Skin blood flow influences near-infrared spectroscopy-derived measurements of tissue oxygenation during heat stress.

Near-infrared (NIR) spectroscopy is a noninvasive optical technique that is increasingly used to assess muscle oxygenation during exercise with the assumption that the contribution of skin blood flow to the NIR signal is minor or nonexistent. We tested this assumption in humans by monitoring forearm tissue oxygenation during selective cutaneous vasodilation induced by locally applied heat (n = 6) or indirect whole body heating (i.e., heating subject but not area surrounding NIR probes; n = 8). Neither perturbation has been shown to cause a measurable change in muscle blood flow or metabolism. Local heating (approximately 41 degrees C) caused large increases in the NIR-derived tissue oxygenation signal [before heating = 0.82 +/- 0.89 optical density (OD), after heating = 18.21 +/- 2.44 OD; P < 0.001]. Similarly, whole body heating (increase internal temperature 0.9 degrees C) also caused large increases in the tissue oxygenation signal (before heating = -0.31 +/- 1.47 OD, after heating = 12.48 +/- 1.82 OD; P < 0.001). These increases in the tissue oxygenation signal were closely correlated with increases in skin blood flow during both local heating (mean r = 0.95 +/- 0.02) and whole body heating (mean r = 0.89 +/- 0.04). These data suggest that the contribution of skin blood flow to NIR measurements of tissue oxygenation can be significant, potentially confounding interpretation of the NIR-derived signal during conditions where both skin and muscle blood flows are elevated concomitantly (e.g., high-intensity and/or prolonged exercise).     

Effects of erythrocyte flexibility on microvascular perfusion and oxygenation during acute anemia

Responses to exchange transfusion using red blood cells (RBCs) with normal and reduced flexibility were studied in the hamster window chamber model during acute moderate isovolemic hemodilution to determine the role of RBC membrane stiffness in microvascular perfusion and tissue oxygenation. Erythrocyte stiffness was increased by 30-min incubation in 0.02% glutaraldehyde solution, and unreacted glutaraldehyde was completely removed. Filtration pressure through 5-microm pore size filters was used to quantify stiffness of the RBCs. Anemic conditions were induced by two isovolemic hemodilution steps using 6% 70-kDa dextran to a hematocrit (Hct) of 18% (moderate hemodilution). The protocol continued with an exchange transfusion to reduce native RBCs to 75% of baseline (11% Hct) with either fresh RBCs (RBC group) or reduced-flexibility RBCs (GRBC group) suspended in 5% albumin at 18% Hct; a plasma expander (6% 70-kDa dextran; Dex70 group) was used as control. Systemic parameters, microvascular perfusion, capillary perfusion [functional capillary density (FCD)], and oxygen levels across the microvascular network were measured by noninvasive methods. RBC deformability for GRBCs was significantly decreased compared with RBCs and moderate hemodilution conditions. The GRBC group had a greater mean arterial blood pressure (MAP) than the RBC and Dex70 groups. FCD was substantially higher for RBC (0.81 +/- 0.07 of baseline) vs. GRBC (0.32 +/- 0.10 of baseline) and Dex70 (0.38 +/- 0.10 of baseline) groups. Microvascular tissue Po(2) was significantly lower for Dex70 and GRBC vs. RBC groups and the moderate hemodilution condition. Results were attributed to decreased oxygen uploading in the lungs and obstruction of tissue capillaries by rigidified RBCs, indicating that the effects impairing RBC flexibility are magnified at the microvascular level, where perfusion and oxygenation may define transfusion outcome.     

Effects of antioxidants on contracting spinotrapezius muscle microvascular oxygenation and blood flow in aged rats

Aged rats exhibit a decreased muscle microvascular O(2) partial pressure (Pmv(O(2))) at rest and during contractions compared with young rats. Age-related reductions in nitric oxide bioavailability due, in part, to elevated reactive O(2) species, constrain muscle blood flow (Qm). Antioxidants may restore nitric oxide bioavailability, Qm, and ameliorate the reduced Pmv(O(2)). We tested the hypothesis that antioxidants would elevate Qm and, therefore, Pmv(O(2)) in aged rats. Spinotrapezius muscle Pmv(O(2)) and Qm were measured, and oxygen consumption (Vm(O(2))) was estimated in anesthetized male Fisher 344 x Brown Norway hybrid rats at rest and during 1-Hz contractions, before and after antioxidant intravenous infusion (76 mg/kg vitamin C and 52 mg/kg tempol). Before infusion, contractions evoked a biphasic Pmv(O(2)) that fell from 30.6 +/- 0.9 Torr to a nadir of 16.8 +/- 1.2 Torr with an "undershoot" of 2.8 +/- 0.7 Torr below the subsequent steady-state (19.7 +/- 1.2 Torr). The principal effect of antioxidants was to elevate baseline Pmv(O(2)) from 30.6 +/- 0.9 to 35.7 +/- 0.8 Torr (P < 0.05) and reduce or abolish the undershoot (P < 0.05). Antioxidants reduced Qm and Vm(O(2)) during contractions (P < 0.05), while decreasing force production 16.5% (P < 0.05) and elevating the force production-to-Vm(O(2)) ratio (0.92 +/- 0.03 to 1.06 +/- 0.6, P < 0.05). Thus antioxidants increased Pmv(O(2)) by altering the balance between muscle O(2) delivery and Vm(O(2)) at rest and during contractions. It is likely that this effect arose from antioxidants reducing myocyte redox below the level optimal for contractile performance and directly (decreased tension) or indirectly (altered balance of vasoactive mediators) influencing O(2) delivery and Vm(O(2)).     

Model analyses of capillary growth and tissue oxygenation during hypoxia

Theoretical analyses were used to determine whether capillary growth is an adaptive response to hypoxia. Parameter values were obtained from models of transverse sections of muscles in which individual fibers were distributed in square-ordered arrays and capillaries were added to the perimeters of individual fibers in the arrays. Increasing the number of capillaries up to 2.0 per fiber increased hypoxic tolerance by 157% above that expected for a Krogh cylinder. However, increasing the number of capillaries from 2.0 to 4.0 per fiber increased hypoxic tolerance by only 18% and, assuming the entire perimeter of each fiber was perfused with blood, increased hypoxic tolerance by only 11% over the value obtained when capillary-to-fiber ratio was 4.0. Capillary growth during normal maturation may result in capillary-to-fiber ratios around 2.0, near the upper limit for producing marked changes in hypoxic tolerance. Therefore, capillary growth may not be an adaptive response to ambient hypoxia because there is little or no gas transport benefit derived from the additional capillaries.     

Ultrasound techniques for measurement of blood flow and tissue motion

This article will review the ability of ultrasound techniques to provide 3D information on arterial geometry, blood flow and tissue motion.3D systems. 3D datasets can be obtained by sequential acquisition of 2D slices. Ideally a transducer is required in which there is full control of beam steering within a 3D volume. This requires a 2D array consisting of several thousand elements. Prototype 2D arrays have been built which provide several 3D datasets per second.Blood velocity measurement. Current Doppler systems estimate only the component of velocity in the direction of the Doppler beam. Lack of knowledge of the direction of blood motion and also other effects associated with 'spectral broadening' limit the accuracy of velocity measurement. Improved accuracy can be obtained using vector Doppler systems using 2 or 3 beam directions; this approach is referred to as 'vector Doppler'.Tissue motion. Doppler techniques can also be used to detect tissue motion (Tissue Doppler Imaging or TDI). Motion of the artery wall can be calculated from the TDI images. It is possible to estimate simultaneously motion for adjacent diameters within the longitudinal plane, and to visualise the relative motion at different parts of the wall.     

Retinal venous oxygen saturation and cardiac output during controlled hemorrhage and resuscitation.

The objective was to test calibration of an eye oximeter (EOX) in a vitiligo swine eye and correlate retinal venous oxygen saturation (Srv(O(2))), mixed venous oxygen saturation (Sv(O(2))), and cardiac output (CO) during robust changes in blood volume. Ten anesthetized adult Sinclair swine with retinal vitiligo were placed on stepwise decreasing amounts of oxygen. At each oxygen level, femoral artery oxygen saturation (Sa(O(2))) and retinal artery oxygen saturation (Sra(O(2))) were obtained. After equilibration on 100% O(2), subjects were bled at 1.4 ml. kg(-1). min(-1) for 20 min. Subsequently, anticoagulated shed blood was reinfused at the same rate. During graded hypoxia, exsanguination, and reinfusion, Sra(O(2)) and Srv(O(2)) were measured by using the EOX, and CO and Sv(O(2)) were measured by using a pulmonary artery catheter. During graded hypoxia, Sra(O(2)) correlated with Sa(O(2)) (r = 0.92). Srv(O(2)) correlated with Sv(O(2)) (r = 0.89) during exsanguination and reinfusion. Sv(O(2)) and Srv(O(2)) correlated with CO during blood removal and resuscitation (r = 0.92). Use of vitiligo retinas improved the calibration of EOX measurements. In this robust hemorrhage model, Srv(O(2)) correlates with CO and Sv(O(2)) across the range of exsanguination and resuscitation.     

Mechanisms of blood flow during pneumatic vest cardiopulmonary resuscitation

Mechanisms of blood flow during cardiopulmonary resuscitation (CPR) were studied in a canine model with implanted mitral and aortic flow probes and by use of cineangiography. Intrathoracic pressure (ITP) fluctuations were induced by a circumferential pneumatic vest, with and without simultaneous ventilation, and by use of positive-pressure ventilation alone. Vascular volume and compression rate were altered with each CPR mode. Antegrade mitral flow was interpreted as left ventricular (LV) inflow, and antegrade aortic flow was interpreted as LV outflow. The pneumatic vest was expected to elevate ITP uniformly and thus produce simultaneous LV inflow and LV outflow throughout compression. This pattern, the passive conduit of "thoracic pump" physiology, was unequivocally demonstrated only during ITP elevation with positive-pressure ventilation alone at slow rates. During vest CPR, LV outflow started promptly with the onset of compression, whereas LV inflow was delayed. At compression rates of 50 times/min and normal vascular filling pressures, the delay was sufficiently long that all LV filling occurred with release of compression. This is the pattern that would be expected with direct LV compression or "cardiac pump" physiology. During the early part of the compression phase, catheter tip transducer LV and left atrial pressure measurements demonstrated gradients necessitating mitral valve closure, while cineangiography showed dye droplets moving from the large pulmonary veins retrograde to the small pulmonary veins. When the compression rate was reduced and/or when intravascular pressures were raised with volume infusion, LV inflow was observed at some point during the compressive phase. Thus, under these conditions, features of both thoracic pump and cardiac pump physiology occurred within the same compression. Our findings are not explained by the conventional conceptions of either thoracic pump or cardiac compression CPR mechanisms alone.     

Effect of microwave radiation on regional blood flow and tissue oxygenation in the brain

The effect of irradiation of the central nervous system by microwaves (MW) at a frequency of 2450 MHz and power 5-40 W on the regulation of cerebral circulation and oxygen supply to the nervous tissue were studied in rabbits. Local irradiation of the exposed cerebral cortex resulted in hyperemia and hyperoxia in the zone of exposure induced by the hyperthermal effect of MW. When the region of the medulla oblongata was irradiated even with low MW power (not leading to hyperthermia), the local circulation and oxygen tension increased in the whole brain, apparently due to the impairment of the regulation of the cerebral blood flow and oxygen supply to the brain tissue.     

Gender dimorphic tissue perfusion response after acute hemorrhage and resuscitation: role of vascular endothelial cell function

Proestrous female rodents are protected from the deleterious effects of trauma-hemorrhage that are observed in males. We hypothesized that the gender dimorphic outcome after trauma-hemorrhage might be related to gender differences in endothelial function and organ perfusion under such conditions. Male and cycle-matched proestrous female Sprague-Dawley rats underwent a midline laparotomy, hemorrhagic shock (40 mmHg for approximately 90 min), and resuscitation (Ringer lactate, 4x shed blood volume over 60 min). Various parameters were measured 2 h after completion of resuscitation. In the first set of animals, the left ventricle was cannulated and heart performance (maximal rate of left ventricular pressure increase) as well as cardiac output and organ perfusion rates were determined with (85)Sr microspheres. In the second set of animals, aortic vessel rings were harvested and relaxation in response to acetylcholine and nitroglycerin was measured. In the third set of animals, in situ isolated small intestine was perfused to measure the response of the splanchnic vessel bed to acetylcholine and nitroglycerin. After trauma-hemorrhage and resuscitation, females maintained cardiac output and demonstrated increased splanchnic and cardiac perfusion compared with males. Moreover, female intestines did not manifest the endothelial dysfunction that was observed in male intestines after hemorrhagic shock. We conclude that proestrous females show improved endothelial function and tissue perfusion patterns after hemorrhagic shock and that this gender-specific response might be a potential mechanism contributing to the beneficial effects of the proestrus stage under such conditions.     

Correlation between tissue oxygenation and erythrocytes elasticity

In this paper, near‐infrared spectroscopy (NIRS) and jumping optical tweezers were used to measure the tissue oxygenation and the elasticity of erythrocytes, respectively. The correlation between tissue oxygenation induced by arterial occlusion test (AOT) and the mechanical properties of individual erythrocytes from a blood sample obtained after AOT was studied. The experimental results show a linear correlation between the oxygenation signal caused by AOT and the elasticity of erythrocytes. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Changes in cerebral oxygenation and blood flow during LBNP in spinal cord-injured individuals.

Spinal cord-injured (SCI) individuals, having a sympathetic nervous system lesion, experience hypotension during sitting and standing. Surprisingly, they experience few syncopal events. This suggests adaptations in cerebrovascular regulation. Therefore, changes in systemic circulation, cerebral blood flow, and oxygenation in eight SCI individuals were compared with eight able-bodied (AB) individuals. Systemic circulation was manipulated by lower body negative pressure at several levels down to -60 mmHg. At each level, we measured steady-state blood pressure, changes in cerebral blood velocity with transcranial Doppler, and cerebral oxygenation using near-infrared spectroscopy. We found that mean arterial pressure decreased significantly in SCI but not in AB individuals, in accordance with the sympathetic impairment in the SCI group. Cerebral blood flow velocity decreased during orthostatic stress in both groups, but this decrease was significantly greater in SCI individuals. Cerebral oxygenation decreased in both groups, with a tendency to a greater decrease in SCI individuals. Thus present data do not support an advantageous mechanism during orthostatic stress in the cerebrovascular regulation of SCI individuals.     

Increased cardiac output and microvascular blood flow during mild hemoconcentration in hamster window model

The effect of small hematocrit (Hct) increases on cardiac index (cardiac output/body wt) and oxygen release to the microcirculation was investigated in the awake hamster window chamber model by means of exchange transfusions of homologous packed red blood cells. Increasing Hct between 8 and 13% from baseline increased cardiac index by 5-31% from baseline (P < 0.05) and significantly lowered systemic blood pressure (P < 0.05). The relationship between Hct and cardiac index is described by a second-order polynomial (R2 = 0.84; P < 0.05) showing that Hct increases up to 20% from baseline increase cardiac index, whereas increases over 20% from baseline decrease cardiac index. Combining this data with measurements of blood pressure allowed to determine total peripheral vascular resistance, which was a minimum at 8-13% Hct increase and was described by a second-order polynomial (R2 = 0.83; P < 0.05). Oxygen measurements in arterioles, venules, and the tissue at 8-13% Hct increase were identical to control; thus, as a consequence of increased flow and oxygen-carrying capacity, oxygen delivery and extraction increased, but the change was not statistically significant. Previous results with the same model showed that the observed effects are related to shear stress-mediated release of nitric oxide, an effect that should be also present in the heart microcirculation, leading to increased blood flow, myocardial oxygen consumption, and contractility. We conclude that a minimum viscosity level is necessary for generating the shear stress required for maintaining normal cardiovascular function.