RESUMEN
OBJECTIVE: Oxygen supply and partial pressure are key determinants of tissue metabolic status and are also regulators of vascular function including production of reactive oxygen species, vascular remodeling, and angiogenesis. The objective of this study was to develop an approach for the determination of oxygen saturation and hematocrit for individual microvessels in trans- and epi-illumination intravital microscopy. METHODS: A spectral approach was used, taking advantage of the availability of commercial imaging systems that allow digital recording of intravital images at a number of predetermined wavelengths within a relatively short time. The dependence of validity and precision of saturation measurements on critical experimental variables (reference spectra, number and selection of wavelengths, exposure time, analysis area, analysis model) was evaluated. In addition, a software approach for two-dimensional analysis of images was developed. RESULTS: Exposure times per wavelength of about 200 ms and use of up to 50 wavelengths evenly spaced from 500 to 598 nm allow automatic discrimination of microvessels from tissue background (segmentation) with reliable determination of oxygen saturation (in trans- and epi-illumination) and hematocrit (in transillumination). CONCLUSIONS: The present imaging spectroscopy approach allows detailed assessment of oxygen transport and other functional parameters at the microcirculatory level.
Asunto(s)
Microscopía por Video , Oximetría , Oxígeno/análisis , Animales , Transporte Biológico/fisiología , Hematócrito , Masculino , Ratones , Ratones Endogámicos BALB C , Microcirculación/fisiología , Consumo de Oxígeno/fisiología , Ratas , Ratas Sprague-Dawley , Análisis Espectral , Circulación Esplácnica/fisiologíaRESUMEN
Matching blood flow to metabolic demand in terminal vascular beds involves coordinated changes in diameters of vessels along flow pathways, requiring upstream and downstream transfer of information on local conditions. Here, the role of information transfer mechanisms in structural adaptation of microvascular networks after a small change in capillary oxygen demand was studied using a theoretical model. The model includes diameter adaptation and information transfer via vascular reactions to wall shear stress, transmural pressure, and oxygen levels. Information transfer is additionally effected by conduction along vessel walls and by convection of metabolites. The model permits selective blocking of information transfer mechanisms. Six networks, based on in vivo data, were considered. With information transfer, increases in network conductance and capillary oxygen supply were amplified by factors of 4.9 +/- 0.2 and 9.4 +/- 1.1 (means +/- SE), relative to increases when information transfer was blocked. Information transfer by flow coupling alone, in which increased shear stress triggers vascular enlargement, gave amplifications of 4.0 +/- 0.3 and 4.9 +/- 0.5. Other information transfer mechanisms acting alone gave amplifications below 1.6. Thus shear-stress-mediated flow coupling is the main mechanism for the structural adjustment of feeding and draining vessel diameters to small changes in capillary oxygen demand.
Asunto(s)
Microcirculación/anatomía & histología , Microcirculación/fisiología , Transducción de Señal/fisiología , Estrés Mecánico , Animales , Arteriolas/fisiología , Presión Sanguínea/fisiología , Simulación por Computador , Hemodinámica/fisiología , Teoría de la Información , Masculino , Modelos Biológicos , Consumo de Oxígeno/fisiología , Ratas , Ratas Wistar , Reología , Vénulas/fisiologíaRESUMEN
Structural reductions in vessel luminal diameters in response to elevated pressure may play a role in the elevation of peripheral resistance generally observed in hypertension. In the present study, a theoretical model is used to simulate the effect of increased driving pressure on flow resistance in microvascular networks. The angioarchitecture (lengths and diameters of all segments, topology) of microvascular networks (n=6) in the rat mesentery was recorded by intravital microscopy. The model simulation of vascular adaptation in response to local wall shear stress, transmural pressure, and tissue PO(2) was used to predict changes in network pressure drop and flow resistance for a given change of driving pressure (DeltaP). For DeltaP increasing from 15% to 190% of the normotensive value, a 3.3-fold increase in flow resistance was observed (structural autoregulation). If vascular reactivity to pressure was suppressed, the resistance increase was abolished. Suppressing pressure sensitivity also led to a rise in mean capillary pressure at normal driving pressure from 23.8+/-7.3 mm Hg to 34+/-6.9 mm Hg. These results indicate that low capillary pressure levels as well as structural autoregulation depend on vascular responses to circumferential wall stress (corresponding to pressure). This tendency of peripheral vascular beds to increase flow resistance for a given increase of bulk flow or driving pressure may amplify and stabilize blood pressure elevation in the development of hypertension.
Asunto(s)
Presión Sanguínea/fisiología , Modelos Cardiovasculares , Circulación Esplácnica/fisiología , Adaptación Fisiológica/fisiología , Animales , Hipertensión/fisiopatología , Masculino , Ratas , Ratas Wistar , Estrés Mecánico , Resistencia Vascular/fisiologíaRESUMEN
Terminal vascular beds continually adapt to changing demands. A theoretical model is used to simulate structural diameter changes in response to hemodynamic and metabolic stimuli in microvascular networks. Increased wall shear stress and decreased intravascular pressure are assumed to stimulate diameter increase. Intravascular partial pressure of oxygen (PO(2)) is estimated for each segment. Decreasing PO(2) is assumed to generate a metabolic stimulus for diameter increase, which acts locally, upstream via conduction along vessel walls, and downstream via metabolite convection. By adjusting the sensitivities to these stimuli, good agreement is achieved between predicted network characteristics and experimental data from microvascular networks in rat mesentery. Reduced pressure sensitivity leads to increased capillary pressure with reduced viscous energy dissipation and little change in tissue oxygenation. Dissipation decreases strongly with decreased metabolic response. Below a threshold level of metabolic response flow shifts to shorter pathways through the network, and oxygen supply efficiency decreases sharply. In summary, the distribution of vessel diameters generated by the simulated adaptive process allows the network to meet the functional demands of tissue while avoiding excessive viscous energy dissipation.
Asunto(s)
Adaptación Fisiológica/fisiología , Mesenterio/irrigación sanguínea , Microcirculación/fisiología , Modelos Cardiovasculares , Animales , Velocidad del Flujo Sanguíneo/fisiología , Presión Sanguínea/fisiología , Viscosidad Sanguínea/fisiología , Simulación por Computador , Hemodinámica/fisiología , Masculino , Oxígeno/metabolismo , Ratas , Ratas Wistar , Flujo Sanguíneo Regional/fisiología , Transducción de Señal/fisiología , Estrés Mecánico , Grado de Desobstrucción Vascular/fisiologíaRESUMEN
The outpatient measurement with the ME-SAM IC system, including the analysis of heart rate, respiration sounds, continuous oxygen saturation and body position, is an important part in a stepped concept for diagnosis and therapy of patients with an obstructive sleep apnea syndrome (OSAS). We investigated the importance of these parameters as well as age and Broca in relation to the continuous positive airway pressure determined in the sleep laboratory. In this study we present the first 60 patients (56 m, 4 f) treated successfully with nCPAP. The mean age was 53.1 +/- 8.6 years, the Broca Index 133.6 +/- 22.9, the respiratory disturbance index (RDI, apnea + hypopnea index) 48.1 +/- 22.7, the mean nocturnal oxygen saturation 90.4 +/- 3.8% and the mean minimal oxygen saturation 84.4 +/- 4.2%. We found positive significant correlation between Broca, RDI and oxygen saturation as well as between Broca and age. The mean effective continuous positive airway pressure was 9.7 +/- 2.1 cm H2O in our group of OSAS patients. With an increase in RDI and with a decrease in age and oxygen saturation we found an significant increase in the effective continuous positive airway pressure. A positive correlation between Broca and air pressure could be seen, but was not significant. Our data show that there is a strong correlation between constitution, the outpatient measurement with MESAM-system and the effective positive airway pressure in patients with a pronounced obstructive sleep apnea syndrome (RDI > 15 and marked clinical symptoms). The knowledge about these relationships should be considered in the nCPAP therapy and during the long term medical care of treated patients.