Paradoxical Increase in Microvascular Resistance During Tachycardia Downstream From a Severe Stenosis in Patients With Coronary Artery Disease

Incremental effect of coronary obstruction on myocardial microvascular dysfunction in type 2 diabetes mellitus patients evaluated by first-pass perfusion CMR study

BACKGROUND

Type 2 diabetes mellitus (T2DM) frequently coexists with obstructive coronary artery disease (OCAD), which are at increased risk for cardiovascular morbidity and mortality. This study aimed to investigate the impact of coronary obstruction on myocardial microcirculation function in T2DM patients, and explore independent predictors of reduced coronary microvascular perfusion.

METHODS

Cardiac magnetic resonance (CMR) scanning was performed on 297 T2DM patients {188 patients without OCAD [T2DM(OCAD -)] and 109 with [T2DM(OCAD +)]} and 89 control subjects. CMR-derived perfusion parameters, including upslope, max signal intensity (MaxSI), and time to maximum signal intensity (TTM) in global and segmental (basal, mid-ventricular, and apical slices) were measured and compared among observed groups. According to the median of Gensini score (64), T2DM(OCAD +) patients were subdivided into two groups. Univariable and multivariable linear regression analyses were performed to identify independent predictors of microcirculation dysfunction.

RESULTS

T2DM(OCAD -) patients, when compared to control subjects, had reduced upslope and prolonged TTM in global and all of three slices (all P < 0.05). T2DM(OCAD +) patients showed a significantly more severe impairment of microvascular perfusion than T2DM(OCAD -) patients and control subjects with a more marked decline upslope and prolongation TTM in global and three slices (all P < 0.05). From control subjects, through T2DM(OCAD +) patients with Gensini score ≤ 64, to those patients with Gensini score > 64 group, the upslope declined and TTM prolonged progressively in global and mid-ventricular slice (all P < 0.05). The presence of OCAD was independently correlated with reduced global upslope (β =  - 0.104, P < 0.05) and global TTM (β = 0.105, P < 0.05) in patients with T2DM. Among T2DM(OCAD +) patients, Gensini score was associated with prolonged global TTM (r = 0.34, P < 0.001).

CONCLUSIONS

Coronary artery obstruction in the context of T2DM exacerbated myocardial microcirculation damage. The presence of OCAD and Gensini score were independent predictors of decreased microvascular function.

TRIAL REGISTRATION

Retrospectively registered.

Perioperative Myocardial Infarction in Free Flap for Head and Neck Reconstruction

INTRODUCTION

Acute myocardial infarction (AMI) is a postoperative complication of major surgical procedures, including free flap surgery. It is the most common cause of postoperative morbidity and mortality. Moreover, patients receiving free flap reconstruction for the head and neck have significant risk factors such as coexisting coronary artery disease (CAD). Our primary aim was to ascertain predictors of perioperative AMI to enable early detection and consequently early treatment of perioperative AMI. Our secondary aim was to determine the group of patients who would be at a high risk for perioperative AMI after free flap surgery.

MATERIALS AND METHODS

This retrospective study enrolled patients who underwent free flap reconstruction surgery at the Division of Plastic and Reconstructive Surgery of Taipei Veterans General Hospital between 2013-01 and 2017-12.

RESULTS

This study included 444 patients and 481 free flap head and neck reconstruction surgeries. Fifteen (3.1%) patients were diagnosed with perioperative AMI. Statistical analysis of the variables revealed that patients with underlying CAD or cerebrovascular accident (CVA) were at a high risk of developing perioperative AMI (odds ratio: 6.89 and 11.11, respectively). The flap failure rate was also higher in patients with perioperative AMI compared with those without perioperative AMI (P = 0.015).

CONCLUSIONS

Patients with underlying diseases, such as CAD or CVA, constituted high-risk groups for perioperative AMI.

Index of microcirculatory resistance: state-of-the-art and potential applications in computational simulation of coronary artery disease

The dysfunction of coronary microcirculation is an important cause of coronary artery disease (CAD). The index of microcirculatory resistance (IMR) is a quantitative evaluation of coronary microcirculatory function, which provides a significant reference for the prediction, diagnosis, treatment, and prognosis of CAD. IMR also plays a key role in investigating the interaction between epicardial and microcirculatory dysfunctions, and is closely associated with coronary hemodynamic parameters such as flow rate, distal coronary pressure, and aortic pressure, which have been widely applied in computational studies of CAD. However, there is currently a lack of consensus across studies on the normal and pathological ranges of IMR. The relationships between IMR and coronary hemodynamic parameters have not been accurately quantified, which limits the application of IMR in computational CAD studies. In this paper, we discuss the research gaps between IMR and its potential applications in the computational simulation of CAD. Computational simulation based on the combination of IMR and other hemodynamic parameters is a promising technology to improve the diagnosis and guide clinical trials of CAD.

Pharmacological and simulated exercise cardiac stress tests produce different ischemic signatures in high-resolution experimental mapping studies

OBJECTIVE

Test the hypothesis that exercise and pharmacological cardiac stressors create different electrical ischemic signatures.

INTRODUCTION

Current clinical stress tests for detecting ischemia lack sensitivity and specificity. One unexplored source of the poor detection is whether pharmacological stimulation and regulated exercise produce identical cardiac stress.

METHODS

We used a porcine model of acute myocardial ischemia in which animals were instrumented with transmural plunge-needle electrodes, an epicardial sock array, and torso arrays to simultaneously measure cardiac electrical signals within the heart wall, the epicardial surface, and the torso surface, respectively. Ischemic stress via simulated exercise and pharmacological stimulation were created with rapid electrical pacing and dobutamine infusion, respectively, and mimicked clinical stress tests of five 3-minute stages. Perfusion to the myocardium was regulated by a hydraulic occluder around the left anterior descending coronary artery. Ischemia was measured as deflections to the ST-segment on ECGs and electrograms.

RESULTS

Across eight experiments with 30 (14 simulated exercise and 16 dobutamine) ischemic interventions, the spatial correlations between exercise and pharmacological stress diverged at stage three or four during interventions (p<0.05). We found more detectable ST-segment changes on the epicardial surface during simulated exercise than with dobutamine (p<0.05). The intramyocardial ischemia formed during simulated exercise had larger ST40 potential gradient magnitudes (p<0.05).

CONCLUSION

We found significant differences on the epicardium between cardiac stress types using our experimental model, which became more pronounced at the end stages of each test. A possible mechanism for these differences was the larger ST40 potential gradient magnitudes within the myocardium during exercise. The presence of microvascular dysfunction during exercise and its absence during dobutamine stress may explain these differences.

The effect of elective implantation of the ABSORB bioresorbable vascular scaffold on coronary microcirculation: Serial assessment using the index of microcirculatory resistance

INTRODUCTION

Stenting with metal stents can affect microcirculatory function. The impact of BVS on the microvascular network has not been studied.

METHODS

A total of 30 patients with bifurcation disease of Medina (X,X,0) scheduled for elective PCI with the ABSORB BVS were studied. Pressure wire studies were performed before and after scaffold implantation and at a mean follow-up of 9 months. At each time point, FFR, IMR, and CFR were calculated using the thermodilution method.

RESULTS

Following scaffold implantation, FFR change from pre-PCI, post-PCI and follow-up was 0.76, 0.92 and 0.91, respectively (P < 0.001 from pre to post-PCI and P = 0.91 from post-PCI to follow-up). There was a statistically significant improvement between pre- and post-procedural IMR (median 27.7 to 17.9, P = 0.02) and CFR (median 2.2 to 2.9, P = 0.02). Median IMR at follow-up (23.6) remained numerically lower than pre-procedure but this was not statistically significant (P = 0.05). Similarly, while median CFR at follow-up remained at post-procedural level (2.9), this effect did not reach statistical significance (P = 0.06).

CONCLUSION

There is an immediate reduction in microvascular resistance after elective BVS implantation but this effect is not sustained long term.

Integrative model of coronary flow in anatomically based vasculature under myogenic, shear, and metabolic regulation

Coronary blood flow is regulated to match the oxygen demand of myocytes in the heart wall. Flow regulation is essential to meet the wide range of cardiac workload. The blood flows through a complex coronary vasculature of elastic vessels having nonlinear wall properties, under transmural heterogeneous myocardial extravascular loading. To date, there is no fully integrative flow analysis that incorporates global and local passive and flow control determinants. Here, we provide an integrative model of coronary flow regulation that considers the realistic asymmetric morphology of the coronary network, the dynamic myocardial loading on the vessels embedded in it, and the combined effects of local myogenic effect, local shear regulation, and conducted metabolic control driven by venous O₂ saturation level. The model predicts autoregulation (approximately constant flow over a wide range of coronary perfusion pressures), reduced heterogeneity of regulated flow, and presence of flow reserve, in agreement with experimental observations. Furthermore, the model shows that the metabolic and myogenic regulations play a primary role, whereas shear has a secondary one. Regulation was found to have a significant effect on the flow except under extreme (high and low) inlet pressures and metabolic demand. Novel outcomes of the model are that cyclic myocardial loading on coronary vessels enhances the coronary flow reserve except under low inlet perfusion pressure, increases the pressure range of effective autoregulation, and reduces the network flow in the absence of metabolic regulation. Collectively, these findings demonstrate the utility of the present biophysical model, which can be used to unravel the underlying mechanisms of coronary physiopathology.

The index of microcirculatory resistance in the physiologic assessment of the coronary microcirculation

The coronary microcirculation plays a critical role in normal cardiac physiology as well as in many disease states. However, methods to evaluate the function of the coronary microvessels have been limited by technical and theoretical issues. Recently, the index of microcirculatory resistance (IMR) has been proposed and validated as a simple and specific invasive method of assessing the coronary microcirculation. By relying on the thermodilution theory and using a pressure-temperature sensor guidewire, IMR provides a measurement of the minimum achievable microcirculatory resistance in a target coronary artery territory, enabling a quantitative assessment of the microvascular integrity. Unlike indices such as coronary flow reserve, IMR is highly reproducible and independent of hemodynamic changes. In ST-elevation myocardial infarction, IMR predicts myocardial recovery and long-term mortality, whereas in patients with stable coronary artery disease, preintervention IMR predicts the occurrence of periprocedural myocardial infarction. Increasingly, research has focused on IMR-guided interventions of the microcirculation, with the aim of preventing and/or treating the microcirculatory dysfunction that commonly accompanies the epicardial coronary disease. In the present review, we will discuss the theoretical and practical basis for IMR, the clinical studies supporting it, and the future lines of research using this novel tool.

Percutaneous coronary intervention increases microvascular resistance in patients with non-ST-elevation acute coronary syndrome

AIMS

In the acute coronary syndrome setting, the interaction between epicardial coronary artery stenosis and microcirculation subtended by the culprit vessel is poorly understood. The purpose of the present study was to assess the immediate impact of percutaneous coronary intervention (PCI) on microvascular resistance (MR) in patients with non-ST-elevation myocardial infarction (NSTEMI).

METHODS AND RESULTS

Thirty-eight patients undergoing PCI for NSTEMI were recruited consecutively. Culprit lesions were stented over a Doppler and pressure-sensor-equipped guidewire. In the presence of epicardial stenosis, MR was calculated by taking collateral flow, as measured by the coronary wedge pressure, into consideration. After removal of epicardial stenosis, MR was calculated simply as distal coronary pressure divided by average peak velocity. When collateral flow was incorporated into the calculation, MR increased significantly from 1.70 ± 0.76 to 2.05 ± 0.72 (p=0.001) after PCI in the whole population. Periprocedural changes (Δ) in absolute values of MR and troponin T correlated significantly (r=0.629, p=0.0001). In patients who developed periprocedural myocardial infarction, MR increased significantly after PCI (1.48 ± 0.73 versus 2.28 ± 0.71, p<0.001). Nevertheless, removal of the epicardial lesion did not change MR in patients without periprocedural MI (1.91±0.73 versus 1.81±0.67, p=0.1).

CONCLUSIONS

When collateral flow is accounted for, removal of epicardial stenosis increases MR in patients with NSTEMI undergoing PCI.

Intracoronary pressure and flow velocity for hemodynamic evaluation of coronary stenoses

Adequate patient selection for percutaneous coronary intervention is of utmost importance to minimize early and late complications. Consequently, objective evidence for myocardial ischemia is mandatory for the management of patients with coronary artery disease, in particular in multivessel disease and intermediate lesions (40-70% diameter stenosis on angiography). The use of sensor-equipped guide wires for the assessment of functional coronary lesion severity has become widespread in the catheterization laboratory. The indices derived from pressure or flow measurements, fractional flow reserve, coronary flow velocity reserve and relative coronary flow velocity reserve show a high agreement with noninvasive stress testing. However, while these indices are based on either intracoronary pressure or flow, they do not investigate the hemodynamics of the coronary circulation entirely, leading to ambiguous outcomes. Only the use of simultaneously measured pressure and flow will avoid any possible misinterpretation of the data.

Improvement in coronary haemodynamics after percutaneous coronary intervention: assessment using instantaneous wave-free ratio

Objective

To determine whether the instantaneous wave-free ratio (iFR) can detect improvement in stenosis significance after percutaneous coronary intervention (PCI) and compare this with fractional flow reserve (FFR) and whole cycle Pd/Pa.

Design

A prospective observational study was undertaken in elective patients scheduled for PCI with FFR ≤0.80. Intracoronary pressures were measured at rest and during adenosine-mediated vasodilatation, before and after PCI. iFR, Pd/Pa and FFR values were calculated using the validated fully automated algorithms.

Setting

Coronary catheter laboratories in two UK centres and one in the USA.

Patients

120 coronary stenoses in 112 patients were assessed. The mean age was 63±10 years, while 84% were male; 39% smokers; 33% with diabetes. Mean diameter stenosis was 68±16% by quantitative coronary angiography.

Results

Pre-PCI, mean FFR was 0.66±0.14, mean iFR was 0.75±0.21 and mean Pd/Pa 0.83±0.16. PCI increased all indices significantly (FFR 0.89±0.07, p<0.001; iFR 0.94±0.05, p<0.001; Pd/Pa 0.96±0.04, p<0.001). The change in iFR after intervention (0.20±0.21) was similar to ΔFFR 0.22±0.15 (p=0.25). ΔFFR and ΔiFR were significantly larger than resting ΔPd/Pa (0.13±0.16, both p<0.001). Similar incremental changes occurred in patients with a higher prevalence of risk factors for microcirculatory disease such as diabetes and hypertension.

Conclusions

iFR and FFR detect the changes in coronary haemodynamics elicited by PCI. FFR and iFR have a significantly larger dynamic range than resting Pd/Pa. iFR might be used to objectively document improvement in coronary haemodynamics following PCI in a similar manner to FFR.

Effect of myocardial contractility on hemodynamic end points under concomitant microvascular disease in a porcine model

In this study, coronary diagnostic parameters, pressure drop coefficient (CDP: ratio of trans-stenotic pressure drop to distal dynamic pressure), and lesion flow coefficient (LFC: ratio of % area stenosis (%AS) to the CDP at throat region), were evaluated to distinguish levels of %AS under varying contractility conditions, in the presence of microvascular disease (MVD). In 10 pigs, %AS and MVD were created using angioplasty balloons and 90-μm microspheres, respectively. Simultaneous measurements of pressure drop, left ventricular pressure (p), and velocity were obtained. Contractility was calculated as (dp/dt)max, categorized into low contractility <900 mmHg/s and high contractility >900 mmHg/s, and in each group, compared between %AS <50 and >50 using analysis of variance. In the presence of MVD, between the %AS <50 and >50 groups, values of CDP (71 ± 1.4 and 121 ± 1.3) and LFC (0.10 ± 0.04 and 0.19 ± 0.04) were significantly different (P < 0.05), under low-contractility conditions. A similar %AS trend was observed under high-contractility conditions (CDP: 18 ± 1.4 and 91 ± 1.4; LFC: 0.08 ± 0.04 and 0.25 ± 0.04). Under MVD conditions, similar to fractional flow reserve, CDP and LFC were not influenced by contractility.

Angiographic and clinical characteristics of type 1 versus type 2 perioperative myocardial infarction

OBJECTIVES

The aim of this study was to analyze clinical and angiographic differences between the two etiologic subtypes of perioperative myocardial infarction (PMI).

BACKGROUND

PMI is believed to occur by either reduced coronary blood flow attributable to acute plaque rupture and thrombosis (type 1) or primary increase in oxygen demand in the setting of stable but stenotic lesions (type 2). Incidence and mortality rates of PMI are substantial, but angiographic and clinical features are not well characterized.

METHODS

Consecutive patients with PMI were classified as "type 1" or "type 2" based on angiographic characteristics of culprit lesions. Clinical and angiographic characteristics of each subtype were compared using statistical analyses.

RESULTS

Of the 54 patients analyzed, 32 (59%) cases had type 1 PMI, whereas 22 others (41%) had type 2 PMI. Compared with type 2 patients, those with type 1 PMI more often had ECG (electrocardiogram) ST elevation (53% versus 23%, P = 0.026), greater peak troponin (15.3 ng/dl versus 5.3 ng/dl, P = 0.035), higher preoperative mean blood pressure (103 mm Hg versus 93 mm Hg, P = 0.009), greater decrease in mean intraoperative blood pressure (-36 mm Hg versus -26 mm Hg, P = 0.015). Type I patients trended toward greater in-hospital mortality (16% versus 5%, P = 0.38) and length of hospitalization (13.5 days versus 9.0 days, P = 0.13).

CONCLUSIONS

These results demonstrate that PMI not only results from "demand ischemia" but also that in nearly 60% of cases the cause is acute plaque rupture. Patients with PMI attributable to plaque rupture suffer more intraoperative hypotension, greater transmural ischemia, larger infarct size, and trended toward worse outcome.

An angiographic technique for coronary fractional flow reserve measurement: in vivo validation

Fractional flow reserve (FFR) is an important prognostic determinant in a clinical setting. However, its measurement currently requires the use of invasive pressure wire, while an angiographic technique based on first-pass distribution analysis and scaling laws can be used to measure FFR using only image data. Eight anesthetized swine were instrumented with flow probe on the proximal segment of the left anterior descending (LAD) coronary arteries. Volumetric blood flow from the flow probe (Qp), coronary pressure (Pa) and right atrium pressure (Pv) were continuously recorded. Flow probe-based FFR (FFRq) was measured from the ratio of flow with and without stenosis. To determine the angiography-based FFR (FFRa), the ratio of blood flow in the presence of a stenosis (QS) to theoretically normal blood flow (QN) was calculated. A region of interest in the LAD arterial bed was drawn to generate time-density curves using angiographic images. QS was measured using a time-density curve and the assumption that blood was momentarily replaced with contrast agent during the injection. QN was estimated from the total coronary arterial volume using scaling laws. Pressure-wire measurements of FFR (FFRp), which was calculated from the ratio of distal coronary pressure (Pd) divided by proximal pressure (Pa), were continuously obtained during the study. A total of 54 measurements of FFRa, FFRp, and FFRq were taken. FFRa showed a good correlation with FFRq (FFRa = 0.97 FFRq +0.06, r(2) = 0.80, p < 0.001), although FFRp overestimated the FFRq (FFRp = 0.657 FFRq + 0.313, r(2) = 0.710, p < 0.0001). Additionally, the Bland-Altman analysis showed a close agreement between FFRa and FFRq. This angiographic technique to measure FFR can potentially be used to evaluate both anatomical and physiological assessments of a coronary stenosis during routine diagnostic cardiac catheterization that requires no pressure wires.

Prognostic value at 5 years of microvascular obstruction after acute myocardial infarction assessed by cardiovascular magnetic resonance

BACKGROUND

Early and late microvascular obstruction (MVO) assessed by cardiovascular magnetic resonance (CMR) are prognostic markers for short-term clinical endpoints after acute ST-elevation myocardial infarction (STEMI). However, there is a lack of studies with long-term follow-up periods (>24 months).

METHODS

STEMI patients reperfused by primary angioplasty (n = 129) underwent MRI at a median of 2 days after the index event. Early MVO was determined on dynamic Gd first-pass images directly after the administration of 0.1 mmol/kg bodyweight Gd-based contrast agent. Furthermore, ejection fraction (EF, %), left ventricular myocardial mass (LVMM) and total infarct size (% of LVMM) were determined with CMR. Clinical follow-up was conducted after a median of 52 months. The primary endpoint was defined as a composite of death, myocardial re-infarction, stroke, repeat revascularization, recurrence of ischemic symptoms, atrial fibrillation, congestive heart failure and hospitalization.

RESULTS

Follow-up was completed by 107 patients. 63 pre-defined events occurred during follow-up. Initially, 74 patients showed early MVO. Patients with early MVO had larger infarcts (mean: 24.9 g vs. 15.5 g, p = 0.002) and a lower EF (mean: 39% vs. 46%, p = 0.006). The primary endpoint occurred in 66.2% of patients with MVO and in 42.4% of patients without MVO (p < 0.05). The presence of early MVO was associated with a reduced event-free survival (log-rank p < 0.05). Early MVO was identified as the strongest independent predictor for the occurrence of the primary endpoint in the multivariable Cox regression analysis adjusting for age, ejection fraction and infarct size (hazard ratio: 2.79, 95%-CI 1.25-6.25, p = 0.012).

CONCLUSION

Early MVO, as assessed by first-pass CMR, is an independent long-term prognosticator for morbidity after AMI.

Effect of heart rate on hemodynamic endpoints under concomitant microvascular disease in a porcine model

Diagnosis of the ischemic power of epicardial stenosis with concomitant microvascular disease (MVD) is challenging during coronary interventions, especially under variable hemodynamic factors like heart rate (HR). The goal of this study is to assess the influence of variable HR and percent area stenosis (%AS) in the presence of MVD on pressure drop coefficient (CDP; ratio of transstenotic pressure drop to the distal dynamic pressure) and lesion flow coefficient (LFC; ratio of %AS to the CDP at the throat region). We hypothesize that CDP and LFC are independent of HR. %AS and MVD were created using angioplasty balloons and 90-μm microspheres, respectively. Simultaneous measurements of pressure drop (DP) and velocity were done in 11 Yorkshire pigs. Fractional flow reserve (FFR), CDP, and LFC were calculated for the groups HR < 120 and HR > 120 beats/min, %AS < 50 and %AS > 50, and additionally for DP < 14 and DP > 14 mmHg, and analyzed using regression and ANOVA analysis. Regression analysis showed independence between HR and the FFR, CDP, and LFC while it showed dependence between %AS and the FFR, CDP, and LFC. In the ANOVA analysis, for the HR < 120 beats/min and HR > 120 beats/min groups, the values of FFR (0.82 ± 0.02 and 0.82 ± 0.02), CDP (83.15 ± 26.19 and 98.62 ± 26.04), and LFC (0.16 ± 0.03 and 0.15 ± 0.03) were not significantly different (P > 0.05). However, for %AS < 50 and %AS > 50, the FFR (0.89 ± 0.02 and 0.75 ± 0.02), CDP (35.97 ± 25.79.10 and 143.80 ± 25.41), and LFC (0.09 ± 0.03 and 0.22 ± 0.03) were significantly different (P < 0.05). A similar trend was observed between the DP groups. Under MVD conditions, FFR, CDP, and LFC were not significantly influenced by changes in HR, while they can significantly distinguish %AS and DP groups.

Non-ST elevation myocardial infarction: a new pathophysiological concept could solve the contradiction between accepted cause and clinical observations

Non-ST elevation myocardial infarction (NSTEMI) and ST elevation infarction have many differences in their appearance and prognosis. A comprehensive search made us form a new hypothesis that a further cause also existsin NSTEMI: an acute, critical increase in the already existing high microvascular resistance in addition to the subtotal coronary artery occlusion. Various findings and studies can be interpreted only by our hypothesis: hemodynamic findings, ECG changes, autopsy reports and clinical observations (different long-time prognosis and different result of acute revascularization therapy in NSTEMI, similarities of NSTEMI with other clinical symptoms where increased microvascular resistance can be supposed without coronary artery disease).

OBJECTIVE

Despite similarities in the underlying pathologic mechanism non-ST elevation myocardial infarction(NSTEMI) and ST elevation infarction (STEMI) have many differences in their clinical presentation and prognosis.

METHOD

A systematic review of the literature about NSTEMI and the blood supply of the myocardium made us form a hypothesis that a further cause also exists in addition to the accepted cause of NSTEMI (subtotal coronaryartery occlusion): an acute, critical increase in an already existing high intramyocardial microvascular resistance.

EVIDENCE

Knowledge about microcirculation disturbances in ischemic heart disease and development of microcirculation damage can be fitted in our hypothesis. Various findings and studies can be interpreted only by our hypothesis: hemodynamic findings, ECG changes, autopsy reports and clinical observations about NSTEMI. The latest ones involve the different long-time prognosis and different result of acute revascularization therapy in STEMI and NSTEMI. Regarding the repolarization changes on the ECG NSTEMI shows similarities with other clinical symptoms where increased intramyocardial microvascular resistance can be supposed without coronary artery disease: false positive exercise stress test, supraventricular tachycardia, left ventricular strain and conduction disturbances.

CONCLUSION

The acute treatment of NSTEMI should aim to improve the blood inflow to the stiff myocardiumand/or impaired microvascular system and decrease the high microvascular resistance.

Assessment of coronary microcirculation in a swine animal model

Coronary microvascular dysfunction has important prognostic implications. Several hemodynamic indexes, such as coronary flow reserve (CFR), microvascular resistance, and zero-flow pressure (P(zf)), were used to establish the most reliable index to assess coronary microcirculation. Fifteen swine were instrumented with a flow probe, and a pressure wire was advanced into the distal left anterior descending artery. Adenosine was used to produce maximum hyperemia. Microspheres were used to create microvascular dysfunction. An occluder was used to produce stenosis. Blood flow from the probe (Q(p)), aortic pressure, distal coronary pressure, and right atrium pressure were recorded. Angiographic flow (Q(a)) was calculated using a time-density curve. Flow probe-based CFR and angiographic CFR were calculated using Q(p) and Q(a), respectively. Flow probe-based (NMR(qh)) and angiographic normalized microvascular resistance (NMR(ah)) were determined using Q(p) and Q(a), respectively, during hyperemia. P(zf) was calculated using Q(p) and distal coronary pressure. Two series of receiver operating characteristic curves were generated: normal epicardial artery model (N model) and stenosis model (S model). The areas under the receiver operating characteristic curves for flow probe-based CFR, angiographic CFR, NMR(qh), NMR(ah), and P(zf) were 0.855, 0.836, 0.976, 0.956, and 0.855 in N model and 0.737, 0.700, 0.935, 0.889, and 0.698 in S model. Both NMR(qh) and NMR(ah) were significantly more reliable than CFR and P(zf) in detecting the microvascular deterioration. Compared with CFR and P(zf), NMR provided a more accurate assessment of microcirculation. This improved accuracy was more prevalent when stenosis existed. Moreover, NMR(ah) is potentially a less invasive method for assessing coronary microcirculation.

Heart rate: a forgotten link in coronary artery disease?

A considerable body of evidence indicates that elevated resting heart rate is an independent, modifiable risk factor for cardiovascular events and mortality in patients with coronary artery disease. Elevated heart rate can produce adverse effects in several ways. Firstly, myocardial oxygen consumption is increased at high heart rates, but the time available for myocardial perfusion is reduced, increasing the likelihood of myocardial ischemia. Secondly, exposure of the large elastic arteries to cyclical stretch is increased at high heart rates. This effect can increase the rate at which components of the arterial wall deteriorate. Elastin fibers, which have an extremely slow rate of turnover in adult life, might be particularly vulnerable. Thirdly, elevated heart rate can predispose the myocardium to arrhythmias, and favor the development and progression of coronary atherosclerosis, by adversely affecting the balance between systolic and diastolic flow. Comparisons of the effects of the specific heart-rate-lowering drug ivabradine with those of β-blockers could help clarify the pathophysiological effects of elevated heart rate. Effective heart rate control among patients with coronary artery disease is uncommon in clinical practice, representing a missed therapeutic opportunity.

Quantification of coronary microvascular resistance using angiographic images for volumetric blood flow measurement: in vivo validation

Structural coronary microcirculation abnormalities are important prognostic determinants in clinical settings. However, an assessment of microvascular resistance (MR) requires a velocity wire. A first-pass distribution analysis technique to measure volumetric blood flow has been previously validated. The aim of this study was the in vivo validation of the MR measurement technique using first-pass distribution analysis. Twelve anesthetized swine were instrumented with a transit-time ultrasound flow probe on the proximal segment of the left anterior descending coronary artery (LAD). Microspheres were injected into the LAD to create a model of microvascular dysfunction. Adenosine (400 μg·kg(-1)·min(-1)) was used to produce maximum hyperemia. A region of interest in the LAD arterial bed was drawn to generate time-density curves using angiographic images. Volumetric blood flow measurements (Q(a)) were made using a time-density curve and the assumption that blood was momentarily replaced with contrast agent during the injection. Blood flow from the flow probe (Q(p)), coronary pressure (P(a)), and right atrium pressure (P(v)) were continuously recorded. Flow probe-based normalized MR (NMR(p)) and angiography-based normalized MR (NMR(a)) were calculated using Q(p) and Q(a), respectively. In 258 measurements, Q(a) showed a strong correlation with the gold standard Q(p) (Q(a) = 0.90 Q(p) + 6.6 ml/min, r(2) = 0.91, P < 0.0001). NMR(a) correlated linearly with NMR(p) (NMR(a) = 0.90 NMR(p) + 0.02 mmHg·ml(-1)·min(-1), r(2) = 0.91, P < 0.0001). Additionally, the Bland-Altman analysis showed a close agreement between NMR(a) and NMR(p). In conclusion, a technique based on angiographic image data for quantifying NMR was validated using a swine model. This study provides a method to measure NMR without using a velocity wire, which can potentially be used to evaluate microvascular conditions during coronary arteriography.

Improved myocardial perfusion in chronic diabetic mice by the up-regulation of pLKB1 and AMPK signaling

Previous studies related impaired myocardial microcirculation in diabetes to oxidative stress and endothelial dysfunction. Thus, this study was aimed to determine the effect of up-regulating pAMPK-pAKT signaling on coronary microvascular reactivity in the isolated heart of diabetic mice. We measured coronary resistance in wild-type and streptozotocin (STZ)-treated mice, during perfusion pressure changes. Glucose, insulin, and adiponectin levels in plasma and superoxide formation, NOx levels and heme oxygenase (HO) activity in myocardial tissue were determined. In addition, the expression of HO-1, 3-nitrotyrosine, pLKB1, pAMPK, pAKT, and peNOS proteins in control and diabetic hearts were measured. Coronary response to changes in perfusion pressure diverged from control in a time-dependent manner following STZ administration. The responses observed at 28 weeks of diabetes (the maximum time examined) were mimicked by L-NAME administration to control animals and were associated with a decrease in serum adiponectin and myocardial pLKB1, pAMPK, pAKT, and pGSK-3 expression. Cobalt protoporphyrin treatment to induce HO-1 expression reversed the microvascular reactivity seen in diabetes towards that of controls. Up-regulation of HO-1 was associated with an increase in adiponectin, pLKB1, pAKT, pAMPK, pGSK-3, and peNOS levels and a decrease in myocardial superoxide and 3-nitrotyrosine levels. In the present study we describe the time course of microvascular functional changes during the development of diabetes and the existence of a unique relationship between the levels of serum adiponectin, pLKB1, pAKT, and pAMPK activation in diabetic hearts. The restoration of microvascular function suggests a new therapeutic approach to even advanced cardiac microvascular derangement in diabetes.

Coronary microvascular resistance: methods for its quantification in humans

Coronary microvascular dysfunction is a topic that has recently gained considerable interest in the medical community owing to the growing awareness that microvascular dysfunction occurs in a number of myocardial disease states and has important prognostic implications. With this growing awareness, comes the desire to accurately assess the functional capacity of the coronary microcirculation for diagnostic purposes as well as to monitor the effects of therapeutic interventions that are targeted at reversing the extent of coronary microvascular dysfunction. Measurements of coronary microvascular resistance play a pivotal role in achieving that goal and several invasive and noninvasive methods have been developed for its quantification. This review is intended to provide an update pertaining to the methodology of these different imaging techniques, including the discussion of their strengths and weaknesses.

A review of methods for assessment of coronary microvascular disease in both clinical and experimental settings

Obstructive disease of the large coronary arteries is the prominent cause for angina pectoris. However, angina may also occur in the absence of significant coronary atherosclerosis or coronary artery spasm, especially in women. Myocardial ischaemia in these patients is often associated with abnormalities of the coronary microcirculation and may thus represent a manifestation of coronary microvascular disease (CMD). Elucidation of the role of the microvasculature in the genesis of myocardial ischaemia and cardiac damage-in the presence or absence of obstructive coronary atherosclerosis-will certainly result in more rational diagnostic and therapeutic interventions for patients with ischaemic heart disease. Specifically targeted research based on improved assessment modalities is needed to improve the diagnosis of CMD and to translate current molecular, cellular, and physiological knowledge into new therapeutic options.

Relationship between microvascular resistance and perfusion in patients with reperfused acute myocardial infarction

Despite its prognostic importance, accurate assessment of microvascular perfusion in patients with ST elevation acute myocardial infarction (STEMI) is difficult. As a new tool, the index of microvascular resistance (IMR) measurement provides us a new opportunity for interrogating microvascular condition after STEMI. In this study, we measured IMR in infarct-related artery (IRA) and explored its relation with other indices which have been suggested to evaluate microvascular perfusion in patients with reperfused STEMI.

METHODS

Forty-two patients with STEMI treated successfully with primary percutaneous coronary intervention (pPCI) were prospectively included. After 48 hours following pPCI, patients were recatheterized and IMR, coronary flow reserve (CFR), systolic and mean coronary wedge pressures (CWPs and CWPm) pressure-derived collateral flow index (CFIp) were measured in IRA by using intracoronary pressure-temperature sensor tipped guide wire. Myocardial blush grade was assessed from the second angiogram. Coronary flow velocity pattern (diastolic deceleration time: DDT) was examined with transthoracic echocardiography 48 hours after pPCI. Percentage of ST-segment recovery was calculated from surface ECG (STR%).

RESULTS

IMR well correlated with CWPs (r = 0.70, P < 0.001), CWPm (r = 0.59, P < 0.001), CFIp (r = 0.65, P < 0.001), CFR (r =-0.50, P = 0.001), and DDT (r =-0.59, P = 0.001). Correlations of IMR to non/semiinvasive indices like myocardial blush grades (MBG) (r =-0.42, P = 0.007) and STR (r =-0.37, P = 0.024) are somewhat weaker.

CONCLUSION

Given its simplicity of measurement, independence from the presence of an epicardial stenosis, and good correlation with all measures of microvascular obstruction used in this study, IMR may prove to be a valuable modality for evaluating the microcirculation.

Effect of simultaneous intracoronary guidewires on the predictive accuracy of functional parameters of coronary lesion severity

The aim of this study was to assess the influence of a second guidewire on the diagnostic accuracy of functional parameters of coronary lesion severity. Sixty-five patients with intermediate coronary lesions underwent myocardial perfusion scintigraphy. Fractional flow reserve (FFR), coronary flow velocity reserve (CFVR), and hyperemic stenosis resistance (HSR) index (HSR = stenosis pressure gradient ÷ velocity) were determined in 77 lesions. Distal pressure and velocity were acquired simultaneously (dual wire) and sequentially (single wire) with two sensor-equipped guidewires. Overall, functional parameters deteriorated from single- to dual-wire assessment. In patients without ischemia, the good diagnostic performance of FFR, CFVR, and HSR deteriorated significantly ( P < 0.001) when assessed by dual wires, with an increase in the number of false-positive results. This trend was more pronounced for HSR, since the presence of a second wire reduced maximal velocity and increased the pressure gradient. The presence of two guidewires, especially across a myocardial perfusion scintigraphy-induced nonsignificant lesion, is associated with overestimation of the hemodynamically assessed lesion severity and, therefore, is likely to have a major impact on clinical decision making. This underscores the advantage of a dual-sensor-equipped guidewire for the evaluation of stenosis severity by combined pressure and velocity measurements.

Changing scenario in chronic ischemic heart disease: therapeutic implications

Over the last 30 years, considerable progress has been made in drug therapy for chronic ischemic heart disease (IHD). Most pharmacologic therapies act by reducing myocardial oxygen consumption, increasing coronary blood flow, improving ventricular relaxation, dilating collateral vessels, and reducing peripheral resistances. Despite "optimal" medical therapy, a large fraction of patients continue to present with angina and myocardial ischemia. Coronary angioplasty (percutaneous transluminal coronary angioplasty) may improve symptoms, but several subgroups of patients, including the elderly, or those who have diabetes mellitus or heart failure, represent high-risk populations that do not always benefit from coronary revascularization. Epidemiologic studies show that life expectancy is longer compared with previous decades and that the prevalence of diabetes is increasing sharply worldwide. In addition, the prevalence of heart failure is growing because of the decreased death rate from acute myocardial infarction and improved survival of patients who have undergone coronary revascularization. We focus on this changing scenario of chronic IHD and propose an innovative approach to the treatment of angina in this context.

Paradoxical coronary microcirculatory constriction during ischemia: a synergic function for nitric oxide and endothelin

A paradoxical microcirculatory constriction has been observed in hearts of patients with ischemia, secondary to coronary stenosis. Here, using the isolated mouse heart (Langendorff), we examined the mechanism of this response, assuming involvement of nitric oxide (NO) and endothelin-1 (ET-1) systems. Perfusion pressure was maintained at 65 mmHg for 70 min (protocol 1), or it was reduced to 30 mmHg over two intervals, between the 20- and 40-min marks (protocol 2) or from the 20-min mark onward (protocol 3). In protocol 1, coronary resistance (CR) remained steady in untreated heart, whereas it progressively increased during treatment with the NO synthesis inhibitor N(G)-nitro-l-arginine methyl ester (L-NAME) (2.7-fold) or the ET(A) antagonist BQ-610 (2.8 fold). The ET(B) antagonist BQ-788 had instead no effect by itself but curtailed vasoconstriction to BQ-610. In protocol 2, hypotension raised CR by 2.2-fold. This response was blunted by reactive oxygen species (ROS) scavengers (mannitol and superoxide dismutase plus catalase) and was converted into vasodilation by l-NAME, BQ-610, or BQ-788. Restoration of normal pressure was followed by vasodilation and vasoconstriction, respectively, in untreated and treated preparations. In protocol 3, CR progressively increased with hypotension in the absence but not presence of L-NAME or BQ-610. We conclude that the coronary vasculature is normally relaxed by two concerted processes, a direct action of NO and ET-1 curtailing an ET(B2)-mediated tonic vasoconstriction through ET(A) activation. The negative feedback mechanism on ET(B2) subsides during hypotension, and the ensuing vasoconstriction is ascribed to ET-1 activating ET(A) and ET(B2) and reactive nitrogen oxide species originating from ROS-NO interaction.

Elevated C-reactive protein levels and coronary microvascular dysfunction in patients with coronary artery disease

AIMS

It is still unknown whether elevated C-reactive protein levels are responsible for coronary microcirculatory dysfunction in patients with coronary artery disease (CAD). This study was aimed at evaluating the association between C-reactive protein levels and endothelium-dependent and endothelium-independent coronary blood flow (CBF) responses in non-culprit arteries of patients with CAD.

METHODS AND RESULTS

We studied 28 patients (14 with normal and 14 with elevated C-reactive protein levels, >5 mg/L) with single-vessel disease and otherwise angiographically normal coronary arteries undergoing percutaneous transluminal coronary angioplasty (PTCA). CBF was measured in the non-PTCA vessel using an intracoronary Doppler guide wire and quantitative coronary angiography at baseline, after intracoronary infusion of substance P and of adenosine, and expressed as per cent change from baseline. The increases in CBF during infusion of substance P and of adenosine were lesser in patients with elevated than in those with normal C-reactive protein levels (34+/-22 vs. 61+/-34%, P=0.04 and 131+/-53 vs. 189+/-89%, P=0.03, respectively). Multivariable analysis identified elevated C-reactive protein level as the only independent predictor of reduced response to substance P (P=0.01) and adenosine (P=0.02).

CONCLUSION

In patients with CAD, evidence of systemic inflammation is independently associated with endothelium-dependent and endothelium-independent coronary microvascular dysfunction, which, in turn, may be critical to precipitate myocardial ischaemia, in particular, in unstable patients.

Coronary microcirculatory vasoconstriction is heterogeneously distributed in acutely ischemic myocardium

The classical model of coronary physiology implies the presence of maximal microcirculatory vasodilation during myocardial ischemia. However, Doppler monitoring of coronary blood flow (CBF) documented severe microcirculatory vasoconstriction during pacing-induced ischemia in patients with coronary artery disease. This study investigates the mechanisms that underlie this paradoxical behavior in nine patients with stable angina and single-vessel coronary disease who were candidates for stenting. While transstenotic pressures were continuously monitored, input CBF (in ml/min) to the poststenotic myocardium was measured by Doppler catheter and angiographic cross-sectional area. Simultaneously, specific myocardial blood flow (MBF, in ml·min−1·g−1) was measured by 133Xe washout. Perfused tissue mass was calculated as CBF/MBF. Measurements were obtained at baseline, during pacing-induced ischemia, and after stenting. CBF and distal coronary pressure values were also measured during pacing with intracoronary adenosine administration. During pacing, CBF decreased to 64 ± 24% of baseline and increased to 265 ± 100% of ischemic flow after adenosine administration. In contrast, pacing increased MBF to 184 ± 66% of baseline, measured as a function of the increased rate-pressure product ( r = 0.69; P < 0.05). Thus, during pacing, perfused myocardial mass drastically decreased from 30 ± 23 to 12 ± 11 g ( P < 0.01). Distal coronary pressure remained stable during pacing but decreased after adenosine administration. Stenting increased perfused myocardial mass to 39 ± 23 g ( P < 0.05 vs. baseline) as a function of the increase in distal coronary pressure ( r = 0.71; P < 0.02). In conclusion, the vasoconstrictor response to pacing-induced ischemia is heterogeneously distributed and excludes a tissue fraction from perfusion. Within perfused tissue, the metabolic demand still controls the vasomotor tone.

Medical versus interventional management of stable angina

Several randomized trials have been performed comparing medical management with surgery, medical management with angioplasty, and angioplasty with surgery for patients with stable angina. They were conducted in parallel with the progress of the revascularization techniques and as such, each of them reflects the prevailing practice of its time. Percutaneous revascularization and, to a lesser extent, coronary surgery have been, and it seems they will continue to be, rapidly evolving specialities. Medical management has also improved and future advances are likely to occur. One of the most remarkable advances in medical management is a new concept in metabolic approach in the treatment of angina pectoris. Despite the complexity of drawing conclusions from trials that compared approaches already outperformed in current practice, in this paper we review the relevant literature in an attempt to outline the comparative efficacy of the available treatment strategies.

Myocardial resistance assessed by guidewire-based pressure-temperature measurement: In vitro validation

By injecting a few cubic centimeters of saline into the coronary artery and using thermodilution principles, mean transit time (T(mn)) of the injectate can be calculated and is inversely proportional to coronary blood flow. Because microvascular resistance equals distal coronary pressure (P(d)) divided by myocardial flow, the product P(d). T(mn) provides an index of myocardial resistance (IMR). In this in vitro study in a physiologic model of the coronary circulation, we compared IMR to true myocardial resistance (TMR) at different degrees of myocardial resistance and at different degrees of epicardial stenosis. Absolute blood flow was varied from 42 to 203 ml/min and TMR varied from 0.39 to 1.63 dynes. sec/cm(5). Inverse mean transit time correlated well to absolute blood flow (R(2) = 0.93). Furthermore, an excellent correlation was found between IMR and TMR (R(2) = 0.94). IMR was independent on the severity of epicardial stenosis and thus specific for myocardial resistance. Thus, using one single guidewire, both fractional flow reserve and IMR can be measured simultaneously as indexes of epicardial and microvascular disease, respectively, enabling separate assessment of both coronary arterial and microvascular disease.

Association between coronary lesion severity and distal microvascular resistance in patients with coronary artery disease

Homogeneity of microvascular resistance in different perfusion areas of the same heart is generally assumed. We investigated the effect of the severity of an epicardial stenosis on microvascular resistance in 27 patients with coronary artery disease and stable angina. All patients had an angiographically normal coronary artery, an artery with an intermediate lesion, and an artery with a severe lesion; the latter was treated with angioplasty. In each patient, distal blood flow velocity and pressure were measured during baseline and maximal hyperemia (induced by intracoronary adenosine) using a Doppler and pressure guide wire, respectively. The ratio of mean distal pressure to average peak blood flow velocity was used as an index for the microvascular resistance (MRv). Within patients, the hyperemic MRv was higher in arteries with more severe stenosis (P = 0.021). After percutaneous transluminal coronary angioplasty (PTCA), the hyperemic MRv decreased (pre-PTCA, 2.6 vs. post-PTCA, 1.9 mmHg.cm(-1)s(-1), P < 0.01) toward the value of the reference artery (1.7 mmHg.cm(-1)s(-1); P = 0.67). We conclude that there is a positive association between coronary lesion severity and variability of distal microvascular resistance that normalizes after angioplasty. This study challenges the concept of uniform distribution of hyperemic MRv that is relevant for the interpretation of both noninvasive and invasive diagnostic tests.

Platelet glycoprotein IIb/IIIa receptor blockade and coronary resistance in unstable angina

OBJECTIVES

We designed a study to explore the effect of glycoprotein (GP) IIb/IIIa blockade on the atherosclerotic plaque and distal coronary vasculature.

BACKGROUND

Platelet GP IIb/IIIa blockers have been proven to be beneficial in acute ischemic syndromes. This effect has also been attributed to the prevention of microvascular obstruction, although the underlying mechanisms have not been fully defined.

METHODS

Eighteen patients with unstable refractory angina pectoris underwent cardiac catheterization and angioplasty. Trans-stenotic and microvascular resistances to flow were measured at baseline, during hyperventilation, and after intracoronary adenosine. Measurements were repeated early after abciximab administration and after successful percutaneous transluminal coronary angioplasty.

RESULTS

Hyperventilation induced an ischemic attack in 12 of 18 patients and increased epicardial (12.8 +/- 16.9 vs. 6.1 +/- 6.1 mm Hg/ml per min, p < 0.05) and microvascular (9.9 +/- 7.5 vs. 6.8 +/- 5.8 mm Hg/ml per min, p < 0.05) coronary resistance. Abciximab had no significant effect on epicardial resistance, although it significantly reduced distal coronary resistance under all study conditions, including baseline (4.8 +/- 4.8 mm Hg/ml per min, p < 0.01), hyperventilation (5.1 +/- 5.4 mm Hg/ml per min, p < 0.01), and intracoronary adenosine (2.7 +/- 3.0 vs. 4.3 +/- 4.3 mm Hg/ml per min, p < 0.05). The hyperventilation test became negative in all patients after abciximab administration.

CONCLUSIONS

These observations confirm the immediate beneficial effects of platelet GP IIb/IIIa blockade with abciximab in acute ischemic syndromes and suggest that improvement of microvascular function may play a central role in the mechanism of action of this drug.

Influence of hemodynamic conditions on fractional flow reserve: parametric analysis of underlying model

Pressure-based fractional flow reserve (FFR) is used clinically to evaluate the functional severity of a coronary stenosis, by predicting relative maximal coronary flow (Q(s)/Q(n)). It is considered to be independent of hemodynamic conditions, which seems unlikely because stenosis resistance is flow dependent. Using a resistive model of an epicardial stenosis (0-80% diameter reduction) in series with the coronary microcirculation at maximal vasodilation, we evaluated FFR for changes in coronary microvascular resistance (R(cor) = 0.2-0.6 mmHg. ml(-1). min), aortic pressure (P(a) = 70-130 mmHg), and coronary outflow pressure (P(b) = 0-15 mmHg). For a given stenosis, FFR increased with decreasing P(a) or increasing R(cor). The sensitivity of FFR to these hemodynamic changes was highest for stenoses of intermediate severity. For P(b) > 0, FFR progressively exceeded Q(s)/Q(n) with increasing stenosis severity unless P(b) was included in the calculation of FFR. Although the P(b)-corrected FFR equaled Q(s)/Q(n) for a given stenosis, both parameters remained equally dependent on hemodynamic conditions, through their direct relationship to both stenosis and coronary resistance.