Diastolic time fraction as a determinant of subendocardial perfusion

Impact of coronary hyperemia on collateral flow correction of coronary microvascular resistance indices

Recently, a novel method to estimate wedge pressure (Pw)-corrected minimal microvascular resistance (MR) was introduced. However, this method has not been validated since, and there are some theoretical concerns regarding the impact of different physiological conditions on the derivation of Pw measurements. This study sought to validate the recently introduced method to estimate Pw-corrected MR in a Doppler-derived study population and to evaluate the impact of different physiological conditions on the Pw measurements and the derivation of Pw-corrected MR. The method to derive "estimated" hyperemic microvascular resistance (HMR) without the need for Pw measurements was validated by estimating the coronary fractional flow reserve (FFRcor) from myocardial fractional flow reserve (FFRmyo) in a Doppler-derived study population (N = 53). From these patients, 24 had hyperemic Pw measurements available for the evaluation of hyperemic conditions on the derivation of Pw and its effect on the derivation of both "true" (with measured Pw) and "estimated" Pw-corrected HMR. Nonhyperemic Pw differed significantly from Pw measured in hyperemic conditions (26 ± 14 vs. 35 ± 14 mmHg, respectively, P < 0.005). Nevertheless, there was a strong linear relationship between FFRcor and FFRmyo in nonhyperemic conditions (R2 = 0.91, P < 0.005), as well as in hyperemic conditions (R2 = 0.87, P < 0.005). There was a strong linear relationship between "true" HMR and "estimated" HMR using either nonhyperemic (R2 = 0.86, P < 0.005) or hyperemic conditions (R2 = 0.85, P < 0.005) for correction. In contrast to a modest agreement between nonhyperemic Pw-corrected HMR and apparent HMR (R2 = 0.67, P < 0.005), hyperemic Pw-corrected HMR showed a strong agreement with apparent HMR (R2 = 0.88, P < 0.005). We validated the calculation method for Pw-corrected MR in a Doppler velocity-derived population. In addition, we found a significant impact of hyperemic conditions on the measurement of Pw and the derivation of Pw-corrected HMR.NEW & NOTEWORTHY The following are what is known: 1) wedge-pressure correction is often considered for the derivation of indices of minimal microvascular resistance, and 2) the Yong method for calculating wedge pressure-corrected index of microvascular resistance (IMR) without balloon inflation has never been validated in a Doppler-derived population and has not been tested under different physiological conditions. This study 1) adds validation for the Yong method for calculated wedge-pressure correction in a Doppler-derived study population and 2) shows significant influence of the physiological conditions on the derivation of coronary wedge pressure.

Research Progress and Clinical Value of Subendocardial Viability Ratio

Cardiovascular disease remains the leading cause of morbidity and mortality worldwide, with ischemic heart disease being a major contributor, either through coronary atherosclerotic plaque-related major vascular disease or coronary microvascular dysfunction. Obstruction of coronary blood flow impairs myocardial perfusion, which may lead to acute myocardial infarction in severe cases. The subendocardial viability ratio, also known as the Buckberg index, is a valuable tool for evaluation of myocardial perfusion because it reflects the balance between myocardial oxygen supply and oxygen demand. The subendocardial viability ratio can effectively evaluate the function of the coronary microcirculation and is associated with arterial stiffness. This ratio also has potential value in predicting adverse cardiovascular events and mortality in various populations. Moreover, the subendocardial viability ratio has demonstrated clinical significance in a range of diseases, including hypertension, aortic stenosis, peripheral arterial disease, chronic kidney disease, diabetes, and rheumatoid arthritis. This review summarizes the applications of the subendocardial viability ratio, its particular progress in the relevant research, and its clinical significance in cardiovascular diseases.

Comparison Between Invasive and Noninvasive Methods to Estimate Subendocardial Oxygen Supply and Demand Imbalance

Background

Estimation of the balance between subendocardial oxygen supply and demand could be a useful parameter to assess the risk of myocardial ischemia. Evaluation of the subendocardial viability ratio (SEVR, also known as Buckberg index) by invasive recording of left ventricular and aortic pressure curves represents a valid method to estimate the degree of myocardial perfusion relative to left ventricular workload. However, routine clinical use of this parameter requires its noninvasive estimation and the demonstration of its reliability.

Methods and Results

Arterial applanation tonometry allows a noninvasive estimation of SEVR as the ratio of the areas directly beneath the central aortic pressure curves obtained during diastole (myocardial oxygen supply) and during systole (myocardial oxygen demand). However, this “traditional” method does not account for the intra‐ventricular diastolic pressure and proper allocation to systole and diastole of left ventricular isometric contraction and relaxation, respectively, resulting in an overestimation of the SEVR values. These issues are considered in the novel method for SEVR assessment tested in this study. SEVR values estimated with carotid tonometry by "traditional” and "new” method were compared with those evaluated invasively by cardiac catheterization. The “traditional” method provided significantly higher SEVR values than the reference invasive SEVR: average of differences±SD= 44±11% (limits of agreement: 23% – 65%). The noninvasive “new” method showed a much better agreement with the invasive determination of SEVR: average of differences±SD= 0±8% (limits of agreement: ‐15% to 16%).

Conclusions

Carotid applanation tonometry provides valid noninvasive SEVR values only when all the main factors determining myocardial supply and demand flow are considered.

Nocturnal blood pressure dip and parapapillary choroidal microvasculature dropout in normal-tension glaucoma

Choroidal microvasculature dropout (CMvD) implies compromised optic nerve head perfusion in glaucoma patients. However, there are conflicting findings whether office-hour systemic blood pressure (BP) is related to the presence of CMvD. The present study investigated which systemic BP parameters, derived from 24-h ambulatory BP monitoring (ABPM), are associated with CMvD as assessed by optical coherence tomography angiography (OCT-A) in normal-tension glaucoma (NTG). This study included 88 eyes of 88 NTG patients who underwent 24-h ABPM and OCT-A imaging. Various systemic BP parameters associated with the presence of CMvD were evaluated using logistic regression analyses. CMvD was detected in 38 NTG eyes (43.2%). NTG eyes with CMvD had nighttime diastolic BP (DBP) dip of greater magnitude and longer duration than eyes without CMvD. In multivariate logistic regression, worse VF mean deviation (MD) (odds ratio [OR] 0.786; P = 0.001), greater nighttime DBP dip “%” (OR 1.051; P = 0.034), and higher daytime peak IOP (OR 1.459; P = 0.013) were significantly associated with the presence of CMvD. Based on our findings that the eyes with CMvD are closely associated with having nighttime DBP dip, NTG patients with CMvD should be recommended to undergo 24-h ABPM.

The effects of capsinoids and fermented red pepper paste supplementation on blood pressure: A systematic review and meta-analysis of randomized controlled trials

BACKGROUND & AIMS

The present systematic review and meta-analysis were conducted to investigate the effects of capsinoids and fermented red pepper paste (FRPP) supplementation on Systolic Blood Pressure (SBP) and Diastolic Blood Pressure (DBP).

METHODS

Relevant studies, published up to May 2020, were searched through PubMed/Medline, Scopus, ISI Web of Science, Embase, and Google Scholar. All randomized clinical trials investigating the effect of capsinoids and FRPP supplementation on blood pressure including SBP and DBP were included.

RESULTS

Out of 335 citations, 7 trials that enrolled 363 subjects were included. Capsinoids and FRPP resulted in significant reduction in DBP (Weighted mean differences (WMD): -1.90 mmHg; 95% CI, -3.72 to -0.09, P = 0.04) but no significant change in SBP (WMD: 0.55 mmHg, 95% CI: -1.45, 2.55, P = 0.588). FRPP had a significant reduction in SBP. Greater effects on SBP were detected in trials, lasted ≥12 weeks, and sample size >50. Capsinoids with dosage ≤200 and FRPP with dosage of 11.9 g significantly decreased DBP.

CONCLUSION

Overall, these data suggest that supplementation with FRPP may play a role in improving SBP and DBP but for capsinoids no effects detected in this analysis on SBP and DBP.

The many faces of myocardial ischaemia and angina

Obstructive disease of the epicardial coronary arteries is the main cause of angina. However, a number of patients with anginal symptoms have normal coronaries or non-obstructive coronary artery disease (CAD) despite electrocardiographic evidence of ischaemia during stress testing. In addition to limited microvascular vasodilator capacity, the coronary microcirculation of these patients is particularly sensitive to vasoconstrictor stimuli, in a condition known as microvascular angina. This review briefly summarizes the determinants and control of coronary blood flow (CBF) and myocardial perfusion. It subsequently analyses the mechanisms responsible for transient myocardial ischaemia: obstructive CAD, coronary spasm and coronary microvascular dysfunction in the absence of epicardial coronary lesions, and variable combinations of structural anomalies, impaired endothelium-dependent and/or -independent vasodilation, and enhanced perception of pain. Lastly, we exemplify mechanism of angina during tachycardia. Distal to a coronary stenosis, coronary dilator reserve is already recruited and can be nearly exhausted at rest distal to a severe stenosis. Increased heart rate reduces the duration of diastole and thus CBF when metabolic vasodilation is no longer able to increase CBF. The increase in myocardial oxygen consumption and resulting metabolic vasodilation in adjacent myocardium without stenotic coronary arteries further acts to divert blood flow away from the post-stenotic coronary vascular bed through collaterals.

Clinical quantitative cardiac imaging for the assessment of myocardial ischaemia

Cardiac imaging has a pivotal role in the prevention, diagnosis and treatment of ischaemic heart disease. SPECT is most commonly used for clinical myocardial perfusion imaging, whereas PET is the clinical reference standard for the quantification of myocardial perfusion. MRI does not involve exposure to ionizing radiation, similar to echocardiography, which can be performed at the bedside. CT perfusion imaging is not frequently used but CT offers coronary angiography data, and invasive catheter-based methods can measure coronary flow and pressure. Technical improvements to the quantification of pathophysiological parameters of myocardial ischaemia can be achieved. Clinical consensus recommendations on the appropriateness of each technique were derived following a European quantitative cardiac imaging meeting and using a real-time Delphi process. SPECT using new detectors allows the quantification of myocardial blood flow and is now also suited to patients with a high BMI. PET is well suited to patients with multivessel disease to confirm or exclude balanced ischaemia. MRI allows the evaluation of patients with complex disease who would benefit from imaging of function and fibrosis in addition to perfusion. Echocardiography remains the preferred technique for assessing ischaemia in bedside situations, whereas CT has the greatest value for combined quantification of stenosis and characterization of atherosclerosis in relation to myocardial ischaemia. In patients with a high probability of needing invasive treatment, invasive coronary flow and pressure measurement is well suited to guide treatment decisions. In this Consensus Statement, we summarize the strengths and weaknesses as well as the future technological potential of each imaging modality.

Effects of myocardial function and systemic circulation on regional coronary perfusion

Cardiac-coronary interaction and the effects of its pathophysiological variations on spatial heterogeneity of coronary perfusion and myocardial work are still poorly understood. This hypothesis-generating study predicts spatial heterogeneities in both regional cardiac work and perfusion that offer a new paradigm on the vulnerability of the subendocardium to ischemia, particularly at the apex. We propose a mathematical and computational modeling framework to simulate the interaction of left ventricular mechanics, systemic circulation, and coronary microcirculation. The computational simulations revealed that the relaxation rate of the myocardium has a significant effect whereas the contractility has a marginal effect on both the magnitude and transmural distribution of coronary perfusion. The ratio of subendocardial to subepicardial perfusion density (Q_endo/Q_epi) changed by -12 to +6% from a baseline value of 1.16 when myocardial contractility was varied by +25 and -10%, respectively; Q_endo/Q_epi changed by 37% when sarcomere relaxation rate, b, was faster and increased by 10% from the baseline value. The model predicts axial differences in regional myocardial work and perfusion density across the wall thickness. Regional myofiber work done at the apex is 30-50% lower than at the center region, whereas perfusion density in the apex is lower by only 18% compared with the center. There are large axial differences in coronary flow and myocardial work at the subendocardial locations, with the highest differences located at the apex region. A mismatch exists between perfusion density and regional work done at the subendocardium. This mismatch is speculated to be compensated by coronary autoregulation.NEW & NOTEWORTHY We present a model of left ventricle perfusion based on an anatomically realistic coronary tree structure that includes its interaction with the systemic circulation. Left ventricular relaxation rate has a significant effect on the regional distribution of coronary flow and myocardial work.

Free-weight versus weight machine resistance exercise on pulse wave reflection and aortic stiffness in resistance-trained individuals

The purpose of this study was to compare the vascular responses to acute free-weight (FW) resistance exercise (RE) versus weight machines (WM). Thirty-two resistance-trained individuals participated in this study. Both modalities involved performing acute RE and a control. Blood pressure and measures of pulse wave reflection were assessed using pulse wave analysis. Aortic stiffness was assessed using carotid-femoral pulse wave velocity (cf-PWV). A repeated measures analysis of variance was used to determine the effects of modality (FW and WM) and condition (acute RE and control) across time (rest and 10-20 min after exercise) on measures of pulse wave reflection and aortic stiffness. Significance was set a priori at p ≤ 0.05. There were no modality by condition by time interactions for any variable, such that the FW and WM modalities responded similarly across time after acute RE (p > 0.05). There were significant (p ≤ 0.05) increases in heart rate, aortic systolic blood pressure, aortic pulse pressure, augmentation index normalized at 75bpm, and decreases in subendocardial viability ratio (SEVR) after acute RE, compared to rest. There was also a significant (p ≤ 0.05) increase in cf-PWV after acute RE, compared to rest. In conclusion, this study demonstrates that acute free-weight and weight-machine RE are associated with transient increases in measures of pulse wave reflection and aortic stiffness, with reductions in myocardial perfusion. These data demonstrate that both modalities result in significant stress on the myocardium during recovery, while simultaneously increasing pressure on the aorta for at least 10-20 min.

A multi-scale model of the coronary circulation applied to investigate transmural myocardial flow

Distribution of blood flow in myocardium is a key determinant of the localization and severity of myocardial ischemia under impaired coronary perfusion conditions. Previous studies have extensively demonstrated the transmural difference of ischemic vulnerability. However, it remains incompletely understood how transmural myocardial flow is regulated under in vivo conditions. In the present study, a computational model of the coronary circulation was developed to quantitatively evaluate the sensitivity of transmural flow distribution to various cardiovascular and hemodynamic factors. The model was further incorporated with the flow autoregulatory mechanism to simulate the regulation of myocardial flow in the presence of coronary artery stenosis. Numerical tests demonstrated that heart rate (HR), intramyocardial tissue pressure (P_im ), and coronary perfusion pressure (P_per ) were the major determinant factors for transmural flow distribution (evaluated by the subendocardial-to-subepicardial (endo/epi) flow ratio) and that the flow autoregulatory mechanism played an important compensatory role in preserving subendocardial perfusion against reduced P_per . Further analysis for HR variation-induced hemodynamic changes revealed that the rise in endo/epi flow ratio accompanying HR decrease was attributable not only to the prolongation of cardiac diastole relative to systole, but more predominantly to the fall in P_im . Moreover, it was found that P_im and P_per interfered with each other with respect to their influence on transmural flow distribution. These results demonstrate the interactive effects of various cardiovascular and hemodynamic factors on transmural myocardial flow, highlighting the importance of taking into account patient-specific conditions in the explanation of clinical observations.

Radial Pulse Spectrum Analysis as Risk Markers to Improve the Risk Stratification of Silent Myocardial Ischemia in Type 2 Diabetic Patients

Diabetic patients with silent myocardial ischemia (SMI) have elevated rates of morbidity and mortality and need intensive care and monitoring. An early predictor of SMI may lead to early diagnosis and medical treatment to prevent progression and adverse clinical events. Therefore, this paper was aimed to evaluate the radial pulse spectrum as risk markers to improve the risk stratification of SMI in type-2 diabetic patients; 195 diabetic patients at high-risk of SMI were enrolled. All patients underwent myocardial perfusion imaging and radial pressure wave measurement. The spectrum analysis of the radial pressure wave was calculated and transformed into Fourier series coefficients Cns and Pns. The risk of SMI (odds ratio: 4.46, 95%, C.I. 1.61–12.4, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$P<0.01$ \end{document}) was raised in diabetic patients classified high-risk group by C2. Multivariable regression analysis showed that C2 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$P<0.05$ \end{document}) and ankle–brachial index [(ABI) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$P<0.05$ \end{document})] were related to SMI (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$R=0.46$ \end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$P<0.05$ \end{document}). The myocardial ischemic score (MIS), combining C2, C3, and P5, the albumin-to-creatinine ratio (ACR), and ABI, presented an excellent risk stratification performance in enrolled patients (odds ratio: 5.78, 95%, C.I. 2.29–14.6, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$P<0.01$ \end{document}). The area under receiver operating characteristic curves for C2, C3, P5, ABI, ACR, and MIS were 0.66, 0.60, 0.68, 0.51, 0.56, and 0.74, respectively, in identifying SMI. This paper demonstrated that C2 was independently associated with the extent of SMI in multivariable regression analysis. Odds ratio and chi-square tests reflected that C2 could be an important marker for the risk stratification of SMI. Furthermore, MIS, adding radial pulse spectrum analysis to ACR and ABI, could significantly improve the risk stratification of SMI in type-2 diabetic patients compared to any single risk factor.

Deleterious Effects of Cold Air Inhalation on Coronary Physiological Indices in Patients With Obstructive Coronary Artery Disease

Background Cold air inhalation during exercise increases cardiac mortality, but the pathophysiology is unclear. During cold and exercise, dual-sensor intracoronary wires measured coronary microvascular resistance ( MVR ) and blood flow velocity ( CBF ), and cardiac magnetic resonance measured subendocardial perfusion. Methods and Results Forty-two patients (62±9 years) undergoing cardiac catheterization, 32 with obstructive coronary stenoses and 10 without, performed either (1) 5 minutes of cold air inhalation (5°F) or (2) two 5-minute supine-cycling periods: 1 at room temperature and 1 during cold air inhalation (5°F) (randomized order). We compared rest and peak stress MVR , CBF , and subendocardial perfusion measurements. In patients with unobstructed coronary arteries (n=10), cold air inhalation at rest decreased MVR by 6% ( P=0.41), increasing CBF by 20% ( P<0.01). However, in patients with obstructive stenoses (n=10), cold air inhalation at rest increased MVR by 17% ( P<0.01), reducing CBF by 3% ( P=0.85). Consequently, in patients with obstructive stenoses undergoing the cardiac magnetic resonance protocol (n=10), cold air inhalation reduced subendocardial perfusion ( P<0.05). Only patients with obstructive stenoses performed this protocol (n=12). Cycling at room temperature decreased MVR by 29% ( P<0.001) and increased CBF by 61% ( P<0.001). However, cold air inhalation during cycling blunted these adaptations in MVR ( P=0.12) and CBF ( P<0.05), an effect attributable to defective early diastolic CBF acceleration ( P<0.05) and associated with greater ST -segment depression ( P<0.05). Conclusions In patients with obstructive coronary stenoses, cold air inhalation causes deleterious changes in MVR and CBF . These diminish or abolish the normal adaptations during exertion that ordinarily match myocardial blood supply to demand.

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.

Baseline subendocardial viability ratio influences left ventricular systolic improvement with cardiac rehabilitation

OBJECTIVE

Subendocardial viability ratio (SEVR), defined as diastolic to systolic pressure-time integral ratio, is a useful tool reflecting the balance between coronary perfusion and arterial load. Suboptimal SEVR creating a supply-demand imbalance may limit favorable cardiac response to cardiac rehabilitation (CR). To explore this hypothesis, we designed a study to analyze the relationship between baseline SEVR and response to CR in patients with coronary artery disease (CAD).

METHODS

In this prospectively study, after baseline arterial tonometry, echocardiography, and cardiopulmonary exercise tests (CPETs), patients undergone 20 sessions of CR. Post-CR echocardiographic and CPET measurements were obtained for comparison.

RESULTS

Final study population was comprised of fifty subjects. Study population was divided into two subgroups by median SEVR value (1.45, interquartile range 0.38). Although both groups showed significant improvements in peak VO2, significant improvements in oxygen pulse (πO2) (from 16.1±3.4 to 19.1±4.8 mL O2.kg-1.beat-1; p<0.001) and stroke volume index (from 31±5 to 35±6 mL; p=0.008) were observed in only the patients in the above-median subgroup. The change in πO2 was also significantly higher in the above-median SEVR subgroup (2.9±3.3 vs. 0.5±2.4; p=0.007).

CONCLUSION

Our study shows that baseline supply-demand imbalance may limit systolic improvement response to CR in patients with CAD.

Open-loop (feed-forward) and feedback control of coronary blood flow during exercise, cardiac pacing, and pressure changes

A control system model was developed to analyze data on in vivo coronary blood flow regulation and to probe how different mechanisms work together to control coronary flow from rest to exercise, and under a variety of experimental conditions, including cardiac pacing and with changes in coronary arterial pressure (autoregulation). In the model coronary flow is determined by the combined action of a feedback pathway signal that is determined by the level of plasma ATP in coronary venous blood, an adrenergic open-loop (feed-forward) signal that increases with exercise, and a contribution of pressure-mediated myogenic control. The model was identified based on data from exercise experiments where myocardial oxygen extraction, coronary flow, cardiac interstitial norepinephrine concentration, and arterial and coronary venous plasma ATP concentrations were measured during control and during adrenergic and purinergic receptor blockade conditions. The identified model was used to quantify the relative contributions of open-loop and feedback pathways and to illustrate the degree of redundancy in the control of coronary flow. The results indicate that the adrenergic open-loop control component is responsible for most of the increase in coronary blood flow that occurs during high levels of exercise. However, the adenine nucleotide-mediated metabolic feedback control component is essential. The model was evaluated by predicting coronary flow in cardiac pacing and autoregulation experiments with reasonable fits to the data. The analysis shows that a model in which coronary venous plasma adenine nucleotides are a signal in local metabolic feedback control of coronary flow is consistent with the available data.

Influence of segmented vessel size due to limited imaging resolution on coronary hyperemic flow prediction from arterial crown volume

Computational predictions of the functional stenosis severity from coronary imaging data use an allometric scaling law to derive hyperemic blood flow (Q) from coronary arterial volume (V), Q = αV(β) Reliable estimates of α and β are essential for meaningful flow estimations. We hypothesize that the relation between Q and V depends on imaging resolution. In five canine hearts, fluorescent microspheres were injected into the left anterior descending coronary artery during maximal hyperemia. The coronary arteries of the excised heart were filled with fluorescent cast material, frozen, and processed with an imaging cryomicrotome to yield a three-dimensional representation of the coronary arterial network. The effect of limited image resolution was simulated by assessing scaling law parameters from the virtual arterial network at 11 truncation levels ranging from 50 to 1,000 μm segment radius. Mapped microsphere locations were used to derive the corresponding relative Q using a reference truncation level of 200 μm. The scaling law factor α did not change with truncation level, despite considerable intersubject variability. In contrast, the scaling law exponent β decreased from 0.79 to 0.55 with increasing truncation radius and was significantly lower for truncation radii above 500 μm vs. 50 μm (P< 0.05). Hyperemic Q was underestimated for vessel truncation above the reference level. In conclusion, flow-crown volume relations confirmed overall power law behavior; however, this relation depends on the terminal vessel radius that can be visualized. The scaling law exponent β should therefore be adapted to the resolution of the imaging modality.

The role of capillary transit time heterogeneity in myocardial oxygenation and ischemic heart disease

Ischemic heart disease (IHD) is characterized by an imbalance between oxygen supply and demand, most frequently caused by coronary artery disease (CAD) that reduces myocardial perfusion. In some patients, IHD is ascribed to microvascular dysfunction (MVD): microcirculatory disturbances that reduce myocardial perfusion at the level of myocardial pre-arterioles and arterioles. In a minority of cases, chest pain and reductions in myocardial flow reserve may even occur in patients without any other demonstrable systemic or cardiac disease. In this topical review, we address whether these findings might be caused by impaired myocardial oxygen extraction, caused by capillary flow disturbances further downstream. Myocardial blood flow (MBF) increases approximately linearly with oxygen utilization, but efficient oxygen extraction at high MBF values is known to depend on the parallel reduction of capillary transit time heterogeneity (CTH). Consequently, changes in capillary wall morphology or blood viscosity may impair myocardial oxygen extraction by preventing capillary flow homogenization. Indeed, a recent re-analysis of oxygen transport in tissue shows that elevated CTH can reduce tissue oxygenation by causing a functional shunt of oxygenated blood through the tissue. We review the combined effects of MBF, CTH, and tissue oxygen tension on myocardial oxygen supply. We show that as CTH increases, normal vasodilator responses must be attenuated in order to reduce the degree of functional shunting and improve blood-tissue oxygen concentration gradients to allow sufficient myocardial oxygenation. Theoretically, CTH can reach levels such that increased metabolic demands cannot be met, resulting in tissue hypoxia and angina in the absence of flow-limiting CAD or MVD. We discuss these predictions in the context of MVD, myocardial infarction, and reperfusion injury.

Impact of anatomical and functional severity of coronary atherosclerotic plaques on the transmural perfusion gradient: a [15O]H2O PET study

BACKGROUND

Myocardial ischaemia occurs principally in the subendocardial layer, whereas conventional myocardial perfusion imaging provides no information on the transmural myocardial blood flow (MBF) distribution. Subendocardial perfusion measurements and quantification of the transmural perfusion gradient (TPG) could be more sensitive and specific for the detection of coronary artery disease (CAD). The current study aimed to determine the impact of lesion severity as assessed by the fractional flow reserve (FFR) on subendocardial perfusion and the TPG using [(15)O]H2O positron emission tomography (PET) imaging in patients evaluated for CAD.

METHODS AND RESULTS

Sixty-six patients with anginal chest pain were prospectively enrolled and underwent [(15)O]H2O myocardial perfusion PET imaging. Subsequently, invasive coronary angiography was performed and FFR obtained in all coronary arteries irrespective of the PET imaging results. Thirty (45%) patients were diagnosed with significant CAD (i.e. FFR ≤0.80), whereas on a per vessel analysis (n = 198), 53 (27%) displayed a positive FFR. Transmural hyperaemic MBF decreased significantly from 3.09 ± 1.16 to 1.67 ± 0.57 mL min(-1) g(-1) (P < 0.001) in non-ischaemic and ischaemic myocardium, respectively. The TPG decreased during hyperaemia when compared with baseline (1.20 ± 0.14 vs. 0.94 ± 0.17, P < 0.001), and was lower in arteries with a positive FFR (0.97 ± 0.16 vs. 0.88 ± 0.18, P < 0.01). A TPG threshold of 0.94 yielded an accuracy to detect CAD of 59%, which was inferior to transmural MBF with an optimal cutoff of 2.20 mL min(-1) g(-1) and an accuracy of 85% (P < 0.001). Diagnostic accuracy of subendocardial perfusion measurements was comparable with transmural MBF (83 vs. 85%, respectively, P = NS).

CONCLUSION

Cardiac [(15)O]H2O PET imaging is able to distinguish subendocardial from subepicardial perfusion in the myocardium of normal dimensions. Hyperaemic TPG is significantly lower in ischaemic myocardium. This technique can potentially be employed to study subendocardial perfusion impairment in more detail. However, the diagnostic accuracy of subendocardial hyperaemic perfusion and TPG appears to be limited compared with quantitative transmural MBF, warranting further study.

Effects of hypocaloric diet, low-intensity resistance exercise with slow movement, or both on aortic hemodynamics and muscle mass in obese postmenopausal women

OBJECTIVE

This study aims to examine the independent and combined impact of hypocaloric diet and low-intensity resistance exercise training (LIRET) on aortic hemodynamics and appendicular skeletal muscle mass (ASM) in obese postmenopausal women.

METHODS

Forty-one obese postmenopausal women (mean [SD] age, 54 [1] y) were randomly assigned to LIRET (n = 13), diet (n = 14), or diet + LIRET (n = 14). Body weight, waist circumference, aortic systolic blood pressure, aortic pulse pressure, augmentation index, subendocardial viability ratio (SEVR; myocardial perfusion), and heart rate (HR) were measured before and after 12 weeks. ASM was assessed by dual-energy x-ray absorptiometry.

RESULTS

Body weight (P < 0.001) and waist circumference (P < 0.01) decreased similarly after diet and diet + LIRET compared with no changes after LIRET. ASM did not change after diet + LIRET, and the decrease observed after diet (P < 0.001) was significant compared with LIRET. Aortic systolic blood pressure decreased similarly after LIRET (P < 0.05), diet (P < 0.01), and diet + LIRET (P < 0.01). Aortic pulse pressure (P < 0.05) decreased similarly after diet and diet + LIRET, but not after LIRET. SEVR (P < 0.01) increased similarly in both diet groups, whereas HR (P < 0.01) decreased only after diet. Changes in SEVR (P < 0.05) and HR (P< 0.01) with diet were different compared with LIRET. The augmentation index did not change in any group.

CONCLUSIONS

Our findings suggest that diet-induced weight loss may reduce cardiovascular risk by improving SEVR via HR and aortic pulse pressure reductions in obese postmenopausal women. LIRET prevents ASM loss associated with hypocaloric diet but has no additive effects on aortic hemodynamics.

Effect of the arteriovenous access for hemodialysis on subendocardial viability ratio, pulse pressure and hospitalizations

BACKGROUND

In some patients the potential benefits of the arteriovenous (AV) access over catheter for hemodialysis seem to be outweighed by global cardiovascular status deterioration.

METHODS

We prospectively evaluated 44 pre-dialysis chronic kidney disease patients submitted to vascular access creation during a follow-up of 25 ± 9 months. We performed pulse wave analysis and biochemical assessment before and 2 months after AV access construction, and we registered premature vascular access thrombosis, and all-cause and cardiovascular hospitalizations throughout follow-up.

RESULTS

We found a statistically significant decrease in the subendocardial viability ratio (SEVR) and pulse pressure (PP) parameters after AV access creation while brain natriuretic peptide significantly increased. Receiver operating characteristic curve analysis identified SEVR ≤113 % evaluated 2 months after vascular access construction as the best cutoff value for predicting all-cause and cardiovascular hospitalizations. Kaplan-Meier analysis showed that a SEVR ≤113 % was associated with all-cause (p = 0.010) and cardiovascular (p = 0.029) hospitalizations; Cox regression analysis verified a 4.9-fold higher risk of all-cause hospitalization in patients with SEVR ≤113 % (p = 0.005).

CONCLUSION

To our best knowledge, this report indicates, for the first time, that despite the decrease in PP parameters, the creation of a vascular access for hemodialysis was also associated with a reduction of SEVR which predicted a worse clinical outcome. We argue that the decrease of pulse pressure after arteriovenous construction may reflect a new hemodynamic set-point after vascular access creation and may not indicate a protective cardiovascular effect.

Increased wave reflection and ejection duration in women with chest pain and nonobstructive coronary artery disease: ancillary study from the Women's Ischemia Syndrome Evaluation

OBJECTIVE

Wave reflections augment central aortic SBP and increase systolic pressure time integral (SPTI) thereby increasing left ventricular (LV) afterload and myocardial oxygen (MVO2) demand. When increased, such changes may contribute to myocardial ischemia and angina pectoris, especially when aortic diastolic time is decreased and myocardial perfusion pressure jeopardized. Accordingly, we examined pulse wave reflection characteristics and diastolic timing in a subgroup of women with chest pain (Women's Ischemia Syndrome Evaluation, WISE) and no obstructive coronary artery disease (CAD).

METHODS

Radial artery BP waveforms were recorded by applanation tonometry, and aortic BP waveforms derived. Data from WISE participants were compared with data from asymptomatic women (reference group) without chest pain matched for age, height, BMI, mean arterial BP, and heart rate.

RESULTS

Compared with the reference group, WISE participants had higher aortic SBP and pulse BP and ejection duration. These differences were associated with increased augmentation index and reflected pressure wave systolic duration. These modifications in wave reflection characteristics were associated with increased SPTI and wasted LV energy (Ew) and a decrease in pulse pressure amplification, myocardial viability ratio, and diastolic pressure time fraction.

CONCLUSION

WISE participants with no obstructive CAD have changes in systolic wave reflections and diastolic timing that increase LV afterload, MVO2 demand, and Ew with the potential to reduce coronary artery perfusion. These alterations in cardiovascular function contribute to an undesirable mismatch in the MVO2 supply/demand that promotes ischemia and chest pain and may contribute to, or increase the severity of, future adverse cardiovascular events.

Perfusion territories subtended by penetrating coronary arteries increase in size and decrease in number toward the subendocardium

Blood flow distribution within the myocardium and the location and extent of areas at risk in case of coronary artery disease are dependent on the distribution and morphology of intramural vascular crowns. Knowledge of the intramural vasculature is essential in novel multiscale and multiphysics modeling of the heart. For this study, eight canine hearts were analyzed with an imaging cryomicrotome, developed to acquire high-resolution spatial data on three-dimensional vascular structures. The obtained vasculature was skeletonized, and for each penetrating artery starting from the epicardium, the dependent vascular crown was defined. Three-dimensional Voronoi tessellation was applied with the end points of the terminal segments as center points. The centroid of end points in each branch allowed classification of the corresponding perfusion territories in subendocardial, midmyocardial, and subepicardial. Subendocardial regions have relatively few territories of about 0.5 ml in volume having their own penetrating artery at the epicardium, whereas the subepicardium is perfused by a multitude of small perfusion territories, in the order of 0.01 ml. Vascular volume density of small arteries up till 400 μm was 3.2% at the subendocardium territories but only 0.8% in the subepicardium territories. Their higher volume density corresponds to compensation for flow impeding forces by cardiac contraction. These density differences result in different scaling law properties of vascular volume and tissue mass per territory type. This novel three-dimensional quantitative analysis may form the basis for patient-specific computational models on coronary perfusion and aid the interpretation of image-based clinical methods for assessing the transmural perfusion distribution.

Flow restoration post revascularization predicted by stenosis indexes: sensitivity to hemodynamic variability

The expected blood flow improvement following a coronary intervention is inversely related to the stenotic-to-normal flow ratio Qs/Qn. Since Qn cannot be measured prior to intervention, treatment decisions rely on stenosis-severity indexes, e.g., area stenosis (%AS), hyperemic stenosis resistance (HSR), and fractional flow reserve (FFR), where treatment cut-off levels have been established for each index based on presence of inducible ischemia. Here, we studied the dependence of these indexes-predicted Qs/Qn under physiological perturbations of stenosis features and of hemodynamic and mechanical conditions. Dynamic coronary flow was simulated based on measured coronary morphometric data and a physics-based computational model. Simulations were used to evaluate the relationship between each index level and Qs/Qn. Under each perturbation, an independence measure (IM) was calculated for each index based on the relative change in Qs/Qn associated with each perturbation. The results show that while %AS prediction of Qs/Qn is largely independent (IM > 90%) of physiological changes in heart rate, venous pressure, and lesion length and location on the epicardial tree, HSR is also independent of changes in left ventricle pressure. FFR-predicted Qs/Qn is also independent of changes in aortic pressure, blood hematocrit, and stenotic vessel stiffness. Nevertheless, independence of all indexes is substantially compromised (IM < 70%) under changes in vasculature stiffness. Specifically, a physiological stiffening elevates Qs/Qn value by 21% at the FFR cut-off value (0.75). These findings suggest that the current FFR cut-off value for treatment of stenotic lesions overestimates the benefit of coronary intervention in patients with a stiffer coronary vasculature (e.g., diabetics, hypertensives).

A NEW METHOD FOR DETERMINING SUBENDOCARDIAL VIABILITY RATIO FROM RADIAL ARTERY PRESSURE WAVES

Aortic subendocardial viability ratio (SEVR), an index of myocardial oxygen demand relative to supply, has been used for the early detection of hemodynamic changes. We aimed to validate a new method for determining SEVR directly from radial pressures. Hemodynamic parameters were measured in 231 outpatients (108 males and 123 females) for physical examination, aged from 20–77 years (45.9 ± 17.3 years), including 210 healthy and 21 hypertensive subjects. Aortic SEVR was obtained using a validated device (SphygmoCor; AtCor Medical, Sydney, Australia), and radial SEVR was obtained using a portable vascular testing device (IIM-2010A; Institute and Intelligent of Machines, Hefei, China). Radial SEVR was strongly related to aortic SEVR (r = 0.824, p < 0.01), with approximately 15.7% lower value. Aortic and radial SEVR had similar independent predictors, including diastolic time fraction (DTF), systolic blood pressure, diastolic blood pressure, age, and height. DTF exerted the most influence on both of them. In healthy subjects, there were significant changes in aortic and radial SEVR between age groups in both males and females (p < 0.05 for both ). Changes in aortic and radial SEVR with aging were parallel though the differences between them increased. These results suggested that the simple and easily obtainable radial SEVR could provide equivalent information to aortic SEVR, and has potential for the primary prevention of cardiovascular disease in health screening.

Diastolic hypotension is an unrecognized risk factor for β-agonist-associated myocardial injury in children with asthma

OBJECTIVES

Tachycardia and diastolic hypotension have been associated with β-2 agonist use. In the setting of β-agonist-induced chronotropy and inotropy, diastolic hypotension may limit myocardial blood flow. We hypothesized that diastolic hypotension is associated with β-agonist use and that diastolic hypotension and tachycardia are associated with biochemical evidence of myocardial injury in children with asthma.

DESIGN

Two patient cohorts were collected. The first, consisting of patients transported for respiratory distress having received at least 10 mg of albuterol, was studied for development of tachycardia and hypotension. The second, consisting of patients who had troponin measured during treatment for status asthmaticus with continuous albuterol, was studied for factors associated with elevated troponin. Exclusion criteria for both cohorts included age younger than 2 years old, sepsis, pneumothorax, cardiac disease, and antihypertensive use. Albuterol dose, other medications, and vital signs were collected. Diastolic and systolic hypotension were defined as an average value below the fifth percentile for age and tachycardia as average heart rate above the 98th percentile for age.

PATIENTS

Ninety of 1,390 children transported for respiratory distress and 64 of 767 children with status asthmaticus met inclusion criteria.

MEASUREMENTS AND MAIN RESULTS

Diastolic hypotension occurred in 56% and 98% of the first and second cohorts, respectively; tachycardia occurred in 94% and 95% of the first and second cohorts, respectively. Diastolic hypotension and tachycardia had a weak linear correlation with albuterol dose (p = 0.02 and p = 0.005, respectively). Thirty-six percent had troponin > 0.1 ng/mL (range, 0-12.6). In multivariate analysis, interaction between diastolic hypotension and tachycardia alone was associated with elevated troponin (p = 0.02).

CONCLUSIONS

Diastolic hypotension and tachycardia are dose-dependent side effects of high-dose albuterol. In high-risk patients with status asthmaticus treated with albuterol, diastolic hypotension and tachycardia are associated with biochemical evidence of myocardial injury. Diastolic hypotension, especially combined with tachycardia, could be a reversible risk factor for myocardial injury related to β-agonist use.

Changes in subendocardial viability ratio with acute high-altitude exposure and protective role of acetazolamide

High-altitude tourism is increasingly frequent, involving also subjects with manifest or subclinical coronary artery disease. Little is known, however, on the effects of altitude exposure on factors affecting coronary perfusion. The aim of our study was to assess myocardial oxygen supply/demand ratio in healthy subjects during acute exposure at high altitude and to evaluate the effect of acetazolamide on this parameter. Forty-four subjects (21 men, age range: 24-59 years) were randomized to double-blind acetazolamide 250 mg bid or placebo. Subendocardial viability ratio and oxygen supply/demand ratio were estimated on carotid artery by means of a validated PulsePen tonometer, at sea level, before and after treatment, and after acute and more prolonged exposure to high altitude (4559 m). On arrival at high altitude, subendocardial viability ratio was reduced in both placebo (from 1.63±0.15 to 1.18±0.17; P<0.001) and acetazolamide (from 1.68±0.25 to 1.35±0.18; P<0.001) groups. Subendocardial viability ratio returned to sea level values (1.65±0.24) after 3 days at high altitude under acetazolamide but remained lower than at sea level under placebo (1.42±0.22; P<0.005 versus baseline). At high altitude, oxygen supply/demand ratio fell both under placebo (from 29.6±4.0 to 17.3±3.0; P<0.001) and acetazolamide (from 32.1±7.0 to 22.3±4.6; P<0.001), its values remaining always higher (P<0.001) on acetazolamide. Administration of acetazolamide may, thus, antagonize the reduction in subendocardial oxygen supply triggered by exposure to hypobaric hypoxia. Further studies involving also subjects with known or subclinical coronary artery disease are needed to confirm a protective action of acetazolamide on myocardial viability under high-altitude exposure.

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.

Synergistic Adaptations to Exercise in the Systemic and Coronary Circulations That Underlie the Warm-Up Angina Phenomenon

Background—

The mechanisms of reduced angina on second exertion in patients with coronary arterial disease, also known as the warm-up angina phenomenon, are poorly understood. Adaptations within the coronary and systemic circulations have been suggested but never demonstrated in vivo. In this study we measured central and coronary hemodynamics during serial exercise.

Methods and Results—

Sixteen patients (15 male, 61±4.3 years) with a positive exercise ECG and exertional angina completed the protocol. During cardiac catheterization via radial access, they performed 2 consecutive exertions (Ex1, Ex2) using a supine cycle ergometer. Throughout exertions, distal coronary pressure and flow velocity were recorded in the culprit vessel using a dual sensor wire while central aortic pressure was recorded using a second wire. Patients achieved a similar workload in Ex2 but with less ischemia than in Ex1 ( P <0.01). A 33% decline in aortic pressure augmentation in Ex2 ( P <0.0001) coincided with a reduction in tension time index, a major determinant of left ventricular afterload ( P <0.001). Coronary stenosis resistance was unchanged. A sustained reduction in coronary microvascular resistance resulted in augmented coronary flow velocity on second exertion (both P <0.001). These changes were accompanied by a 21% increase in the energy of the early diastolic coronary backward-traveling expansion, or suction, wave on second exercise ( P <0.05), indicating improved microvascular conductance and enhanced left ventricular relaxation.

Conclusions—

On repeat exercise in patients with effort angina, synergistic changes in the systemic and coronary circulations combine to improve vascular–ventricular coupling and enhance myocardial perfusion, thereby potentially contributing to the warm-up angina phenomenon.

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.

Increased basal coronary blood flow as a cause of reduced coronary flow reserve in diabetic patients

A reduced coronary flow reserve (CFR) has been demonstrated in diabetes, but the underlying mechanisms are unknown. We assessed thermodilution-derived CFR after 5-min intravenous adenosine infusion through a pressure-temperature sensor-tipped wire in 30 coronary arteries without significant lumen reduction in 30 patients: 13 with and 17 without a history of diabetes. We determined CFR as the ratio of basal and hyperemic mean transit times (Tmn); fractional flow reserve (FFR) as the ratio of distal and proximal pressures at maximal hyperemia to exclude local macrovascular disease; and an index of microvascular resistance (IMR) as the distal coronary pressure at maximal hyperemia divided by the inverse of the hyperemic Tmn. We also assessed insulin resistance by the homeostasis model assessment (HOMA) index. FFR was normal in all investigated arteries. CFR was significantly lower in diabetic vs. nondiabetic patients [median (interquartile range): 2.2 (1.4–3.2) vs. 4.1 (2.7–4.4); P = 0.02]. Basal Tmn was lower in diabetic vs. nondiabetic subjects [median (interquartile range): 0.53 (0.25–0.71) vs. 0.64 (0.50–1.17); P = 0.04], while hyperemic Tmn and IMR were similar. We found significant correlations at linear regression analysis between logCFR and the HOMA index ( r2 = 0.35; P = 0.0005) and between basal Tmn and the HOMA index ( r2 = 0.44; P < 0.0001). In conclusion, compared with nondiabetic subjects, CFR is lower in patients with diabetes and epicardial coronary arteries free of severe stenosis, because of increased basal coronary flow, while hyperemic coronary flow is similar. Basal coronary flow relates to insulin resistance, suggesting a key role of cellular metabolism in the regulation of coronary blood flow.

Abnormal shortened diastolic time length at increasing heart rates in patients with abnormal exercise-induced increase in pulmonary artery pressure

Background

The degree of pulmonary hypertension is not independently related to the severity of left ventricular systolic dysfunction but is frequently associated with diastolic filling abnormalities. The aim of this study was to assess diastolic times at increasing heart rates in normal and in patients with and without abnormal exercise-induced increase in pulmonary artery pressure (PASP). Methods. We enrolled 109 patients (78 males, age 62 ± 13 years) referred for exercise stress echocardiography and 16 controls. The PASP was derived from the tricuspid Doppler tracing. A cut-off value of PASP ≥ 50 mmHg at peak stress was considered as indicative of abnormal increase in PASP. Diastolic times and the diastolic/systolic time ratio were recorded by a precordial cutaneous force sensor based on a linear accelerometer.

Results

At baseline, PASP was 30 ± 5 mmHg in patients and 25 ± 4 in controls. At peak stress the PASP was normal in 95 patients (Group 1); 14 patients (Group 2) showed an abnormal increase in PASP (from 35 ± 4 to 62 ± 12 mmHg; P < 0.01). At 100 bpm, an abnormal (< 1) diastolic/systolic time ratio was found in 0/16 (0%) controls, in 12/93 (13%) Group 1 and 7/14 (50%) Group 2 patients (p < 0.05 between groups).

Conclusion

The first and second heart sound vibrations non-invasively monitored by a force sensor are useful for continuously assessing diastolic time during exercise. Exercise-induced abnormal PASP was associated with reduced diastolic time at heart rates beyond 100 beats per minute.

Altered coronary vascular control during cold stress in healthy older adults

Cardiovascular-related mortality increases in the cold winter months, particularly in older adults. Previously, we reported that determinants of myocardial O(2) demand, such as the rate-pressure product, increase more in older adults compared with young adults during cold stress. The aim of the present study was to determine if aging influences the coronary hemodynamic response to cold stress in humans. Transthoracic Doppler echocardiography was used to noninvasively measure peak coronary blood velocity in the left anterior descending artery before and during acute (20 min) whole body cold stress in 10 young adults (25 ± 1 yr) and 11 older healthy adults (65 ± 2 yr). Coronary vascular resistance (diastolic blood pressure/peak coronary blood velocity), coronary perfusion time fraction (coronary perfusion time/R-R interval), and left ventricular wall stress were calculated. We found that cooling (via a water-perfused suit) increased left ventricular wall stress, a primary determinant of myocardial O(2) consumption, in both young and older adults, although the magnitude of this increase was nearly twofold greater in older adults (change of 9.1 ± 3.5% vs. 17.6 ± 3.2%, P < 0.05, change from baseline in young and older adults and young vs. older adults). Despite the increased myocardial O(2) demand during cooling, coronary vasodilation (decreased coronary vascular resistance) occurred only in young adults (3.22 ± 0.23 to 2.85 ± 0.18 mmHg·cm(-1)·s(-1), P < 0.05) and not older adults (3.97 ± 0.24 to 3.79 ± 0.27 mmHg·cm(-1)·s(-1), P > 0.05). Consistent with a blunted coronary vascular response, absolute coronary perfusion time tended to decrease (P = 0.13) and coronary perfusion time fraction decreased (P < 0.05) during cooling in older adults but not young adults. Collectively, these data suggest that older adults demonstrate an altered coronary hemodynamic response to acute cold stress.

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".

Cardiac oxygen supply is compromised during the night in hypertensive patients

The enhanced heart rate and blood pressure soon after awaking increases cardiac oxygen demand, and has been associated with the high incidence of acute myocardial infarction in the morning. The behavior of cardiac oxygen supply is unknown. We hypothesized that oxygen supply decreases in the morning and to that purpose investigated cardiac oxygen demand and oxygen supply at night and after awaking. We compared hypertensive to normotensive subjects and furthermore assessed whether pressures measured non-invasively and intra-arterially give similar results. Aortic pressure was reconstructed from 24-h intra-brachial and simultaneously obtained non-invasive finger pressure in 14 hypertensives and 8 normotensives. Supply was assessed by Diastolic Time Fraction (DTF, ratio of diastolic and heart period), demand by Rate-Pressure Product (RPP, systolic pressure times heart rate, HR) and supply/demand ratio by A_dia/A_sys, with A_dia and A_sys diastolic and systolic areas under the aortic pressure curve. Hypertensives had lower supply by DTF and higher demand by RPP than normotensives during the night. DTF decreased and RPP increased in both groups after awaking. The DTF of hypertensives decreased less becoming similar to the DTF of normotensives in the morning; the RPP remained higher. A_dia/A_sys followed the pattern of DTF. Findings from invasively and non-invasively determined pressure were similar. The cardiac oxygen supply/demand ratio in hypertensive patients is lower than in normotensives at night. With a smaller night-day differences, the hypertensives’ risk for cardiovascular events may be more evenly spread over the 24 h. This information can be obtained noninvasively.

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.

Endotoxemia decreases matching of regional blood flow and O2 delivery to O2 uptake in the porcine left ventricle

Heterogeneity of regional coronary blood flow is caused in part by heterogeneity in O(2) demand in the normal heart. We investigated whether myocardial O(2) supply/demand mismatching is associated with the myocardial depression of sepsis. Regional blood flow (microspheres) and O(2) uptake ([(13)C]acetate infusion and analysis of resultant NMR spectra) were measured in about nine contiguous tissue samples from the left ventricle (LV) in each heart. Endotoxemic pigs (n = 9) showed hypotension at unchanged cardiac output with a fall in LV stroke work and first derivative of LV pressure relative to controls (n = 4). Global coronary blood flow and O(2) delivery were maintained. Lactate accumulated in arterial blood, but net lactate extraction across the coronary bed was unchanged during endotoxemia. When LV O(2) uptake based on blood gas versus NMR data were compared, the correlation was 0.73 (P = 0.007). While stable over time in controls, regional blood flows were strongly redistributed during endotoxin shock, with overall flow heterogeneity unchanged. A stronger redistribution of blood flow with endotoxin was associated with a larger fall in LV function parameters. Moreover, the correlation of regional O(2) delivery to uptake fell from r = 0.73 (P < 0.001) in control to r = 0.18 (P = 0.25, P = 0.009 vs. control) in endotoxemic hearts. The results suggest a redistribution of LV regional coronary blood flow during endotoxin shock in pigs, with regional O(2) delivery mismatched to O(2) demand. Mismatching may underlie, at least in part, the myocardial depression of sepsis.

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.

Why is the subendocardium more vulnerable to ischemia? A new paradigm

Myocardial ischemia is transmurally heterogeneous where the subendocardium is at higher risk. Stenosis induces reduced perfusion pressure, blood flow redistribution away from the subendocardium, and consequent subendocardial vulnerability. We propose that the flow redistribution stems from the higher compliance of the subendocardial vasculature. This new paradigm was tested using network flow simulation based on measured coronary anatomy, vessel flow and mechanics, and myocardium-vessel interactions. Flow redistribution was quantified by the relative change in the subendocardial-to-subepicardial perfusion ratio under a 60-mmHg perfusion pressure reduction. Myocardial contraction was found to induce the following: 1) more compressive loading and subsequent lower transvascular pressure in deeper vessels, 2) consequent higher compliance of the subendocardial vasculature, and 3) substantial flow redistribution, i.e., a 20% drop in the subendocardial-to-subepicardial flow ratio under the prescribed reduction in perfusion pressure. This flow redistribution was found to occur primarily because the vessel compliance is nonlinear (pressure dependent). The observed thinner subendocardial vessel walls were predicted to induce a higher compliance of the subendocardial vasculature and greater flow redistribution. Subendocardial perfusion was predicted to improve with a reduction of either heart rate or left ventricular pressure under low perfusion pressure. In conclusion, subendocardial vulnerability to a acute reduction in perfusion pressure stems primarily from differences in vascular compliance induced by transmural differences in both extravascular loading and vessel wall thickness. Subendocardial ischemia can be improved by a reduction of heart rate and left ventricular pressure.

Significance of microvascular remodelling for the vascular flow reserve in hypertension

Vascular flow reserve (VFR) is the relative increase in tissue perfusion from the resting state to a state with maximum vasodilatation. Longstanding hypertension reduces the VFR, which in turn reduces the maximum working capacity of the tissue. In principle, both inward arteriolar remodelling and rarefaction of the microvascular network may contribute to this reduction. These processes are known to occur simultaneously in the microcirculation of the hypertensive individual and both cause a reduction in the luminal trans-sectional area available for perfusion. Which of them is the main factor responsible for the reduction in VFR is, however, not known. Here we present simulations performed on large microvascular networks to assess the VFR in various situations. Particular attention is paid to the VFR in networks in which the vessels have structurally adapted to a sustained increase in pressure by inward eutrophic remodelling (IER), i.e. by redistributing the same amount of wall material around a smaller lumen. Collectively, the results indicate that the IER may not per se be the main factor responsible for the hypertensive reduction in VFR. Rather, it may be explained by the presence of arteriolar and capillary rarefaction.