The study was conducted using a convenience sample of mattresses that were in regular use in 1 hospital in Norway between July and August 2018. The following mattresses, with the year they were put into use in parentheses, were included:
• standard compact foam hospital mattress (2011)
• standard compact foam hospital mattress (2014)
• standard compact foam hospital mattress with egg-crated surface (2018)
• new viscoelastic overlay (2018) placed on a standard hospital mattress from 2014
• worn viscoelastic full mattress (no date available).
The standard mattresses had a thickness of 13 cm, a weight limit of 120 to 130 kg, and density of 35 kg/m3. The manufacturer of all standard mattresses was Puls Brann (Norway); since 2018, the standard foam has an egg-crate surface layer.
Both viscoelastic mattresses had 7-cm thick viscoelastic foam with open cells and a high density of 85 to 110 kg/m3. The mattresses had a polyurethane shield and a weight limit of 100 kg. The full viscoelastic mattress had an 8-cm, high-quality foam layer as well. Both viscoelastic mattresses used in this study were manufactured by Tempur MED (Denmark).
Volunteer sample. Using convenience sampling methods, including mail and posters, 20 hospital employees were recruited to participate. All were healthy, mobile volunteers, and the only exclusion criterion was body temperature above 37.5°C.
All mattresses were tested with volunteers lying in the flat supine position for 10 minutes on all 5 surfaces. The order of mattress selection for each participant was random. The website www.randomization.com was used for the random order, and each order was put in a sealed envelope. Most participants were dressed in their work uniform but some were dressed in pants and a T-shirt.
The participants and the researcher were not blinded due to the nature of the study.
Variables/measures. The researcher used a paper form to document the room temperature in Celsius, participant characteristics (sex, age in years, height in centimeters, weight in kilograms, body temperature in Celsius, oxygen saturation [%] measured using the Mindray VS 900, and blood pressure measured by Mindray VS 900 [China] in mm Hg). These measurements were obtained before the participants laid down on the mattresses. Software for the pressure mapping mat was installed on the researcher’s computer, and the interface pressure (mm Hg) and sensing area (cm2) in the supine position was measured at baseline (0 minutes) and after 10 minutes. After 10 minutes, participants completed 1 question for the comfort and 1 question for the mobility levels for each mattress. The comfort and mobility level questions were answered using a 5-point Likert scale (1 = very poor and 5 = very good).
Pressure mapping system. The Force Sensing Array pressure mapping bed system from BodiTrak (Vista Medical, Canada) was used to measure pressure distribution of the participant’s body. The mat was of bed size (193 × 76 cm). The mapping system measured pressure from 0 to 100 mm Hg. The mat had 1809 sensors (67 × 27). Pressure above 90 mm Hg in an area indicated risk of pressure ulcer.13 Pressure mapping is an objective, noninvasive method to measure interface pressure and pressure redistribution.
Procedures. The pressure mat was placed on top of the mattresses covered with a thin loose sheet. All participants were allowed to have a pillow under their head for comfort. Five (5) minutes is required before a measure should be done to allow for the person’s acclimation.14 After the 10-minute pressure mapping session, participants were asked to move from side to side to assess their mobility. The researcher was present at all times and used a stopwatch app on a mobile phone as a timer.
Ethics. The study was approved by the hospital’s privacy protection officer. All participants received oral and written information about the purpose of the study and provided written informed consent. Participant anonymity and confidentiality was maintained by aggregating the data. The list of participants’ names was shredded when the data collection was done. The study involved minimal risk of harm as all participants were healthy volunteers and laid on the mattresses for a short period.
Statistics. All data were entered into Excel 2013 (Microsoft, USA) and SPSS version 25 (IBM, USA). Descriptive statistics of participants’ characteristics and sensing area (cm2) (which refers to the area of the mat whose readings are at or above 0) and mean interface pressure (mm Hg) were entered, and the sensor’s data from scanning were exported from the software into Excel and SPSS. The mean numbers of cells reaching 60 mm Hg, 80 mm Hg, 90 mm Hg, and 100 mm Hg after 10 minutes were calculated. The authors eliminated pressure values of 0 to avoid underestimation of pressure. Nonparametric statistics method was used due to the small sample. Kruskal-Wallis test was used to compare type of mattress with mean interface pressure and mean number of cells (60–100 mm Hg). Spearman correlation was used to examine associations between comfort level and ease of mobility. The correlation value ranged from ± 1.00, and large strength of relationship was considered for r = .50 to 1.0.15 The Mann-Whitney U test was used to compare mean interface pressure between persons weighing more than and less than 100 kg.