Study design. An experimental, cross-sectional study was conducted in a university laboratory setting. A repeated measures design was utilized, with each person participating in both the experimental and control procedures. To prevent any cross-over effect, the order of the 2 procedures was randomized by drawing envelopes containing 2 different orders, and a washout period of 2 hours was provided between the procedures. Each procedure commenced with a 15-minute rest (baseline) period and ended with 20 minutes of rest (post-loading).
Participants. People with SCI were recruited for this study from the greater Chicago area using a convenience sampling method. Flyers describing the study were posted on public bulletin boards at different outpatient clinics, and potential participants were encouraged to contact the research team. Inclusion criteria stipulated participants must be 18 to 65 years old, have a SCI with a neurologic level of injury at T10 or above of more than 1 year, and use a wheelchair for primary mobility. People with a pressure injury, diabetes mellitus, and/or hypertension or other vascular or pulmonary diseases were excluded. Independent patient demographic and injury information variables collected included age, gender, body mass index (BMI), duration of SCI, and American Spinal Injury Association (ASIA) Impairment Scale (AIS) scores were used to assess the sensory and motor levels affected by the SCI. For example, Grade A means the impairment is complete (no motor or sensory function exists below the level of injury) and Grade B stipulates impairment is incomplete (ie, sensory function, but not motor function, is preserved below the first normal level above the level of injury).
Procedure. The experimental procedure consisted of 2 protocols: the AP protocol followed by the post-AP protocol, with 30 minutes of rest in between. During the AP protocol, participants were asked to lay supine for 40 minutes on an low-profile AP overlay (Dabir Surface Inc) that was placed over a 2-inch foam OR pad (Alimed Inc) (see Figure 1A). A 10-minute inflation-deflation cycle was applied throughout this protocol (4 inflation/deflation cycles). Compared to typical AP overlays and mattresses, the low-profile AP overlay evaluated in this study is less than an inch in height when fully inflated, and the air cells are an inch in width, providing periodic micropressure relief. The AP overlay then was removed for the 30 minutes of rest. During the post-AP protocol, participants lay on the OR pad only for 40 minutes.
The control procedure consisted of only 1 protocol. During this protocol, participants lay on the OR pad only for 40 minutes. The difference between the control protocol and the post-AP protocol was the lack of AP protocol 30 minutes before the control protocol. To ensure consistency throughout the study, participants were asked to lay on their right side during all rest periods, including 15 minutes of baseline, 30 minutes rest between AP protocol and post-AP protocol, and 20 minutes post-loading. Pillows and towels were provided throughout both procedures to maintain patient posture; the biggest positioning difficulty encountered was the external rotation of the participants’ legs. A rolled towel was provided lateral to the leg to ensure the area of skin in contact with the overlay or pad was consistent throughout both procedures.
Study protocols were approved by the University of Illinois at Chicago Institutional Review Board. All participants provided written informed consent before study participation.
IP. The 2 study outcome measures were IP and SBF. Whole-body IP was measured using the XSENSOR X3 pressure mapping system (XSENSOR Technology Corp) (see Figure 1B). The IP data from the 3 protocols (AP, post-AP, and control) were collected during the 40 minutes of laying supine at 1 Hz. The x-sensor pressure mapping software was used for IP data acquisition, and data for selected areas and frames (as detailed in data analysis section) were exported to a Microsoft Excel spreadsheet for further analyses. Area and time frame details are described in data analysis section.
SBF. SBF was measured using the laser Doppler flowmetry system (Moor Instruments Inc). Two (2) silicone soft flat probes (VP11SC, see Figure 1C) were taped to the highest pressure points (determined by the pressure map readings while participants were laying supine on top of the OR pad with pressure map in between) at the sacrum and left heel. The silicone soft flat probes are flexible and can deform to stay in total contact to the skin. The SBF data were collected throughout the baseline, 3 protocols, and post-loading periods for both experimental and control procedures. PowerLab (ADInstruments Inc) was used for SBF data acquisition at 20Hz, and spreadsheets of the data were generated via the LabChart software (ADInstruments Inc) for further analyses. One (1) measurement was taken for both outcome measures during both experimental and control procedures for each participant.
Data analysis. Two (2) variables — peak IP (the highest value among the 9 adjoining sensors located at the highest pressure point) and averaged IP (the averaged value of these 9 sensors) — were calculated from data collected at the highest pressure points at the sacrum and left heel. Once the 9 sensors were identified for the highest pressure points, the peak IP and the averaged IP of each frame were exported to an Excel spreadsheet for further analyses.
To examine the effect of AP use, the mean values of peak IP, averaged IP, and SBF during the inflation and deflation cycles of the AP protocol were compared to values from the 40 minutes of the control protocol. To examine the effect of AP after removal, the mean values of peak IP, averaged IP, and SBF during the 40 minutes of post-AP protocol also were compared to the control protocol outcomes. The first and last minute of the 40 minutes laying supine on OR pad and the first and last minute of the 5 minutes inflation/deflation cycle were excluded from the analyses to eliminate the artifact caused by transition between different postures or inflation/deflation cycle of the AP overlay.
To further examine the effect of AP on SBF after removal, 4 SBF variables derived from the SBF data collected during the rest periods of baseline (the 15 minutes before loading) and post-loading were compared between the experimental and the control procedures: baseline SBF, peak reactive SBF, normalized peak reactive SBF, and the total reactive SBF. Figure 2 shows the SBF response recorded from 1 of the participants during the post-loading period of the control protocol at the heel and includes baseline SBF, peak reactive SBF, and total reactive SBF. Baseline SBF was averaged over the 15 minutes of rest at the beginning of experimental and control protocol. A polynomial equation was used to determine the SBF response during the post-loading periods.20 The peak reactive SBF is the highest point on the fit curve. Normalized peak reactive SBF was calculated using the following equation:
(peak reactive SBF – baseline SBF)/baseline SBF x 100%
Total reactive SBF is the total amount of increase in SBF from baseline (the interval between the fit curve and the baseline value). The formula used to calculate variables from the fit curve was based on previous literature.20 Based on previous literature,21,22 the amount of increase in SBF during the post-loading period indicated the severity of tissue ischemia during the loading period. The more severe the tissue ischemia during the weight-bearing condition, the larger the SBF increase (normalized peak reactive SBF or total reactive SBF) during the post-loading period.
All data were transferred from Excel spreadsheets to IBM SPSS, version 25.0 for statistical analyses. Descriptive analyses were performed for all variables to determine need for parametric or nonparametric analyses. To compare the mean value of peak IP and averaged IP among inflation and deflation cycles of AP, and post-AP and control protocols, repeated measures analysis of variance (ANOVA) was used. To compare the mean SBF among inflation and deflation cycles of AP, and post-AP and control protocols, the nonparametric Friedman test was employed. If statistical significance was noted using repeated measures ANOVA or the Friedman test, paired t tests and Wilcoxon signed rank tests were used for pair-wise comparison with Bonferroni correction at alpha level 0.0125, respectively. To compare the SBF response during the post-loading period between the experimental and control procedure, Wilcoxon signed rank tests were performed.