Heterogeneity Of Blood Flow Research Articles

Heterogeneity of Cerebral Blood Flow: a Fractal Approach

We demonstrate the heterogeneity of regional cerebral blood flow using a fractal approach and single-photon emission computed tomography (SPECT). Tc-99m-labelled ethylcysteine dimer was injected intravenously in 10 healthy controls and in 10 patients with dementia of frontal lobe type. The head was imaged with a gamma camera and transaxial, sagittal and coronal slices were reconstructed. Two hundred fifty-six symmetrical regions of interest (ROIs) were drawn onto each hemisphere of functioning brain matter. Fractal analysis was used to examine the spatial heterogeneity of blood flow as a function of the number of ROIs. Relative dispersion (= coefficient of variation of the regional flows) was fractal-like in healthy subjects and could be characterized by a fractal dimension of 1.17 +/- 0.05 (mean +/- SD) for the left hemisphere and 1.15 +/- 0.04 for the right hemisphere, respectively. The fractal dimension of 1.0 reflects completely homogeneous blood flow and 1.5 indicates a random blood flow distribution. Patients with dementia of frontal lobe type had a significantly lower fractal dimension of 1.04 +/- 0.03 than in healthy controls. Within the limits of spatial resolution of SPECT, the heterogeneity of brain blood flow is well characterized by a fractal dimension. Fractal analysis may help brain scientists to assess age-, sex- and laterality-related anatomic and physiological changes of brain blood flow and possibly to improve precision of diagnostic information available for patient care.

The 400 microsphere per piece "rule" does not apply to all blood flow studies.

Microsphere experiments are useful in measuring regional organ perfusion as well as heterogeneity of blood flow within organs and correlation of perfusion between organ pieces at different time points. A 400 microspheres/piece "rule" is often used in planning experiments or to determine whether experiments are valid. This rule is based on the statement that 400 microspheres must lodge in a region for 95% confidence that the observed flow in the region is within 10% of the true flow. The 400 microspheres precision rule, however, only applies to measurements of perfusion to a single region or organ piece. Examples, simulations, and an animal experiment were carried out to show that good precision for measurements of heterogeneity and correlation can be obtained from many experiments with <400 microspheres/piece. Furthermore, methods were developed and tested for correcting the observed heterogeneity and correlation to remove the Poisson "noise" due to discrete microsphere measurements. The animal experiment shows adjusted values of heterogeneity and correlation that are in close agreement for measurements made with many or few microspheres/piece. Simulations demonstrate that the adjusted values are accurate for a variety of experiments with far fewer than 400 microspheres/piece. Thus the 400 microspheres rule does not apply to many experiments. A "rule of thumb" is that experiments with a total of at least 15,000 microspheres, for all pieces combined, are very likely to yield accurate estimates of heterogeneity. Experiments with a total of at least 25,000 microspheres are very likely to yield accurate estimates of correlation coefficients.

Fractal heterogeneity of peripheral blood flow: Implications for hematogenous metastases

To determine how inhomogeneities in blood perfusion might affect the number of metastases that develop within an individual with cancer.Experiments with lung metastases in mice, involving 320 treatment groups and 3165 mice, were reviewed. Inhomogeneities in the distribution of metastases amongst identically treated mice were analyzed by calculating the relative dispersion and clumping index.The relative dispersion exhibited fractal self-similarity on change of scale, and paralleled the effects observed with pulmonary blood flow. Clustering of metastases was also apparent: a minority of mice developed relatively large numbers of metastases; a majority of mice developed few metastases.Clustering of lung metastases occurred within groups of identically treated mice, and could be attributed to inhomogeneous blood perfusion. Consequently, the number of metastases in any individual was highly variable and correlated only partly with malignant potential. Inhomogeneities in blood flow favored the development of relatively few metastases, such that solitary or nil metastasis should occur more frequently than expected from chance alone.

Characterization of the frequency distribution for human hematogenous metastases: evidence for clustering and a power variance function.

When groups of mice are injected with cells from a metastasizing tumor, a minority of individuals within a given group tends to sustain disproportionately larger numbers of metastases relative to the remaining group members. This clustering of metastases obeys a power function relationship, sigma2 = amub, between the variance sigma2 and the mean number of lung metastases per animal mu (a and b constant). To see whether such clustering occurs with human lung, brain, and liver metastases, a meta-analysis of clinical and pathological series was performed. Thirty-three published series were identified that provided data regarding the numbers of organ metastases sustained by 5582 people. The data were grouped according to the primary tumor, site of metastasis and method of detection of metastases. Clustering of metastases within individuals of each subgroup (similar to the murine systems) was demonstrated by variance to mean ratios greater than 1, and by a strong correlation to the variance to mean power function (a approximately 0.49, b approximately 2.24, r2 = 96%, p < 10(-6)). The cause of this clustering remains unclear, but it may in part relate to heterogeneities in regional blood flow. As a consequence of this clustering, limited metastases would be expected to occur more frequently than predicted from random chance-providing for some optimism in the management of limited metastasis. As well, the frequency distribution for metastases revealed certain scaling symmetries, likely reflective of the underlying mechanisms of metastasis, that could be of interest to both clinicians and experimentalists working with metastasis.

Nodular regenerative hyperplasia of the liver: SPECT evaluation with Tc-99m stannous colloid and Tc-99m neoglycoalbumin

The authors describe a case of nodular regenerative hyperplasia of the liver and its findings on liver SPECT with Tc-99m radiocolloid and Tc-99m neoglycoalbumin. A 22-year-old woman with systemic lupus erythematosus was hospitalized for intractable thrombocytopenia. A diagnostic work-up including CT revealed multiple liver nodules and portal hypertension with splenomegaly, suggesting hypersplenism as a cause of the thrombocytopenia. Liver SPECT with Kupffer cell–oriented Tc-99m stannous colloid and hepatocyte-oriented Tc-99m neoglycoalbumin showed increased uptake in the liver nodules, indicating the presence of Kupffer cells and hepatocytes. The patient underwent splenectomy and wedge liver biopsy. Pathologic analysis of the liver specimen revealed many hyperplastic nodules and sinusoidal congestion without cirrhosis. Based on these histopathologic features, the diagnosis of nodular regenerative hyperplasia of the liver was made. FIGURE 1Fig. 1: (A) CT shows no appreciable liver nodules. Contrast-enhanced liver nodules are demonstrated in (B) the arterial-dominant and (C) the portal-dominant phases of the sequential contrast-enhanced liver CT. (D) CT during arterioportography shows many contrast-enhanced liver nodules. The largest nodule is seen in the left medial segment. These findings indicate that these nodules had arterial and portal blood supplies. Although most nodules were described to be hypodense as seen on plain CT (1), all nodules in this case are isodense.FIGURE 2Fig. 2: Planar (A) anterior and (B) posterior images with Tc-99m stannous colloid show a heterogeneous distribution and focal increased uptake. Splenomegaly with increased splenic uptake is also noted. According to previous reports, patchy liver uptake, focal photopenic areas with focal patchy uptake within the nodule, and photon-deficient defects were described as the findings of nodular regenerative hyperplasia on radiocolloid liver scintigraphy (2,3).FIGURE 3Fig. 3: Transaxial SPECT images with Tc-99m stannous colloid show focal increased uptake in the liver. The obvious focal uptake (arrow) corresponds to the nodule seen in the left medial segment on the contrast-enhanced CT. Because Smith (3) has raised the possibility of this finding, the focal increased uptake could be related to hyperplasia of Kupffer cells and to possible heterogeneous blood flow (4).FIGURE 4Fig. 4: Transaxial SPECT images with Tc-99m neoglycoalbumin, which is a specific ligand for the asialoglycoprotein (galactose-terminated glycoprotein) receptor on the surface of the hepatocyte (5), show focal increased uptake in the same areas as in the Tc-99m stannous colloid SPECT. The obvious focal uptake (arrow) corresponds to the nodule seen in the left medial segment on Tc-99m stannous colloid SPECT. Because the presence of both Kupffer cells and virtually normal hepatocytes in the nodules were shown, these SPECT findings suggested that the hepatic nodules could have a benign character, although it was difficult to recognize small nodules and confluent nodules on the SPECT images. Multiple benign liver nodules associated with systemic lupus erythematosus and portal hypertension raised the possibility of nodular regenerative hyperplasia of the liver. The patient underwent splenectomy to treat the thrombocytopenia, and a wedge liver biopsy was performed. Histopathologic examination of the liver specimen revealed many noncirrhotic hyperplastic liver nodules without perinodular fibrosis, the sizes of which ranged from 1 mm to 2 cm in diameter. Microscopic examination showed that the the nodules consisted of hyperplastic normal hepatocytes. Perivascular fibrosis was also seen. However, fibrous septa or infiltration of the inflammatory cells, which were suggestive of cirrhosis, were not seen. The diagnosis of nodular regenerative hyperplasia of the liver was made based on these histopathologic findings (4).

Postmortem diffusion of autoradiographic blood flow tracers

The heterogeneity of blood flow in the brain under normo- and pathophysiological conditions, as well as during functional activation, has stimulated an interest in the use of autoradiography as a technique for the measurement of local cerebral blood flow. [ 14 C ]iodoantipyrine is the most prevalent tracer for the autoradiographic measurement of local cerebral blood flow since it is inert, nonvolatile, and is readily diffusible through the blood–brain barrier. The ability to diffuse freely in cerebral tissue, however, can lead to significant errors if the time duration between when the animal is sacrificed and when the tissue is frozen becomes appreciable, leading to significant postmortem diffusion of the tracer. Using an in vitro technique, the bulk diffusion coefficient for [ 14 C ]iodoantipyrine was measured in brain tissue (2.1×10 −6 cm 2/s). Cerebral blood flow was measured with [ 14 C ]iodoantipyrine in anesthetized rats. At the end of the radiotracer infusion, the brain was freeze-captured using a device consisting of two rapidly spinning stainless steel blades that were pneumatically driven through the head, freezing the tissue several hundred milliseconds following sacrifice. Autoradiograms from these brains exhibit considerable heterogeneity in blood flow. Computer simulations of the effect of tracer diffusion on these autoradiograms show significant degradation of the images highlighting the importance of very rapid postmortem freezing.

ET-1 Induces Pancreatitis-like Microvascular Deterioration and Acinar Cell Injury

Using in vivo microscopy red blood cell (RBC) velocities, functional capillary density (FCD) and capillary diameters were estimated after inducing acute pancreatitis by intraductal infusion of sodium taurocholate (0.8 ml; 4%) or after topical superfusion of the pancreas with ET-1 (100 pmol). Sodium taurocholate mediated a significant decrease in RBC velocities between 50 and 70%, transient decrease in capillary diameters by 10%, and a sustained decrease in FCD between 60 and 70% paralleled by a dramatic heterogeneity in blood flow. Topical superfusion of the exteriorized pancreas with ET-1 caused a significant decrease in RBC velocities between 65 and 75%, a sustained decrease in capillary diameters by 10%, and a decrease in FCD by 45% accompanied by an increase in flow heterogeneity. Following sodium taurocholate infusion pancreas histology revealed a severe edema and sublobular acinar cell necrosis, while topical ET-1 application displayed a severe edema of the pancreas with focal acinar cell necrosis. Thus, ET-1 mediated a deterioration of the pancreatic microcirculation, which is similar to the microcirculatory failure found in sodium taurocholate-induced experimental pancreatitis and was associated with focal acinar cell necrosis. We are thus inclined to hypothesize that endothelin released by injured endothelial cells during acute biliary pancreatitis promotes microcirculatory failure and ischemia in acute pancreatitis, eventually leading to acinar cell necrosis.

Heterogeneity of myocardial blood flow in man

Heterogeneity of myocardial blood flow in man

How much blood flow is required by the myocardium?

The myocardium of the left ventricle exhibits spatial heterogeneity of blood flow under physiological conditions. This study was designed to investigate, whether oxygen supply is jeopardized in low flow areas (blood flow < 50% of mean) under physiological conditions and whether areas of high flow (> 150% of mean) exhibit perfusion in excess of demand ("luxury perfusion"). The study was performed in anesthetized and ventilated beagle dogs. Local blood flow was reduced by mechanically narrowing of the r. circumflexus of the left coronary artery; myocardial blood flow was measured by the tracer-microsphere technique, free concentrations cellular adenosine by the SAH-technique, regional metabolism of substrates by the desoxyglucose-technique. Low flow areas exhibited normal oxygenation of the myocardium, while in high flow areas no luxury perfusion could be demonstrated. Myocardial blood flow and metabolism demonstrate significant spatial heterogeneity. There appears to be no absolute threshold of blood flow, where regional myocardial ischemia develops. Probably biochemical evidence of myocardial ischemia is determined by a local ratio of oxygen supply and demand.

Blood flow distribution and tissue allometry in channel catfish

Blood flow (as percentage of cardiac output) in fasted channel catfish acclimated to 21°C was directed primarily to white muscle (72%) followed by head kidney (5·7%), red muscle (5·5%), trunk kidney (3·1%), liver (2·2%), swim bladder (1·4%) and skin (1·1%). The stomach, intestines, pyloric caeca, gonads, brain, abdominal fat and spleen contained &lt;0·5% of blood flow. There was considerable interfish variation among blood flow distribution to visceral organs with substantial spatial heterogeneity of blood flow to white muscle. The spatial heterogeneity of flow to muscle prevented accurate estimation of total flow to this tissue based on the microsphere deposition of a few sub‐samples. Instead, a novel approach, based on the whole animal counting of the eviscerated carcass was used to measure blood flow to white muscle. The scaling relationships for tissue mass in catfish (63–1873 g) followed the allometric equation (aWb) and tended to exhibit negative allometry, with organ weight decreasing in proportion to body weight. The b values for most tissues ranged between 0·83 and 1·0. The relative mass of the brain showed the greatest decline and with a b value of 0·32. The results, together with previous data on cardiac output, permitted calculation of organ blood flow rates in channel catfish. © 1999 The Fisheries Society of the British Isles

Regulation of sinusoidal perfusion: in vivo methodology and control by endothelins.

Considerable attention has recently been focused on the phenomenon of active constriction of sinusoids as a mechanisms for regulating perfusion of the liver. Although many methods for estimating liver blood flow have been used in the past, the ability to directly study vascular responses in the sinusoids required the spatial and temporal resolution provided by intravital microscopy. Although techniques for viewing microvessels in thin tissues such as the mesentery or cremaster muscle have been available for many years, our current ability to fully use intravital microscopy to study microvascular responses and related metabolic parameters in thick tissues such as the liver has resulted from recent advances in fluorescence microscopy. Intravital microscopy can be used in in vivo or isolated perfused liver studies to assess changes in sinusoidal perfusion. Additional information concerning the relationship between microvascular changes and metabolic parameters in the liver can be simultaneously obtained by exploiting various recent advances in the design of fluorescent indicators. These techniques have allowed the mechanisms regulating sinusoid perfusion to be studied in great detail. It is now clear that sinusoids constrict in vivo in a graded and reversible manner in response to specific mediators such as endothelins. This constriction is modulated by dilators such as nitric oxide and carbon monoxide, which are also generated within the sinusoids. It is likely that poorly regulated sinusoid constriction contributes to liver injury and long-term development of increased intrahepatic vascular resistance. This response is mediated by alterations in the expression of endothelin receptor subtypes and eventually by phenotypic transformation of the hepatic stellate cells. In addition, local mismatch in the stress-induced induction of vasodilator and vasoconstrictor influences lead to an increase in the local heterogeneity of blood flow and oxygen supply. This heterogeneous perfusion contributes to the development of focal ischemia and progression of injury. Taken together, the results reviewed here indicate that the sinusoid is an important site of regulation of liver blood flow and that dysregulation of sinusoidal perfusion leads to propagation of liver injury.

A three-dimensional model for arterial tree representation, generated by constrained constructive optimization

The computational method of constrained constructive optimization (CCO) has been generalized in two important respects: (1) arterial model trees are now grown within a convex, three-dimensional piece of tissue and (2) terminal flow variability has been incorporated into the model to account for the heterogeneity of blood flow observed in real vascular beds. Although no direct information from topographic anatomy enters the model, computer-generated CCO trees closely resemble corrosion casts of real arterial trees, both on a visual basis and with regard to morphometric parameters. Terminal flow variability was found to induce transitions in the connective structure early in the trees' development. The present generalization of CCO offers — for the first time — the possibility to generate optimized arterial model trees in three dimensions, representing a realistic geometrical substrate for hemodynamic simulation studies. With the implementation of terminal flow variability the model is ready to simulate processes such as the adaptation of arterial diameters to changes in blood flow rate or the formation of different patterns of angiogenesis induced by changing needs of blood supply.

Positive end-expiratory pressure redistributes perfusion to dependent lung regions in supine but not in prone lambs.

To examine the influence of positive end-expiratory pressure (PEEP) and posture on the distribution of pulmonary blood flow. Experimental study. University animal laboratory. Seven anesthetized and mechanically-ventilated lambs. Four conditions were studied in random order: prone or supine position, with or without 5 cm H2O PEEP. The distribution of pulmonary blood flow was assessed using fluorescent-labeled microspheres (15 microm) in small (approximately 1.7 cm3) lung regions. Pulmonary blood flow heterogeneity was evaluated using the coefficient of variation of blood flow of the lung regions. The number of regions analyzed were 1290+/-154 (SD) per animal. PEEP increased pulmonary blood flow heterogeneity in the supine position (47.0+/-7.7% to 54.1+/-7.3%, p < .01, paired Student's t-test), but not in the prone position (40.4+/-8.1% to 39.6+/-11.5). Dorsal to ventral (gravitational) flow gradients were present only in the supine position, and increased with PEEP (-7.2%/cm vs. -10.4% cm, p< .001). PEEP redistributes pulmonary perfusion to dependent lung regions in supine, but not in prone, anesthetized and mechanically-ventilated sheep.

Blood flow heterogeneity in the heart.

Local deposition density of microspheres is heterogeneous in histologically homogeneous myocardium under physiological conditions. The underlying biological heterogeneity must be distinguished from a methodological heterogeneity which depends preferentially on the number of microspheres injected, blood flow to a particular myocardial region and sample mass. As the variables space (spat), time (temp), and method (meth) are independent of each other, the observed (obs) variability may be approximated using the coefficients of variation (CV) of the individual variables: CVobs = (CV2spat+CV2temp+CV2meth)0.5. Studies in which these different variables have been quantified indicate that the largest fraction of the observed variability of microsphere deposition density is contributed by spatial flow heterogeneity which exists independent of the myocardial layer. Spatial flow heterogeneity increases with decreasing sample mass and decreasing mean flow. Fractal and autocorrelation analyses have shown that adjacent myocardial flows are spatially correlated and nonrandom. Local blood flow was shown to correlate with various metabolic and transport rates, while no differences were found between low and high flow regions with respect to several metabolic markers of tissue hypoxia. In conclusion, the evidence available to date indicates that 1) in histologically homogeneous myocardium there exists a spatial blood flow heterogeneity which 2) is temporally stable, 3) resolution dependent, 4) largely layer-independent, 5) nonrandom, and 6) related to local aerobic metabolism.

Automatic full field analysis of perfusion images gained by scanning laser Doppler flowmetry

BACKGROUNDScanning laser Doppler flowmetry (SLDF) enables the measurement of the laser Doppler frequency shift in retinal tissue. This process allows the quantification of retinal and optic nerve head perfusion in...

Estimation of blood flow heterogeneity in human skeletal muscle using intravascular tracer data: importance for modeling transcapillary exchange.

Distributed models of blood-tissue exchange are widely used to measure kinetic events of various solutes from multiple tracer dilution experiments. Their use requires, however, a careful description of blood flow heterogeneity along the capillary bed. Since they have mostly been applied in animal studies, direct measurement of the heterogeneity distribution was possible, e.g., with the invasive microsphere method. Here we apply distributed modeling to a dual tracer experiment in humans, performed using an intravascular (indocyanine green dye, subject to distribution along the vascular tree and confined to the capillary bed) and an extracellular ([3H]-D-mannitol, tracing passive transcapillary transfer across the capillary membrane in the interstitial fluid) tracer. The goal is to measure relevant parameters of transcapillary exchange in human skeletal muscle. We show that assuming an accurate description of blood flow heterogeneity is crucial for modeling, and in particular that assuming for skeletal muscle the well-studied cardiac muscle blood flow heterogeneity is inappropriate. The same reason prevents the use of the common method of estimating the input function of the distributed model via deconvolution, which assumes a known blood flow heterogeneity, either defined from literature or measured, when possible. We present a novel approach for the estimation of blood flow heterogeneity in each individual from the intravascular tracer data. When this newly estimated blood flow heterogeneity is used, a more satisfactory model fit is obtained and it is possible to reliably measure parameters of capillary membrane permeability-surface product and interstitial fluid volume describing transcapillary transfer in vivo.

Haemodynamic and metabolic changes induced by repeated episodes of hypoxia in pigs.

Repeated hypoxia and surgical trauma trigger a potent neuroendocrine response and their association is thought to play a pivotal role in the pathogenesis of multi-organ dysfunction. We investigated the cardiovascular and metabolic responses to repeated acute hypoxia in anaesthetised and surgically instrumented pigs. Under ketamine-midazolam anaesthesia, 15 pigs were surgically instrumented for measurements of cardiac output, vascular pressures and organ blood flows. Lactate production and O2 uptake were determined in the brain, liver, kidney and intestine. Ten animals were subjected to two 12-min periods of ventilatory hypoxia (FIO2 = 7%) followed by re-oxygenation and 5 animals underwent 120-min normoxic ventilation (Control group). Both hypoxic challenges produced a comparable release of catecholamines that was associated with increased cardiac output and redistribution of blood flow away from the intestinal and renal areas towards the brain and the liver; O2 uptake was markedly reduced in the intestine (-56 +/- 10%, P < 0.05) and least affected in the brain and the kidney (-19 +/- 12% and -23 +/- 21%, respectively). During the second hypoxic test, lethal cardiovascular depression occurred in 5 animals; these non-survivors demonstrated impaired hyperdynamic response and incomplete recovery of intestinal O2 uptake during the first hypoxia/reoxygenation test. In the Control group, normoxic ventilation was not associated with significant haemodynamic and metabolic changes. Intraoperative hypoxia causes marked heterogeneity in organ blood flow and metabolism. The inability to develop a hyperdynamic cardiovascular response during a first hypoxic event, as well as a persistent intestinal O2 debt following re-oxygenation, predict the occurrence of death during the second hypoxic insult.

Coronary reserve of high- and low-flow regions in the dog heart left ventricle.

Left ventricular myocardial blood flow is spatially heterogeneous. The hypothesis we tested was whether myocardial areas with a steady-state flow <0.5 times mean flow are underperfused and areas with flow > 1.5 times mean flow are overperfused. In anesthetized beagle dogs (n=10), the relationship between local blood flow versus S-adenosylhomocysteine (SAH) concentration, a measure of the free intracellular adenosine concentration, and lactate, a measure of the myocardial NADH/NAD+ ratio, were determined under control conditions and after coronary constriction. Control local myocardial blood flow was 0.99+/-0.46 mL x min(-1) x g(-1), with a coefficient of variation of 0.36+/-0.12 (n=256 per heart; sample wet mass, 125+/-30 mg). Tissue concentrations of SAH (3.4+/-2.5 nmol/g) and lactate (1.88+/-0.80 micromol/g) were not elevated in low-flow samples. However, after coronary artery constriction, poststenotic blood flow decreased from 1.00+/-0.27 to 0.49+/-0.22 mL x min(-1) x g(-1) (P<0.04), with significant correlation between local SAH and flow (r=-0.59) and lactate and flow (r = -0.50). Although nearly all samples from control high-flow regions showed increased SAH concentrations if relative flow after stenosis was <1.0, control low-flow samples frequently displayed low SAH concentrations. The percent reduction in flow determined the changes in the local SAH and lactate concentration, independent of the local control blood flow. When the coronary inflow is unrestricted, the oxygen supply to control low-flow regions meets metabolic demand. Flow to control high-flow regions reflects a higher local demand rather than overperfusion. Thus, blood flow heterogeneity most likely reflects differences in aerobic metabolism.

A dual hypoxic marker technique for measuring oxygenation change within individual tumors

Rodent tumour models have been the 'workhorse' for tumour oxygenation research and for investigating radiobiological hypoxic fraction. Because of the intertumour heterogeneity of blood flow and related parameters, most studies have pooled information derived from several different tumours to establish the statistical significance of specific measurements. But it is the oxygenation status of and its modulation in individual tumours that has important prognostic significance. In that regard, the bioreducible hypoxic marker technique was tested for its potential to quantify oxygenation changes within individual tumours. Beta-D-iodinated azomycin galactoside (IAZG) and beta-D-iodinated azomycin xylopyranoside (IAZXP) were each radiolabelled with Iodine-125 and iodine-131 for measurements of animal tumour oxygenation. The tumour-blood (T/B) ratio of marker radioactivity in mice after the renal excretion of unbound marker (at 3 h and longer times) had been shown to be proportional to radiobiological hypoxic fraction. When markers labelled with both radioisotopes were administered simultaneously to EMT-6 tumour-bearing scid mice, T/B ratios were found to vary by up to 300% between different tumours, with an average intratumour variation of only approximately 4%. When the markers were administered 2.5-3.0 h apart, changes in T/B ratios of 8-25% were observed in 10 out of 28 (36%) tumours. Changes to both higher and lower hypoxic fraction were observed, suggestive of acute or cycling hypoxia. When 0.8 mg g(-1) nicotinamide plus carbogen was administered to increase tumour oxygenation, reductions in T/B ratios (mean deltaT/B approximately 38%) were observed in all tumours. Similar results were obtained with Dunning rat prostate carcinomas growing in Fischer x Copenhagen rats whose T/B ratios of IAZG and radiobiological hypoxic fractions are significantly lower. These studies suggest that fluctuating hypoxia can account for at least 25% of the total hypoxic fraction in some tumours and that correlations between bioreducible marker avidity and related tumour properties will be optimal when the independent assays are performed over the same time period. This dual hypoxic marker technique should prove useful for investigating both spontaneous and induced oxygenation changes within individual rodent tumours.

Preserved relative dispersion but blunted stimulation of mean flow, absolute dispersion, and blood volume by insulin in skeletal muscle of patients with essential hypertension.

We examined the integrity of the effects of insulin on mean muscle blood flow, flow heterogeneity, and blood volume in essential hypertension. Positron emission tomography, combined with [15O]H2O and [15O]CO as tracers for direct measurement of blood flow and volume in skeletal muscle, and a new bayesian iterative reconstruction algorithm allowing pixel-by-pixel quantitation of blood flow and flow dispersion, were used. Measurements were performed basally after an overnight fast and under normoglycemic hyperinsulinemic conditions in 11 newly diagnosed, untreated mildly hypertensive men (age, 35 +/- 1 years; body mass index, 25.2 +/- 0.4 kg/m2, blood pressure 141 +/- 4/96 +/- 2 mm Hg, mean +/- SE) and 11 matched normotensive men. Insulin-stimulated whole body glucose uptake was significantly decreased in the hypertensive men (41 +/- 4 mumol/kg per minute) compared with the normotensive (59 +/- 4 mumol/kg per minute, P < 0.005) men. Mean blood flow in skeletal muscle was significantly lower in the hypertensive than the normal subjects basally (1.7 +/- 0.2 versus 2.7 +/- 0.4 mL/0.1 kg per minute, P < 0.05) and during hyperinsulinemia (2.3 +/- 0.2 versus 4.2 +/- 0.8, P < 0.05). The flow response to insulin (0.6 +/- 0.2 versus 1.9 +/- 0.5 mL/0.1 kg per minute, hypertensive versus normal subjects, P < 0.05) was also significantly blunted. Muscle blood volume was significantly lower in the hypertensive than in the normal subjects, both basally (3.0 +/- 0.2 versus 3.5 +/- 0.2 mL/0.1 kg, P < 0.05) and during hyperinsulinemia (3.1 +/- 0.2 versus 4.0 +/- 0.2 mL/0.1 kg muscle, P < 0.02). The increase in muscle blood volume by insulin was significant in the normal (P < 0.05) but not the hypertensive subjects. Regional pixel-by-pixel analysis within femoral muscles revealed significant spatial heterogeneity of blood flow. Insulin increased absolute dispersion of blood flow significantly more in the normal subjects than in the hypertensive subjects (P < 0.05). True flow heterogeneity, as judged from the coefficients of variation (relative dispersion), was comparable between the groups basally and during hyperinsulinemia. We conclude that mean flow, its absolute dispersion, and blood volume exhibit insulin resistance in patients with essential hypertension.