Of 6155 people assessed for eligibility, 1972 were randomised from 10 of the 11 hospitals between January 2001 and April 2004 (). One patient was randomised twice and therefore excluded, providing an intention to treat population of 1971 people. The two groups were well balanced for baseline characteristics ().
Flow of participants through trial
Baseline characteristics of participants allocated to alternating pressure overlay or alternating pressure mattress. Values are numbers (percentages) unless stated otherwise
Overall, 207 (10.5%) people developed a total of 305 new pressure ulcers, most of which (n = 297) were grade 2 (97.4%). Eight people developed grade 3 ulcers: three in the overlay group and five in the mattress group. More of the acute patients developed a new pressure ulcer than the elective patients; medical or elderly patients were more likely to develop a new pressure ulcer than vascular or orthopaedic patients, and more people with an existing grade 2 ulcer than without at randomisation developed a new ulcer ().
Development of new pressure ulcers by minimisation factors
One hundred and six (10.7%) people in the overlay group and 101 (10.3%) people in the mattress group developed one or more new grade 2 pressure ulcers. The difference in the proportions of people with a new pressure ulcer (overlay - mattress) was 0.4% (95% confidence interval - 2.3% to 3.1%). In the adjusted analysis the odds ratio for developing a new pressure ulcer using the overlay compared with the mattress was 0.94 (95% confidence interval 0.68 to 1.29) indicating no difference between the surfaces in the odds of developing a new pressure ulcer (P = 0.70, logistic regression; ).
Adjusted analysis of proportion of patients developing a new pressure ulcer of grade 2 or worse
Factors that had a significant effect on the proportion of people developing a new pressure ulcer were admission for an acute condition, the presence of a wound, skin trauma or non-blanching erythema on any site at baseline, age, haemoglobin level, and diabetes ().
No statistically significant differences between the groups were found for development of a pressure ulcer within 30 days (10.0% overlay v 9.3% mattress, P = 0.58, χ2 test) or median time to development of a new pressure ulcer for the mattress group (56 days 95% confidence interval 48 days, upper limit not estimable); the median for the overlay group had not been reached (P = 0.76, log rank test). The groups had similar total areas for ulcers, with most new ulcers being small (median area 1.1 cm2, range 0.1-68.1 cm2). No evidence was found of a difference in median ulcer area between the groups (1.2 cm2 overlay v 1.1 cm2 mattress, P = 0.98, Mann-Whitney U test).
In total 416 people requested one or more changes for comfort and other device related reasons: 230 (23.3%) people in the overlay group and 186 (18.9%) in the mattress group. Significantly more people allocated overlays requested a change (difference in proportions 4.4%, 95% confidence interval 0.7% to 7.9%; P = 0.02, χ2 test).
Healing of existing pressure ulcers
Overall, 113 people had pressure ulcers at randomisation and, of these, 39 (35%) healed during the trial: 34% (n = 20) in the overlay group and 35% (n = 19) in the mattress group. No statistically significant differences were found between the groups for median time to healing (20 days in both groups, P = 0.86, log rank test) or the maximum final ulcer grade per patient (P = 0.96, Cochran-Armitage test for trend).
In total, 349 (17.7%) participants were not placed on the assigned surface at baseline: of these a device was unavailable for 43% (n = 151) and an alternative surface was allocated for 19% (n = 66). More participants (n = 10) randomised to an overlay than to a mattress (n = 2) were not placed on the assigned surface because of a clinical decision. In addition, ward staff initiated the change in surface for 149 (15.1%) people in the overlay group and 142 (14.5%) people in the mattress group. Some evidence was found of ward nurse preference for the replacement surface, with people perceived to be at high risk but assigned to overlay provided with a replacement mattress at baseline or subsequently changed to a replacement mattress. Furthermore, mattresses were more likely than overlays to be removed from a trial patient and allocated to a patient considered needier. The results of the cost effectiveness analysis are reported separately.6