High-resolution imaging reveals a limit in spatial resolution of blood flow measurements by microspheres

The history of the microsphere method for measuring blood flows with special reference to myocardial blood flow: a personal memoir

We use many types of equipment and technologies to make our measurements but give little thought to how they developed. Evolution was once described as a series of recoils from blind alleys, and this is exemplified by the gradual development of the microsphere method of measuring blood flows. The microsphere method is one of the most frequently used methods for measuring blood flow to organs and portions of organs. The method can measure myocardial blood flow with reasonable accuracy (within 10%) down to samples weighing >50 mg but probably will not do so for samples weighing 1-10 mg. Microspheres with diameters from 10 to 15 μm provide the best compromise between accurate flow measurement and retention in tissue. Radioactive labels have been almst entirely replaced by fluorescent labels, but colored microspheres and neutron-activated labels are also used.NEW & NOTEWORTHY The contributions of the various individuals who developed the microsphere method of measuring regional blood flows and how these advances took place are brought to light in this paper.

Measurement and modeling of coronary blood flow

Ischemic heart disease that comprises both coronary artery disease and microvascular disease is the single greatest cause of death globally. In this context, enhancing our understanding of the interaction of coronary structure and function is not only fundamental for advancing basic physiology but also crucial for identifying new targets for treating these diseases. A central challenge for understanding coronary blood flow is that coronary structure and function exhibit different behaviors across a range of spatial and temporal scales. While experimental studies have sought to understand this feature by isolating specific mechanisms, in tandem, computational modeling is increasingly also providing a unique framework to integrate mechanistic behaviors across different scales. In addition, clinical methods for assessing coronary disease severity are continuously being informed and updated by findings in basic physiology. Coupling these technologies, computational modeling of the coronary circulation is emerging as a bridge between the experimental and clinical domains, providing a framework to integrate imaging and measurements from multiple sources with mathematical descriptions of governing physical laws. State-of-the-art computational modeling is being used to combine mechanistic models with data to provide new insight into coronary physiology, optimization of medical technologies, and new applications to guide clinical practice.

Microsphere skimming in the porcine coronary arteries: Implications for flow quantification

Particle skimming is a phenomenon where particles suspended in fluid flowing through vessels distribute disproportionately to bulk fluid volume at junctions. Microspheres are considered a gold standard of intra-organ perfusion measurements and are used widely in studies of flow distribution and quantification. It has previously been hypothesised that skimming at arterial junctions is responsible for a systematic over-estimation of myocardial perfusion from microspheres at the subendocardium. Our objective is to integrate coronary arterial structure and microsphere distribution, imaged at high resolution, to test the hypothesis of microsphere skimming in a porcine left coronary arterial (LCA) network. A detailed network was reconstructed from cryomicrotome imaging data and a Poiseuille flow model was used to simulate flow. A statistical approach using Clopper-Pearson confidence intervals was applied to determine the prevalence of skimming at bifurcations in the LCA. Results reveal that microsphere skimming is most prevalent at bifurcations in the larger coronary arteries, namely the epicardial and transmural arteries. Bifurcations at which skimming was identified have significantly more asymmetric branching parameters. This finding suggests that when using thin transmural segments to quantify flow from microspheres, a skimming-related deposition bias may result in underestimation of perfusion in the subepicardium, and overestimation in the subendocardium.

A quantitative high resolution voxel-wise assessment of myocardial blood flow from contrast-enhanced first-pass magnetic resonance perfusion imaging: microsphere validation in a magnetic resonance compatible free beating explanted pig heart model

AIMS

To assess the feasibility of high-resolution quantitative cardiovascular magnetic resonance (CMR) voxel-wise perfusion imaging using clinical 1.5 and 3 T sequences and to validate it using fluorescently labelled microspheres in combination with a state of the art imaging cryomicrotome in a novel, isolated blood-perfused MR-compatible free beating pig heart model without respiratory motion.

METHODS AND RESULTS

MR perfusion imaging was performed in pig hearts at 1.5 (n = 4) and 3 T (n = 4). Images were acquired at physiological flow ('rest'), reduced flow ('ischaemia'), and during adenosine-induced hyperaemia ('stress') in control and coronary occlusion conditions. Fluorescently labelled microspheres and known coronary myocardial blood flow represented the reference standards for quantitative perfusion validation. For the comparison with microspheres, the LV was divided into 48 segments based on a subdivision of the 16 AHA segments into subendocardial, midmyocardial, and subepicardial subsegments. Perfusion quantification of the time-signal intensity curves was performed using a Fermi function deconvolution. High-resolution quantitative voxel-wise perfusion assessment was able to distinguish between occluded and remote myocardium (P < 0.001) and between rest, ischaemia, and stress perfusion conditions at 1.5 T (P < 0.001) and at 3 T (P < 0.001). CMR-MBF estimates correlated well with the microspheres at the AHA segmental level at 1.5 T (r = 0.94, P < 0.001) and at 3 T (r = 0.96, P < 0.001) and at the subendocardial, midmyocardial, and subepicardial level at 1.5 T (r = 0.93, r = 0.9, r = 0.88, P < 0.001, respectively) and at 3 T (r = 0.91, r = 0.95, r = 0.84, P < 0.001, respectively).

CONCLUSION

CMR-derived voxel-wise quantitative blood flow assessment is feasible and very accurate compared with microspheres. This technique is suitable for both clinically used field strengths and may provide the tools to assess extent and severity of myocardial ischaemia.

Detection and quantification methods of monocyte homing in coronary vasculature with an imaging cryomicrotome

Cellular imaging modalities are important for revealing the behavior and role of monocytes in response to neovascularization progression in coronary artery disease. In this study we aimed to develop methods for high-resolution three-dimensional (3D) imaging and quantification of monocytes relative to the entire coronary artery network using a novel episcopic imaging modality. In a series of ex vivo experiments, human umbilical vein endothelial cells and CD14+ monocytes were labeled with fluorescent live cell tracker probes and infused into the coronary artery network of excised rat hearts by a Langendorff perfusion method. Coronary arteries were subsequently infused with fluorescent vascular cast material and processed with an imaging cryomicrotome, whereby each heart was consecutively cut (5 μm slice thickness) and block face imaged at appropriate excitation and emission wavelengths. The resulting image stacks yielded 3D reconstructions of the vascular network and the location of cells administered. Successful detection and quantification of single cells and cell clusters were achieved relative to the coronary network using customized particle detection software. These methods were then applied to an in vivo rabbit model of chronic myocardial ischemia in which autologous monocytes were isolated from peripheral blood, labeled with a fluorescent live cell tracker probe and re-infused into the host animal. The processed 3D image stacks revealed homing of monocytes to the ischemic myocardial tissue. Monocytes detected in the ischemic tissue were predominantly concentrated in the mid-myocardium. Vessel segmentation identified coronary collateral connections relative to monocyte localization. This study established a novel imaging platform to efficiently determine the localization of monocytes in relation to the coronary microvascular network. These techniques are invaluable for investigating the role of monocyte populations in the progression of coronary neovascularization in animal models of chronic and sub-acute myocardial ischemia.

Assessment of renal function; clearance, the renal microcirculation, renal blood flow, and metabolic balance

Historically, tools to assess renal function have been developed to investigate the physiology of the kidney in an experimental setting, and certain of these techniques have utility in evaluating renal function in the clinical setting. The following work will survey a spectrum of these tools, their applications and limitations in four general sections. The first is clearance, including evaluation of exogenous and endogenous markers for determining glomerular filtration rate, the adaptation of estimated glomerular filtration rate in the clinical arena, and additional clearance techniques to assess various other parameters of renal function. The second section deals with in vivo and in vitro approaches to the study of the renal microvasculature. This section surveys a number of experimental techniques including corticotomy, the hydronephrotic kidney, vascular casting, intravital charge coupled device videomicroscopy, multiphoton fluorescent microscopy, synchrotron-based angiography, laser speckle contrast imaging, isolated renal microvessels, and the perfused juxtamedullary nephron microvasculature. The third section addresses in vivo and in vitro approaches to the study of renal blood flow. These include ultrasonic flowmetry, laser-Doppler flowmetry, magnetic resonance imaging (MRI), phase contrast MRI, cine phase contrast MRI, dynamic contrast-enhanced MRI, blood oxygen level dependent MRI, arterial spin labeling MRI, x-ray computed tomography, and positron emission tomography. The final section addresses the methodologies of metabolic balance studies. These are described for humans, large experimental animals as well as for rodents. Overall, the various in vitro and in vivo topics and applications to evaluate renal function should provide a guide for the investigator or physician to understand and to implement the techniques in the laboratory or clinic setting.

Hyperemic stress myocardial perfusion cardiovascular magnetic resonance in mice at 3 Tesla: initial experience and validation against microspheres

BACKGROUND

Dynamic first pass contrast-enhanced myocardial perfusion is the standard CMR method for the estimation of myocardial blood flow (MBF) and MBF reserve in man, but it is challenging in rodents because of the high temporal and spatial resolution requirements. Hyperemic first pass myocardial perfusion CMR during vasodilator stress in mice has not been reported.

METHODS

Five C57BL/6 J mice were scanned on a clinical 3.0 Tesla Achieva system (Philips Healthcare, Netherlands). Vasodilator stress was induced via a tail vein catheter with an injection of dipyridamole. Dynamic contrast-enhanced perfusion imaging (Gadobutrol 0.1 mmol/kg) was based on a saturation recovery spoiled gradient echo method with 10-fold k-space and time domain undersampling (k-t PCA). One week later the mice underwent repeat anaesthesia and LV injections of fluorescent microspheres at rest and at stress. Microspheres were analysed using confocal microscopy and fluorescence-activated cell sorting.

RESULTS

Mean MBF at rest measured by Fermi-function constrained deconvolution was 4.1 ± 0.5 ml/g/min and increased to 9.6 ± 2.5 ml/g/min during dipyridamole stress (P = 0.005). The myocardial perfusion reserve was 2.4 ± 0.54. The mean count ratio of stress to rest microspheres was 2.4 ± 0.51 using confocal microscopy and 2.6 ± 0.46 using fluorescence. There was good agreement between cardiovascular magnetic resonance CMR and microspheres with no significant difference (P = 0.84).

CONCLUSION

First-pass myocardial stress perfusion CMR in a mouse model is feasible at 3 Tesla. Rest and stress MBF values were consistent with existing literature and perfusion reserve correlated closely to microsphere analysis. Data were acquired on a 3 Tesla scanner using an approach similar to clinical acquisition protocols, potentially facilitating translation of imaging findings between rodent and human studies.

3D Imaging of vascular networks for biophysical modeling of perfusion distribution within the heart

One of the main determinants of perfusion distribution within an organ is the structure of its vascular network. Past studies were based on angiography or corrosion casting and lacked quantitative three dimensional, 3D, representation. Based on branching rules and other properties derived from such imaging, 3D vascular tree models were generated which were rather useful for generating and testing hypotheses on perfusion distribution in organs. Progress in advanced computational models for prediction of perfusion distribution has raised the need for more realistic representations of vascular trees with higher resolution. This paper presents an overview of the different methods developed over time for imaging and modeling the structure of vascular networks and perfusion distribution, with a focus on the heart. The strengths and limitations of these different techniques are discussed. Episcopic fluorescent imaging using a cryomicrotome is presently being developed in different laboratories. This technique is discussed in more detail, since it provides high-resolution 3D structural information that is important for the development and validation of biophysical models but also for studying the adaptations of vascular networks to diseases. An added advantage of this method being is the ability to measure local tissue perfusion. Clinically, indices for patient-specific coronary stenosis evaluation derived from vascular networks have been proposed and high-resolution noninvasive methods for perfusion distribution are in development. All these techniques depend on a proper representation of the relevant vascular network structures.

A quantitative pixel-wise measurement of myocardial blood flow by contrast-enhanced first-pass CMR perfusion imaging: microsphere validation in dogs and feasibility study in humans

OBJECTIVES

The aim of this study was to evaluate fully quantitative myocardial blood flow (MBF) at a pixel level based on contrast-enhanced first-pass cardiac magnetic resonance (CMR) imaging in dogs and in patients.

BACKGROUND

Microspheres can quantify MBF in subgram regions of interest, but CMR perfusion imaging may be able to quantify MBF and differentiate blood flow at a much higher resolution.

METHODS

First-pass CMR perfusion imaging was performed in a dog model with local hyperemia induced by intracoronary adenosine. Fluorescent microspheres were the reference standard for MBF validation. CMR perfusion imaging was also performed on patients with significant coronary artery disease (CAD) by invasive coronary angiography. Myocardial time-signal intensity curves of the images were quantified on a pixel-by-pixel basis using a model-constrained deconvolution analysis.

RESULTS

Qualitatively, color CMR perfusion pixel maps were comparable to microsphere MBF bull's-eye plots in all animals. Pixel-wise CMR MBF estimates correlated well against subgram (0.49 ± 0.14 g) microsphere measurements (r = 0.87 to 0.90) but showed minor underestimation of MBF. To reduce bias due to misregistration and minimize issues related to repeated measures, 1 hyperemic and 1 remote sector per animal were compared with the microsphere MBF, which improved the correlation (r = 0.97 to 0.98), and the bias was close to zero. Sector-wise and pixel-wise CMR MBF estimates also correlated well (r = 0.97). In patients, color CMR stress perfusion pixel maps showed regional blood flow decreases and transmural perfusion gradients in territories served by stenotic coronary arteries. MBF estimates in endocardial versus epicardial subsectors, and ischemic versus remote sectors, were all significantly different (p < 0.001 and p < 0.01, respectively).

CONCLUSIONS

Myocardial blood flow can be quantified at the pixel level (∼32 μl of myocardium) on CMR perfusion images, and results compared well with microsphere measurements. High-resolution pixel-wise CMR perfusion maps can quantify transmural perfusion gradients in patients with CAD.

Cardiovascular magnetic resonance imaging in small animals

Noninvasive imaging studies involving small animals are becoming increasingly important in preclinical pharmacological, genetic, and biomedical cardiovascular research. Especially small animal magnetic resonance imaging (MRI) using high field and clinical MRI systems has gained significant importance in recent years. Compared to other imaging modalities, like computer tomography, MRI can provide an excellent soft tissue contrast, which enables the characterization of different kinds of tissues without the use of contrast agents. In addition, imaging can be performed with high spatial and temporal resolution. Small animal MRI cannot only provide anatomical information about the beating murine heart; it can also provide functional and molecular information, which makes it a unique imaging modality. Compared to clinical MRI examinations in humans, small animal MRI is associated with additional challenges. These included a smaller size of all cardiovascular structures and a up to ten times higher heart rate. Dedicated small animal monitoring devices make a reliable cardiac triggering and respiratory gating feasible. MRI in combination with molecular probes enables the noninvasive imaging of biological processes at a molecular level. Different kinds of iron oxide or gadolinium-based contrast agents can be used for this purpose. Compared to other molecular imaging modalities, like single photon emission computed tomography (SPECT) and positron emission tomography (PET), MRI can also provide imaging with high spatial resolution, which is of high importance for the assessment of the cardiovascular system. The sensitivity for detection of MRI contrast agents is however lower compared to sensitivity of radiation associated techniques like PET and SPECT. This chapter is divided into the following sections: (1) "Introduction," (2) "Principals of Magnetic Resonance Imaging," (3) "MRI Systems for Preclinical Imaging and Experimental Setup," and (4) "Cardiovascular Magnetic Resonance Imaging."

Rhodamine 800 as a near-infrared fluorescent deposition flow tracer in rodent hearts

We investigated the use of a near-infrared (NIR) fluorescent dye, Rhodamine 800 (Rhod800, λ(exc) = 693 nm, λ(em) > 720 nm) as a flow-dependent molecular tracer for NIR spectroscopy and high-resolution cardiac imaging. Rhod800 accumulates in isolated mitochondria in proportion to the mitochondrial membrane potential (ΔΨ). However, in the intact myocardium, Rhod800 binding is ΔΨ-independent. Rat hearts were perfused in a Langendorff mode with Krebs-Henseleit buffer containing 45-nM Rhod800 at normal (100%), increased (150%), or reduced (50%) baseline coronary flow (CF) per gram, for 30 to 60 min. In a different group of hearts, the left anterior descending artery (LAD) was occluded prior to Rhod800 infusion to create a flow deficit area. Rhod800 deposition was analyzed by: 1. absorbance spectroscopy kinetics in the Rhod800-perfused hearts, 2. Rhod800 absorbance and fluorescence imaging in the short-axis heart slices, and 3. dynamic epicardial/subepicardial fluorescence imaging of Rhod800 in KCl-arrested hearts, with a spatial resolution of ∼ 200 μm. Rhod800 deposition was proportional to the perfusate volume (CF and perfusion time) and there was no Rhod800 loss during the washout period. In the LAD-ligated hearts, Rhod800 fluorescence was missing from the no-flow, LAD-dependent endocardial and epicardial/subepicardial area. We concluded that Rhod800 can be used as a deposition flow tracer for dynamic cardiac imaging.

Fluorescence imaging to quantify the fluorescent microspheres in cardiac tissue

To quantify the fluorescent microsphere (FM) content in cardiac tissue, which is an indicative of blood flow, fluorescence imaging of both sides of the pig heart slice was employed. Despite the light scattering inside the tissue and contributions from multiple tissue layers to the total emission, it is shown that the fluorescence intensity at any pixel is proportional to the FM content and the fluorescence image may be transformed to the image of the FM concentration. A convenient standard for the emission-FM concentration transformation is proposed. The approach has several advantages in comparison with the traditional "digestion & extraction" method such as: non-destructiveness, high spatial resolution, high throughput, repeatability and simplicity of operation.

Improved detection of fluorescently labeled microspheres and vessel architecture with an imaging cryomicrotome

Due to spectral overlap, the number of fluorescent labels for imaging cryomicrotome detection was limited to 4. The aim of this study was to increase the separation of fluorescent labels. In the new imaging cryomicrotome, the sample is cut in slices of 40 μm. Six images are taken for each cutting plane. Correction for spectral overlap is based on linear combinations of fluorescent images. Locations of microspheres are determined by using the system point spread function. Five differently colored microspheres were injected in vivo distributed over two major coronaries, the left anterior descending and left circumflex artery. Under absence of collateral flow, microspheres outside of target perfusion territories were not found and the procedure did not generate false positive detection when spectral overlap was relevant. In silico-generated microspheres were used to test the effect of background image, transparency correction, and color separation. The percentage of microspheres undetected was 2.3 ± 0.8% in the presence and 1.5 ± 0.4% in the absence of background structures with a density of 900 microspheres per color per cm³. The image analysis method presented here, allows for an increased number of experimental conditions that can be investigated in studies of regional myocardial perfusion.

Cardiovascular MRI in small animals

Imaging studies of cardiovascular disease in small rodents have become a prerequisite in preclinical cardiovascular research. Transgenic and gene-knockout models of cardiovascular diseases enables the investigation of the influence of single genes or groups of genes on disease pathogenesis. In addition, experimental and genetically altered models provide valuable in vivo platforms to investigate the efficacy of novel drugs and contrast agents. Owing to the excellent soft tissue contrast, high spatial and temporal resolution, as well as the tomographic nature of MRI, anatomy and function can be assessed with unique accuracy and reproducibility. Furthermore, using novel targeted MRI contrast agents, molecular changes associated with cardiovascular disease can be investigated in the same imaging session. This review focuses on recent advances in hardware, imaging sequences and probe design.

MR in mouse models of cardiac disease

Transgenic and knockout mice can be used to study the genes and basic mechanisms involved in heart disease, and have therefore assumed a central role in modern cardiac research. MRI and MRS techniques have recently been developed for mice that enable the quantitative or semi-quantitative in vivo assessment of cardiac anatomy, function, perfusion, infarction, Ca(2+) influx, and metabolism. With these techniques, the normal mouse heart has been shown to be well suited as a model of human cardiac disease. The roles of individual genes in normal cardiac physiology have recently been studied by MR, including the role of neuronal nitric oxide synthase in beta-adrenergic stimulation, the roles of the inducible nitric oxide synthase and myoglobin in function, dilation, and energetics, and the role of cardiac troponin I in contractility. Furthermore, with a mouse model of myocardial infarction, the roles of the angiotensin II type 2 receptor, xanthine oxidase inhibitors, blood coagulation factor XIII, and inducible nitric oxide synthase in post-infarct function and remodeling have been further elucidated. Non-invasive in vivo MRI and MRS in mice provide a unique and powerful means for phenotyping genetically engineered mice and can improve our understanding of the roles of specific genes and proteins in cardiac physiology and pathophysiology.

Partitioning locomotor energy use among and within muscles Muscle blood flow as a measure of muscle oxygen consumption

Linking the mechanics and energetics of locomotion in vertebrates has been hampered by a lack of information regarding the energy use of individual skeletal muscles in vivo. Here, we present a review of the available data concerning the relationship between the rates of skeletal muscle blood flow and oxygen consumption(V̇O2). In active muscle, during aerobically supported exercise, there is a linear relationship between these variables, irrespective of the muscle fiber type and intensity of exercise through most of the aerobic exercise range. We conclude that the rate of blood flow is the best available indicator of aerobic metabolic rate in multiple individual muscles or regions of muscles during locomotion. The practical considerations of using the injectable microsphere technique to measure muscle blood flow in this context are discussed.

Comparing microsphere deposition and flow modeling in 3D vascular trees

Blood perfusion in organs has been shown to be heterogeneous in a number of cases. At the same time, a number of models of vascular structure and flow have been proposed that also generate heterogeneous perfusion. Although a relationship between local perfusion and vascular structure has to exist, no model has yet been validated as an accurate description of this relationship. A study of perfusion and three-dimensional (3D) arterial structure in individual rat kidneys is presented, which allows comparison between local measurements of perfusion and model-based predictions. High-resolution computed tomography is used to obtain images of both deposited microspheres and of an arterial cast in the same organ. Microsphere deposition is used as an estimate of local perfusion. A 3D cylindrical pipe model of the arterial tree is generated based on an image of the arterial cast. Results of a flow model are compared with local microsphere deposition. High correlation (r(2) > 0.94) was observed between measured and modeled flows through the vascular tree segments. However, the relative dispersion of the microsphere perfusion measurement was two- to threefold higher than perfusion heterogeneity calculated in the flow model. Also, there was no correlation in the residual deviations between the methods. This study illustrates the importance of comparing models of local perfusion with in vivo measurements of perfusion in the same biologically realistic vascular tree.

Retrospective determination of the area at risk for reperfused acute myocardial infarction with T2-weighted cardiac magnetic resonance imaging: histopathological and displacement encoding with stimulated echoes (DENSE) functional validations

BACKGROUND

The aim of this study was to determine whether edema imaging by T2-weighted cardiac magnetic resonance (CMR) imaging could retrospectively delineate the area at risk in reperfused myocardial infarction. We hypothesized that the size of the area at risk during a transient occlusion would be similar to the T2-weighted hyperintense region observed 2 days later, that the T2-weighted hyperintense myocardium would show partial functional recovery after 2 months, and that the T2 abnormality would resolve over 2 months.

METHODS AND RESULTS

Seventeen dogs underwent a 90-minute coronary artery occlusion, followed by reperfusion. The area at risk, as measured with microspheres (9 animals), was comparable to the size of the hyperintense zone on T2-weighted images 2 days later (43.4+/-3.3% versus 43.0+/-3.4% of the left ventricle; P=NS), and the 2 measures correlated (R=0.84). The infarcted zone was significantly smaller (23.1+/-3.7; both P<0.001). To test whether the hyperintense myocardium would exhibit partial functional recovery over time, 8 animals were imaged on day 2 and 2 months later. Systolic strain was mapped with displacement encoding with stimulated echoes. Edema, as detected by a hyperintense zone on T2-weighted images, resolved, and regional radial systolic strain partially improved from 4.9+/-0.7 to 13.1+/-1.5 (P=0.001) over 2 months.

CONCLUSIONS

These findings are consistent with the premise that the T2 abnormality depicts the area at risk, a zone of reversibly and irreversibly injured myocardium associated with reperfused subendocardial infarctions. The persistence of postischemic edema allows T2-weighted CMR to delineate the area at risk 2 days after reperfused myocardial infarction.

Knockout of the neural and heart expressed gene HF-1b results in apical deficits of ventricular structure and activation

OBJECTIVE

Knockout of the neural and cardiac expressed transcription factor HF-1b causes electrophysiological abnormalities including fatal ventricular arrhythmias that occur with increasing frequency around the 4th week of postnatal life. This study addresses factors that may contribute to conduction disturbance in the ventricle of the HF-1b knockout mouse. Disruptions to gap junctional connexin40 (Cx40) have been reported in distal (i.e., apically located), but not proximal His-Purkinje conduction tissues of the HF-1b knockout mouse. This abnormality in myocardial Cx40 led us to address whether 4-week-old HF-1b knockout postnates display other disruptions to ventricular structure and function.

METHODS

Western blotting and immunoconfocal quantification of Cx43 and coronary arteriole density and function were undertaken in the ventricle. Electrical activation was described by optical mapping.

RESULTS

Western blotting and immunoconfocal microscopy indicated that overall levels of Cx43 (p<0.001) and percent of Cx43 localized in intercalated disks (p<0.001) were significantly decreased in the ventricular myocardium of knockouts relative to wildtype littermate controls. Analysis of the reduction in Cx43 level by basal and apical territories revealed that the decrease was most pronounced in the lower, apical half of the ventricle of knockouts relative to controls (p<0.001). Myocyte size also showed a significant decrease in the knockout, that was more marked within the apical half of the ventricle (p<0.05). Optical recordings of ventricular activation indicated apically localized sectors of slowed conduction in knockout ventricles not occurring in controls that could be correlated directly to tissues showing reduced Cx43. These discrete sectors of abnormal conduction in the knockout heart were resolved following point stimulation of the ventricular epicardium and thus were not explained by dysfunction of the His-Purkinje system. To further probe base-to-apex abnormalities in the HF-1b knockout ventricle, we analyzed coronary arterial structure and function. These analyses indicated that relative to controls, the apical ventricular territory of the HF-1b knockout had reductions in the density of small resistance vessels (p<0.01) and deficits in arterial function as assayed by bead perfusion (p<0.01).

CONCLUSION

The HF-1b knockout ventricle displays abnormalities in Cx43 level, myocyte size, activation spread and coronary arterial structure and function. These abnormalities tend to be more pronounced in the apical territory of the ventricle and seem likely to be factors contributing to the pathological disturbance of cardiac conduction that characterizes the heart of the HF-1b knockout mouse.

Regional myocardial perfusion under exchange transfusion with liposomal hemoglobin: in vivo and in vitro studies using rat hearts

The purpose of this study was to test the hypothesis that exchange transfusion with liposomal hemoglobin (LH) reduces the microheterogeneity of regional myocardial flows while sustaining cardiac function. Neo Red Cell mixed with albumin was used as the LH solution, in which the LH volume fraction was 17∼18% and hemoglobin density was nearly two-thirds smaller than in rat blood. Regional myocardial flows in left ventricular free walls were measured by tracer digitalradiography (100-μm resolution) in anesthetized rats with or without 50% blood-LH exchange transfusion. Within-layer flow distributions showed lower heterogeneity with ( n = 8) than without ( n = 8) LH transfusion. No extravasation of hemoglobin was confirmed by 3,3-diaminobenzidin staining ( n = 2). Carotid flow increased by 68% due to LH transfusion, whereas arterial pressure and heart rate remained unchanged. On the other hand, cross-circulated rat hearts ( n = 7) were used to evaluate the effects of 50% blood-LH exchange on coronary flow and tone preservation under 300-beats/min pacing and 100-mmHg perfusion pressure. Blood-LH exchange caused a 71% increase of coronary flow and 10% decrease of percent flow increase during hyperemia after 30-s flow interruption. Myocardial O2 supply and consumption increased by 9% and 10%, respectively, whereas myocardial O2 extraction remained unchanged. The large increases of in vivo carotid flow and coronary flow in cross-circulated hearts due to LH coperfusion could be explained by the reduction of apparent flow viscosity. These results suggest that under LH coperfusion, the microheterogeneity of myocardial flows decreases with increased coronary flow while fairly preserving coronary tone and cardiac function.