Analysis of the pressure perturbation due to the introduction of a measuring probe under an elastic garment.

Attention is drawn to the increase of the pressure on the skin due to the introduction of a measuring probe under an elastic garment. The increase of pressure is explained by looking at the force equilibrium on the transducer. Making some assumptions on the geometry and the conditions that actually prevail at the interface, this mechanical phenomenon is expressed mathematically. Expressions for estimating the order of magnitude of the relative pressure perturbation are derived both for the case of a cylindrical probe and for a thin plate probe with the same curvature as the limb. Thanks to the geometrical assumptions, these expressions are independent of the mechanical properties of the probe and the soft tissue. As in some of these formulas the magnitude of the indentation appears, a measurement is necessary for estimating the error. Other expressions, only taking in account the dimensions of the limb and the probe, give a rougher estimation of the error. It is shown through a few practical examples that the error cannot be disregarded and cannot be calculated precisely, which means that this measuring technique is unreliable. The same mathematical expressions are used to show that pressure paddings effectively increase the pressure. It is also shown that the introduction of an optical fibre under the garment, used in laser Doppler flowmetry, creates an important additional pressure on the skin.