Alterations in vasomotor control of coronary resistance vessels in remodelled myocardium of swine with a recent myocardial infarction

Coronary blood flow in heart failure: cause, consequence and bystander

Heart failure is a clinical syndrome where cardiac output is not sufficient to sustain adequate perfusion and normal bodily functions, initially during exercise and in more severe forms also at rest. The two most frequent forms are heart failure of ischemic origin and of non-ischemic origin. In heart failure of ischemic origin, reduced coronary blood flow is causal to cardiac contractile dysfunction, and this is true for stunned and hibernating myocardium, coronary microembolization, myocardial infarction and post-infarct remodeling, possibly also for the takotsubo syndrome. The most frequent form of non-ischemic heart failure is dilated cardiomyopathy, caused by genetic mutations, myocarditis, toxic agents or sustained tachyarrhythmias, where alterations in coronary blood flow result from and contribute to cardiac contractile dysfunction. Hypertrophic cardiomyopathy is caused by genetic mutations but can also result from increased pressure and volume overload (hypertension, valve disease). Heart failure with preserved ejection fraction is characterized by pronounced coronary microvascular dysfunction, the causal contribution of which is however not clear. The present review characterizes the alterations of coronary blood flow which are causes or consequences of heart failure in its different manifestations. Apart from any potentially accompanying coronary atherosclerosis, all heart failure entities share common features of impaired coronary blood flow, but to a different extent: enhanced extravascular compression, impaired nitric oxide-mediated, endothelium-dependent vasodilation and enhanced vasoconstriction to mediators of neurohumoral activation. Impaired coronary blood flow contributes to the progression of heart failure and is thus a valid target for established and novel treatment regimens.

Pulmonary vasodilation by phosphodiesterase 5 inhibition is enhanced and nitric oxide independent in early pulmonary hypertension after myocardial infarction

Myocardial infarction (MI) may result in pulmonary hypertension (PH). Inhibition of phosphodiesterase 5 (PDE5), the enzyme responsible for the breakdown of cGMP in vascular smooth muscle, has become part of the contemporary therapeutic armamentarium for pulmonary arterial hypertension and may also be beneficial for PH secondary to MI. Nitric oxide (NO) is an important activator of cGMP synthesis and can be enhanced in early PH and decreased in severe PH. In the present study, we investigated if PDE5 inhibition ameliorates pulmonary hemodynamics in swine with PH secondary to MI and whether NO is essential. The PDE5 inhibitor EMD360527 was administered in awake, chronically instrumented swine with or without MI. At rest, PDE5 inhibition produced pulmonary vasodilation as evidenced by a decrease in pulmonary vascular resistance, which was more pronounced in MI ( n = 5) compared with normal swine ( n = 10, P ≤ 0.01) and was accompanied by an increase in stroke volume in MI swine. Both pulmonary vasodilation and increased stroke volume were maintained during exercise, suggesting that this therapy may improve exercise capacity in patients with PH secondary to MI. Interestingly, prior inhibition of NO significantly enhanced ( P ≤ 0.01) pulmonary vasodilation by PDE5 inhibition in both normal ( n = 8) and MI swine ( n = 5, P ≤ 0.05 vs. normal). This suggests that the increased vasodilator responses to PDE5 inhibition after MI were not due to an increase in NO-induced cGMP production. These observations indicate that PDE5 inhibition represents an interesting pharmacotherapeutic approach in early PH after a recent MI to prevent overt PH. NEW & NOTEWORTHY This research article is the first to describe that pulmonary vasodilation to phosphodiesterase 5 inhibition is enhanced and nitric oxide independent in resting and exercising swine with pulmonary hypertension as a result of myocardial infarction. This suggests that phosphodiesterase 5 inhibition can normalize pulmonary hemodynamics in postcapillary pulmonary hypertension after a recent myocardial infarction and may improve exercise capacity.

Severe familial hypercholesterolemia impairs the regulation of coronary blood flow and oxygen supply during exercise

Accelerated development of coronary atherosclerosis is a defining characteristic of familial hypercholesterolemia (FH). However, the recent data highlight a significant cardiovascular risk prior to the development of critical coronary stenosis. We, therefore, examined the hypothesis that FH produces coronary microvascular dysfunction and impairs coronary vascular control at rest and during exercise in a swine model of FH. Coronary vascular responses to drug infusions and exercise were examined in chronically instrumented control and FH swine. FH swine exhibited ~tenfold elevation of plasma cholesterol and diffuse coronary atherosclerosis (20-60 % plaque burden). Similar to our recent findings in the systemic vasculature in FH swine, coronary smooth muscle nitric oxide sensitivity was increased in vivo and in vitro with maintained endothelium-dependent vasodilation in vivo in FH. At rest and during exercise, FH swine exhibited increased myocardial O2 extraction resulting in reduced coronary venous SO2 and PO2 versus control. During exercise in FH swine, the transmural distribution of coronary blood flow was unchanged; however, a shift toward anaerobic cardiac metabolism was revealed by increased coronary arteriovenous H(+) concentration gradient. This shift was associated with a worsening of cardiac efficiency (relationship between cardiac work and O2 consumption) in FH during exercise owing, in part, to a generalized reduction in stroke volume which was associated with increased left atrial pressure in FH. Our data highlight a critical role for coronary microvascular dysfunction as a contributor to impaired myocardial O2 balance, cardiac ischemia, and impaired cardiac function prior to the development of critical coronary stenosis in FH.

What can we learn about treating heart failure from the heart's response to acute exercise? Focus on the coronary microcirculation

Coronary microvascular function and cardiac function are closely related in that proper cardiac function requires adequate oxygen delivery through the coronary microvasculature. Because of the close proximity of cardiomyocytes and coronary microvascular endothelium, cardiomyocytes not only communicate their metabolic needs to the coronary microvasculature, but endothelium-derived factors also directly modulate cardiac function. This review summarizes evidence that the myocardial oxygen balance is disturbed in the failing heart because of increased extravascular compressive forces and coronary microvascular dysfunction. The perturbations in myocardial oxygen balance are exaggerated during exercise and are due to alterations in neurohumoral influences, endothelial function, and oxidative stress. Although there is some evidence from animal studies that the myocardial oxygen balance can partly be restored by exercise training, it is largely unknown to what extent the beneficial effects of exercise training include improvements in endothelial function and/or oxidative stress in the coronary microvasculature and how these improvements are impacted by risk factors such as diabetes, obesity, and hypercholesterolemia.

Cardiovascular remodelling in coronary artery disease and heart failure

Remodelling is a response of the myocardium and vasculature to a range of potentially noxious haemodynamic, metabolic, and inflammatory stimuli. Remodelling is initially functional, compensatory, and adaptive but, when sustained, progresses to structural changes that become self-perpetuating and pathogenic. Remodelling involves responses not only of the cardiomyocytes, endothelium, and vascular smooth muscle cells, but also of interstitial cells and matrix. In this Review we characterise the remodelling processes in atherosclerosis, vascular and myocardial ischaemia-reperfusion injury, and heart failure, and we draw attention to potential avenues for innovative therapeutic approaches, including conditioning and metabolic strategies.

Exercise training in adverse cardiac remodeling

Cardiac remodeling in response to a myocardial infarction or chronic pressure-overload is an independent risk factor for the development of heart failure. In contrast, cardiac remodeling produced by regular physical exercise is associated with a decreased risk for heart failure. There is evidence that exercise training has a beneficial effect on disease progression and survival in patients with cardiac remodeling and dysfunction, but concern has also been expressed that exercise training may aggravate pathological remodeling and dysfunction. Here we present studies from our laboratory into the effects of exercise training on pathological cardiac remodeling and dysfunction in mice. The results indicate that even in the presence of a large infarct, exercise training exerts beneficial effects on the heart. These effects were mimicked in part by endothelial nitric oxide synthase (eNOS) overexpression and abrogated by eNOS deficiency, demonstrating the importance of nitric oxide signaling in mediating the cardiac effects of exercise. Exercise prior to a myocardial infarction was also cardioprotective. In contrast, exercise tended to aggravate pathological cardiac remodeling and dysfunction in the setting of pressure-overload produced by an aortic stenosis. These observations emphasize the critical importance of the underlying pathological stimulus for cardiac hypertrophy and remodeling, in determining the effects of exercise training. Future studies are needed to define the influence of exercise type, intensity and duration in different models and severities of pathological cardiac remodeling. Together such studies will aid in optimizing the therapy of exercise training in the setting of cardiovascular disease.

Phosphodiesterase 5 inhibition-induced coronary vasodilation is reduced after myocardial infarction

The balance between the production and removal of cGMP in coronary vascular smooth muscle is of critical importance in determining coronary vasomotor tone and thus in the regulation of coronary blood flow. cGMP production by soluble guanylyl cyclase is activated by nitric oxide (NO), whereas cGMP breakdown occurs through phosphodiesterase 5 (PDE5). We hypothesized that myocardial infarction (MI) alters the balance between the production and removal of cGMP in the coronary vasculature and thereby alters the control of coronary vasomotor tone. Chronically instrumented swine with and without a 2-wk-old MI were exercised on a treadmill in the absence and presence of the PDE5 inhibitor EMD-360527 (300 μg·kg−1·min−1 iv). Inhibition of PDE5 produced coronary resistance vessel dilation, which was more pronounced at rest than during exercise in normal swine. PDE5 gene expression was markedly reduced in coronary resistance vessels isolated from the remote myocardium of MI swine, which was accompanied by a similarly marked attenuation of coronary vasodilation by PDE5 inhibition in MI swine. The coronary vasoconstriction produced by inhibition of NO synthesis with Nω-nitro-l-arginine (20 mg/kg iv) was only slightly smaller in swine with MI. Interestingly, inhibition of NO synthesis reduced the vasodilator response to subsequent PDE5 inhibition in normal swine but not in MI swine. Conversely, PDE5 inhibition enhanced the coronary vasoconstriction produced by NO synthesis inhibition in normal swine but not in MI swine, suggesting that downregulation of PDE5 mitigated the loss of NO vasodilator influence. In conclusion, the expression and vasoconstrictor influence of PDE5 are markedly attenuated in coronary resistance vessels in the remote myocardium after MI, which appears to serve as a compensatory mechanism to mitigate the loss of NO vasodilator influence.

Reduced vascular smooth muscle BK channel current underlies heart failure-induced vasoconstriction in mice

Excessively increased peripheral vasoconstriction is a hallmark of heart failure (HF). Here, we show that in mice with systolic HF post-myocardial infarction, the myogenic tone of third-order mesenteric resistance vessels is increased, the vascular smooth muscle (VSM) membrane potential is depolarized by ~20 mV, and vessel wall intracellular [Ca(2+)] is elevated relative to that in sham-operated control mice. Despite the increased [Ca(2+)], the frequency and amplitude of spontaneous transient outward currents (STOCs), mediated by large conductance, Ca(2+)-activated BK channels, were reduced by nearly 80% (P<0.01) and 25% (P<0.05), respectively, in HF. The expression of the BK α and β1 subunits was reduced in HF mice compared to controls (65 and 82% lower, respectively, P<0.01). Consistent with the importance of a reduction in BK channel expression and function in mediating the HF-induced increase in myogenic tone are two further findings: a blunting of paxilline-induced increase in myogenic tone in HF mice compared to controls (0.9 vs. 10.9%, respectively), and that HF does not alter the increased myogenic tone of BK β1-null mice. These findings identify electrical dysregulation within VSM, specifically the reduction of BK currents, as a key molecular mechanism sensitizing resistance vessels to pressure-induced vasoconstriction in systolic HF.

Heart failure with preserved ejection fraction: chronic low-intensity interval exercise training preserves myocardial O2 balance and diastolic function

We have previously reported chronic low-intensity interval exercise training attenuates fibrosis, impaired cardiac mitochondrial function, and coronary vascular dysfunction in miniature swine with left ventricular (LV) hypertrophy (Emter CA, Baines CP. Am J Physiol Heart Circ Physiol 299: H1348-H1356, 2010; Emter CA, et al. Am J Physiol Heart Circ Physiol 301: H1687-H1694, 2011). The purpose of this study was to test two hypotheses: 1) chronic low-intensity interval training preserves normal myocardial oxygen supply/demand balance; and 2) training-dependent attenuation of LV fibrotic remodeling improves diastolic function in aortic-banded sedentary, exercise-trained (HF-TR), and control sedentary male Yucatan miniature swine displaying symptoms of heart failure with preserved ejection fraction. Pressure-volume loops, coronary blood flow, and two-dimensional speckle tracking ultrasound were utilized in vivo under conditions of increasing peripheral mean arterial pressure and β-adrenergic stimulation 6 mo postsurgery to evaluate cardiac function. Normal diastolic function in HF-TR animals was characterized by prevention of increased time constant of isovolumic relaxation, normal LV untwisting rate, and enhanced apical circumferential and radial strain rate. Reduced fibrosis, normal matrix metalloproteinase-2 and tissue inhibitors of metalloproteinase-4 mRNA expression, and increased collagen III isoform mRNA levels (P < 0.05) accompanied improved diastolic function following chronic training. Exercise-dependent improvements in coronary blood flow for a given myocardial oxygen consumption (P < 0.05) and cardiac efficiency (stroke work to myocardial oxygen consumption, P < 0.05) were associated with preserved contractile reserve. LV hypertrophy in HF-TR animals was associated with increased activation of Akt and preservation of activated JNK/SAPK. In conclusion, chronic low-intensity interval exercise training attenuates diastolic impairment by promoting compliant extracellular matrix fibrotic components and preserving extracellular matrix regulatory mechanisms, preserves myocardial oxygen balance, and promotes a physiological molecular hypertrophic signaling phenotype in a large animal model resembling heart failure with preserved ejection fraction.

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".

Prostanoids suppress the coronary vasoconstrictor influence of endothelin after myocardial infarction

Myocardial infarction (MI) is associated with endothelial dysfunction resulting in an imbalance in endothelium-derived vasodilators and vasoconstrictors. We have previously shown that despite increased endothelin (ET) plasma levels, the coronary vasoconstrictor effect of endogenous ET is abolished after MI. In normal swine, nitric oxide (NO) and prostanoids modulate the vasoconstrictor effect of ET. In light of the interaction among NO, prostanoids, and ET combined with endothelial dysfunction present after MI, we investigated this interaction in control of coronary vasomotor tone in the remote noninfarcted myocardium after MI. Studies were performed in chronically instrumented swine (18 normal swine; 13 swine with MI) at rest and during treadmill exercise. Furthermore, endothelial nitric oxide synthase (eNOS) and cyclooxygenase protein levels were measured in the anterior (noninfarcted) wall of six normal and six swine with MI. eNOS inhibition with N(ω)-nitro-L-arginine (L-NNA) and cyclooxygenase inhibition with indomethacin each resulted in coronary vasoconstriction at rest and during exercise, as evidenced by a decrease in coronary venous oxygen levels. The effect of l-NNA was slightly decreased in swine with MI, although eNOS expression was not altered. Conversely, in accordance with the unaltered expression of cyclooxygenase-1 after MI, the effect of indomethacin was similar in normal and MI swine. L-NNA enhanced the vasodilator effect of the ET(A/B) receptor blocker tezosentan but exclusively during exercise in both normal and MI swine. Interestingly, this effect of L-NNA was blunted in MI compared with normal swine. In contrast, whereas indomethacin increased the vasodilator effect of tezosentan only during exercise in normal swine, indomethacin unmasked a coronary vasodilator effect of tezosentan in MI swine both at rest and during exercise. In conclusion, the present study shows that endothelial control of the coronary vasculature is altered in post-MI remodeled myocardium. Thus the overall vasodilator influences of NO as well as its inhibition of the vasoconstrictor influence of ET on the coronary resistance vessels were reduced after MI. In contrast, while the overall prostanoid vasodilator influence was maintained, its inhibition of ET vasoconstrictor influences was enhanced in post-MI remote myocardium.

Reversal of isoprenaline-induced cardiac remodeling by rutaecarpine via stimulation of calcitonin gene-related peptide production

Dysfunction of capsaicin-sensitive sensory nerves is involved in cardiac remodeling, and rutaecarpine has been shown to exert a beneficial effect on cardiac function through activating the sensory nerves. This study was conducted to explore the potential inhibitory effect of rutaecarpine on cardiac remodeling and the underlying mechanisms. A rat cardiac remodeling model was established by injection of isoprenaline (5 mg/kg per day, s.c.) for 10 days. Rutaecarpine (10 or 40 mg/kg, i.g.) was coadministrated with isoprenaline to evaluate the effect of rutaecarpine on cardiac remodeling. After echocardiographic analysis was performed, blood samples were collected to quantify calcitonin gene-related peptide (CGRP), dorsal root ganglia were isolated for examining CGRP mRNA expression, and the hearts were weighed and saved for evaluating the parameters related to apoptosis and hypertrophy. Isoprenaline significantly increased the ratio of left ventricle weight to body weight, the cross-sectional area of cardiomyocytes, cardiac apoptosis, and collagen deposition concomitantly with decreased CGRP production, which were reversed by rutaecarpine treatment. The beneficial effects of rutaecarpine were attenuated by pretreatment with capsaicin, which selectively depleted CGRP. These results suggest that rutaecarpine was able to reverse isoprenaline-induced cardiac remodeling through stimulating CGRP production.

Exercise training does not improve cardiac function in compensated or decompensated left ventricular hypertrophy induced by aortic stenosis

There is ample evidence that regular exercise exerts beneficial effects on left ventricular (LV) hypertrophy, remodeling and dysfunction produced by ischemic heart disease or systemic hypertension. In contrast, the effects of exercise on pathological LV hypertrophy and dysfunction produced by LV outflow obstruction have not been studied to date. Consequently, we evaluated the effects of 8 weeks of voluntary wheel running in mice (which mitigates post-infarct LV dysfunction) on LV hypertrophy and dysfunction produced by mild (mTAC) and severe (sTAC) transverse aortic constriction. mTAC produced ~40% LV hypertrophy and increased myocardial expression of hypertrophy marker genes but did not affect LV function, SERCA2a protein levels, apoptosis or capillary density. Exercise had no effect on global LV hypertrophy and function in mTAC but increased interstitial collagen, and ANP expression. sTAC produced ~80% LV hypertrophy and further increased ANP expression and interstitial fibrosis and, in contrast with mTAC, also produced LV dilation, systolic as well as diastolic dysfunction, pulmonary congestion, apoptosis and capillary rarefaction and decreased SERCA2a and ryanodine receptor (RyR) protein levels. LV diastolic dysfunction was likely aggravated by elevated passive isometric force and Ca(2+)-sensitivity of myofilaments. Exercise training failed to mitigate the sTAC-induced LV hypertrophy and capillary rarefaction or the decreases in SERCA2a and RyR. Exercise attenuated the sTAC-induced increase in passive isometric force but did not affect myofilament Ca(2+)-sensitivity and tended to aggravate interstitial fibrosis. In conclusion, exercise had no effect on LV function in compensated and decompensated cardiac hypertrophy produced by LV outflow obstruction, suggesting that the effect of exercise on pathologic LV hypertrophy and dysfunction depends critically on the underlying cause.

'Integrative Physiology 2.0': integration of systems biology into physiology and its application to cardiovascular homeostasis

Since the completion of the Human Genome Project and the advent of the large scaled unbiased '-omics' techniques, the field of systems biology has emerged. Systems biology aims to move away from the traditional reductionist molecular approach, which focused on understanding the role of single genes or proteins, towards a more holistic approach by studying networks and interactions between individual components of networks. From a conceptual standpoint, systems biology elicits a 'back to the future' experience for any integrative physiologist. However, many of the new techniques and modalities employed by systems biologists yield tremendous potential for integrative physiologists to expand their tool arsenal to (quantitatively) study complex biological processes, such as cardiac remodelling and heart failure, in a truly holistic fashion. We therefore advocate that systems biology should not become/stay a separate discipline with '-omics' as its playing field, but should be integrated into physiology to create 'Integrative Physiology 2.0'.

Sodium-calcium exchange mediated contraction in left anterior descending and left ventricular branch arteries

We tested the hypothesis that the de-endothelialized artery rings from the left anterior descending (LAD) coronary artery and its left ventricular branch (LVB) differ in their contractile responses to Na⁺–Ca²⁺-exchanger (NCX) mediated Ca²⁺-entry, muscarinic receptor activation with carbachol, and sarco/endoplasmic reticulum Ca²⁺ pump (SERCA) inhibition with thapsigargin. In LVB, the force of contraction (in N/g tissue) produced by the NCX mediated Ca²⁺-entry (17.5 ± 1.4) and carbachol (18 ± 1.5) was only slightly smaller than that due to membrane depolarization with KCl (24.0 ± 1.0). In contrast, in LAD the force of contraction produced with NCX (8.7 ± 0.7) and carbachol (6.1 ± 1.1) was much smaller than with KCl (15.7 ± 0.7). Thapsigargin also contracted LVB with greater force than LAD. When isolated microsomes were used, the binding to the muscarinic receptor antagonist quinuclidinyl benzilate was greater in LVB than in LAD. Microsomes were also used for Western blots. The intensities of signals for both SERCA and NCX were greater in LVB than in LAD. These biochemical observations were consistent with the contractile experiments. Thus, it appears that the differences between LAD and the resistance arteries may begin as early as LVB.

Ventricular remodeling and function: insights using murine echocardiography

Extracellular matrix disturbances play an important role in the development of ventricular remodeling and failure. Genetically modified mice with abnormalities in the synthesis and degradation of extracellular matrix have been generated, in particular mice with deletion or overexpression of matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases (TIMPs). Echocardiography is ideally suited to serially evaluate left ventricular (LV) size and function, thus defining the progression of LV remodeling and failure. This Review describes the echocardiographic parameters that may provide insights into the development of ventricular remodeling and heart failure. The application of echocardiography to study LV remodeling and function after myocardial infarction and LV pressure-overload in wild-type mice and mice deficient or overexpressing MMPs or TIMPs is then detailed. Finally, using the example of mice deficient in nitric oxide synthase 3, a cautionary example is given illustrating discrepancies between the cardiac echocardiographic phenotype and modifications of the extracellular matrix.

Prevention of myofilament dysfunction by beta-blocker therapy in postinfarct remodeling

BACKGROUND

Myofilament contractility of individual cardiomyocytes is depressed in remote noninfarcted myocardium and contributes to global left ventricular pump dysfunction after myocardial infarction (MI). Here, we investigated whether beta-blocker therapy could restore myofilament contractility.

METHODS AND RESULTS

In pigs with a MI induced by ligation of the left circumflex coronary artery, beta-blocker therapy (bisoprolol, MI+beta) was initiated on the first day after MI. Remote left ventricular subendocardial biopsies were taken 3 weeks after sham or MI surgery. Isometric force was measured in single permeabilized cardiomyocytes. Maximal force (F(max)) was lower, whereas Ca(2+) sensitivity was higher in untreated MI compared with sham (both P<0.05). The difference in Ca(2+) sensitivity was abolished by treatment of cells with the beta-adrenergic kinase, protein kinase A. beta-blocker therapy partially reversed F(max) and Ca(2+) sensitivity to sham values and significantly reduced passive force. Despite the lower myofilament Ca(2+) sensitivity in MI+beta compared with untreated myocardium, the protein kinase A induced reduction in Ca(2+) sensitivity was largest in cardiomyocytes from myocardium treated with beta-blockers. Phosphorylation of beta-adrenergic target proteins (myosin binding protein C and troponin I) did not differ among groups, whereas myosin light chain 2 phosphorylation was reduced in MI, which coincided with increased expression of protein phosphatase 1. beta-blockade fully restored the latter alterations and significantly reduced expression of protein phosphatase 2a.

CONCLUSIONS

beta-blockade reversed myofilament dysfunction and enhanced myofilament responsiveness to protein kinase A in remote myocardium after MI. These effects likely contribute to the beneficial effects of beta-blockade on global left ventricular function after MI.

Coronary structure and perfusion in health and disease

Blood flow is distributed through the heart muscle via a system of vessels forming the coronary circulation. The perfusion of the myocardium can be hampered by atherosclerosis creating localized obstructions in the epicardial vessels or by microvascular disease. In early stages of the disease, these impediments to blood flow are offset by dilation of the resistance vessels, which normally compensates for a decrease in perfusion pressure or increased metabolism. However, this dilatory reserve can become exhausted, which in general occurs first at the deeper layers of the heart wall where intramural vessels are subjected to compressive forces related to heart contraction. In the catheterization laboratory, guide wires of 0.33 mm diameter are available that are equipped with a pressure and flow velocity sensor at the tip, which can be positioned distal to the stenosis. These signals provide information about the impediment of the stenosis on coronary flow and allow for the evaluation of the status of the microcirculation. However, the interpretation of these signals is strongly model-dependent and therefore it is of paramount importance to develop realistic models reflecting the anatomy and unique physiology of the coronary circulation.