Regulation of Microvascular Blood Flow : a clinical and experimental study based on laser Doppler perfusion imaging

Abstract: Local blood flow reflexes in the foot skin of healthy controls and in young diabetic patients with or without nerve dysfunction have been studied using laser Doppler perfusion imaging (LDPI). A neurophysiological follow-up study on nerve dysfunction is presented as a complerhent to the work on blood flow regulation in the young diabetics.An enhanced high-resolution LDPI (EHR-LDPI), intended for visualization and interpretation of flow dynamics in separate microvessels, has been adapted and evaluated in in vitro tube models and in an in vivo tissue model (hamster cheek pouch). By focusing the laser beam to 40 µm in the focal plane and reducing the step length to 25 µm, full format images (4096 measurement sites) of microvascular tissue areas as small as 1,5 x 1,5 mm were created.The objectives of the work were to study if the vasoconstrictor response seen during change in posture is a mechanism elicited by a rise in venous pressure, but also to investigate if young diabetic patients with nerve dysfunction have an impairment in the local regulation of foot skin blood flow (postural vasoconstriction and hyperemic response) compared to diabetics without nerve dysfunction. An additional aim was to elucidate whether abnormal nerve conduction is retarded or even prevented by tight metabolic control in patients with type 1 diabetes mellitus.The experimental studies aimed to improve the resolution of the EHR-LDPI system, to evaluate the system flow response in an in vitro model and to evaluate the performance, the limitations and the future potentials by studying flow dynamics in a tissue containing separate microvessels. It was concluded that:(1) The LDPI recorded skin perfusion during variations in venous stasis and posture, adding information on flow distribution changes. The difference in flow distribution seen suggested an additive regulatory mechanism to a venoarteriolar reflex during change in posture.(2) Subclinical nerve conduction defects were more common than microvascular abnormalities as measured by LDPI in the present models in young diabetic patients. Although, no signs of established retinopathy or nephropathy in this patient group, resting skin blood flow abnormalities were present, and these findings were related to high HbA1c-levels.(3) Tight longwterm metabolic control, with HbA1c values less than 6,5%, could retard nerve dysfunction in patients with type 1 diabetes mellitus and a mean disease duration of 12 years.(4) Using EHR-LDPI a decrease in signal level was obtained as the tube diameter increased, although the algorithm scaled linearly with velocity and was found not to be sensitive to hematocrit variations. Individual microvessel diameters could be estimated, which on average resulted in a difference of 11 µm compared to microscopic measurements.(5) A dynamic overview of the vascular tree with volumetric flow estimate as well as RBC velocities of separate vessels was obtained. The need for further focusing of the beam and reduction of the step length appeared to be important tasks to solve in order to get a more accurate vessel diameter determination and to refine the volumetric flow estimate.