Overlays or mattresses to prevent pressure sores?

In this issue of the BMJ (pp 1413, 1416) the Pressure Trial Group reports the results of a randomised controlled trial of two methods of preventing pressure ulcers and an economic analysis of that trial.¹,² The European Pressure Advisory Panel defines a pressure ulcer as an area of localised damage to the skin and underlying tissue caused by pressure, shear, friction, or a combination of these.³ Regardless of aetiology the problem of pressure ulcers in poorly mobile patients is common and can be encountered in many health settings, including at home, in community care, and in intensive care.^4-6 Incidence varies from 0% to 17% for home care, 2.2% to 23.9% for long term care, 0.4% to 38% for hospitals, and 8% to 79% for intensive care.⁵,⁶

Pressure ulcers are a source of distress to patients, are costly to manage, and can lead to litigation.⁴ Their severity is scored by standardised grading systems.³ Many of the predictive risk scores have reasonable sensitivity but limited specificity and may only be useful for particular groups of patients.³,⁵ These imperfect tools mean that moderately expensive preventive technologies are applied to many more patients than will actually require them. The limited ability to target these interventions decreases their cost effectiveness.

The problem of pressure ulcers is given little space in established textbooks of pathology, medicine, surgery, or intensive care—perhaps reflecting a lack of interest by doctors.³,⁷ A recent literature review identified only three randomised controlled trials in the intensive care literature over a 20 year period,³ and a Cochrane review identified only 41 randomised controlled trials across all clinical settings, including accident and emergency departments as well as operating theatres.⁴ The systematic review concluded that the relative effectiveness of alternating pressure surfaces was unknown.

In this context we should welcome the PRESSURE trial, which investigated two preventive technologies which have a large difference in acquisition costs. This was a large randomised controlled trial of almost 2000 poorly mobile patients at high risk of developing pressure ulcers in a variety of settings but excluding intensive care. The interventions compared were alternating pressure mattresses and alternative pressure overlays (the mattresses costing four times as much to buy as the overlays). The trial was methodologically rigorous, using a variety of scores to identify patients at risk and to grade their ulcers. It also used appropriate tools to adjust for factors that might bias outcomes.

This study found little difference between the two devices. Notably, fewer patients expressed dissatisfaction with mattresses than with overlays. The trial also provided further empirical evidence of important risk factors for pressure ulcers.

The lack of difference between these technologies is, perhaps, unsurprising. Given the similarity of the technologies the expected 50% reduction in pressure ulcers may have been too optimistic. Moreover, 349 patients (18%) did not receive the intended device, and 600 patients (30%) were changed from the mattress to which they were randomised. A per protocol analysis might have provided additional insights into the relative efficacy of these devices.⁸ Choice of trial size often involves pragmatic considerations as well as statistical science.⁹

The economic evaluation conducted as part of this trial also used robust methods, and in many respects the authors went as far as they could with the data available.² The sample size of the trial meant that at conventional 5% significance levels there was no evidence of a difference in either costs or days free from pressure ulcers. This is a common problem with randomised controlled trials, which are rarely adequately powered with respect to economic outcomes.

One of the limitations with the economic evaluation was the reliance on a single clinical outcome: days free from pressure ulcers. It is unclear whether this measure captures all the benefits that might be important to patients. The economic evaluation still provides useful information for decision makers, however, in finding that the alternating pressure mattresses have an approximately 85% chance of being considered cost effective compared with the alternating pressure overlays.² This means that a decision to use alternating pressure mattresses has abouta1in7 chance of being wrong.

The PRESSURE trial and its economic analysis, despite some limitations, is welcome and will help healthcare providers to make better decisions when buying alternating pressure devices. It also raises the standard for research in the neglected but important field of preventing and treating pressure ulcers.