Portable Bioimpedance System for Early Detection and Prevention of Pressure Injuries

Joshua R. Harvey, Worcester Polytechnic Institute


Pressure injuries (pressure ulcers, bed sores) are a localized injury to skin and/or underlying tissues as a result of excessive pressure with or without shear force or friction. These injuries are painful, increase risk of secondary infection, have prolonged healing times (months), and cost the U.S. healthcare system 26.8 billion dollars annually. While these injuries are considered preventable, pressure injuries have an overall prevalence of 12.3% in U.S. healthcare facilities with 40% of all pressure injury incidents being facility acquired. The current clinical standard of rotating patients every two hours to offload tissues and performing visual skin assessments is not sufficient for preventing all pressure injuries. The susceptibility to pressure injury formation varies between individuals, and visual skin assessments cannot detect tissue damage below the skin surface. Objective and non-invasive methods for detecting early signs of tissue damage, such as ultrasound and subepidermal moisture scanners, have shown promise in detecting subcutaneous damage. However, these spot scan techniques require trained personnel for interpretation and can lag the pressure loading event by more than 2 days. As part of an ongoing effort to develop an objective and wearable noninvasive monitor to detect early changes related to pressure injury formation, a low-cost, battery-powered bioimpedance sensor was developed and tested on ex vivo and in vivo animal models. The developed sensor is capable of measuring typical human skin impedances with errors of less than 6% as determined using a human skin equivalent electrical model. Continuous bioimpedance-based monitors need to be capable of measuring accurately during changing skin conditions, such as changes in temperature and sweating. The effect of changes in skin microclimate on bioimpedance measurements was investigated using an ex vivo porcine dermis model. We found that increased tissue hydration, simulated by washing porcine dermis in saline, can significantly impact impedance magnitude and phase angle measurements. Finally, an in vivo pressure ulcer rat model was used to determine the relationship between bioimpedance measures, tissue loading intensities, and clinical ulcer staging. Changes between bioimpedance measures before and immediately following the pressure loading event had moderate (0.55) to strong (0.8) linear and rank order correlation with tissue loading intensity (pressure x time). Thresholds were determined to separate clinical ulcer stages via bioimpedance measures taken 3 days prior to visual skin assessment and found that the majority of ulcers could be properly categorized into ulcer/no ulcer groups. These results indicate that bioimpedance is a promising parameter in the development of a continuous noninvasive system for early pressure ulcer detection.