Short-term prone positioning is associated with decreased SV, CO and SBF and increased calculated SVR index.

The Monte-Carlo method is employed to simulate the illumination of a blood slab by a continuous laser. It is assumed that the geometry of the medium is bidimensional and that scattering or absorption takes place only when a photon strikes a red blood cell. The parameters involved in the calculations concern the photons free path lengths between two collisions, the scattering angles and the absorption probability at collision. These parameters are assessed according to experimental or literature data. Fortran programs allow the computation of diffuse and collimated transmittances (Td and Tc, respectively), of transmittance measured with an optic fiber Tf and of reflectance R. The variations of Tc and Tf with blood thickness are in accordance with established laws. Moreover, measured and computed reflectances change with hematocrit ratio in a similar way. This work demonstrates that the Monte-Carlo method is a simple reliable tool which can be used to provide a realistic model of laser penetration in complex biological structures. Moreover, this method will permit investigations in laser tomo-spectrometry by providing a useful simulation of the interaction of ultrashort light pulses with biological media.