An evaluation of the effects of localised skin cooling on microvascular, inflammatory, structural, and perceptual responses to sustained mechanical loading of the sacrum: A study protocol

Abstract

This study protocol aims to investigate how localised cooling influences the skin's microvascular, inflammatory, structural, and perceptual tolerance to sustained mechanical loading at the sacrum, evaluating factors such as morphology, physiology, and perceptual responses. The protocol will be tested on individuals of different age, sex, skin tone and clinical status, using a repeated-measure design with three participants cohorts: i) young healthy (n = 35); ii) older healthy (n = 35); iii) spinal cord injured (SCI, n = 35). Participants will complete three testing sessions during which their sacrum will be mechanically loaded (60 mmHg; 45 min) and unloaded (20 min) with a custom-built thermal probe, causing pressure-induced ischemia and post-occlusive reactive hyperaemia. Testing sessions will differ by the probe's temperature, which will be set to either 38°C (no cooling), 24°C (mild cooling), or 16°C (strong cooling). We will measure skin blood flow (via Laser Doppler Flowmetry; 40 Hz); pro- and anti-inflammatory biomarkers in skin sebum (Sebutape); structural skin properties (Optical Coherence Tomography); and ratings of thermal sensation, comfort, and acceptance (Likert Scales); throughout the loading and unloading phases. Changes in post-occlusive reactive hyperaemia will be considered as the primary outcome and data will be analysed for the independent and interactive effects of stimuli's temperature and of participant group on within- and between-subject mean differences (and 95% Confidence Intervals) in peak hyperaemia, by means of a 2-way mixed model ANOVA (or Friedman). Regression models will also be developed to assess the relationship between absolute cooling temperatures and peak hyperaemia. Secondary outcomes will be within- and between-subject mean changes in biomarkers' expression, skin structural and perceptual responses. This analysis will help identifying physiological and perceptual thresholds for the protective effects of cooling from mechanically induced damage underlying the development of pressure ulcers in individuals varying in age and clinical status.

Fig 1. Experimental design.

We have designed a clinically relevant experiment in healthy young participants and in groups at-risk of PUs (i.e., elderly and spinal cord injured, SCI), which will determine how different levels of cooling [i.e. no cooling (38°C), mild cooling (16°C), and strong cooling (16°C)] alter the skin’ microvascular, inflammatory, structural, and perceptual responses to sustained pressure-induced ischemia and reactive hyperaemia. From an applied standpoint, the research will identify physiological and perceptual cooling thresholds (i.e. level of cooling, modulations via age and clinical status), which could be used as design parameters for the development of user-centred medical devices and thermal wearables. LDF: Laser Doppler Flowmetry; OCT: Optical Coherence Tomography.

Fig 2. Standardise pressure protocol delivered over the sacrum.

The thermal probe will be used to deliver a standardised 60mmHg pressure protocol to evoke pronounced tissue ischaemia under 3 thermal conditions, i.e. a control skin temperature evoking no cooling (i.e. 38°C) and two cooling temperatures of 24°C and 16°C. During the protocol, a series of non-invasive measurements will be conducted [i.e. skin blood flow via LDF; inflammatory biomarker sampling from skin sebum; structural and functional imaging via Optical Coherence Tomography (OCT); perceptual assessment of subjective thermal sensation, comfort, and acceptance] at different time points (identified in the diagram by ↑).