns	ns
Oasis block	ns	P < 0·0001	P < 0·0001
Action® block	P < 0·0001	P < 0·0001	P < 0·0001

There was no significant difference at the heel between the two offloading devices (the Oasis and the prototype). These two devices generated significantly less pressure at the heel compared with the VEG mat, the Action^® heel block and the theatre table mattress. The median pressure for the offloading devices was 0 mmHg at the heel.

The theatre mattress and Action^® VEG mat applied no pressure to the Achilles tendon because the heel lifted the tendon off the surface. The prototype elevation device applied significantly less pressure to the Achilles tendon than the Action heel block and the Oasis block.

The average pressure applied by both the prototype and the Action^® VEG mat at the lateral malleolus was zero, being significantly lower than both the Action^® heel block and the Oasis.

The prototype elevation device applied more pressure than all other devices at the calf, because of the way the weight of the leg was distributed. The median pressure exerted by the elevation device was 53·7 mmHg compared with 48·4 mmHg exerted by the theatre table. This was not statistically significant.

DISCUSSION

Pressure ulcers are a common, costly and largely preventable problem in most Western countries (10). Approximately, 25% of all hospital acquired pressure ulcers begin in the operating theatre with the deep tissue injury occurring at the time of surgery (7). The ulcer is not visible till several days or weeks later. The injury is due to pressure and for this reason heel offloading is considered the best way to prevent heel pressure ulcers (1). There are many devices on the market, but they generally fall into three design groups. These devices may:

• 1offload the heel by supporting the Achilles tendon (Oasis heel block),
• 2cradle the heel in a gel or foam (Action^® heel block) and
• 3provide a surface which distributes the load and decreases peak pressures (Action^® VEG pad).

There is no doubt that offloading is the best way to prevent heel pressure ulcers, but the issue then becomes how to achieve it without creating other problems. Offloading the heel with current offloading devices allows the knee to drop back unsupported. This causes popliteal vein compression, particularly in supine anaesthetised patients 9, 11. This in turn is thought to increase the likelihood of DVT 9, 11. Offloading can create high pressure on the Achilles tendon.

The offloading devices generate no pressure on the heel. The Oasis does this at the expense of high pressures on the Achilles tendon. The elevation device also generated the increased pressures on the Achilles tendon, but they are low, well below capillary filling pressure. The Action^® VEG mat, the theatre table and the elevation device created higher pressures on the calf (43, 48 and 53 mmHg, respectively, and not significantly different from each other) compared with the other devices. Gefen showed that muscle withstands loads of around 50 mmHg for long periods (4). As discussed above, the pressures over the Achilles tendon and the bony prominences are significantly higher than would be predicted using the Reswick and Rogers interface pressure curve potentially pushing pressure over bony prominences further into the unacceptable range. This is in contradistinction to the pressure over the calf muscle, where pressures are not amplified (there is no bony interface). Higher pressure on the calf was anticipated when trying to distribute the weight of the ankle and foot more evenly.

It is possible to design a device as recommended in the EPUAP/NPUAP consensus document. Such a device does generate a more favourable distribution of pressure than those tested. It is considered likely that avoiding hyperextension of the knee will result in lower incidence of perioperative DVT 9, 11.