Relation between diastolic perfusion time and coronary artery stenosis during stress-induced myocardial ischemia

The Cardiovascular Continuum extended: Aging effects on the aorta and microvasculature

The ‘Cardiovascular Continuum’ was described by Dzau and colleagues in 2006 to explain the development over many years of coronary disease with its complications, then end-stage heart failure. The Continuum identified different points along the way where the process could be interrupted by drug therapies or interventions, then described the trials that have been undertaken over the last three decades to establish their value. The approach summarized the major steps in cardiology through modern times, but it had an emphasis on coronary atherosclerosis in prosperous nations, and did not account fully for the problems of aging, which occur in all societies. Aging of the aorta and elastic arteries causes arterial stiffening and leads to development of cardiac failure and microvascular disease in highly perfused organs such as the brain and kidneys. The ‘Vascular Aging Continuum’ which we introduce, dovetails with the late phases of the Cardiovascular Continuum and provides a more comprehensive explanation, especially for vascular diseases in nations with little atherosclerosis. It will become more common in the Western World where attention to risk factors and widespread use of statins are responsible for a decrease in atherosclerotic disease, prolongation of life, and dominance of macrovascular and microvascular arterial disease, as well as of cardiac failure.

Pre-diabetes and arterial stiffness in uraemic patients

BACKGROUND

In order to address factors of relevance for new onset diabetes mellitus and cardiovascular disease after kidney transplantation, we investigated the presence of pre-diabetes, arterial stiffness and endothelial dysfunction in patients with end-stage renal disease (ESRD) accepted for kidney transplantation.

METHODS

Pre-diabetes and an insulin sensitivity index were estimated by an oral glucose tolerance test in 66 consecutive uraemic patients, without diabetes, being on the waiting list for the first renal transplantation. Mean age was 43 +/- 13 years. Duration of ESRD was 32 +/- 27 months. A control group consisted of 14 healthy subjects. Arterial stiffness was measured by aorta pulse wave velocity (PWV) and aorta augmentation index (AIX). Endothelial function was evaluated by flow-mediated vasodilatation (FMD) and plasma concentrations of von Willebrand factor antigen (vWF). Mean arterial blood pressure (MAP) was measured in supine resting position.

RESULTS

Twenty-seven uraemic patients (41%) had pre-diabetes (IFG+IGT), and 39 had normal glucose tolerance. The uraemic patients were more insulin resistant with lower insulin sensitivity index compared to healthy controls (6.1 +/- 3 vs. 15 +/- 7, P < 0.0001) but with no difference between patients with and without pre-diabetes. HbA1c and fasting plasma glucose was comparable in uraemic patients with and without pre-diabetes. PWV was higher in pre-diabetic compared to normoglycaemic uraemic patients (9.1 +/- 3 vs. 7.3 +/- 2 m/s, P = 0.03) and healthy controls (9.1 +/- 3 vs. 6.7 +/- 1, P = 0.01), while AIX did not differ (24.9 +/- 13 vs. 23.2 +/- 12 vs. 17 +/- 16, P = NS). Presence of pre-diabetes was positively associated to PWV in a univariate analysis. Multivariable analysis revealed age and MAP as independent predictors of PWV in uraemic patients. FMD and vWF were impaired in uraemic patients compared to healthy controls (3 +/- 4 vs. 7 +/- 3, P = 0.007 and 1.8 +/- 0.7 vs. 0.96 +/- 0.3 kIU/L, P = 0.0002, respectively) but with no difference between the two groups of uraemic patients. In conclusion, a high prevalence of pre-diabetes, impaired insulin resistance, increased arterial stiffness of aorta as well as impaired augmentation index and vasodilatation was demonstrated in uraemic patients prior to kidney transplantation. Increased arterial stiffness of aorta and augmentation index were independently associated with age and blood pressure.

Hypertension and heart failure: a dysfunction of systole, diastole or both?

The pathological myocardial hypertrophy associated with hypertension contains the seed for further maladaptive development. Increased myocardial oxygen consumption, impaired epicardial coronary perfusion, ventricular fibrosis and remodelling, abnormalities in long-axis function and torsion, cause, to a varying degree, a mixture of systolic and diastolic abnormalities. In addition, chronotropic incompetence and peripheral factors such as lack of vasodilator reserve and reduced arterial compliance further affect cardiac output particularly on exercise. Many of these factors are common to hypertensive heart failure with a normal ejection fraction as well as systolic heart failure. There is increasing evidence that these apparently separate phenotypes are part of a spectrum of heart failure differing only in the degree of ventricular remodelling and volume changes. Furthermore, dichotomizing heart failure into systolic and diastolic clinical entities has led to a paucity of clinical trials of therapies for heart failure with a normal ejection fraction. Therapies aimed at reversing myocardial fibrosis, and targets outside the heart such as enhancing vasodilator reserve and improving chronotropic incompetence deserve further study and may improve the exercise capacity of hypertensive heart failure patients. Hypertension heart disease with heart failure is simply not a dysfunction of systole and diastole. Other peripheral factors including heart rate and vasodilator response with exercise may deserve equal attention in an attempt to develop more effective treatments for this disorder.

Increased diastolic time fraction as beneficial adjunct of α1-adrenergic receptor blockade after percutaneous coronary intervention

The effect of α1-receptor blockade with urapidil on coronary blood flow and left ventricular function has been attributed to relief of diffuse coronary vasoconstriction following percutaneous coronary intervention (PCI). We hypothesized that an increase in diastolic time fraction (DTF) contributes to the beneficial action of urapidil. In eleven patients with a 63% (SD 13) diameter stenosis, ECG, aortic pressure (Pa) and distal intracoronary pressure (Pd), and blood flow velocity were recorded at baseline and throughout adenosine-induced hyperemia. Measurements were obtained before and after PCI and after subsequent α1-receptor blockade with urapidil (10 mg ic). DTF was determined from the ECG and the Pa waveform. Functional parameters such as coronary flow velocity reserve, fractional flow reserve, and an index of hyperemic microvascular resistance (HMR) were assessed. Urapidil administration after PCI induced an upward shift in the DTF-heart rate relationship, resulting in a 3.1% (SD 2.7) increase in hyperemic DTF at a constant heart rate ( P < 0.005) due to a shorter duration of systole. Hyperemic Pa and Pd decreased, respectively, by 6.1% (SD 6.6; P < 0.05) and 5.7% (SD 5.8; P < 0.01) after α1-blockade. Although epicardially measured functional parameters were on average not altered by α1-blockade due to concurrent changes in pressure and heart rate, HMR decreased by urapidil in those patients where coronary pressure remained constant. In conclusion, α1-receptor blockade after PCI produced a modest but significant prolongation of DTF at a given heart rate, thereby providing an adjunctive beneficial mechanism for improving subendocardial perfusion, which critically depends on DTF.

Cardiovascular aging and exercise in healthy older adults

OBJECTIVE

Physical inactivity in an aging population is a major contributing factor to the rising numbers of older persons with chronic illnesses and disabilities. The purpose of this article is to review the relationship between physical inactivity and age-associated changes to the cardiovascular system, and provide guidance on prescribing exercise to healthy older persons in order to mitigate the adverse effects of cardiovascular aging.

DESIGN

Interpretive review of the literature.

RESULTS

A number of structural and functional changes occur in the cardiovascular system with advancing age, many of which are mediated by changes in vascular stiffness. These changes lead not only to cardiovascular events and strokes, but also to frailty, functional decline, and cognitive impairment. A substantial proportion of the decline in aerobic capacity with age may result from physical inactivity. Guidelines for the prescription of aerobic, resistance, and balance training for otherwise healthy older persons are provided.

CONCLUSIONS

Lack of physical activity is a major risk factor for the epidemic of chronic disease and disability facing an aging population. Many age-associated changes in cardiovascular function result from physical inactivity. The benefits of regular exercise include prevention of cardiovascular events, disability, and cognitive impairment. Age is not a contraindication to exercise, which can usually be initiated safely in older persons.

Aortic stiffness and the balance between cardiac oxygen supply and demand: the Rotterdam Study

OBJECTIVES

Aortic stiffness is an independent predictor of cardiovascular morbidity and mortality. We investigated whether aortic stiffness, estimated as aortic pulse wave velocity, is associated with decreased perfusion pressure estimated as the cardiac oxygen supply potential.

METHODS

Aortic stiffness and aortic pressure waves, reconstructed from finger blood pressure waves, were obtained in 2490 older adults within the framework of the Rotterdam Study, a large population-based study. Cardiac oxygen supply and demand were estimated using pulse wave analysis techniques, and related to aortic stiffness by linear regression analyses after adjustment for age, sex, mean arterial pressure and heart rate.

RESULTS

Cardiac oxygen demand, estimated as the Systolic Pressure Time Index and the Rate Pressure Product, increased with increasing aortic stiffness [0.27 mmHg s (95% confidence interval: 0.21; 0.34)] and [42.2 mmHg/min (95% confidence interval: 34.1; 50.3)], respectively. Cardiac oxygen supply potential estimated as the Diastolic Pressure Time Index decreased [-0.70 mmHg s (95% confidence interval: -0.86; -0.54)] with aortic stiffening. Accordingly, the supply/demand ratio Diastolic Pressure Time Index/Systolic Pressure Time Index -1.11 (95% confidence interval: -0.14; -0.009) decreased with increasing aortic stiffness.

CONCLUSION

Aortic stiffness is associated with estimates of increased cardiac oxygen demand and a decreased cardiac oxygen supply potential. These results may offer additional explanation for the relation between aortic stiffness and cardiovascular morbidity and mortality.

Reduced aortic distensibility and coronary flow velocity reserve in diabetes mellitus patients with a negative coronary angiogram

BACKGROUND

Structural and functional abnormalities of the aortic wall and disturbances of the coronary circulation with presumed microvascular complications have been reported in patients with diabetes mellitus.

OBJECTIVES

To simultaneously establish the coronary flow velocity reserve (CFVR) and aortic distensibility indexes in type 2 diabetes mellitus patients who have normal epicardial coronary arteries by stress transesophageal echocardiography (STEE).

METHODS

The elastic properties of the descending aorta and the CFVR were evaluated simultaneously in 18 type 2 diabetes mellitus patients who had negative coronary angiograms. These results were compared with those of 21 nondiabetic subjects with normal epicardial coronary arteries and 24 patients with left anterior descending coronary artery (LAD) stenosis. STEE was used for the evaluation of elastic moduli of the descending aorta. The CFVR was calculated as the ratio of the average peak diastolic flow velocity during hyperemia to that at rest.

RESULTS

The CFVR of diabetic patients with normal epicardial coronary arteries and those with LAD stenosis was similarly decreased compared with the controls (2.10+/-0.63 and 1.78+/-0.47 versus 2.76+/-1.25, P<0.05 and P<0.001, respectively). The elastic modulus (in 103 mmHg) was similarly increased in patients with diabetes mellitus and normal epicardial coronary arteries, and in those with LAD stenosis, compared with the control subjects (0.94+/-0.82 and 0.91+/-0.59 versus 0.49+/-0.19, P<0.05 and P<0.05, respectively).

CONCLUSIONS

It may be stated that reduced aortic distensibility (increased elastic modulus) and the CFVR were demonstrated simultaneously during STEE in diabetic patients compared with nondiabetic subjects with negative coronary angiograms.

Mechanical factors in arterial aging: a clinical perspective

The human arterial system in youth is beautifully designed for its role of receiving spurts of blood from the left ventricle and distributing this as steady flow through peripheral capillaries. Central to such design is "tuning" of the heart to arterial tree; this minimizes aortic pressure fluctuations and confines flow pulsations to the larger arteries. With aging, repetitive pulsations (some 30 million/year) cause fatigue and fracture of elastin lamellae of central arteries, causing them to stiffen (and dilate), so that reflections return earlier to the heart; in consequence, aortic systolic pressure rises, diastolic pressure falls, and pulsations of flow extend further into smaller vessels of vasodilated organs (notably the brain and kidney). Stiffening leads to increased left ventricular (LV) load with hypertrophy, decreased capacity for myocardial perfusion, and increased stresses on small arterial vessels, particularly of brain and kidney. Clinical manifestations are a result of diastolic LV dysfunction with dyspnea, predisposition to angina, and heart failure, and small vessel degeneration in brain and kidney with intellectual deterioration and renal failure. While aortic stiffening is the principal cause of cardiovascular disease with age in persons who escape atherosclerotic complications, it is not a specific target for therapy. The principal target is the smooth muscle in distributing arteries, whose relaxation has little effect on peripheral resistance but causes substantial reduction in the magnitude of wave reflection. Such relaxation is achieved through regular exercise and with the vasodilating drugs that are used in modern treatment of hypertension and cardiac failure.

The pathophysiology of chronic ischemic heart disease

With our success in management of acute coronary syndromes (ACS), aging population and epidemics of diabetes and obesity, the management of patients with chronic coronary artery disease is becoming an increasing important part of clinical practice. Although the rates of death and myocardial infarction (MI) in this group of patients are not high as a group, a subgroup has very high risk but many have poor quality of life related to limiting angina. The purpose of this article is to review the pathophysiology of the chronic angina syndrome to provide an improved understanding of the basis for the comprehensive management required to yield patient benefits. Where possible, targets for novel therapeutic approaches are highlighted.

Proteolytic N-terminal Truncation of Cardiac Troponin I Enhances Ventricular Diastolic Function

Besides the core structure conserved in all troponin I isoforms, cardiac troponin I (cTnI) has an N-terminal extension that contains phosphorylation sites for protein kinase A under beta-adrenergic regulation. A restricted cleavage of this N-terminal regulatory domain occurs in normal cardiac muscle and is up-regulated during hemodynamic adaptation (Z.-B. Yu, L.-F. Zhang, and J.-P. Jin (2001) J. Biol. Chem. 276, 15753-15760). In the present study, we developed transgenic mice overexpressing the N-terminal truncated cTnI (cTnI-ND) in the heart to examine its biochemical and physiological significance. Ca(2+)-activated actomyosin ATPase activity showed that cTnI-ND myofibrils had lower affinity for Ca(2+) than controls, similar to the effect of isoproterenol treatment. In vivo and isolated working heart experiments revealed that cTnI-ND hearts had a significantly faster rate of relaxation and lower left ventricular end diastolic pressure compared with controls. The higher baseline relaxation rate of cTnI-ND hearts was at a level similar to that of wild type mouse hearts under beta-adrenergic stimulation. The decrease in cardiac output due to lowered preload was significantly smaller for cTnI-ND hearts compared with controls. These findings indicate that removal of the N-terminal extension of cTnI via restricted proteolysis enhances cardiac function by increasing the rate of myocardial relaxation and lowering left ventricular end diastolic pressure to facilitate ventricular filling, thus resulting in better utilization of the Frank-Starling mechanism.

Diastolic time fraction as a determinant of subendocardial perfusion

Diastolic time fraction (DTF) has been recognized as an important determinant for subendocardial perfusion, but microsphere studies in which DTF was the independent variable are practically absent. In 21 anesthetized goats, the left coronary main stem was artificially perfused at controlled pressure. DTF was varied by pacing the heart, vagus stimulation, or administration of dobutamine. Regional coronary flow was measured with fluorescent microspheres under full adenosine dilation. Perfusion pressure (P(c)) was defined as mean coronary arterial pressure minus minimal left ventricular pressure. Regional flow conductances (flow/P(c)) were as follows: for the subendocardium, C(endo) = -0.103 + 0.197 DTF + 0.00074 P(c) (P < 0.001); for the midmyocardium, conductance = -0.048 + 0.126 DTF + 0.00049 P(c) (P < 0.001); and for the subepicardium, C(epi) was not significant. C(endo)-DTF relations demonstrated a finite value for DTF at which flow is zero, implying that, at physiological pressures, systolic subendocardial flow limitation extends into diastole. The DTF corresponding to an equal conductance in subendocardium and subepicardium (DTF1) was inversely related to P(c): DTF1 = 0.78 - 0.003 P(c) (P < 0.01). When heart rate and P(c) were held constant and dobutamine was administered (5 goats), contractility doubled and DTF increased by 39%, resulting in an increase of C(endo) of 40%. It is concluded that 1) DTF is a determinant of subendocardial perfusion, 2) systolic compression exerts a flow-limiting effect into diastole, and 3) corresponding to clinical findings on inducible ischemia we predict that, under hyperemic conditions, C(endo) < C(epi) if P(c) is lower than approximately 75% of a normal aortic pressure and heart rate >80 beats/min.

Fréquence cardiaque et ischémie myocardique expérimentale

Every increase in heart rate represents a poor prognostic factor in cardiology, and multiple arguments have now led to the belief that reducing heart rate is a major therapeutic challenge. A comparison of the pharmacological effects of If current inhibitors such as zatebradine, and more recently ivabradine (Procoralan) and beta-blockers, have demonstrated experimentally that reductions in heart rate and myocardial contractile force contribute equally to the reduction in myocardial oxygen consumption in the normal heart. Conversely, at a similar level of reduction in heart rate, the lack of a concomitant negative inotropic effect with ivabradine affords longer diastolic perfusion times than beta-blockers. In other words, a negative inotropic effect is deleterious when an increase in coronary blood flow is required. Hence, if the anti-ischaemic effects afforded by an If current inhibitor and a beta-blocker are roughly comparable, the former are clearly of higher benefit than beta-blockers in the treatment of myocardial dysfunction accompanying cardiac ischaemia-reperfusion, especially myocardial stunning.

[Relationship between the subendocardial viability ratio and risk factors for ischemic heart disease]

Ischemic heart disease is one of the major causes of sudden death in Japanese workers. Hypercholesteremia, hypertension, obesity, and the smoking habit are considered to be risk factors for cardiovascular events. Generally the subendocardium is thought to be more sensitive to a shortage of blood supply than the subepicardium. Buckberg et al. have demonstrated that the ratio of the area of the diastolic phase (diastolic pressure time index: DPTI) to that of the systolic phase (time tension index: TTI) in the central aortic profile has a close correlation with the blood supply to the subendocardium. This ratio was designated as the subendocardial viability ratio: SEVR (DPTI/TTI). We examined the relationships between the SEVR, as measured by SphygmoCor (AtCor Medical Ltd.), and the data from the health examination of 178 males working for a steel company. A significantly low SEVR was observed in people with the smoking habit, a high pulse rate, obesity, abnormality of blood fat components, or hyperglycemia. Employees in the normal SEVR group (SEVR: 140% or more) n = 120 and low SEVR group (SEVR: 139% or less) n = 58 were compared. The results suggested that employees with low SEVR were older and had a higher BMI, higher pulse rate, higher total cholesterol, higher triglyceride, and higher fasting plasma glucose. They also had lower HDL-cholesterol. In the low SEVR group, the percentage of employees with hyperlipidemia or hyperglucosemia was significantly higher than that in the normal SEVR group. Results of stepwise regression analysis indicated that the pulse rate and age were significant and independent predictive factors for SEVR. These data on SEVR calculated from the wave pattern of the central artery are considered to be useful parameters for evaluating the risk of myocardial ischemia and for guiding health promotion.

Contributions of heart rate and contractility to myocardial oxygen balance during exercise

The respective contributions of heart rate (HR) reduction and left ventricular (LV) negative inotropy to the effects of antianginal drugs are debated. Accordingly, eight instrumented dogs were investigated during exercise at spontaneous and paced HR (250 beats/min) after administration of either saline, atenolol, or ivabradine (selective pacemaker current channel blocker). During exercise, atenolol and ivabradine (both 1 mg/kg iv) similarly reduced HR (-30% from 222 +/- 5 beats/min), and LV mean ejection wall stress was not altered. LV dP/dt(max) was reduced by atenolol but not ivabradine. Diastolic time (DT) was increased by atenolol versus saline (195 +/- 6 vs. 123 +/- 4 ms, respectively) and to a greater extent by ivabradine (233 +/- 11 ms). Myocardial oxygen consumption (MVo(2)) was lower under ivabradine and atenolol versus saline (6.7 +/- 0.6 and 4.7 +/- 0.4 vs. 8.1 +/- 0.6 ml/min, respectively, P < 0.05). Under pacing, DT and MVo(2) were similar between ivabradine and saline but significantly reduced with atenolol. Thus HR reduction and negative inotropy equally contribute to the reduction in MVo(2) during exercise in the normal heart. The negative inotropy limits the increase in DT afforded by HR reduction.

Exercise training improves diastolic perfusion time in patients with coronary artery disease

Exercise training elicits an improvement in work capacity and in left-ventricular function in patients with coronary artery disease. An improvement in myocardial oxygen supply accounts for these effects. The aim of this study was to test the hypothesis that exercise training could favorably influence diastolic perfusion time, a major determinant of subendocardial perfusion. Twenty-two male patients with coronary artery disease were randomized to a training or control group. At the study entry and after one year, all patients underwent an exercise stress test. After one year, rest heart rate was lower and diastolic perfusion time was higher in the training group but not in the control group. At peak of exercise, diastolic perfusion time increased and ST-segment depression decreased significantly in the training group but not in the control group. A significant relation was found between the R-R interval and the diastolic perfusion time either before or after training, with a difference in the intercepts of two regressions. Training shifted updown-line regression, effecting a higher value of diastolic perfusion time for a given value of heart rate. Thus, training increases diastolic perfusion time, independently from the effect on heart rate. This mechanism may contribute to the improvement of myocardial perfusion.

Stratification of single-vessel coronary stenosis by ischemic threshold at the onset of wall motion abnormality during continuous monitoring of left ventricular function by semisupine exercise echocardiography

We studied the relation between the ischemic threshold at the onset of wall motion abnormality on exercise echocardiography (EE) and the severity of coronary stenosis in patients with 1-vessel coronary artery disease (CAD). We screened 216 consecutive patients who underwent coronary angiography and EE for suspected CAD. Ninety-five (74 men; age, 56 +/- 12 years) satisfied the study criteria, that is, the presence of 1-vessel disease or no evidence of CAD on angiography and a normal baseline echocardiogram. Eighty-seven patients had 1-vessel CAD on angiography, and exercise-induced wall motion abnormality occurred in 73 (77%). Optimal cutoff values of percent diameter stenosis and minimal lumen diameter for predicting a positive EE were 61% (sensitivity and specificity of 76%) and 1.12 mm (sensitivity and specificity of 74%). Among patients with positive EE, heart rate-blood pressure product at ischemic threshold was correlated with quantitative coronary stenosis (r = -0.72, P <.001). The ischemic threshold from continuous monitoring of left ventricular function during semisupine EE is correlated with the severity of coronary stenosis among patients with 1-vessel disease and a normal resting echocardiogram.

Short-term variability of pulse pressure and systolic and diastolic time in heart transplant recipients

In heart transplant recipients (HTR), short-term systolic blood pressure variability is preserved, whereas heart rate variability is almost abolished. Heart period is the sum of left ventricular ejection time (LVET) and diastolic time (DT). In the present time-domain prospective study, we tested the hypothesis that short-term fluctuations in aortic pulse pressure (PP) in HTR were related to fluctuations in LVET. Seventeen male HTR (age 48 +/- 6 yr) were studied 16 +/- 11 mo after transplantation. Aortic root pressure was obtained over a 15-s period using a micromanometer both at rest (n = 17) and following the cold pressor test (CPT, n = 14). There was a strong positive linear relationship between beat-to-beat LVET and beat-to-beat PP in all patients at rest and in 13 of 14 patients following CPT (each P < 0.01). The slope of this relationship showed little scatter both at rest (0.34 +/- 0.07 mmHg/ms) and following CPT (0.35 +/- 0.09 mmHg/ms, P = not significant). Given the essentially fixed heart period, DT varied inversely with LVET. As a result, in 13 of 17 HTR at rest and in 12 of 14 HTR following CPT, there was a negative linear relationship between beat-to-beat PP and DT. In conclusion, our short-term time-domain study demonstrated a strong positive linear relationship between LVET and blood pressure variability in male HTR. We also identified a subgroup of HTR in whom there was a mismatch between PP and DT.

Pulsatile hemodynamics in hypertension

The hemodynamics of hypertension and antihypertensive therapy have generally been approached in terms of the steady-flow load on the heart. Recent evidence, however, suggests that the pulsatile component of hemodynamic load may play a fundamental role in both the development and progression of hypertensive cardiovascular disease and its clinical sequelae. Pulse pressure, a correlate of conduit vessel stiffness, has been shown to be an important independent predictor of clinical events in hypertensive patients and in the general population. Unrecognized effects on pulsatile hemodynamics may account for the differential effects of various agents on left ventricular mass and events. A better understanding of abnormalities in pulsatile load in hypertension will facilitate risk stratification in and treatment of patients with hypertension.

Diastolic time during exercise-induced myocardial ischemia in patients with myocardial infarction

The relation between diastolic time and myocardial perfusion defect redistribution of the infarct-related region was studied during upright bicycle exercise with thallium-201 scintigraphy in 37 patients with recent anterior myocardial infarction. In addition to the higher incidence of residual stenosis of the infarct-related artery, a disproportionate shortening of diastolic time in patients with myocardial perfusion defect redistribution permitted further reduction of subendocardial blood flow during exercise.