Spatial heterogeneity of local blood flow and metabolite content in dog hearts

Spatial organization of acute myocardial ischemia

INTRODUCTION

Myocardial ischemia is a pathological condition initiated by supply and demand imbalance of the blood to the heart. Previous studies suggest that ischemia originates in the subendocardium, i.e., that nontransmural ischemia is limited to the subendocardium. By contrast, we hypothesized that acute myocardial ischemia is not limited to the subendocardium and sought to document its spatial distribution in an animal preparation. The goal of these experiments was to investigate the spatial organization of ischemia and its relationship to the resulting shifts in ST segment potentials during short episodes of acute ischemia.

METHODS

We conducted acute ischemia studies in open-chest canines (N=19) and swines (N=10), which entailed creating carefully controlled ischemia using demand, supply or complete occlusion ischemia protocols and recording intramyocardial and epicardial potentials. Elevation of the potentials at 40% of the ST segment between the J-point and the peak of the T-wave (ST40%) provided the metric for local ischemia. The threshold for ischemic ST segment elevations was defined as two standard deviations away from the baseline values.

RESULTS

The relative frequency of occurrence of acute ischemia was higher in the subendocardium (78% for canines and 94% for swines) and the mid-wall (87% for canines and 97% for swines) in comparison with the subepicardium (30% for canines and 22% for swines). In addition, acute ischemia was seen arising throughout the myocardium (distributed pattern) in 87% of the canine and 94% of the swine episodes. Alternately, acute ischemia was seen originating only in the subendocardium (subendocardial pattern) in 13% of the canine episodes and 6% of the swine episodes (p<0.05).

CONCLUSIONS

Our findings suggest that the spatial distribution of acute ischemia is a complex phenomenon arising throughout the myocardial wall and is not limited to the subendocardium.

Fractal regional myocardial blood flows pattern according to metabolism, not vascular anatomy

Regional myocardial blood flows are markedly heterogeneous. Fractal analysis shows strong near-neighbor correlation. In experiments to distinguish control by vascular anatomy vs. local vasomotion, coronary flows were increased in open-chest dogs by stimulating myocardial metabolism (catecholamines + atropine) with and without adenosine. During control states mean left ventricular (LV) myocardial blood flows (microspheres) were 0.5-1 ml·g(-1)·min(-1) and increased to 2-3 ml·g(-1)·min(-1) with catecholamine infusion and to ∼4 ml·g(-1)·min(-1) with adenosine (Ado). Flow heterogeneity was similar in all states: relative dispersion (RD = SD/mean) was ∼25%, using LV pieces 0.1-0.2% of total. During catecholamine infusion local flows increased in proportion to the mean flows in 45% of the LV, "tracking" closely (increased proportionately to mean flow), while ∼40% trended toward the mean. Near-neighbor regional flows remained strongly spatially correlated, with fractal dimension D near 1.2 (Hurst coefficient 0.8). The spatial patterns remain similar at varied levels of metabolic stimulation inferring metabolic dominance. In contrast, adenosine vasodilation increased flows eightfold times control while destroying correlation with the control state. The Ado-induced spatial patterns differed from control but were self-consistent, inferring that with full vasodilation the relaxed arterial anatomy dominates the distribution. We conclude that vascular anatomy governs flow distributions during adenosine vasodilation but that metabolic vasoregulation dominates in normal physiological states.

Normal range and regional heterogeneity of myocardial perfusion in healthy human myocardium: assessment on dynamic perfusion CT using 128-slice dual-source CT

Information about myocardial perfusion in healthy hearts is essential for evaluating patients with ischemic heart disease. The purpose of this study was to determine the range and regional variability of myocardial perfusion in normal volunteers on dynamic perfusion computed tomography (CT). Myocardial perfusion was assessed in 19 healthy volunteers (age 33-60 years; 11 men) at rest and during adenosine-induced hyperemia using a 128-slice dual-source CT scanner. Data were quantified as cc/cc/min for the transmural myocardium based on a 17-segment American Heart Association model. Mean myocardial blood flows (MBF) were 1.73 ± 0.33 cc/cc/min during adenosine-induced hyperemia, 0.83 ± 0.21 cc/cc/min at rest, and perfusion reserve was 2.20 ± 0.53. Regional variability was 17 ± 5% for hyperemic perfusion, 18 ± 7% for resting, and 21 ± 6 % for perfusion reserve. Although statistically insignificant, perfusion in the septum was lower at rest and during hyperemia than in other regions. Women tended to have lower perfusion during hyperemia (1.65 ± 0.40 vs. 1.79 ± 0.28 cc/cc/min, P = 0.40), and higher perfusion at rest than men (0.91 ± 0.27 vs. 0.77 ± 0.15 cc/cc/min, P = 0.23), resulting in lower perfusion reserve (1.86 ± 0.31 vs. 2.45 ± 0.53, P = 0.11). This small cohort of healthy volunteers study reveals normal myocardial perfusion parameter on dynamic perfusion CT as follows: mean MBF is 1.73 ± 0.33 cc/cc/min during hyperemia, 0.83 ± 0.21 cc/cc/min at rest, and perfusion reserve is 2.20 ± 0.53. And the study also demonstrates considerable regional heterogeneity of the myocardial perfusion.

Interstudy Reproducibility of Quantitative Perfusion Cardiovascular Magnetic Resonance

PURPOSE

To determine the interstudy reproducibility of quantitative first-pass perfusion cardiovascular magnetic resonance with comparison of 2 previously described analysis techniques. There is no published data on the interstudy reproducibility of perfusion cardiovascular magnetic resonance which can be used to determine the significance of longitudinal changes in myocardial perfusion after pharmacologic or therapeutic interventions with defined sample sizes.

METHODS

Sixteen subjects (7 normal volunteers, 9 patients with coronary artery disease) had rest and adenosine stress perfusion cardiovascular magnetic resonance studies on two separate visits. A short axis slice was studied on each visit using a fast low-angle shot sequence. The global and regional myocardial perfusion reserve indices were calculated using 2 methods: model based constrained deconvolution with the Fermi function, and normalized upslopes. Reproducibility was defined as the standard deviation of the measurement differences, divided by the mean (coefficient of variation).

RESULTS

The reproducibility of global myocardial perfusion reserve indices was 21% in normal volunteers, which was similar to that in patients with coronary artery disease (CAD) (23%, p = .88). The reproducibility of regional myocardial perfusion reserve indices was 28% (p = .45 vs. global analysis). The reproducibility of global MPRi was superior with Fermi deconvolution compared with normalized upslopes (21% vs. 41%, p = .02).

CONCLUSION

At this stage of clinical development, the reproducibility of quantitative perfusion cardiovascular magnetic resonance is good, and superior using Fermi deconvolution in preference to upslope analysis.

Hyperdynamic Myocardial Response to Beta-Adrenergic Stimulation in Patients With Chest Pain and Normal Coronary Arteries

OBJECTIVES

The goal of this study was to test the hypothesis that an abnormal response to beta-adrenergic stimulation may play a role in the pathophysiology of chest pain in patients with normal coronary arteries.

BACKGROUND

The mechanism of angina-like (AL) chest pain in patients with angiographically normal coronary arteries remains controversial.

METHODS

Fifty-eight patients with AL pain and a normal coronary angiogram underwent dobutamine echocardiography (DE) to evaluate regional wall motion and intraventricular flow velocities (IFV). Control patients consisted of 22 matched patients free of angina and coronary artery disease. Abnormal IFV were defined as dagger-shaped Doppler spectrum > or =3 m/s.

RESULTS

Dobutamine-induced regional wall motion abnormalities did not develop in any of the patients. An IFV > or = 3 m/s was found in 28 patients (48%) with AL pain but in only 4 (18%) control patients (p < 0.05). In the subgroup of patients with AL pain and IFV > or =3 m/s, plasma renin concentration (PRC) was higher as compared with those with IFV <3 m/s (18 +/- 17 pg/ml vs. 9 +/- 6 pg/ml, p < 0.05). There were no differences in plasma ADR, NADR, or angiotensin-converting enzyme levels. Fourteen patients with angina and IFV > or =3 underwent control DE and blood sampling after 6 weeks treatment with 10 mg of bisoprolol. In these patients, a decrease in IFV (from 3.4 +/- 0.35 m/s to 2.46 +/- 0.64 m/s, p < 0.001) and a decrease in angina score (from 5.4 +/- 1.5 to 0.6 +/- 1.4, p < 0.001) were observed at follow-up.

CONCLUSIONS

The present data suggest that an exaggerated myocardial response to beta-adrenergic stimulation plays a role in the mechanisms of chest pain in some patients with normal coronary arteries.

Regional differences of myocardial infarct development and ischemic preconditioning

The spatial and temporal development of myocardial infarction depends on the area at risk (AAR), the severity and duration of blood flow reduction (energy supply) as well as on heart rate and regional wall function (energy demand). Both supply and demand can vary within the AAR of a given heart, potentially resulting in differences in infarct development. We therefore retrospectively analyzed infarct size (IS, %AAR, TTC) in 24 anesthetized pigs in vivo following 90 min hypoperfusion and 120 min reperfusion of the LAD coronary artery, which supplies parts of the LV septum (LVS) and anterior free wall (LVAFW). The total LAD perfusion territory averaged 49.8 +/- 14.2 (SD) g (49.2 +/- 8.4% of LV); 61.4 +/- 8.1% of the AAR was LVAFW. IS within the LVS was 25.3 +/- 15.1%, while IS within the LVAFW was 16.6 +/-10.1% (p<0.05). While ischemic blood flow (radiolabeled microspheres) did not differ between LVS (0.05 +/- 0.02 ml/min/g) and LVAFW (0.05 +/- 0.03 ml/min/g), perivascular connective tissue (56 +/- 9 vs. 38+/-7 microm(2), p < 0.05) and the capillary-to-myocyte distance (1.65 +/- 0.23 vs. 1.18 +/- 0.23 mm, p < 0.05) were larger in LVS than in LVAFW. Interestingly, IS in LVS (9.3 +/- 9.6%, n = 24) and LVAFW (9.2 +/- 9.1%) were reduced to the same absolute extent by ischemic preconditioning with one cycle of 10 min ischemia and 15 min reperfusion, suggesting that a similar regional difference exists also in the protection afforded by ischemic preconditioning. The mechanism(s) for that remain(s) to be established.

CONCLUSION

In pigs, regional differences in infarct development and protection from it exist in the LAD perfusion territory, which are independent of ischemic blood flow but apparently related to pre-existing structural differences.

Myocardial O2 consumption in porcine left ventricle is heterogeneously distributed in parallel to heterogeneous O2 delivery

Myocardial blood flow is unevenly distributed, but the cause of this heterogeneity is unknown. Heterogeneous blood flow may reflect heterogeneity of oxygen demand. The aim of the present study was to assess the relation between oxygen consumption and blood flow in small tissue regions in porcine left ventricle. In seven male, anesthetized, open-chest pigs, local oxygen consumption was quantitated by computational model analysis of the incorporation of 13C in glutamate via the tricarboxylic acid cycle during timed infusion of [13C]acetate into the left anterior descending coronary artery. Blood flow was measured with radioactive microspheres before and during acetate infusion. High-resolution nuclear magnetic resonance 13C spectra were obtained from extracts of tissue samples (159 mg mean dry wt) taken at the end of the acetate infusion. Mean regional myocardial blood flow was stable [5.0 ± 1.6 (SD) and 5.0 ± 1.4 ml·min−1·g dry wt−1 before and after 30 min of acetate infusion, respectively]. Mean left ventricular oxygen consumption measured with the NMR method was 18.6 ± 7.7 μmol·min−1·g dry wt−1 and correlated well ( r = 0.85, P = 0.02, n = 7) with oxygen consumption calculated from blood flow, hemoglobin, and blood gas measurements (mean 22.8 ± 4.7 μmol·min−1·g dry wt−1). Local blood flow and oxygen consumption were significantly correlated ( r = 0.63 for pooled normalized data, P < 0.0001, n = 60). We calculate that, in the heart at normal workload, the variance of left ventricular oxygen delivery at submilliliter resolution is explained for 43% by heterogeneity in oxygen demand.

Myocardial density and composition: a basis for calculating intracellular metabolite concentrations

Systems for describing myocardial cellular metabolism with appropriate thermodynamic constraints on reactions have to be on the basis of estimates of intracellular and mitochondrial concentrations of metabolites as driving forces for reactions. This requires that tissue composition itself must be modeled, but there is marked inconsistency in the literature and no full data set on hearts of any species. To formulate a self-consistent set of information on the densities, contents, or concentrations of chemical components and volumes of tissue spaces, we drew on information mostly on rats. From the data on densities, volumes, volume fractions, and mass fractions observed mainly on left ventricular myocardium, cytoplasm, and mitochondria and from morphometric data on cellular components and the vasculature, we constructed a matrix based on conservation laws for density, volume, and constituent composition. The four constituents were water, protein, fat, and solutes (or ash). To take into account the variances in the observed data sets, we used a constrained nonlinear least squares optimization to minimize the differences between the final results and the data sets. The results provide a detailed estimate of cardiac tissue composition, previously unavailable, for the translation of whole tissue concentrations or concentrations per gram protein into estimated local concentrations that are relevant to reaction processes. An example is that the concentrations of phosphocreatine and ATP in cytosolic water space are twice as high as their mean tissue concentrations. This conservation optimization method is applicable to any tissue or organ.

The mechanical and metabolic basis of myocardial blood flow heterogeneity

Precise measurements of regional myocardial blood flow heterogeneity had to be developed before one could seek causation for the heterogeneity. Deposition techniques (particles or molecular microspheres) are the most precise, but imaging techniques have begun to provide high enough resolution to allow in vivo studies. Assigning causation has been difficult. There is no apparent association with the regional concentrations of energy-related enzymes or substrates, but these are measures of status, not of metabolism. There is statistical correlation between flow and regional substrate uptake and utilization. Attribution of regional flow variation to vascular anatomy or to vasomotor control appears not to be causative on a long-term basis. The closest relationships appear to be with mechanical function, but one cannot say for sure whether this is related to ATP hydrolysis at the crossbridge or associated metabolic reactions such as calcium uptake by the sarcoplasmic reticulum.

Validation of minimally invasive measurement of myocardial perfusion using electron beam computed tomography and application in human volunteers

OBJECTIVES

To measure myocardial perfusion using an estimate of intramyocardial vascular volume obtained by electron beam computed tomography (EBCT) in an animal model; to assess the feasibility and validity of measuring regional myocardial perfusion in human volunteers using the techniques developed and validated in the animal studies.

METHODS

Measurements of myocardial perfusion with EBCT employing intravenous contrast injections were compared with radioactive microsphere measurements (flow 57 to 346 ml/100 g/min) in seven closed chest dogs. Fourteen human volunteers then underwent EBCT scans using intravenous contrast injections.

RESULTS

Mean (SEM) global intramyocardial vascular volume by EBCT was 7.6 (1.1)%. The correlation between global EBCT (y) and microsphere (x) perfusion was y = 0.59x + 15.56 (r = 0.86) before, and y = 0.72x + 6. 06 (r = 0.88) after correcting for intramyocardial vascular volume. Regional perfusion correlation was y = 0.75x + 23.84 (r = 0.82). Corresponding improvements in agreement between the two techniques were also seen using Bland-Altman plots. In the human subjects, mean resting global myocardial flow was 98 (6) ml/100 g/min, with homogeneous flow across all regions. In 10 of these subjects, perfusion was studied during coronary vasodilatation using intravenous adenosine. Global flow increased from 93 (5) ml/100 g/min at rest to 250 (19) ml/100 g/min during adenosine (p < 0.001), with an average perfusion reserve ratio of 2.8 (0.2). Similar changes in regional perfusion were observed and were uniform throughout all regions, with a mean regional perfusion reserve ratio of 2.8 (0.3).

CONCLUSIONS

Accounting for intramyocardial vascular volume improves the accuracy of EBCT measurements of myocardial perfusion when using intravenous contrast injections. The feasibility of providing accurate measurements of global and regional myocardial perfusion and perfusion reserve in people using this minimally invasive technique has also been demonstrated.

Spatial heterogeneity of myocardial blood flow presages salvage versus necrosis with coronary artery reperfusion in conscious baboons

BACKGROUND

Myocardial blood flow distribution is known to be heterogeneous. It is also known that not all of the area at risk (AAR) infarcts with coronary artery occlusion (CAO) and coronary artery reperfusion (CAR). The goal of the present study was to determine whether the proportion of AAR that is salvaged or infarcted can be predicted by the pre-CAO level of myocardial blood flow, which varies considerably in individual samples as a result of natural heterogeneity.

METHODS AND RESULTS

The effects of 90-minute CAO followed by 5- to 7-day CAR were examined in six conscious baboons instrumented with aortic and left atrial catheters and coronary artery occluders. AAR was determined by dual perfusion. Myocardial blood flow was measured by radioactive microspheres before and after CAO and CAR. The AAR was cut into small pieces (0.21 +/- 0.01 g) and separated into two categories; salvaged (n = 252) or infarcted (n = 133). Analysis of myocardial blood flow distribution revealed two distinct populations (P < .01); infarcted tissues demonstrated higher pre-CAO myocardial blood flow than salvaged tissues. Importantly, 50% of the salvaged tissue samples were characterized by pre-CAO myocardial blood flows of < 0.90 mL.min-1.g-1 compared with 29% for infarcted samples, whereas 51% of infarcted samples were characterized by pre-CAO myocardial blood flows of > 1.12 mL.min-1.g-1 compared with 22% of salvaged samples. Endocardial analyses were qualitatively similar to transmural analyses.

CONCLUSIONS

This study suggests that heterogeneity of pre-CAO myocardial blood flows can predict the proportion of myocardium salvaged by CAR and can further explain the spatial heterogeneity of infarction that occurs after CAR, potentially independent of CAR injury.

Local continuity of myocardial blood flow studied by monochromatic synchrotron radiation-excited x-ray fluorescence spectrometry

We have developed a monochromatic synchrotron radiation-excited system for two-dimensional mapping of x-ray fluorescence evoked from heavy element-loaded microspheres, which can evaluate myocardial blood flow in small contiguous regions with a small methodological error: 10.8 +/- 2.4% of the average of difference of the dual flow for 7- to 10-mg myocardial tissue (4 dogs). The fractal D value obtained from the slope of the log relative dispersion-log mass plot was 1.21 +/- 0.08 for a voxel size of 7 to 1260 mg (5 dogs) and that for a voxel size of 2.5 to 40 mg (1.12 +/- 0.06) was smaller than that for a voxel size of 40 to 1280 mg (1.25 +/- 0.14, P < .05, ANOVA, 4 dogs). The distance-correlation coefficient relation for paired myocardial regions was attenuated (correlation analysis), and the correlation coefficients between the original grouping and the two aggregates of the adjacent regions were dissociated (extended correlation analysis) under reduction of coronary perfusion pressure (6 dogs). Suppression of myocardial contraction with lidocaine (3 dogs) and vasodilation with adenosine partly improved the distance-correlation coefficient relation under reduced coronary perfusion pressure. Thus, an x-ray fluorescence system designed for precise flow measurement shows that the fractal nature of local flow distribution can be extended into regions smaller than previously reported, that in these regions the flow becomes more homogeneous, and that the self similarity and continuity of local flow are attenuated by the reduction of coronary perfusion pressure and improved by contractile suppression and coronary vasodilation.

Correlation of heterogeneous blood flow and fatty acid uptake in the normal dog heart

Blood flow heterogeneity in normal myocardium may be caused by heterogeneous metabolic demand. We studied, from 80 tissue samples of the left ventricle (LV) of eight anesthetised, open-chest dogs (with prior beta-blockade (metoprolol) in four dogs), the radioactivity of 201Thallium-chloride (201Tl), an indicator of blood flow, and of the fatty acid 131-Iodine-heptadecanoic acid (131I-HDA), an indicator of metabolic demand, 3 min after intravenous injection. Global LV uptake (in percent of injected dose x 10(-2), per g tissue; mean +/- SD) was 4.94 +/- 0.71 for 201Tl and 4.48 +/- 0.58 for 131I-HDA in the dogs without beta-blockade, and 2.08 +/- 0.26 and 1.69 +/- 0.20, respectively, in dogs with beta-blockade (p < 0.05). Beta-blockade thus decreased the fraction of cardiac output delivered to the LV, concurrently with a decreased heart rate and arterial blood pressure (p < 0.05) and, thus, global metabolic demand and fatty acid uptake. Regional radioactivities per gram were normalized for mean LV radioactivities and heterogeneity was expressed as the coefficient of variation (CV). For pooled data (n = 320) in dogs without beta-blockade, regional 201Tl and 131I-HDA radioactivities varied from a factor of 0.1 to 1.6 and 0.3 to 1.8 of mean radioactivities, with a CV of 22.9 and 19.4%, respectively, and correlated (r = 0.77, p < 0.005). For pooled data (n = 320) in dogs with beta-blockade, regional 201Tl and 131I-HDA radioactivities varied from a factor of 0.2 to 1.5 and 0.2 to 1.6 of mean radioactivity and CV was 23.6% and 24.8%, respectively; r = 0.92 (p < 0.005). The endo/epi ratio for both radioactivities exceeded unity in each dog. In normal myocardium, blood flow and fatty acid uptake are thus heterogeneous, both transmurally and circumferentially, and matched, concomitantly with coupling of global blood flow to global metabolic demand and fatty acid uptake. This supports the idea that heterogeneous myocardial O2 supply reflects heterogeneous metabolic demand.

Heterogeneities in regional volumes of distribution and flows in rabbit heart

The heterogeneity of volumes of distribution in the heart influences the rates of uptake and washout of substrates and metabolites; thus it is important to evaluate their variability in the normal heart. Several tracers were injected intravenously into anesthetized adult closed-chest rabbits, and time was allowed for equilibration in the heart. Tracer microspheres were injected into the left ventricular cavity at the apex for the measurement of regional flows, the chest was opened, another set of microspheres was injected, and the heart was frozen rapidly in situ with liquid nitrogen-cooled Freon-22. Each heart was divided into 72 pieces of less than 0.1 g weight, and the tracer content of each was determined by multichannel gamma-counting and the water content by desiccation. The regional myocardial flows were (closed chest) 0.62 +/- 0.16 ml.g-1.min-1 and (open chest) 0.63 +/- 0.37 ml.g-1.min-1. The volumes of distribution (ml/g) for the 432 pieces for six rabbits, given as mean +/- SD (% coefficient of variation), were as follows: for plasma, VP = 0.11 +/- 0.03 (26%); erythrocytes, VRBC = 0.041 +/- 0.015 (37%); vascular space, VV = 0.15 +/- 0.04 (26%); extracellular space, VECF = 0.33 +/- 0.05 (15%); interstitial space, VISF = 0.21 +/- 0.03 (15%); and water space, VW -0.79 +/- 0.022 (2.8%). Regional hematocrits were 77% +/- 9% of the large-vessel hematocrits.

Regional myocardial flow heterogeneity explained with fractal networks

There is marked heterogeneity in regional myocardial blood flow. To explain how the distribution of flow broadens with an increase in the spatial resolution of the measurement, we developed fractal models for vascular networks. A dichotomous branching network of vessels represents the arterial tree and connects to a similar venous network. A small difference in vessel lengths and radii between the two daughter vessels, with the same degree of asymmetry at each branch generation, predicts the dependence of the relative dispersion (mean +/- SD) on spatial resolution of the perfusion measurement reasonably well. When the degree of asymmetry increases with successive branching, a better fit to data on sheep and baboons results. When the asymmetry is random, a satisfactory fit is found. These models show that a difference in flow of 20% between the daughter vessels at a branch point gives a relative dispersion of flow of approximately 30% when the heart is divided into 100-200 pieces. Although these simple models do not represent anatomic features accurately, they provide valuable insight on the heterogeneity of flow within the heart.