Tissue Blood Flow Responses to External Pressure Using LDF and PPG : Testing a System Developed for Pressure Ulcer Research

Abstract: Background: Pressure ulcers are a problem for immobile individuals, and having pressure ulcers impact and restrict the daily and are often associated with pain. Pressure ulcers occur frequently and cause high costs for the health care system. The prevention of pressure ulcers by focussing on different mattresses and overlays aimed to reduce the interface pressure or the pressure exposure of the tissue. The problem is the poor evaluation of this type of equipment. There are important factors regarding pressure ulcer development, pressure, shear, temperature and humidity. People are affected by external pressure in different ways and therefore it is preferable to measure the effect of pressure as a complement to the pressure measurement and thus we consider blood flow measurements to be a suitable method.Aims: The aim of Study I, the first part in this thesis was to investigate the existence of sacral tissue blood flow at different depths in response to external pressure in elderly individuals as a part of evaluation of a. newly developed system. The aim of Study II, the second part was to evaluate a multiparametric system combining LDF and photoplethysmography into a single probe, for the simultaneous measurement of blood flow at different depths in the sacral tissue when the tissue is exposed to external load. This new system will be used to facilitate the understanding of pressure ulcer formation.Methods: To be able to observe tissue blood flow, the non-invasive optical methods laser Doppler flowmetry and photoplethysmography were used. In this thesis a newly developed prototype probe was used, combining the two methods. Green light and infrared light were used in the PPG instrument for penetrating the depths of approximately 2 mm, 8 mm and 20 mm depths. A HeNe laser was used to measure the superficial skin blood flow, <1 mm depth. The prototype probe, made of silicone was fixed in a stiff 10×10 cm plate.Seventeen active individuals over the age of 60 were recruited for the two studies. In Study I, the subject´s sacral blood flow and tissue thickness (using ultrasound) were measured in unloaded position and in supine position loading the area with their own body weight. In Study II, the sacral area was provoked with external load at 37.5 mmHg and 50.0 mmHg and the relative change in blood flow at different depths was observed before, during and after load.Results: Study I showed that the sacral tissue in elderly individuals is highly affected by load and is compressed by 60.3 ± 11.9%. The mean sacral tissue thickness was 26 ± 13 mm in unloaded tissue and 10 ± 6 mm in loaded tissue. Correlations were found between BMI and tissue thickness: both TTunload r=0.68 (p=0.003) and TTload r=0.68 (p=0.003). Almost all subjects had affected blood flow superficially but only occasionally deeper in the tissue and findings may indicate that the blood flow is occluded in the superficial layer before it is occluded deeper in the tissue structure. The most common response in Study II was an increase in blood flow while loading. In those occasions when the blood flow decreased, it was mostly affected at the skin surface and the reactive hyperaemia occurred more frequently in the superficial tissue structures. The blood flow responses may be different in the different tissue layers.Conclusions: The newly developed system was found to be suitable for measuring tissue blood flow at different depths; however the prototype probe had some limitations that will be solved in the further development of the system into a thin flexible probe with ability to measure a larger area.