Spectral changes in skin blood flow during pressure manipulations or sympathetic stimulation.

Skin blood flow is commonly determined by laser Doppler flowmetry (LDF). It has been suggested that pathophysiological conditions can be assessed by analysis of specific frequency domains of the LDF signals. We tested whether physiological stimuli that activate myogenic and neurogenic mechanisms would affect relevant portions of the laser Doppler spectrum. LDF sensors were placed on the right forearm of 14 healthy volunteers for myogenic (six females) and 13 for neurogenic challenge (five females). Myogenic responses were tested by positioning the arm ∼50° above/below heart level. Neurogenic responses were tested by immersing the left hand into an ice slurry with and without topical application of local anaesthetic. Short-time Fourier analyses were computed over the range of 0.06 to 0.15 Hz for myogenic and 0.02 to 0.06 Hz for neurogenic. No significant differences in spectral density were observed (P = 0.40) in the myogenic range with arm above (7 ± 54 × 10-4 dB) and below heart (7 ± 14 × 10-4 dB). Neurogenic spectral density showed no significant increase from baseline to cold pressor test (0.0017 ± 0.0013 and 0.0038 ± 0.0039 dB; P = 0.087, effect size 0.47). After application of anaesthetic, neurogenic spectral density was unchanged between the baseline and cold pressor test (0.0014 ± 0.0025 and 0.0006 ± 0.0005 dB; P = 0.173). These results suggest that changes in the myogenic and neurogenic spectral density of LDF signals did not fully reflect the skin vascular function activated by pressure manipulation and sympathetic stimulation. Therefore, LDF myogenic and neurogenic spectral density data should be interpreted with caution.

Clinical Risk Factors of Perioperative Pressure Injury in Older Adult Patients with a Hip Fracture.

OBJECTIVE: To examine the clinical risk factors of perioperative pressure injury (PrI) in older adults with a hip fracture, including preoperative chronic comorbidities and postoperative complications. METHODS: In this retrospective study, the authors queried the PearlDiver Patient Records database between January 2011 and January 2020. Data from 54,194 patients without preexisting PrI were included for analyses. Patients were separated into two groups: (1) one or more perioperative PrI and (2) no PrI. Clinical factors as outcome variables include 21 comorbidities and 10 complications. RESULTS: Univariate analyses were computed to compare the variables between groups, and two logistic regression models were developed to find comorbidity predictors and complication predictors. Of all patients, 1,362 (2.5%) developed one or more perioperative PrI. Patients with perioperative PrIs were more likely to be older men. One-year mortality for patients with perioperative PrI was 2.5 times that of patients without PrI. The regression models showed that predictors of perioperative PrI are malnutrition, hypoalbuminemia, frailty, peripheral vascular disease, dementia, urinary tract infection, perioperative red blood cell transfusion, and atrial fibrillation. CONCLUSIONS: Screening for these comorbidities and complications may assist in determining the risk of PrI in older adults undergoing hip fracture surgery. Determining PrI risk enables the appropriate prevention strategies to be applied perioperatively.

Emerging Role of Quantitative Ultrasound-Based Imaging Techniques for Characterizing Rotator Cuff Tears: A Scoping Review.

Rotator cuff myosteatosis following cuff tears is very common and one of the most important prognostic factors in clinical management. Quantitative ultrasound-based imaging techniques (QUBIT) are frequently used along with magnetic resonance imaging (MRI) to evaluate rotator cuff fatty degeneration. However, the examination of rotator cuff tissue integrity by QUBIT is lacking a standardized imaging protocol and procedural methodologies. In this scoping review, we synthesized the current state of QUBIT against the reference imaging modalities in patients with rotator cuff tears. The literature search was extracted from 963 studies, with 22 studies included in the final review in accordance with the preferred reporting items for systematic reviews and meta-analyses extensions for scoping reviews. The selected studies included human participants and focused on measuring at least one prognostic or diagnostic factor using ultrasonography-based imaging with reference to MRI. The findings suggest both conventional B-mode ultrasound and shear wave elastography imaging were comparable to MRI-based imaging techniques for the evaluation of fatty infiltration and rotator cuff tear characterization. This review establishes guidelines for reporting shoulder-specific QUBIT aimed at developing a standardized imaging protocol. The objective was to enhance the diagnostic and prognostic capabilities of QUBIT in the clinical setting.

Predictors of one year pressure injury outcomes in hospitalized spinal cord injured veterans with one stage 3 or 4 pressure injury.

OBJECTIVE: The purpose of this study is to investigate predictors of pressure injury (PrI) outcomes at one year after discharge for Veterans with spinal cord injury (SCI) hospitalized with a Stage 3 or 4 PrI. DESIGN/SETTING/PARTICIPANTS: This is a retrospective medical record review from one VA Health Care System SCI unit. Participants were Veterans with traumatic or non-traumatic SCI admitted with one Stage 3 or 4 pelvic PrI treated medically (e.g. without flap surgery). Logistic models were used to select the significant predictors of PrI healing outcomes. OUTCOME MEASURE: One year after discharge wound outcomes (healed vs non-healed) for Veterans with SCI hospitalized with a Stage 3 or 4 PrI. RESULTS: A total of 62 hospitalizations were included for analyses resulting in 33 healed and 29 non-healed PrIs. Three significant predictors of non-healed PrI outcomes included use of pressure mapping during hospitalization, greater PrI depth, and usage of alginate dressings. Two significant predictors of healed PrI outcomes included the use of animal-based tissue and hydrocolloid dressings. Area under curve of this logistic regression model was 79.98%. CONCLUSION: The clinical decision of having a patient pressure mapped predicts that the PrI may not heal at one year of discharge. Pressure mapping protocol correlated with another variable that could not serve as a predictor by itself, including using powered pressure relief techniques. The three PrI treatment predictors may represent characteristics of the PrI itself, rather than the efficacy of the product. Further investigation on these clinical decision-making factors is warranted to ensure efficient and cost-effective treatment strategies for individuals with SCI hospitalized with PrIs.

Pressure Injury on Poststroke Admission Assessment to Skilled Nursing Facilities: Risk Factors, Management, and Impact on Rehabilitation.

OBJECTIVE: To describe pressure injury (PrI) prevalence, comorbidities, and rehabilitation utilization among older adults with stroke at skilled nursing facilities' (SNFs') admission assessment. DESIGN: Retrospective cohort. SETTING AND PARTICIPANTS: Older Medicare beneficiaries (>65 years old) with stroke admitted to SNFs. METHODS: We extracted data between 2013 and 2014 using the Master Beneficiary Summary, Medicare Provider Analysis and Review, and Minimum Data Set 3.0. PI data were assessed during admission assessment. RESULTS: Of the 65,330 older adults poststroke admitted to SNFs, 11% had at least 1 PrI present on admission assessment. Individuals who were non-Hispanic Black, with a longer hospital stay, from lower socioeconomic status, with higher proportions of comorbidities (eg, underweight, urinary and bowel incontinence, diabetes, congestive heart failure, arrhythmias, and infections), and higher functional impairments were likely to present with a PrI at SNF admission assessment. Compared with individuals with superficial PrI, individuals with deep PrI were more likely to be young-old (<75 years), non-Hispanic Black, from lower socioeconomic status, present with a shorter hospital stay, an intensive care unit stay, with higher functional impairments, skin integrity issues, system failure, and infections. Compared to those without PrI or superficial PrI, individuals with any-stage PrI or deep PrI were more likely to be cotreated by physical and occupational therapist and less likely to receive individual therapy. Those with PrI poststroke had low documented turning and repositioning rates than those without PrI. CONCLUSIONS AND IMPLICATIONS: Identifying modifiable risk factors to prevent PrIs poststroke in SNFs will facilitate targeted preventative interventions and improve wound care efficacy and rehabilitation utilization for optimized patient outcomes. Identifying residents with a higher risk of PrI during acute care discharge and providing early preventive care during post-acute care would possibly decrease costs and improve outcome quality.

Utilisation of skin blood flow as a precursor for pressure injury development in persons with acute spinal cord injury: A proof of concept.

People with spinal cord injury (SCI) are at high risk of developing a pressure injury. It is unclear why some people with SCI develop pressure injury while others with similar predisposing risk factors do not during acute hospitalisation. This may hinder healthcare utilisation to prevent pressure injuries. The purpose of the study was to examine the proof-of-concept objective bedside skin blood flow measurements before a pressure injury develops in spinal cord injured patients during acute hospitalisation. This was an observational study. All participants had acute traumatic SCI and were pressure injury-free upon enrollment. Skin blood flow patterns were collected at both heels under two circumstances: localised pressure for reactive hyperemia, and localised heating for heat hyperemia. Our results showed that reactive and heat hyperemia were successfully induced in all eleven participants. Two participants developed pressure injury and nine did not have pressure injury at discharge. Heat hyperemia was smaller in participants with pressure injury. No difference was observed in reactive hyperemia between the groups. In conclusion, skin blood flow measurements could be obtained at bedside during acute hospitalisation of SCI for the purpose of research. Further examination of a larger group is warranted to determine clinical use of heat hyperemia pattern as predictor for pressure injury development.

Healthcare utilization and outcomes of spinal cord injured veterans with stage 3-4 pressure injuries.

BACKGROUND: Pressure injuries (PI) are a significant source of morbidity for individuals with spinal cord injury/disease (SCI/D). They are also associated with significant healthcare resource utilization including prolonged hospitalizations. However, the long-term outcomes in terms of wound recurrence-free survival, hospital readmission rates, and all-cause mortality in this population remain largely unknown. OBJECTIVE: To examine the clinical characteristics, healthcare utilization and outcomes of SCI Veterans hospitalized at the VA North Texas Health Care System (VANTHCS) SCI inpatient unit with stage 3 and 4 PI, and compare these between those who received a myocutaneous flap surgery (flap patients (FP)) and those treated medically (non-flap patients (NFP)). METHODS: A retrospective chart review was conducted of all adult patients admitted to the VANTHCS SCI/D unit with stage 3 or 4 pelvic PI between 1/1/2013 and 12/31/2018. Healthcare utilization and outcome information was extracted for pre-specified time points. RESULTS: 78 patients met criteria (113 hospitalizations; 27 FP; 51 NFP). Average length of stay (LOS) was 122 days; FP had a significantly higher LOS than NFP (P = 0.01). Average number of consults was 24. Estimated cost per hospitalization was $175,198. Readmission rate within 30 days was 12.39%. The mortality rate within 1 year of discharge was 21.57% for NFP, as opposed to 3.70% in the FP group. Only 5.00% of NFP wounds were healed at discharged with sustained healing at 1 year, significantly less than FP wounds (55.26%, P < 0.01). CONCLUSIONS: Despite the high investment in terms of healthcare utilization, outcomes in terms of wound healing are poor. Additionally, nearly 22% of NFP died within one year of discharge. This calls into question the utility of prolonged hospitalizations for PI in the SCI/D population in terms of wound treatment efficacy, healthcare costs, and patient morbidity/mortality.

Assessing Skin Blood Flow and Interface Pressure in Patients With Spinal Cord Injury Provided an Alternating Pressure Overlay: A Cross-sectional Study

The effects of an alternating pressure (AP) overlay on the skin are not fully understood. PURPOSE: This study was conducted among persons with spinal cord injury (SCI) to examine skin blood flow (SBF) and interface pressure (IP) during and after AP overlay use. METHODS: In this cross-sectional, repeated measures study, persons eligible for participation were clinic outpatients from a large metropolitan area in the midwest United States who were 18 to 65 years old with a SCI with a neurologic level of injury at T10 or above for more than 1 year and used a wheelchair for primary mobility. Persons with a current pressure injury, diabetes mellitus, and/or hypertension or other vascular or pulmonary diseases were excluded. Data regarding age, gender, body mass index (BMI), duration of SCI, and American Spinal Injury Association Impairment Scale scores were collected. The experimental study involved 3 protocols: the AP protocol (participants lay supine for 40 minutes on an operating room [OR] pad with a low-profile AP that used a 10-minute inflation-deflation cycle); the post-AP protocol (participants lay on the 2-inch foam OR pad for 40 minutes), with 30 minutes of rest in between; and the control protocol, comprised of 40-minutes of laying supine on the OR pad. Each participant served as his/her own control. Outcome variables included 1) peak IP (the highest value among adjoining sensors located at the highest pressure point); 2) averaged IP (the averaged value of the sensors), calculated from pressure mapping system data from the sacrum and left heel; and SBF, measured using a laser Doppler flowmetry system. Descriptive analyses were performed for all variables to determine need for parametric or nonparametric analyses. The mean value of peak IP, averaged IP among inflation and deflation cycles of AP, and post-AP and control protocols were compared using repeated measures analysis of variance (ANOVA). Mean SBF among inflation and deflation cycles of AP and post-AP and control protocols were compared using the nonparametric Friedman test, and Wilcoxon signed rank tests were used to compare the SBF responses during the post-loading period. If the results of repeated measures ANOVA or Friedman tests were statistically significant, paired t tests and Wilcoxon signed rank tests were used for pairwise comparison with Bonferroni correction at alpha level 0.0125, respectively. RESULTS: Among the 15 participants (11 men, 4 women; age 41.77 ± 14.49 [range 20-62] years; BMI 26.81 ± 4.13 [range 22-37]; injury duration 17 ± 14.62 [range 1-48] years; mostly (11) African American), peak IP decreased during the AP deflation at sacrum (51.47 ± 30.18 mm Hg vs. 114.13 ± 60.97 mm Hg; P = .002) and heel (26.79 ± 12.91 mm Hg vs. 53.05 ± 18.22 mm Hg; P = 0 .001), and SBF increased at the heel (27.92 ± 32.15 vs. 10.43 ± 11.16 au; P = .006) but was not significant at the sacrum (15.54 ± 15.33 au vs. 11.96 ± 10.26 au, P = .023). Peak IP decreased during post-AP at the sacrum (104.62 ± 58.17 mm Hg; P = .002) but not at the heel (47.69 ± 16.21 mm Hg; P = .097). SBF increased during post-AP at the sacrum (15.78 ± 15.82 au; P = .012) but not at the heel (16.31 ± 29.18 au, P = .427). CONCLUSION: An AP overlay redistributed IP and increased SBF at the sacrum and heel during use, and its effect 40 minutes after removal was observed only at the sacrum. Studies, including evaluating the lasting effect of AP on weight-bearing tissue protection at different anatomical locations, are needed.

Implementing local cooling to increase skin tolerance to ischemia during normal seating in people with spinal cord injury.

The purpose of this study was to investigate the effectiveness of local cooling in reducing reactive hyperemia after ischemia at the ischial tuberosities for people with spinal cord injury (SCI) during normal seating. The degree of the reactive hyperemic response is indicative of the extent of cellular stress caused by the ischemia. We hypothesized that reactive hyperemic skin blood flow (SBF) responses will be lower when local cooling is implemented by the wheelchair seat cushion. This study used a repeated measures design, and each subject underwent two conditions: normal seating with temperature control 'on' (cooling) and 'off' (non-cooling) for 30 min. Twenty-three participants with traumatic SCI were recruited. SBF and skin temperature were collected before, during and after seating. SBF signals were processed with short-time Fourier analyses to examine the underlying vascular control mechanisms, including the following (corresponding frequency bands): metabolic (0.0095-0.02 Hz), neurogenic (0.02-0.05 Hz), and myogenic (0.05-0.15 Hz) spectral densities. Our results showed that with cooling, skin temperature decreased (range -0.4 ~ -3.1 °C, p = 0.002), and reactive hyperemia parameters (normalized peak SBF and perfusion area) were reduced (p = 0.02, p = 0.033, respectively). In addition, changes in normalized peak SBF (non-cooling - cooling) was moderately correlated with changes in normalized metabolic and neurogenic spectral densities. Our findings suggested that local cooling has a positive effect on reducing the cellular stress caused by ischemia during normal seating. Metabolic and neurogenic SBF control mechanisms may play a minor role. Further exploration of the effect of temperature control on pressure injury prevention is warranted.

Increased skin blood flow during low intensity vibration in human participants: Analysis of control mechanisms using short-time Fourier transform.

AIM: Investigate the immediate effect of low intensity vibration on skin blood flow and its underlying control mechanisms in healthy human participants. MATERIALS AND METHODS: One-group pre-post design in a university laboratory setting. Nine adults underwent two bouts of 10-minute vibration (30Hz, peak acceleration 0.4g). Outcome measures include skin blood flow, and skin temperature on the right foot. To examine the control mechanisms underlying the vibration-induced blood flow response, SHORT-TIME Fourier analyses were computed to obtain the spectral densities for three frequency bands: metabolic (0.0095-0.02Hz), neurogenic (0.02-0.06Hz), and myogenic (0.06-0.15Hz). Non-parametric Friedman's tests were computed to compare changes of the outcome measures and control mechanisms over the course of vibration. RESULTS: Vibration increased skin blood flow during both bouts of vibration, however the effect did not last after vibration was terminated. Myogenic spectral density increased during both bouts of vibration, whereas the metabolic and neurogenic spectral densities increased only during the 2nd bout of vibration. Interestingly, only the metabolic spectral density remained elevated after vibration ended. CONCLUSION: Low intensity vibration produced acute increases in skin blood flow mediated in part by vascular control mechanisms of myogenic origin. Further investigation is warranted to determine whether low intensity vibration induces similar increases in skin blood flow in populations prone to developing chronic non-healing wounds, such as spinal cord injury and diabetes.

Predictive validity of the Spinal Cord Injury Pressure Ulcer Scale (SCIPUS) in acute care and inpatient rehabilitation in individuals with traumatic spinal cord injury.

OBJECTIVE: To evaluate the validity of the Spinal Cord Injury Pressure Ulcer Scale (SCIPUS) during acute care and inpatient rehabilitation following spinal cord injury (SCI) by determining critical cutoff points and assessing the ability to predict risk for pressure ulceration (PrU). DESIGN: Retrospective. METHODS: Sensitivity, specificity, and area under the curve (AUC) for the receiver operating characteristic were determined for the scale's ability to predict PrU 2-3 and 5-7 days after administrating the SCIPUS during acute care, and 5-7 and 14-21 days after administrating the SCIPUS during inpatient rehabilitation. RESULTS: During acute hospitalization, SCIPUS's ability to assess risk for PrUs within 2-3 days was determined at cutoff score of 15 with 100% sensitivity and 75% specificity, AUC = 0.85. The scale was unable to assess PrU risk at 5-7 days, AUC < 0.6 at cutoff score of 13. During inpatient rehabilitation, the scale was unable to assess PrU risk at 5-7 and 14-21 days, AUC < 0.6 at cutoff score of 9. CONCLUSIONS: The SCIPUS could predict PrU occurring within 2-3 days following administration during acute, but unable to predict over a longer term within acute or inpatient rehabilitation. Improved PrU risk assessment following SCI may be possible with modification to the SCIPUS.

Relative contributions of interface pressure, shear stress, and temperature on ischemic-induced, skin-reactive hyperemia in healthy volunteers: a repeated measures laboratory study.

Although the primary risk factors for pressure ulcer development - pressure, shear, skin temperature, moisture, and friction - have been identified for decades, the relative contribution of each to this risk remains unclear. To confirm the results of and expand upon earlier research into the relative contributions of interface pressures, shear stress, and skin temperature among 4 healthy volunteers, a study involving 6 additional healthy 40- to 75-year-old volunteers was conducted and results of the 2 studies were pooled. All 3 variables (interface pressures, shear stress, and skin temperature) were systematically and randomly varied. In the prone position, volunteers each underwent 18 test conditions representing different combinations of temperature (28˚ C, 32˚ C, 36˚ C), pressure (8.0 and 13.3 kPa), and shear (0, 6.7, and 14.0 kPa) using a computer-controlled indenter applied to the sacrum for 20 minutes exerting weights of 100 g and 200 g to induce 0.98 N and 1.96 N of shear force, respectively. Each condition was tested twice, resulting in a total of 360 trials. Magnitude of postload reactive hyperemia as an index of ischemia was measured by laser Doppler flowmetry. Fixed effects regression models were used to predict 3 different indices of reactive hyperemic magnitude. Friedman tests were performed to compare the reactive hyperemia among 3 different skin temperatures or shear stresses under the same amount of localized pressure. In all regression models, pressure and temperature were highly significant predictors of the extent of reactive hyperemia (P <0.0001 and P <0.0001, respectively); the contributions of shear stress were not statistically significant (P = 0.149). With higher temperature, reactive hyperemia increased significantly, especially at greater localized pressure and shear stress, and the difference was more profound between 32˚ C and 36˚ C than between 28˚ C and 32˚ C. These results confirm that, in laboratory settings, temperature is an important factor in tissue ischemia. Additional studies examining the relative importance of pressure, shear, and temperature and potential effects of lowering temperature on tissue ischemia in healthy volunteers and patients at risk for pressure ulcer development are warranted. Because deformation at weight-bearing areas often results in blood flow occlusion, actively lowering the temperature may reduce the severity of ischemia and lower pressure ulcer risk. In this study, shear did not appear to contribute to ischemia in the dermal tissues when assessed using laser Doppler; further work is needed to examine its effect on deeper layers, particularly with regard to nonischemic mechanisms.

Effectiveness of local cooling for enhancing tissue ischemia tolerance in people with spinal cord injury.

OBJECTIVE: To investigate the effects of localized cooling and cooling rate on pressure-induced ischemia for people with and without neurological deficits. DESIGN: A 2 × 3 mixed factorial design with two groups: (1) people with spinal cord injury (SCI) and (2) people without neurological deficits (control), and three test conditions: (1) pressure only, (2) pressure with fast cooling (-4°C/min), and (3) pressure with slow cooling (-0.33°C/min). SETTING: University laboratory. PARTICIPANTS: Fourteen controls and 14 individuals with SCI. INTERVENTIONS: Pressure on the sacrum was 0.4 kPa for 5 minutes, then 8 kPa for 20 minutes, and finally 0.4 kPa for 15 minutes. Fast and slow cooling to 25°C applied during 8 kPa of pressure. OUTCOME MEASURES: Reactive hyperemia and its spectral densities in the metabolic, neurogenic, and myogenic frequency ranges. RESULTS: In controls, reactive hyperemia was greater in pressure only as compared with both cooling conditions. No change was noted in all spectral densities in both cooling conditions, and only neurogenic spectral density increased without cooling. In subjects with SCI, no difference was noted in reactive hyperemia among conditions. However, metabolic and myogenic spectral densities increased without cooling and all spectral densities increased with slow cooling. No change was noted in all spectral densities with fast cooling. CONCLUSION: Local cooling reduced the severity of ischemia in controls. This protective effect may be masked in subjects with SCI due to chronic microvascular changes; however, spectral analysis suggested local cooling may reduce metabolic vasodilation. These findings provide evidence towards the development of support surfaces with temperature control for weight-bearing soft tissues.

Hybrid equation/agent-based model of ischemia-induced hyperemia and pressure ulcer formation predicts greater propensity to ulcerate in subjects with spinal cord injury.

Pressure ulcers are costly and life-threatening complications for people with spinal cord injury (SCI). People with SCI also exhibit differential blood flow properties in non-ulcerated skin. We hypothesized that a computer simulation of the pressure ulcer formation process, informed by data regarding skin blood flow and reactive hyperemia in response to pressure, could provide insights into the pathogenesis and effective treatment of post-SCI pressure ulcers. Agent-Based Models (ABM) are useful in settings such as pressure ulcers, in which spatial realism is important. Ordinary Differential Equation-based (ODE) models are useful when modeling physiological phenomena such as reactive hyperemia. Accordingly, we constructed a hybrid model that combines ODEs related to blood flow along with an ABM of skin injury, inflammation, and ulcer formation. The relationship between pressure and the course of ulcer formation, as well as several other important characteristic patterns of pressure ulcer formation, was demonstrated in this model. The ODE portion of this model was calibrated to data related to blood flow following experimental pressure responses in non-injured human subjects or to data from people with SCI. This model predicted a higher propensity to form ulcers in response to pressure in people with SCI vs. non-injured control subjects, and thus may serve as novel diagnostic platform for post-SCI ulcer formation.

The relative contributions of interface pressure, shear stress, and temperature on tissue ischemia: a cross-sectional pilot study.

Tissue ischemia is thought to play a major role in the development of pressure ulcers. Pressure, shear, and temperature are acknowledged contributors, but the relative magnitude of each factor is largely unknown. A cross-sectional pilot study was conducted on the sacrums of four healthy volunteers to estimate the relative contributions of each variable by systematically varying and assessing the resulting level of ischemia in the skin tissue. Using a repeated measures design, 21 combinations of temperature (28˚C, 32˚C, and 36˚C); pressure (0 kPa, 8.0 kPa, and 13.3 kPa), corresponding to 0 mm Hg, 60 mm Hg, and 100 mm Hg; and shear stress (0 kPa, 6.7 kPa, and 14.0 kPa), corresponding to 0 mm Hg, 50 mm Hg, and 100 mm Hg (practical testing values), were tested twice, for a total of 168 trials. Using laser Doppler flowmetry, the magnitude of post-load reactive hyperemia was used as an index of ischemia. Fixed Effects and Ranks linear regression models were developed to predict three different indices of reactive hyperemic magnitude with pressure, shear stress, and temperature as the variables. Pressure and temperature were always highly significant predictors of the extent of reactive hyperemia (P <0.0001 for Perfusion Area, peak minus baseline blood flow, and Normalized Peak blood flow), and the contributions of shear stress were insignificant (P = 0.5351 for Perfusion Area, P = 0.6403 for Peak minus Baseline blood flow, and P = 0.8941 for Normalized Peak blood flow). Depending upon the model, comparison of coefficients suggested that an increase of 1.0˚C contributes as much to reactive hyperemia in the skin as 12 mm Hg to 15 mm Hg of interface pressure (coefficient ratios of temperature/pressure are 14.33 for Perfusion Area, 11.77 for Peak minus Baseline, and 12.97 for Normalized Peak, respectively). The findings also indicate that post-load metabolic repayment varied with temperature only at higher pressures, suggesting protective vasodilation was able to keep pace with mild compression. If confirmed in subsequent studies, the results suggest that managing both skin pressure and temperature may reduce the risk of ischemia.

Effects of local cooling on sacral skin perfusion response to pressure: implications for pressure ulcer prevention.

People with spinal cord injuries are at high risk for developing pressure ulcers. Increased skin temperature is one of the extrinsic causative factors for this multi-factorial disease. Previous animal studies revealed that local skin cooling reduced the severity of ulceration, and cooling is widely used in plastic surgery and organ transplants for tissue preservation. The objectives of this pilot study were to develop test protocols and instrumentation and to investigate the effect of local cooling on skin perfusion response to pressure on young healthy human subjects. Reactive hyperemia was quantified in this study to compare the effects of pressure with and without cooling. Reactive hyperemia is a normal physiological response occurring after vessel occlusion. Laser Doppler flowmetry was used to measure skin blood flow. Time-dependent spectral analysis was used to analyze and decompose the blood flow data into frequency ranges associated with specific blood flow control mechanisms. The study used a repeated measures design with two test conditions: 8 kPa of pressure with and without cooling to 25 degrees C. We hypothesized that local cooling would reduce the post-ischemic reactive hyperemic response induced by the rigid indenter. Time series results showed that normalized peak perfusion response was significantly lower with cooling (p=0.019). Time-dependent spectral analysis results suggested that both metabolic and myogenic responses contribute to this protective effect. Findings from our study on humans were consistent with previous animal studies. Additional studies on individuals with spinal cord injury are planned to further evaluate the cooling effect in a high-risk population.