Coronary microcirculation: physiology and pharmacology

Coronary microvascular dysfunction in childhood: An emerging pathological entity and its clinical implications

Coronary microvascular dysfunction (CMD) encompasses a spectrum of structural and functional alterations in coronary microvasculature resulting in impaired coronary blood flow and consequent myocardial ischemia without obstruction in epicardial coronary artery. The pathogenesis of CMD is complex involving both functional and structural alteration in the coronary microcirculation. In adults, CMD is predominantly discussed in context with anginal chest pain or existing ischemic heart disease and its risk factors. The presence of CMD suggests increased risk of adverse cardiovascular events independent of coronary atherosclerosis. Coronary microvascular dysfunction is also known in children but is rarely recognized due to paucity of concommitent coronary artery disease. Thus, its clinical presentation, underlying mechanism of impaired microcirculation, and prognostic significance are poorly understood. In this review article, we will overview variable CMD reported in children and delineate its emerging clinical significance.

The role of coronary microvascular dysfunction in the pathogenesis of heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) is a common condition with few effective therapies and hence represents a major healthcare burden. The clinical syndrome of HFpEF can be caused by varying pathophysiological processes, with coronary microvascular dysfunction (CMD) proposed as one of the aetiologies, although confirming causality has been challenging. CMD is characterised by the inability of the coronary vasculature to augment blood flow in response to a physiological stressor and has been established as the driver of angina in patients with non-obstructed coronaries (ANOCA), and this has subsequently led to efficacious endotype-directed therapies. CMD is also highly prevalent among sufferers of HFpEF and may represent a novel treatment target for this particular endotype of this condition. This review aims to discuss the role of the microcirculation in the healthy heart how it's dysfunction may precipitate HFpEF and explore the current diagnostic tools available. We also discuss the gaps in evidence and where we believe future research should be focussed.

Mixture of Doxycycline, ML-7 and L-NAME Restores the Pro- and Antioxidant Balance during Myocardial Infarction-In Vivo Pig Model Study

The restoration of blood flow to the ischemic myocardium inflicts ischemia/reperfusion (I/R) heart injury (IRI). The main contributors to IRI are increased oxidative stress and subsequent excessive production of ROS, increased expression of NOS and peroxinitate, activation of MMPs, and enhanced posttranslational modifications of contractile proteins, which make them more susceptible to proteolytic degradation. Since the pathophysiology of IRI is a complex issue, and thus, various therapeutic strategies are required to prevent or reduce IRI and microvascular dysfunction, in the current study we proposed an innovative multi-drug therapy using low concentrations of drugs applied intracoronary to reach microvessels in order to stabilize the pro- and antioxidant balance during a MI in an in vivo pig model. The ability of a mixture of doxycycline (1 μM), ML-7 (0.5 μM), and L-NAME (2 μM) to modulate the pro- and antioxidative balance was tested in the left ventricle tissue and blood samples. Data showed that infusion of a MIX reduced the total oxidative status (TOS), oxidative stress index (OSI), and malondialdehyde (MDA). It also increased the total antioxidant capacity, confirming its antioxidative properties. MIX administration also reduced the activity of MMP-2 and MMP-9, and then decreased the release of MLC1 and BNP-26 into plasma. This study demonstrated that intracoronary administration of low concentrations of doxycycline in combination with ML-7 and L-NAME is incredibly efficient in regulating pro- and antioxidant balance during MI.

Tachykinins Play a Major Role in Micro and Macrovascular Complications in Type 2 Diabetes Patients

Diabetes Mellitus is a metabolic disorder, which is characterized by an increase in blood glucose levels. The defects in the secretion or action of insulin are the major cause of diabetes. Increase in the blood glucose level exerts a negative effect on the normal functions of the body organs and this leads to the dysfunctions of cells and tissue and causes vascular complications in diabetic patients. Several studies indicate that neuropeptides are released from the neurosensory cells which are mainly known as tachykinins which provoke major vascular complications in diabetic patients. Tachykinins are known as pro-inflammatory peptides which increase vascular complications and vascular permeability. The duration and severity of diabetes disease increase the risk of vascular complication in patients. The aim of this review is to elaborate the role of tachykinins in microvascular and macrovascular complications in diabetic patients. The study concluded that tachykinins increase micro and macrovascular complications in diabetic patients.

Current understanding of the contribution of lactate to the cardiovascular system and its therapeutic relevance

Research during the past decades has yielded numerous insights into the presence and function of lactate in the body. Lactate is primarily produced via glycolysis and plays special roles in the regulation of tissues and organs, particularly in the cardiovascular system. In addition to being a net consumer of lactate, the heart is also the organ in the body with the greatest lactate consumption. Furthermore, lactate maintains cardiovascular homeostasis through energy supply and signal regulation under physiological conditions. Lactate also affects the occurrence, development, and prognosis of various cardiovascular diseases. We will highlight how lactate regulates the cardiovascular system under physiological and pathological conditions based on evidence from recent studies. We aim to provide a better understanding of the relationship between lactate and cardiovascular health and provide new ideas for preventing and treating cardiovascular diseases. Additionally, we will summarize current developments in treatments targeting lactate metabolism, transport, and signaling, including their role in cardiovascular diseases.

Coronary microvascular dysfunction: A review of recent progress and clinical implications

The coronary microcirculation plays a cardinal role in regulating coronary blood flow to meet the changing metabolic demands of the myocardium. Coronary microvascular dysfunction (CMD) refers to structural and functional remodeling of the coronary microcirculation. CMD plays a role in the pathogenesis of obstructive and non-obstructive coronary syndromes as well as myocardial diseases, including heart failure with preserved ejection fraction (HFpEF). Despite recent diagnostic advancements, CMD is often under-appreciated in clinical practice, and may allow for the development of novel therapeutic targets. This review explores the diagnosis and pathogenic role of CMD across a range of cardiovascular diseases, its prognostic significance, and the current therapeutic landscape.

Diagnostic performance and prognostic impact of coronary angiography-based Index of Microcirculatory Resistance assessment: A systematic review and meta-analysis

BACKGROUND

The Index of Microcirculatory Resistance (IMR), measured with a pressure-thermistor tipped coronary guidewire has been established as a gold standard for coronary microvascular assessment. Angiography-based IMR (angio-IMR) is a novel method to derive IMR without intracoronary instrumentation or the need for adenosine.

METHODS

PubMed and Embase databases were systemically searched in November 2021 for studies that measured angio-IMR. The primary outcomes were pooled sensitivity and specificity as well as the area under the curve (AUC) of the summary receiver operating characteristic curve using IMR as a reference standard.

RESULTS

A total of 129 records were initially identified and 8 studies were included in the final analysis. Overall, 1653 lesions were included in this study, of which 733 were in patients presenting with ST-segment elevation myocardial infarction. Angio-IMR yielded high diagnostic performance predicting wire-based IMR with pooled sensitivity = 0.81 (95% confidence interval: 0.76, 0.85), specificity = 0.80 (0.72, 0.86), and AUC = 0.86 (0.82, 0.88), which was similar irrespective of patient presentation. When the clinical outcome was compared between high versus low angio-IMR in patients presenting with myocardial infarction, high angio-IMR predicted an increased risk of major adverse cardiac events (MACE).

CONCLUSION

Our study found that coronary angio-IMR has relatively high diagnostic performance as well as prognostic values predicting MACE, supporting its use in clinical practice.

Preliminary study: myocardial T1 relaxation time in patients with ischemic findings and normal findings on coronary angiography

OBJECTIVE

The aim of this study is to evaluate the myocardium structure in patients with chest pain who were determined to have moderate and/or high risk for cardiac ischemic heart disease (IHD) but who had normal findings on conventional coronary angiography by using native cardiac magnetic resonance imaging (CMRI) T1 mapping and comparing with healthy volunteers.

METHODS

A total of 50 patients and 30 healthy volunteers who underwent CMRI were included in our prospective study. Patients whose clinical findings were compatible with stable angina pectoris, with moderate and/or high risk for IHD, but whose conventional coronary angiography was normal, were our patient group. Native T1 values were measured for 17 myocardial segments (segmented based on American Heart Association recommendations) by two radiologists independently. The data obtained were statistically compared with the sample t-test.

RESULTS

Myocardial native T1 values were found to be significantly prolonged in the patient group compared with the control group (p<0.05). Inter-observer reliability for native T1 value measurements of groups was high for both patient and control groups (α = 0.92 for the patient group and 0.96 for the control group).

CONCLUSION

Findings suggestive of ischemia were detected by T1 mapping in the myocardium of our patients. For this reason, it is recommended that this patient group should be included in early diagnosis and close follow-up assessments for IHD.

Ischemic Heart Disease Pathophysiology Paradigms Overview: From Plaque Activation to Microvascular Dysfunction

Ischemic heart disease still represents a large burden on individuals and health care resources worldwide. By conventions, it is equated with atherosclerotic plaque due to flow-limiting obstruction in large-medium sized coronary arteries. However, clinical, angiographic and autoptic findings suggest a multifaceted pathophysiology for ischemic heart disease and just some cases are caused by severe or complicated atherosclerotic plaques. Currently there is no well-defined assessment of ischemic heart disease pathophysiology that satisfies all the observations and sometimes the underlying mechanism to everyday ischemic heart disease ward cases is misleading. In order to better examine this complicated disease and to provide future perspectives, it is important to know and analyze the pathophysiological mechanisms that underline it, because ischemic heart disease is not always determined by atherosclerotic plaque complication. Therefore, in order to have a more complete comprehension of ischemic heart disease we propose an overview of the available pathophysiological paradigms, from plaque activation to microvascular dysfunction.

Pathophysiological coronary and microcirculatory flow alterations in aortic stenosis

Regulation of coronary blood flow is maintained through a delicate balance of ventriculoarterial and neurohumoral mechanisms. The aortic valve is integral to the functions of these systems, and disease states that compromise aortic valve integrity have the potential to seriously disrupt coronary blood flow. Aortic stenosis (AS) is the most common cause of valvular heart disease requiring medical intervention, and the prevalence and associated socio-economic burden of AS are set to increase with population ageing. Valvular stenosis precipitates a cascade of structural, microcirculatory, and neurohumoral changes, which all lead to impairment of coronary flow reserve and myocardial ischaemia even in the absence of notable coronary stenosis. Coronary physiology can potentially be normalized through interventions that relieve severe AS, but normality is often not immediately achievable and probably requires continued adaptation. Finally, the physiological assessment of coronary artery disease in patients with AS represents an ongoing challenge, as the invasive physiological measures used in current cardiology practice are yet to be validated in this population. This Review discusses the key concepts of coronary pathophysiology in patients with AS through presentation of contemporary basic science and data from animal and human studies.

Coronary Microcirculation in Aortic Stenosis

Aortic stenosis is a heterogeneous disorder. Variations in the pathological and physiological responses to pressure overload are incompletely understood and generate a range of flow and pressure gradient patterns, which ultimately cause varying microvascular effects. The impact of cardiac-coronary coupling depends on these pressure and flow effects. In this article, we explore important concepts concerning cardiac physiology and the coronary microcirculation in aortic stenosis and their impact on myocardial remodeling, aortic valve flow patterns, and clinical progression.

Coronary microvascular dysfunction in diabetes mellitus

The significance, mechanisms and consequences of coronary microvascular dysfunction associated with diabetes mellitus are topics into which we have insufficient insight at this time. It is widely recognized that endothelial dysfunction that is caused by diabetes in various vascular beds contributes to a wide range of complications and exerts unfavorable effects on microcirculatory regulation. The coronary microcirculation is precisely regulated through a number of interconnected physiological processes with the purpose of matching local blood flow to myocardial metabolic demands. Dysregulation of this network might contribute to varying degrees of pathological consequences. This review discusses the most important findings regarding coronary microvascular dysfunction in diabetes from pre-clinical and clinical perspectives.

Influence of increased heart rate and aortic pressure on resting indices of functional coronary stenosis severity

Baseline assessment of functional stenosis severity has been proposed as a practical alternative to hyperemic indices. However, intact autoregulation mechanisms may affect intracoronary hemodynamics. The aim of this study was to investigate the effect of changes in aortic pressure (Pa) and heart rate (HR) on baseline coronary hemodynamics and functional stenosis assessment. In 15 patients (55 ± 3% diameter stenosis) Pa, intracoronary pressure (Pd) and flow velocity were obtained at control, and during atrial pacing at 120 bpm, increased Pa (+30 mmHg) with intravenous phenylephrine (PE), and elevated Pa while pacing at sinus heart rate (PE + sHR). We derived rate pressure product (RPP = systolic Pa × HR), baseline microvascular resistance (BMR = Pd/velocity), and stenosis resistance [BSR = (Pa − Pd)/velocity] as well as whole-cycle Pd/Pa. Tachycardia (120 ± 1 bpm) raised RPP by 74% vs. control. Accordingly, BMR decreased by 27% (p < 0.01) and velocity increased by 36% (p < 0.05), while Pd/Pa decreased by 0.05 ± 0.02 (p < 0.05) and BSR remained similar to control. Raising Pa to 121 ± 3 mmHg (PE) with concomitant reflex bradycardia increased BMR by 26% (p < 0.001) at essentially unchanged RPP and velocity. Consequently, BSR and Pd/Pa were only marginally affected. During PE + sHR, velocity increased by 21% (p < 0.01) attributable to a 46% higher RPP (p < 0.001). However, BMR, BSR, and Pd/Pa remained statistically unaffected. Nonetheless, the interventions tended to increase functional stenosis severity, causing Pd/Pa and BSR of borderline lesions to cross the diagnostic threshold. In conclusion, coronary microvascular adaptation to physiological conditions affecting metabolic demand at rest influences intracoronary hemodynamics, which may lead to altered basal stenosis indices used for clinical decision-making.

Lipoprotein(a) as a predictor of poor collateral circulation in patients with chronic stable coronary heart disease

As a mechanism compensating for obstructive coronary artery disease, coronary collateral circulation (CCC) has attracted cardiologists for a long time to explore its potential impact. In the present study, Chinese patients suffering from ≥95% coronary stenosis, as diagnosed by angiography, have been investigated for the correlation between CCC and lipoprotein(a) [Lp(a)] levels. A cohort of 654 patients was divided into four categories according to Rentrop grades 0, 1, 2, and 3. Lp(a) levels were divided into model 1, discretized with critical values of 33 and 66%, and model 2, discretized with a cutoff value of 30.0 mg/dL. Furthermore, we evaluated the correlation between CCC and serum Lp(a) levels. The four groups had significantly different Lp(a) levels (25.80±24.72, 18.99±17.83, 15.39±15.80, and 8.40±7.75 mg/dL; P<0.001). In model 1, concerning R0, the risk in the third Lp (a) tertile (OR=3.34, 95%CI=2.32-4.83) was greater than that in the first tertile. In model 2, concerning R0, the risk in Lp(a) >30.0 group (OR=6.77, 95%CI=4.44-10.4) was greater than that of Lp(a) <30.0 mg/dL. The worst condition of CCC can be predicted independently by Lp(a) levels. In addition to clinical usage, Lp(a) levels can also be utilized as biological markers.

Targeting the dominant mechanism of coronary microvascular dysfunction with intracoronary physiology tests

The coronary microcirculation plays a key role in modulating blood supply to the myocardium. Several factors like myocardial oxygen demands, endothelial and neurogenic conditions determine its function. Although there is available evidence supporting microvascular dysfunction as an important cause of myocardial ischaemia, with both prognostic and symptomatic implications, its diagnosis and management in clinical practice is still relegated to a second plane. Both diagnostic and therapeutic approaches are hampered by the broadness of the concept of microvascular dysfunction, which fails addressing the plurality of mechanisms leading to dysfunction. Normal microcirculatory function requires both structural integrity of the microcirculatory vascular network and preserved signalling pathways ensuring adequate and brisk arteriolar resistance shifts in response to myocardial oxygen demands. Pathological mechanisms affecting these requirements include structural remodelling of microvessels, intraluminal plugging, extravascular compression or vasomotor dysregulation. Importantly, not every diagnostic technique provides evidence on which of these pathophysiological mechanisms is present or predominates in the microcirculation. In this paper we discuss the mechanisms of coronary microvascular dysfunction and the intracoronary tools currently available to detect it, as well as the potential role of each one to unmask the main underlying mechanism.

Depth-resolved 3D visualization of coronary microvasculature with optical microangiography

In this study, we propose a novel implementation of optical coherence tomography-based angiography combined with ex vivo perfusion of fixed hearts to visualize coronary microvascular structure and function. The extracorporeal perfusion of Intralipid solution allows depth-resolved angiographic imaging, control of perfusion pressure, and high-resolution optical microangiography. The imaging technique offers new opportunities for microcirculation research in the heart, which has been challenging due to motion artifacts and the lack of independent control of pressure and flow. With the ability to precisely quantify structural and functional features, this imaging platform has broad potential for the study of the pathophysiology of microvasculature in the heart as well as other organs.

Fractal analysis of the ischemic transition region in chronic ischemic heart disease using magnetic resonance imaging

OBJECTIVES

To introduce a novel hypothesis and method to characterise pathomechanisms underlying myocardial ischemia in chronic ischemic heart disease by local fractal analysis (FA) of the ischemic myocardial transition region in perfusion imaging.

METHODS

Vascular mechanisms to compensate ischemia are regulated at various vascular scales with their superimposed perfusion pattern being hypothetically self-similar. Dedicated FA software ("FraktalWandler") has been developed. Fractal dimensions during first-pass (FD_first-pass) and recirculation (FD_{recirculation}) are hypothesised to indicate the predominating pathomechanism and ischemic severity, respectively.

RESULTS

Twenty-six patients with evidence of myocardial ischemia in 108 ischemic myocardial segments on magnetic resonance imaging (MRI) were analysed. The 40th and 60th percentiles of FD_first-pass were used for pathomechanical classification, assigning lesions with FD_first-pass ≤ 2.335 to predominating coronary microvascular dysfunction (CMD) and ≥2.387 to predominating coronary artery disease (CAD). Optimal classification point in ROC analysis was FD_first-pass = 2.358. FD_{recirculation} correlated moderately with per cent diameter stenosis in invasive coronary angiography in lesions classified CAD (r = 0.472, p = 0.001) but not CMD (r = 0.082, p = 0.600).

CONCLUSIONS

The ischemic transition region may provide information on pathomechanical composition and severity of myocardial ischemia. FA of this region is feasible and may improve diagnosis compared to traditional noninvasive myocardial perfusion analysis.

KEY POINTS

• A novel hypothesis and method is introduced to pathophysiologically characterise myocardial ischemia. • The ischemic transition region appears a meaningful diagnostic target in perfusion imaging. • Fractal analysis may characterise pathomechanical composition and severity of myocardial ischemia.

Obesity Related Coronary Microvascular Dysfunction: From Basic to Clinical Practice

Obesity related coronary microvascular disease is a medical entity which is not yet fully elucidated. The pathophysiological basis of coronary microcirculatory dysfunction consists of a heterogeneous group of disorders with individual morphologic/functional/clinical presentation and prognosis. Coronary microcirculatory changes include mechanisms connected with vascular dysfunction, as well as extravascular and vasostructural changes in responses to neural, mechanical, and metabolic factors. Cardiometabolic changes that include obesity, dyslipidemia, diabetes mellitus type II, and hypertension are associated with atherosclerosis of epicardial coronary arteries and/or microvascular coronary dysfunction, with incompletely understood underlying mechanisms. In obesity, microvascular disease is mediated via adipokines/cytokines causing chronic, subclinical inflammation with (a) reduced NO-mediated dilatation, (b) changed endothelial- and smooth muscle-dependent vasoregulating mechanisms, (c) altered vasomotor control with increased sympathetic activity, and (d) obesity related hypertension with cardiomyocytes hypertrophy and impaired cardiac vascular adaptation to metabolic needs. From a clinical point of view it can present itself in acute or chronic form with different prognosis, as a practice problem for real-life diagnosis and treatment.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Histomorphometric evaluation of the small coronary arteries in rats exposed to industrial noise

Morphological changes induced by industrial noise (IN) have been experimentally observed in several organs. Histological observations of the coronary arteries showed prominent perivascular tissue and fibrosis among IN-exposed rats. The effects on the small arteries are unknown. Objective: To evaluate the histomorphometric changes induced by IN on rat heart small arteries. Methods: Twenty Wistar rats exposed to IN during a maximum period of seven months and 20 age-matched controls were studied. Hearts were transversely sectioned from ventricular apex to atria and a mid-ventricular fragment was selected for analysis. The histological images were obtained with an optical microscope using 400× magnifications. A total of 634 arterial vessels (298 IN-exposed and 336 controls) were selected. The mean lumen-to-vessel wall (L/W) and mean vessel wall-to-perivascular tissue (W/P) ratios were calculated using image J software. Results: There were no differences between exposed and control animals in their L/W ratios (p = 0.687) and time variations in this ratio were non-significant (p = 0.110). In contrast, exposed animals showed lower W/P ratios than control animals (p < 0.001), with significant time variations (p = 0.004). Conclusions: Industrial noise induced an increase in the perivascular tissue of rat small coronary arteries, with significant development of periarterial fibrosis.

Three-dimensional microvasculature in rat and human hearts using a non-injection Ca2+-ATPase method on thick and ultra-thick sections

Currently there are no methods available for staining rat and human myocardial microvasculature on thick sections that would allow for specific staining and differentiation of arterioles, venules, and capillaries. A non-injection technique is described that allows for labeling of the microvascular bed (MVB) in formalin-fixed pieces of the myocardium from humans and the white rat Rattus norvegicus, as well as human full-mount pericardium. Vessel staining is based on the activity of phosphatases (ATPases) and the precipitation of the released phosphate with calcium ions at high pH (pH 10.5-11.5). The resulting precipitate subsequently is converted to black or brown lead sulfide. The specificity of this reaction to vessels of the MVB allows arterioles, venules, capillaries, and pre- and postcapillaries to be clearly visualized in thick (60-100 µm) and ultra-thick (300-500 µm) sections against an unstained background of muscle and connective tissue. In addition, smooth muscle cells of arterioles are also stained allowing for differentiation between arteriolar and venular beds. These observations have not been reported in rat or human myocardium using other methods. This procedure should benefit studies of coronary microcirculation in experimental and pathological conditions, as well as in pharmacological investigations.

MODELING OF CORONARY ARTERY BALLOON-ANGIOPLASTY USING EQUIVALENT ELECTRICAL CIRCUIT

Arterial thrombosis and atherosclerosis result in chronic total or partial occlusion of the coronary artery. Coronary artery disease (CAD) destroys some parts of the heart muscle tissue and is the leading cause of human deaths in the industrialized world. In this study, cardiovascular system is simulated by 42 compartments and then a coronary set (including artery, venous, myocardium and capillaries) is added to the model. Each vessel is modeled by three parameters, such as resistor, capacitor and inductor. These three parameters are variable with respect to the radius of the vessel. In this paper, first of all, aortic and coronary flow under healthy condition is studied. The obtained results are in complete agreement with experimental outcomes. Then cardiovascular system behavior in coronary artery stenosis condition is investigated. Finally, the effect of intra-coronary balloon pump on heart attack risk and also on stabilization of patient's emergency condition by mathematical simulation is analyzed. The results of modeling show that the balloon pumping of coronary artery is an advantageous way in rendering primary cure to patients. The proposed model, in addition, has implications for investigation of effects of different diseases on the cardiovascular system. It also has the potential to model different treatment methods on heart's performance and, as a result, recommend new methods in order to cure variant cardiovascular diseases.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Myocardial oxygenation is maintained during hypoxia when combined with apnea - a cardiovascular MR study

Oxygenation-sensitive (OS) cardiovascular magnetic resonance (CMR) is used to noninvasively measure myocardial oxygenation changes during pharmacologic vasodilation. The use of breathing maneuvers with OS CMR for diagnostic purposes has been recently proposed based on the vasodilatory effect of Co₂, which can be enhanced by the additive effect of mild hypoxia. This study seeks to investigate this synergistic concept on coronary arteriolar resistance with OS CMR. In nine anesthetized swine, normoxemic and mild hypoxemic arterial partial pressure of oxygen (Pao₂) levels (100 and 80 mmHg) were targeted with three arterial partial pressure of carbon dioxide (Paco₂) levels of 30, 40, and 50 mmHg. During a 60-sec apnea from the set baselines, OS T2*-weighted gradient echo steady-state free precession (SSFP) cine series were obtained in a clinical 1.5T magnetic resonance imaging (MRI) system. Arterial blood gases were acquired prior to and after apnea. Changes in global myocardial signal intensity (SI) were measured. Although a greater drop in arterial oxygen saturation (SaO₂) was observed in the hypoxemic baselines, myocardial SI increased or was maintained during apnea in all levels (n = 6). An observed decrease in left ventricular blood pool SI was correlated with the drop in SaO₂. Corrected for the arterial desaturation, the calculated SI increase attributable to the increase in myocardial blood flow was greater in the hypoxemic levels. Both the changes in Paco₂ and Pao₂ were correlated with myocardial SI changes at normoxemia, yet not at hypoxemic levels. Using OS CMR, we found evidence that myocardial oxygenation is preserved during hypoxia when combined with Co₂-increasing maneuvers, indicating synergistic effects of hypoxemia and hypercapnia on myocardial blood flow.

Stereological study of the diabetic heart of male rats

The present study aimed to quantitatively compare the normal and diabetic hearts of rats using stereological methods. Diabetic and control rats received streptozotocin (60 mg/kg) and no treatments, respectively. On the 56^th day, the hearts were removed and their total volume was estimated using isotropic Cavalieri method. The total volume of the connective tissues and vessels, total length and diameter of the vessels, total number of cardiomyocytes nuclei, and the mean volume of the cardiomyocytes were estimated, as well. In comparison to the control animals, 60 and 43% increase was observed in the total volume of the connective tissue and microvessels of the diabetic rats, respectively (P<0.05). The percent of the vessel profiles with the diameter of 2-4 µm was decreased, while the percent of the vessel profiles with the diameter of 4.1-8 µm was increased in the diabetic hearts (P<0.05). No significant difference was found in the vessels with more than 8 µm diameters. The total number of the cardiomyocytes' nuclei and the number-weighted mean volume were respectively decreased by 37 and 64% in the diabetic group (P<0.01). A significant difference was observed between the two groups concerning the left ventricle volume to body weight ratio as an index for ventricular hypertrophy (P<0.05), while no difference was found regarding the right ventricle to body weight ratio. It can be concluded that diabetes can induce structural changes, including loss and/or atrophy of the cardiomyocytes, accompanied with increase in the connective tissue in the rats' hearts.

Ultrasound-mediated drug delivery for cardiovascular disease

INTRODUCTION

Ultrasound (US) has been developed as both a valuable diagnostic tool and a potent promoter of beneficial tissue bioeffects for the treatment of cardiovascular disease. These effects can be mediated by mechanical oscillations of circulating microbubbles, or US contrast agents, which may also encapsulate and shield a therapeutic agent in the bloodstream. Oscillating microbubbles can create stresses directly on nearby tissue or induce fluid effects that effect drug penetration into vascular tissue, lyse thrombi or direct drugs to optimal locations for delivery.

AREAS COVERED

The present review summarizes investigations that have provided evidence for US-mediated drug delivery as a potent method to deliver therapeutics to diseased tissue for cardiovascular treatment. In particular, the focus will be on investigations of specific aspects relating to US-mediated drug delivery, such as delivery vehicles, drug transport routes, biochemical mechanisms and molecular targeting strategies.

EXPERT OPINION

These investigations have spurred continued research into alternative therapeutic applications, such as bioactive gas delivery and new US technologies. Successful implementation of US-mediated drug delivery has the potential to change the way many drugs are administered systemically, resulting in more effective and economical therapeutics, and less-invasive treatments.

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.

Nitric oxide-mediated relaxation to lactate of coronary circulation in the isolated perfused rat heart

The objective of this study was to analyze the effects of lactate on coronary circulation. Rat hearts were perfused in a Langendorff preparation, and the coronary response to lactate (3-30 mM) was recorded after precontracting coronary vasculature with 11-dideoxy-1a,9a-epoxymethanoprostaglandin F2α (U46619), in the presence or the absence of the inhibitor of nitric oxide synthesis, N-omega-nitro-l-arginine methyl ester (l-NAME, 10 M), the blocker of Ca-dependent potassium channels, tetraethylammonium (TEA, 10 M), or the blocker of adenosine triphosphate-sensitive potassium channels, glybenclamide (10 M). The effects of lactate were also studied in isolated segments of rat coronary arteries that were precontracted with U46619, with or without endothelium. In perfused hearts, lactate induced concentration-dependent coronary vasodilatation and a reduction in myocardial contractility (left ventricular developed pressure and dP/dt) without altering the heart rate. Coronary vasodilatation in response to lactate was reduced by l-NAME but unaffected by TEA or glybenclamide. The effects of lactate on myocardial contractility were unchanged by l-NAME, TEA, or glybenclamide. In isolated coronary artery segments, lactate also produced relaxation, an effect attenuated by removing the endothelium. Together these findings suggest that lactate exerts coronary vasodilatory effects through the release of endothelial nitric oxide, independently of potassium channels. These findings may be relevant for the regulation of coronary circulation when lactate levels are elevated.

Minimal effect of collateral flow on coronary microvascular resistance in the presence of intermediate and noncritical coronary stenoses

Depending on stenosis severity, collateral flow can be a confounding factor in the determination of coronary hyperemic microvascular resistance (HMR). Under certain assumptions, the calculation of HMR can be corrected for collateral flow by incorporating the wedge pressure (P(w)) in the calculation. However, although P(w) > 25 mmHg is indicative of collateral flow, P(w) does in part also reflect myocardial wall stress neglected in the assumptions. Therefore, the aim of this study was to establish whether adjusting HMR by P(w) is pertinent for a diagnostically relevant range of stenosis severities as expressed by fractional flow reserve (FFR). Accordingly, intracoronary pressure and Doppler flow velocity were measured a total of 95 times in 29 patients distal to a coronary stenosis before and after stepwise percutaneous coronary intervention. HMR was calculated without (HMR) and with P(w)-based adjustment for collateral flow (HMR(C)). FFR ranged from 0.3 to 1. HMR varied between 1 and 5 and HMR(C) between 0.5 and 4.2 mmHg·cm(-1)·s. HMR was about 37% higher than HMR(C) for stenoses with FFR < 0.6, but for FFR > 0.8, the relative difference was reduced to 4.4 ± 3.4%. In the diagnostically relevant range of FFR between 0.6 and 0.8, this difference was 16.5 ± 10.4%. In conclusion, P(w)-based adjustment likely overestimates the effect of potential collateral flow and is not needed for the assessment of coronary HMR in the presence of a flow-limiting stenosis characterized by FFR between 0.6 and 0.8 or for nonsignificant lesions.

Differential effects of androgens on coronary blood flow regulation and arteriolar diameter in intact and castrated swine

Background

Low endogenous testosterone levels have been shown to be a risk factor for the development of cardiovascular disease and cardiovascular benefits associated with testosterone replacement therapy are being advocated; however, the effects of endogenous testosterone levels on acute coronary vasomotor responses to androgen administration are not clear. The objective of this study was to compare the effects of acute androgen administration on in vivo coronary conductance and in vitro coronary microvascular diameter in intact and castrated male swine.

Methods

Pigs received intracoronary infusions of physiologic levels (1–100 nM) of testosterone, the metabolite 5α-dihydrotestosterone, and the epimer epitestosterone while left anterior descending coronary blood flow and mean arterial pressure were continuously monitored. Following sacrifice, coronary arterioles were isolated, cannulated, and exposed to physiologic concentrations (1–100 nM) of testosterone, 5α-dihydrotestosterone, and epitestosterone. To evaluate effects of the androgen receptor on acute androgen dilation responses, real-time PCR and immunohistochemistry for androgen receptor were performed on conduit and resistance coronary vessels.

Results

In vivo, testosterone and 5α-dihydrotestosterone produced greater increases in coronary conductance in the intact compared to the castrated males. In vitro, percent maximal dilation of microvessels was similar between intact and castrated males for testosterone and 5α-dihydrotestosterone. In both studies epitestosterone produced significant increases in conductance and microvessel diameter from baseline in the intact males. Androgen receptor mRNA expression and immunohistochemical staining were similar in intact and castrated males.

Conclusions

Acute coronary vascular responses to exogenous androgen administration are increased by endogenous testosterone, an effect unrelated to changes in androgen receptor expression.

The multi-scale modelling of coronary blood flow

Coronary flow is governed by a number of determinants including network anatomy, systemic afterload and the mechanical interaction with the myocardium throughout the cardiac cycle. The range of spatial scales and multi-physics nature of coronary perfusion highlights a need for a multiscale framework that captures the relevant details at each level of the network. The goal of this review is to provide a compact and accessible introduction to the methodology and current state of the art application of the modelling frameworks that have been used to study the coronary circulation. We begin with a brief description of the seminal experimental observations that have motivated the development of mechanistic frameworks for understanding how myocardial mechanics influences coronary flow. These concepts are then linked to an overview of the lumped parameter models employed to test these hypotheses. We then outline the full and reduced-order (3D and 1D) continuum mechanics models based on the Navier–Stokes equations and highlight, with examples, their application regimes. At the smaller spatial scales the case for the importance of addressing the microcirculation is presented, with an emphasis on the poroelastic approach that is well-suited to bridge an existing gap in the development of an integrated whole heart model. Finally, the recent accomplishments of the wave intensity analysis and related approaches are presented and the clinical outlook for coronary flow modelling discussed.

QUANTITATIVE DOUBLE-TRACER DIGITALRADIOGRAPHY BASED ON DESMETHYLIMIPRAMINE DEPOSITION: APPLICATION TO STUDIES ON MYOCARDIAL PERFUSION HETEROGENEITY

Desmethylimipramine (DMI), an α2-adrenergic antagonist, is a nearly ideal deposition tracer for evaluating the myocardial flow distribution with the least artifactual effects on microcirculation. Myocardial retentions of tritium- and iodine-125-labeled DMI (HDMI, IDMI) were confirmed to be satisfactory; the retentions of IDMI and HDMI at 1 min were 95 and 91% respectively in isolated Tyrode-perfused rabbit hearts (n=6) at a perfusion rate of 8.1 ml/min/g, and 98 and 96% respectively, in blood-perfused rat hearts (n=4) at a perfusion rate of 3.1 ml/min/g. Using these tracers combined with subtraction digitalradiography, it allowed the assessing of changes of myocardial flow distribution with 400×400 μ m 2 resolution. In blood-perfused rat hearts (n=4), the validity of this method was verified by the strong cross-correlation between regional densities of two tracers injected simultaneously (r=0.94) and the regression line having a slope close to one. Furthermore, in Tyrode-perfused rabbit hearts, the flow distributions were evaluated before and after decreasing perfusion rate moderately by 34% (n=7) and severely by 70% (n=7). Severe flow reduction increased the coefficient of variation of tracer density (CV) significantly from 19 to 25%, but CV did not change with moderate flow reduction (20 vs. 19%). Regional densities of two tracers were cross-correlated still substantially under severe low-flow perfusion (r=0.84). Accordingly, flow differences between originally high- and low-flow regions were enlarged under severe flow reduction. In conclusion, double-tracer digitalradiography based on the DMI deposition will be a potent method for the analysis of flow heterogeneity at microvascular levels.

Coronary pressure-flow relations as basis for the understanding of coronary physiology

Recent technological advancements in the area of intracoronary physiology, as well as non-invasive contrast perfusion imaging, allow to make clinical decisions with respect to percutaneous coronary interventions and to identify microcirculatory coronary pathophysiology. The basic characteristics of coronary hemodynamics, as described by pressure-flow relations in the normal and diseased heart, need to be understood for a proper interpretation of these physiological measurements. Especially the hyperemic coronary pressure-flow relation, as well as the influence of cardiac function on it, bears great clinical significance. The interaction of a coronary stenosis with the coronary pressure-flow relation can be understood from the stenosis pressure drop-flow velocity relationship. Based on these relationships the clinically applied concepts of coronary flow velocity reserve, fractional flow reserve, stenosis resistance and microvascular resistance are discussed. Attention is further paid to the heterogeneous nature of myocardial perfusion, the vulnerability of the subendocardium and the role of collateral flow on hyperemic coronary pressure-flow relations. This article is part of a Special Issue entitled "Coronary Blood Flow".

Angiographic estimation of atherosclerotic disease burden in a coronary artery fed by collaterals: a potential pitfall in decision for revascularization

Despite the remarkable advances in revascularization strategies made during the last decade, a significant proportion of patients are excluded from either percutaneous coronary intervention or coronary artery bypass grafting because of unsuitable coronary anatomy. Diffuse severe coronary artery disease, small vessel caliber, chronic total occlusions, or extremely calcified vessels are frequent reasons for deferring revascularization with either percutaneous coronary intervention or coronary artery bypass grafting. We present a case concerning a middle-aged asymptomatic patient who was treated successfully with percutaneous coronary intervention due to a chronic total occlusion lesion of the left anterior descending artery. Coronary angiography is an inadequate method for the estimation of the burden of atherosclerotic disease in an artery fed by collaterals. Assessment of any residual antegrade flow, and ipsilateral and contralateral collateral filling of the segments distal to the occlusion with invasive or noninvasive techniques, could affect the appropriate decision-making by physicians.

Theoretical models for coronary vascular biomechanics: progress & challenges

A key aim of the cardiac Physiome Project is to develop theoretical models to simulate the functional behaviour of the heart under physiological and pathophysiological conditions. Heart function is critically dependent on the delivery of an adequate blood supply to the myocardium via the coronary vasculature. Key to this critical function of the coronary vasculature is system dynamics that emerge via the interactions of the numerous constituent components at a range of spatial and temporal scales. Here, we focus on several components for which theoretical approaches can be applied, including vascular structure and mechanics, blood flow and mass transport, flow regulation, angiogenesis and vascular remodelling, and vascular cellular mechanics. For each component, we summarise the current state of the art in model development, and discuss areas requiring further research. We highlight the major challenges associated with integrating the component models to develop a computational tool that can ultimately be used to simulate the responses of the coronary vascular system to changing demands and to diseases and therapies.

Age impairs Flk-1 signaling and NO-mediated vasodilation in coronary arterioles

Impairment of flow-induced vasodilation in coronary resistance arterioles may contribute to the decline in coronary vasodilatory reserve that occurs with advancing age. This study investigated the effects of age on flow-induced signaling and activation of nitric oxide (NO)-mediated vasodilation in coronary resistance arterioles. Coronary arterioles were isolated from young (approximately 6 mo) and old (approximately 24 mo) male Fischer-344 rats to assess vasodilation to flow, vascular endothelial growth factor (VEGF), and ACh. Flow- and VEGF-induced vasodilation of coronary arterioles was impaired with age (P<or=0.05); however, ACh-induced vasodilation was preserved with age. NG-nitro-L-arginine methyl ester (L-NAME) (1x10(-5) M) eliminated vasodilation to flow, VEGF, and ACh, indicating dependence of these responses on NO. SU-1498, an inhibitor of vascular endothelial growth factor receptor 2 (VEGFR, also known as Flk-1), abolished age-related differences in flow-induced vasodilation. Flow-stimulated phosphorylation of Flk-1 in coronary arterioles from young but not old rats and Flk-1 protein was reduced in coronary arterioles from old rats compared with those from young rats. Flow stimulated phosphorylation of endothelial nitric oxide synthase (eNOS) in coronary arterioles from both young and old rats. VEGF induced phosphorylation of both protein kinase B (Akt) and eNOS in coronary arterioles. VEGF-induced phosphorylation of Akt, but not eNOS, was significantly reduced in arterioles from old rats compared with arterioles from young rats. Wortmannin, an inhibitor of phosphatidylinositol (PI) 3-kinase, eliminated age-related differences in both flow- and VEGF-induced vasodilation. These results indicate that impairment of Flk-1/PI3-kinase signaling contributes to the reduction of flow-induced vasodilation in coronary arterioles with advancing age.

Windkesselness of coronary arteries hampers assessment of human coronary wave speed by single-point technique

A novel single-point technique to calculate local arterial wave speed (SPc) has recently been presented and applied in healthy human coronary arteries at baseline flow. We investigated its applicability for conditions commonly encountered in the catheterization laboratory. Intracoronary pressure (P(d)) and Doppler velocity (U) were recorded in 29 patients at rest and during adenosine-induced hyperemia in a distal segment of a normal reference vessel and downstream of a single stenosis before and after revascularization. Conduit vessel tone was minimized with nitroglycerin. Microvascular resistance (MR) and SPc were calculated from P(d) and U. In the reference vessel, SPc decreased from 21.5 m/s (SD 8.0) to 10.5 m/s (SD 4.1) after microvascular dilation (P < 0.0001). SPc was substantially higher in the presence of a proximal stenosis and decreased from 34.4 m/s (SD 18.2) at rest to 27.5 m/s (SD 13.4) during hyperemia (P < 0.0001), with a concomitant reduction in P(d) by 20 mmHg and MR by 55.4%. The stent placement further reduced hyperemic MR by 26% and increased P(d) by 26 mmHg but paradoxically decreased SPc to 13.1 m/s (SD 7.7) (P < 0.0001). Changes in SPc correlated strongly with changes in MR (P < 0.001) but were inversely related to changes in P(d) (P < 0.01). In conclusion, the single-point method yielded erroneous predictions of changes in coronary wave speed induced by a proximal stenosis and distal vasodilation and is therefore not appropriate for estimating local wave speed in coronary vessels. Our findings are well described by a lumped reservoir model reflecting the "windkesselness" of the coronary arteries.