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The aim of our study was to determine the cause of fluctuations in microvascular blood flow measured by laser Doppler perfusion monitoring (LDPM) in microvessels of the hamster skin fold preparation. LDPM flow fluctuations (flow motion) were compared to simultaneous records of diameter changes obtained from arterioles and venules. Time-varying coherence analysis was used to quantify the frequency of flow and diameter changes in awake hamsters in control conditions and during sodium nitroprusside-induced vasodilation. Power spectrum analysis of LDPM signals of order 4 and 3 arterioles indicated frequencies of 0.06 +/- 0.01 and 0.08 +/- 0.02 Hz, respectively. Order 3 arterioles exhibited significant temporal coherence between flow motion and vasomotion signals at a frequency of the order of 0.1 Hz. There was lack of coherence when vasomotion was absent in arterioles. Order 4 arterioles had several coherent frequencies in addition to that around 0.1 Hz. In conclusion, time-variant coherence analysis indicated that spontaneous fluctuations in flow are caused by vasomotion of order 3 arterioles with frequency around 0.1 Hz that appears to coordinate local control processes in the microcirculation. Additional frequency components present in LDPM signals are unrelated to vasomotion.

A mathematical model of a microvasculature was used to study the effects of myogenic and flow-dependent stimuli on the characteristics of vasomotion and microvascular perfusion regulation. The model includes three branching orders of arterioles derived from in vivo observations and incorporates a mechanism for terminal arteriolar closure during vasomotion. Simulations were performed to evaluate the effect of vasodilation and vasoconstriction on vasomotion pattern, and the changes in arteriolar effective diameter and flow in response to arterial blood pressure variations triggering the regulatory mechanisms. Vasomotion patterns were studied in the hamster cutaneous muscle, visualized by fluorescent microscopy, in control conditions and after injection of acetylcholine (Ach) or NG-monomethyl-L-arginine (L-NMMA). We have found that vasomotion may be caused by different combinations of feedback mechanisms, including a strong rate-dependent myogenic response or a strong flow-dependent mechanism with no rate-dependent response. Decreasing the rate-dependent component of the myogenic mechanism and increasing the time constant of the flow-dependent mechanism causes vessel stabilization and disappearance of vasomotion. In hamster microcirculation, Ach decreased vasomotion frequency and increased vasomotion amplitude and arteriolar effective diameter, whereas L-NMMA caused a slight increase in vasomotion frequency and decrease in effective diameter. Model simulations, under dilatory and constrictory stimuli, confirmed these results. Moreover, the model predicted that mean blood flow is maintained closer to normal despite arterial pressure changes (+/-15% flow changes versus +/-50% pressure variations) when the vessels were in nonoscillatory than when they are in oscillatory state. In conclusion, a large variety of vasomotion patterns affect arteriolar resistance and microvessel perfusion in skeletal muscle. Furthermore, in the presence of vasomotion the network exhibits a poorer aptitude for regulating blood flow during arterial pressure changes (i.e., worse autoregulation) than the nonoscillatory network.

The effects of arginine (L-arg), promoter of nitric oxide (NO) production and NO synthesis inhibitors, NG-monomethyl-L-arginine (L-NMMA) and N omega-nitro-L-arginine (L-NNA), on arteriolar responses and capillary perfusion after 30 min ischemia were studied in the cheek pouch preparation under pentobarbital anesthesia and intravenous drug infusion. Capillary density, venular leukocyte sticking, and vessel diameters were investigated by fluorescence microscopy. Damage due to photoactivation of intravascular dyes was investigated by injecting fluorescent dextran 150,000 MW prior to and after ischemia reperfusion. No difference was found indicating that effects were independent from exposure time to photoactivated dyes. Capillary perfusion reduction was always present after reperfusion in untreated, L-NMMA-treated, and L-NNA-treated animals, with increased venular leukocytes adhesion. Arteriolar vasomotion was induced by L-NMMA treatment. Capillary perfusion recovered in L-arg-treated hamsters, where capillary blood flow velocity was lower than in L-NMMA group and the number of adhering leukocytes was lower than in untreated controls, L-NMMA, and L-NNA groups. It is concluded that L-arg determines perfusion with increased blood flow heterogeneity while inhibition of NO preserves capillary perfusion causing appearance of vasomotion in the arterial network.

An enhanced high-resolution laser Doppler imager (EHR-LDI), configured to fit the demands of a measurement area containing separate microvessels, was evaluated for perfusion measurements in hamster cheek pouch preparations during ischemia, reperfusion, and pharmacologically induced vasodilation and vasoconstriction. Measurements in separate microvessels where the laser beam was smaller than the vessel diameter were referred to as red blood cell (RBC) velocity estimates, as previously validated in vitro, whereas a relative flow index, RFI (mean RBC velocity/tissue area), was introduced as a volumetric flow measure. Microvessel diameter and RBC velocity changes during ischemia, reperfusion, as well as during vasoconstriction and vasodilation correlated to the data obtained from the microscope. Correspondingly, during the described provocations anticipated volumetric flow changes were registered as changes in the RFI. When data on intravessel RBC velocity profiles are presented they reflect a parabolic flow profile usually seen in this size microvessel. The EHR-LDI appears a promising tool for investigation of the microvasculature, as it almost simultaneously provides information on relative changes of both in vivo RBC velocity and volumetric flow (RFI), although the latter estimate needs to be further refined.

An enhanced high-resolution laser Doppler imaging (EHR-LDI) technique intended for visualization of separate microvessels was evaluated by use of in vitro flow models. In EHR-LDI, a laser beam focused to a half-power diameter less than 40 microm successively scans the tissue under study in steps of 25 microm. Spatial blood flow variations within microvascular structures of 1.5 x 1.5 mm are rendered by 64 x 64 measurement sites. Individual microvessel diameters could be estimated and an average difference of 11 microm compared to microscopic measurements was obtained. For the flow algorithm used, the LDI output signal was found to scale linearly with average velocity (0-3.5 mm/s) when a plastic tube of inner diameter 175 microm was perfused with human blood (correlation coefficient 0.99). The LDI output signal was further found insensitive to hematocrit variations in the range 16-44%. Due to the limited laser light penetration in blood, a reduction in the LDI output signal was observed as the inner tube diameters were successively changed from 280 to 1400 microm.

A mathematical model was used to study the origin of complex self-sustained diameter oscillations in multibranched microvascular networks. The model includes three branching levels (order 3, 2, and 1 arterioles) of a microvascular network derived from in vivo observation in the hamster dorsal cutaneous muscle. The main biomechanical aspects covered by the model are (1) the dependence of the elastic and active wall stress on the inner radius and (2) the static and dynamic myogenic response. Simulations on isolated arterioles indicate that self-sustained periodic diameter oscillations may occur at constant transmural pressure. Conversely, simulations on the entire network reveal different oscillatory patterns, including periodic, quasiperiodic, and chaotic fluctuations. Chaos in the model is revealed by the presence of a broad noise-like component in the frequency spectrum and by the sensitivity dependence of model results on small perturbations. Our results suggest that, owing to the intrinsic nonlinearity of the system, a contracting mechanism, such as the myogenic response, may induce different oscillatory patterns. The change from periodic to chaotic oscillations may be a consequence of a modest variation in a parameter (systemic pressure or arterial resistance) not necessarily related to pathophysiological conditions. Accordingly, our in vivo observations in the skeletal muscle showed that in some instances arteriolar vasomotion is converted from regular to highly irregular patterns in basal conditions. Vasomotion is found to affect mean blood flow compared with the nonoscillatory steady state. Chaotic oscillations tend to maintain a constant ratio of blood flows entering into bifurcation vessels, whereas periodic vasomotion determines a different flow distribution at branches.

The effects of hypovolemic shock on the time-dependent diameter changes of small arteries and arterioles were studied in the hamster skin fold window preparation. This experimental model permits the visualization of the microvasculature without the effects of acute surgery, anesthesia, and exposure. In these conditions, all the arterial microvessels showed vasomotion, while the venules and small veins, that were also studied, did not show rhythmic diameter changes. Hemorrhage was induced by the withdrawal of blood through a chronically implanted arterial catheter. The mean arterial blood pressure was reduced to 40 mm Hg in 20 min, and was maintained at this value for an additional 30-min period. Reinfusion of the withdrawn blood was made at 50 min. During the shock period, vasomotion disappeared in all arterial vessels. The small arteries and arterioles, A1 (70-100 micron, mean diameter), A2 (40-70 micron, md), and A3 (15-40 micron, md), contracted by 20 +/- 7, 33 +/- 10, and 34 +/- 11% of the control mean diameter, respectively. A4 terminal arterioles (less than 15 micron, md) dilated after the onset of bleeding; their rhythmic diameter changes subsequently stopped and their mean diameter increased by 75 +/- 7% of the original value. V1 small veins (150-200 micron, md) contracted during shock, while V2 (35-55 micron, md), V3 (25-35 micron, md), and V4 (15-25 micron, md) venous vessels did not show any significant change. Reinfusion of shed blood caused the reappearance of vasomotion; control vasomotion patterns recovered after reinfusion. Our results indicate that the microcirculatory responses to hypovolemic shock are dependent on the vessel type; this inhomogeneous reactivity may be due to the different responsiveness of microvessels to the mechanisms elicited by hemorrhage.

The effects of systemic injections of vasoactive substances were studied in the micro-circulation of the hamster skin fold window preparation, which can be observed without anesthesia, exposure, and acute surgical procedures. The effects were characterized by the continuous measurement of the diameter of the arterial microvessels ranging from 100 to 8 micron. Power spectrum analysis was utilized to determine the frequency and the amplitude of the fundamental component of spontaneous diameter changes. Epinephrine and norepinephrine increased the frequency of vasomotion and reduced mean diameter at low dosages. Phentolamine reduced the frequency of vasomotion and increased mean diameter. Propranolol increased the frequency of vasomotion and did not significantly change mean diameter. Adenosine and verapamil suppressed vasomotion and increased mean diameter. These results are explained by postulating that low-dosage alpha-adrenergic receptor stimulation facilitates the spontaneous discharge of smooth muscle cells; beta-adrenergic receptor stimulation has the opposite effect, whereas beta-adrenergic receptor inhibition also enhances the vasomotor effect. Calcium entry blockers abolish the rhythmic discharge. This explanation of the activity of the various substances supports the hypothesis that the spontaneous vasomotion of the arterial microvessels is related to the intrinsic property of smooth muscle cells.