Microsphere maps of regional blood flow and regional ventilation.

Systematically mapped samples cut from lungs previously labeled with intravascular and aerosol microspheres can be used to create high-resolution maps of regional perfusion and regional ventilation. With multiple radioactive or fluorescent microsphere labels available, this methodology can compare regional flow responses to different interventions without partial volume effects or registration errors that complicate interpretation of in vivo imaging measurements. Microsphere blood flow maps examined at different levels of spatial resolution have revealed that regional flow heterogeneity increases progressively down to an acinar level of scale. This pattern of scale-dependent heterogeneity is characteristic of a fractal distribution network, and it suggests that the anatomic configuration of the pulmonary vascular tree is the primary determinant of high-resolution regional flow heterogeneity. At approximately 2-cm(3) resolution, the large-scale gravitational gradients of blood flow per unit weight of alveolar tissue account for <5% of the overall flow heterogeneity. Furthermore, regional blood flow per gram of alveolar tissue remains relatively constant with different body positions, gravitational stresses, and exercise. Regional alveolar ventilation is accurately represented by the deposition of inhaled 1.0-microm fluorescent microsphere aerosols, at least down to the approximately 2-cm(3) level of scale. Analysis of these ventilation maps has revealed the same scale-dependent property of regional alveolar ventilation heterogeneity, with a strong correlation between ventilation and blood flow maintained at all levels of scale. The ventilation-perfusion (VA/Q) distributions obtained from microsphere flow maps of normal animals agree with simultaneously acquired multiple inert-gas elimination technique VA/Q distributions, but they underestimate gas-exchange impairment in diffuse lung injury.     

Contributions of ventilation and perfusion inhomogeneities to the VA/Q distribution.

The anatomic distributions of ventilation (VA) and perfusion (Q) in prone and supine dogs have been described in the literature. These data also provide frequency distributions, i.e., the distribution of lung units as a function of VA or Q. A comprehensive distribution that encompasses these two distributions is described, and the properties of the comprehensive distribution that determine the width of the VA/Q distribution are identified. Using data on the VA and Q distributions taken from various sources in the literature, we estimated the widths of the VA/Q distributions. The widths estimated from the independent data on the VA and Q distributions agree well with the widths obtained from gas exchange data. The analysis provides information about the relative contributions of the VA and Q distributions to the width of the VA/Q distribution. In the prone dog, the VA and Q distributions, as described by the available data, have different length scales, and we argue that these distributions are therefore not highly correlated. As a result, the variance of the VA/Q distributions is approximately the sum of the variances of the VA and Q distributions. Two-thirds of the variance in VA/Q is a result of nonuniform Q, and one-third is a result of nonuniform VA. In the supine dog, the variance of VA is larger than in the prone dog because of a vertical gradient and the variance of Q is larger, in part, because of a vertical gradient. Because the magnitudes of the vertical gradients of VA and Q are about equal, the vertical gradient of VA/Q is small, and these components of the VA and Q inhomogeneities contribute little to the width of the VA/Q distribution. The other components of Q inhomogeneity cause the additional variance of VA/Q in the supine dog.     

Fractal nature of regional ventilation distribution.

High-resolution measurements of pulmonary perfusion reveal substantial spatial heterogeneity that is fractally distributed. This observation led to the hypothesis that the vascular tree is the principal determinant of regional blood flow. Recent studies using aerosol deposition show similar ventilation heterogeneity that is closely correlated with perfusion. We hypothesize that ventilation has fractal characteristics similar to blood flow. We measured regional ventilation and perfusion with aerosolized and injected fluorescent microspheres in six anesthetized, mechanically ventilated pigs in both prone and supine postures. Adjacent regions were clustered into progressively larger groups. Coefficients of variation were calculated for each cluster size to determine fractal dimensions. At the smallest size lung piece, local ventilation and perfusion are highly correlated, with no significant difference between ventilation and perfusion heterogeneity. On average, the fractal dimension of ventilation is 1.16 in the prone posture and 1. 09 in the supine posture. Ventilation has fractal properties similar to perfusion. Efficient gas exchange is preserved, despite ventilation and perfusion heterogeneity, through close correlation. One potential explanation is the similar geometry of bronchial and vascular structures.     

Differential responses of regional sympathetic activity and blood flow to visceral afferent stimulation

The sympathetic nervous system is essential for the cardiovascular responses to stimulation of visceral afferents. It remains unclear how the reflex-evoked sympathetic output is distributed to different vascular beds to initiate the hemodynamic changes. In the present study, we examined changes in regional sympathetic nerve activity and blood flows in anesthetized cats. Cardiovascular reflexes were induced by either electrical stimulation of the right splanchnic nerve or application of 10 microg/ml of bradykinin to the gallbladder. Blood flows were measured using colored microspheres or the Transonic flow meter system. Sympathetic efferent activity was recorded from the left splanchnic, inferior cardiac, and tibial nerves. Stimulation of visceral afferents decreased significantly blood flows in the celiac (from 49 +/- 4 to 25 +/- 3 ml/min) and superior mesenteric (from 35 +/- 4 to 23 +/- 2 ml/min) arteries, and the vascular resistance in the splanchnic bed was profoundly increased. Consistently, stimulation of visceral afferents decreased tissue blood flows in the splanchnic organs. By contrast, activation of visceral afferents increased significantly blood flows in the coronary artery and portal vein but did not alter the vascular resistance of the femoral artery. Furthermore, stimulation of visceral afferents increased significantly sympathetic efferent activity in the splanchnic (182 +/- 44%) but not in the inferior cardiac and tibial nerves. Therefore, this study provides substantial new evidence that stimulation of abdominal visceral afferents differentially induces sympathetic outflow to the splanchnic vascular bed.     

CT-measured regional specific volume change reflects regional ventilation in supine sheep.

Computer tomography (CT) imaging techniques permit the noninvasive measurement of regional lung function. Regional specific volume change (sVol), determined from the change in lung density over a tidal breath, should correlate with regional ventilation and regional lung expansion measured with other techniques. sVol was validated against xenon (Xe)-CT-specific ventilation (sV) in four anesthetized, intubated, mechanically ventilated sheep. Xe-CT used expiratory gated axial scanning during the washin and washout of 55% Xe. sVol was measured from the tidal changes in tissue density (H, houndsfield units) of lung regions using the relationship sVol = [1,000(Hi - He)]/[He(1,000 + Hi)], where He and Hi are expiratory and inspiratory regional density. Distinct anatomical markings were used to define corresponding lung regions of interest between inspiratory, expiratory, and Xe-CT images, with an average region of interest size of 1.6 +/- 0.7 ml. In addition, sVol was compared with regional volume changes measured directly from the positions of implanted metal markers in an additional animal. A linear relationship between sVol and sV was demonstrated over a wide range of regional sV found in the normal supine lung, with an overall correlation coefficient (R(2)) of 0.66. There was a tight correlation (R(2) = 0.97) between marker-measured volume changes and sVol. Regional sVol, which involves significantly reduced exposure to radiation and Xe gas compared with the Xe-CT method, represents a safe and efficient surrogate for measuring regional ventilation in experimental studies and patients.     

Ovine fetal electrocortical activity and regional cerebral blood flow

Fetuses of 12 near-term sheep were prepared for microsphere determination of cerebral blood flow. Experiments were performed 5 days postsurgery. The regional blood flows were measured in successive high (HV), low (LV) and high voltage electrocorticographic states. Comparisons were made between the observations made in the LV and averaged flanking HV cycles. Total cerebral blood flow was 95 +/- 8, 119 +/- 11 and 100 +/- 9 ml/min/100 g in HV, LV and HV, respectively. Low voltage electrocortical activity increased average cerebral blood flow by 22% (P less than 0.01). Significant changes were seen in all regions except the occipital cortex. The maximum change was observed in the thalamus in which the flows were 152 +/- 23, 243 +/- 35 and 138 +/- 20 ml/min/per 100 g tissue, respectively. The increase was 68% (P less than 0.001). The percent changes seen in the cerebrum are as follows: Frontal grey + 18%, frontal white + 22%, parietal white + 22%, temporal + 18%. A + 17% change was seen in the cord (P less than 0.03). It is concluded that in low voltage electrocortical activity all of the brain, except the occipital region, shows an increase in cerebral blood flow. This is probably secondary to a variance in cerebral activity. This preparation may be useful in localizing function in the fetal brain.     

Effects of surfactant depletion on regional pulmonary metabolic activity during mechanical ventilation

Inflammation during mechanical ventilation is thought to depend on regional mechanical stress. This can be produced by concentration of stresses and cyclic recruitment in low-aeration dependent lung. Positron emission tomography (PET) with (18)F-fluorodeoxyglucose ((18)F-FDG) allows for noninvasive assessment of regional metabolic activity, an index of neutrophilic inflammation. We tested the hypothesis that, during mechanical ventilation, surfactant-depleted low-aeration lung regions present increased regional (18)F-FDG uptake suggestive of in vivo increased regional metabolic activity and inflammation. Sheep underwent unilateral saline lung lavage and were ventilated supine for 4 h (positive end-expiratory pressure = 10 cmH(2)O, tidal volume adjusted to plateau pressure = 30 cmH(2)O). We used PET scans of injected (13)N-nitrogen to compute regional perfusion and ventilation and injected (18)F-FDG to calculate (18)F-FDG uptake rate. Regional aeration was quantified with transmission scans. Whole lung (18)F-FDG uptake was approximately two times higher in lavaged than in nonlavaged lungs (2.9 ± 0.6 vs. 1.5 ± 0.3 10(-3)/min; P < 0.05). The increased (18)F-FDG uptake was topographically heterogeneous and highest in dependent low-aeration regions (gas fraction 10-50%, P < 0.001), even after correction for lung density and wet-to-dry lung ratios. (18)F-FDG uptake in low-aeration regions of lavaged lungs was higher than that in low-aeration regions of nonlavaged lungs (P < 0.05). This occurred despite lower perfusion and ventilation to dependent regions in lavaged than nonlavaged lungs (P < 0.001). In contrast, (18)F-FDG uptake in normally aerated regions was low and similar between lungs. Surfactant depletion produces increased and heterogeneously distributed pulmonary (18)F-FDG uptake after 4 h of supine mechanical ventilation. Metabolic activity is highest in poorly aerated dependent regions, suggesting local increased inflammation.     

Effect of acute exercise stress in cardiac hypertrophy: I. correlation of regional blood flow and qualitative ultrastructural changes.

Ultrastructural myocardial cell changes were determined in eight miniswine after the development of pressure-overload hypertrophy induced by supra-valvular aortic constriction. Four miniswine served as control animals. Regional myocardial blood flows were measured at rest and during exercise stress with radioactive microspheres after two days and one month of aortic constriction. Exercise stress, causing the heart rate to increase to 85 percent of its maximum, was imposed twice weekly for 7 minutes on four pressure-overloaded animals and the four control animals to elicit differences between the control and experimental groups that might not occur at rest. After one month of pressure overload the swine were killed and myocardial samples were processed for electron microscopy. Ultrastructural changes similar to those in hypertrophied hearts were present throughout the left ventricular walls of the pressure-overloaded animals. Other changes consistent with ischemic injury were present in the subendocardial regions of pressure-overloaded animals subjected to exercise stress. These changes included disorganization of myofibrils, disintegration and broadening of Z-bands, swelling and aggregation of mitochondria, electron-dense deposits in mitochondria, decreased cristal density and vacuolization of mitochondria, intracellular edema, margination and clumping of nuclear chromatin, and a decrease of glycogen granules. Regional ischemia in the subendocardium of these animals was confirmed by functional studies which showed decreased regional myocardial blood flow to the subendocardium during exercise and S-T segment elevation for the first 2-10 days after inducing pressure overload. The ischemia, as shown by flow studies, during exercise stress persisted in the compensatory stage of hypertrophy although S-T segments returned to normal. Thus, the combined effect of pressure overload and exercise stress can produce focal subendocardial ischemia in the compensated, hypertrophied heart.     

Changes in regional myocardial volume during the cardiac cycle: implications for transmural blood flow and cardiac structure

Although previous studies report a reduction in myocardial volume during systole, myocardial volume changes during the cardiac cycle have not been quantitatively analyzed with high spatiotemporal resolution. We studied the time course of myocardial volume in the anterior mid-left ventricular (LV) wall of normal canine heart in vivo (n = 14) during atrial or LV pacing using transmurally implanted markers and biplane cineradiography (8 ms/frame). During atrial pacing, there was a significant transmural gradient in maximum volume decrease (4.1, 6.8, and 10.3% at subepi, midwall, and subendo layer, respectively, P = 0.002). The rate of myocardial volume increase during diastole was 4.7 +/- 5.8, 6.8 +/- 6.1, and 10.8 +/- 7.7 ml.min(-1).g(-1), respectively, which is substantially larger than the average myocardial blood flow in the literature measured by the microsphere method (0.7-1.3 ml.min(-1).g(-1)). In the early activated region during LV pacing, myocardial volume began to decrease before the LV pressure upstroke. We conclude that the volume change is greater than would be estimated from the known average transmural blood flow. This implies the existence of blood-filled spaces within the myocardium, which could communicate with the ventricular lumen. Our data in the early activated region also suggest that myocardial volume change is caused not by the intramyocardial tissue pressure but by direct impingement of the contracting myocytes on the microvasculature.     

Temperature-dependence of cardiac output and regional blood flow in rainbow trout, Salmo gairdneri Richardson

Cardiac output, blood flow distribution and regional perfusion were determined in free-swimming rainbow trout acclimated to 6, 12 and 18°C, using the indicator dilution and microsphere methods. Cardiac output (ml min⁻¹ kg⁻¹) increased linearly with increasing temperature, while circulation time decreased. Blood flow distribution (% of cardiac output) to the spleen, liver, kidney, gall bladder and gastro-intestinal tract was significantly reduced at 18°C relative to 6°C-acclimated fish. White muscle received the largest fraction of cardiac output, and blood flow distribution to white muscle increased significantly with increasing acclimation temperature. Blood perfusion (ml h⁻¹ g⁻¹) of various organs and red muscle was not influenced by acclimation temperature, while white muscle perfusion increased with increasing temperature. These results demonstrate physiological adaptation of the cardiovascular system of rainbow trout to changes in acclimation temperature.     

Contributions of pulmonary perfusion and ventilation to heterogeneity in VA/Q measured by PET

Treppo, Steven, Srboljub M. Mijailovich, and José G. Venegas. Contributions of pulmonary perfusion and ventilation toheterogeneity in A/measured by PET. J. Appl. Physiol. 82(4): 1163-1176, 1997. To estimate the contributions of the heterogeneity in regionalperfusion () and alveolar ventilation(A) to that of ventilation-perfusionratio (A/), we haverefined positron emission tomography (PET) techniques to image localdistributions of andA per unit of gas volume content(s and sA,respectively) and VA/ indogs. sA was assessed in two ways:1) the washout of ¹³NN tracer after equilibrationby rebreathing (sA_i), and2) the ratio of an apneic image after a bolus intravenousinfusion of ¹³NN-saline solution to an image collectedduring a steady-state intravenous infusion of the same solution(sA_p).sA_p was systematically higher than sA_i in allanimals, and there was a high spatial correlation betweens andsA_p in both body positions(mean correlation was 0.69 prone and 0.81 supine) suggesting thatventilation to well-perfused units was higher than to those poorlyperfused. In the prone position, the spatial distributions ofs, sA_p, and A/ were fairlyuniform with no significant gravitational gradients; however, in thesupine position, these variables were significantly more heterogeneous,mostly because of significant gravitational gradients (15, 5.5, and10%/cm, respectively) accounting for 73, 33, and 66% of thecorresponding coefficient of variation (CV)² values. Weconclude that, in the prone position, gravitational forces in blood andlung tissues are largely balanced out by dorsoventral differences inlung structure. In the supine position, effects of gravity andstructure become additive, resulting in substantial gravitationalgradients in s andsA_p, with the higherheterogeneity inA/ caused by agravitational gradient in s, only partially compensated by that in sA.

     

Gestational jet lag predisposes to later-life skeletal and cardiac disease

Circadian rhythm disturbance (CRD) increases the risk of disease, e.g. metabolic syndrome, cardiovascular disease, and cancer. In the present study, we investigated later life adverse health effects triggered by repeated jet lag during gestation. Pregnant mice were subjected to a regular light-dark cycle (CTRL) or to a repeated delay (DEL) or advance (ADV) jet lag protocol. Both DEL and ADV offspring showed reduced weight gain. ADV offspring had an increased circadian period, and an altered response to a jet lag was observed in both DEL and ADV offspring. Analysis of the bones of adult male ADV offspring revealed reduced cortical bone mass and strength. Strikingly, analysis of the heart identified structural abnormalities and impaired heart function. Finally, DNA methylation analysis revealed hypermethylation of miR17-92 cluster and differential methylation within circadian clock genes, which correlated with altered gene expression. We show that developmental CRD affects the circadian system and predisposes to non-communicable disease in adult life.     

Altered regional blood flow responses to submaximal exercise in older rats.

Maximal aerobic capacity and the ability to sustain submaximal exercise (Ex) declines with advancing age. Whether altered muscle blood flow (BF) plays a mechanistic role in these effects remains to be resolved. The present investigation determined the effects of aging on the hemodynamic and regional BF response to submaximal Ex in rats. Heart rate (HR), mean arterial pressure (MAP), and BF to different organs (kidneys, splanchnic organs, and 28 hindlimb muscles) were determined at rest and during submaximal treadmill Ex (20 m/min, 5% grade) with radiolabeled microspheres in young (Y; 6-8 mo old, 339 +/- 8 g, n = 9) and old (O; 27-29 mo old, 504 +/- 18 g, n = 7) Fischer 344 x Brown Norway rats. Results demonstrated that HR, MAP, and BF to the pancreas, small and large intestine, and total hindlimb musculature were similar between Y and O rats at rest. BF to the kidneys, spleen, and stomach were 33, 60, and 43% lower, respectively, in O compared with Y rats. BF to the total hindlimb musculature increased (P < 0.05) during Ex and was similar for both Y and O rats (Y: 16 +/- 3 to 124 +/- 7 vs. O: 20 +/- 3 to 137 +/- 12 ml.min-1.100 g-1). However, in O vs. Y rats, BF was reduced in 6 (highly oxidative) and elevated in 8 (highly glycolytic) of the 28 individual hindquarter muscles or muscle parts examined (P < 0.05). During Ex, BF to the spleen and stomach decreased (P < 0.05) from rest in Y rats, whereas BF decreased in the kidneys, pancreas, spleen, stomach, as well as the small and large intestines of O rats. In conclusion, these data demonstrate that, despite similar increases in total hindlimb BF in Y and O rats during submaximal Ex, there is a profound BF redistribution from highly oxidative to highly glycolytic muscles.