[Coronary reserve capacity and maximum oxygen consumption of the human heart]

History and Development of Coronary Flow Reserve and Fractional Flow Reserve for Clinical Applications

We discuss the historical development of clinical coronary physiology, emphasizing coronary flow reserve (CFR) and fractional flow reserve (FFR). Our analysis focuses on the clinical motivations and technologic advances that prompted and enabled the application of physiology for patient diagnosis. CFR grew from the general concepts of physiologic and coronary reserve, linking the anatomic severity of a lesion to its impact on hyperemic flow. FFR developed from existing models relating pressure measurements to the potential for flow to increase after removing a stenosis. Because pressure measurements have proved easier and more robust than flow measurements, FFR has become the dominant metric.

Dipyridamole stress echocardiography

Dipyridamole stress is the forerunner and prototype of pharmacological stress echo tests in the diagnosis of coronary artery disease. The safety of this test has been conclusively demonstrated as a result of extensive experience in large-scale multicenter projects. The diagnostic accuracy of dipyridamole stress echo is comparable to dobutamine and largely a function of the employed dose. Higher dosages (up to 0.84 mg/kg) are being required to achieve good sensitivity. The prognostic value has been shown to be independent and additive to clinical, exercise echocardiogram, and angiographic data. The test positive response should be titrated on the basis of severity, extent, and timing of induced dyssynergy with low positivity being associated to more anatomically and functionally severe forms of disease. Multicenter, randomized, prospective, international studies on cost-effectiveness directly comparing a noninvasive strategy centered on stress echo versus an invasive strategy centered on coronary angiography are currently ongoing.

Magnetization exchange in capillaries by microcirculation affects diffusion-controlled spin-relaxation: a model which describes the effect of perfusion on relaxation enhancement by intravascular contrast agents

The effect of perfusion on relaxation time in tissue has only been considered for first-pass kinetics of NMR-signal after application of contrast agents. The importance of perfusion on relaxation has not yet been studied for steady state conditions, i.e., when the intravascular relaxation rate is constant in time. The aim of this study is to develop a model in which T1 relaxation is derived as a function of perfusion and intracapillary volume fraction (regional blood volume). Tissue is considered to be two-compartment system, which consists of intracapillary and extravascular space. Intracapillary relaxation differs from relaxation in the arterial system due to diffusion-exchange of magnetization from extravascular to intracapillary space. Perfusion tends to attenuate this difference and thus counteracts the effect on intracapillary relaxation. Relaxation in the extravascular and intracapillary magnetization are linked by diffusion. This dependence is presented in analytical form and a generic equation is derived. AT1 experiment is considered in which all spins of tissue and blood are inverted at the beginning. Calculations are performed for the fast exchange model of tissue. Perfusion increases relaxation enhancement of intravascular contrast agents. This effect is considerable in highly perfused tissue like myocardium. The dependence of relaxation on perfusion implies an overestimation of the regional blood volume when the calculation of the latter is based on tissue models that neglect perfusion. The model presented here is applied to predict the effect of perfusion on T1 imaging with FLASH-pulse sequences because this technique has been proven to be a powerful method to obtain T1 maps within a short time interval. For the fast exchange model, two algorithms are suggested that determine perfusion and regional blood volume from T1 imaging in the presence and absence of intravascular contrast agents.

The influence of decreased left-ventricular afterload on cardiac morphology in hypertrophied rat hearts

The influence of afterload reduction on cardiac morphology was investigated in Goldblatt rats (hypertensive Wistar rats) with left-ventricular hypertrophy. The measurements were performed on a modified heart-lung preparation in which left-ventricular afterload was reduced either 0%, 15%-30% or 35%-50%. Goldblatt rats without heart-lung preparation served as control group. The aim of this study was to investigate the effects of decreased left-ventricular afterload on the myocardial morphology. The hearts were prepared for light and electron microscopy. For ultrastructural investigations, we developed a mitochondrial alteration score and graded the myocytes according to the degree of ischemic injury. The changes ranged from score 0 (normal) to score 4 (most severe). The mean score of the group with marked afterload reduction (35%-50%) was significantly increased compared to all other groups. Our data suggest that marked afterload reduction has a negative impact on the myocardial ultrastructure in hypertension-induced left-ventricular hypertrophy. Light microscopically, we found a correlation between myocardial cell diameter and blood pressure and a significant difference in arterial wall thickness between normotensive and hypertensive rats.

Significance of coronary circulation in hypertensive heart disease for development and prevention of heart failure

Coronary hemodynamics (coronary blood flow, coronary reserve, myocardial oxygen consumption) were analyzed in both experimental and clinical essential hypertension. Significant reduced coronary reserve was found in hypertensive patients with left ventricular hypertrophy. Extracoronary reasons for these phenomena were ruled out. Considerable thickening of the coronary resistance vessels (medial hypertrophy) in hypertensive hypertrophy associated with a marked increase in the wall thickness/radius ratio was considered sufficient to explain the impairment of coronary flow. After long-term pharmacotherapy there was normalization of both medial hypertrophy and coronary reserve. This small-vessel abnormality correlates well with clinical findings in hypertensive heart disease (angina and electrocardiographic changes despite normal coronary arteriogram). Moreover, this structural adaptation of the small vessels may carry the inherent risk of an impaired oxygen supply to the hypertrophied myocardium. Thus, late cardiac failure of the hypertrophied heart in hypertension may be attributed, in part, to this microcirculation disorder. Conversely, reversal of left ventricular hypertrophy and of hypertrophy of vascular smooth muscle by specific pharmacotherapy can be considered a possible approach to the rational prevention of cardiac failure in hypertensive patients. For future investigations, controlled clinical trials are needed to confirm these findings with regard to prevention of heart failure, and pharmacotherapeutic studies are necessary to define the optimal drug regimen for reversal of vascular smooth muscle hypertrophy.

Abnormal coronary vasomotion during exercise in patients with normal coronary arteries and reduced coronary flow reserve

A reduced coronary flow reserve has been reported in patients with ischemialike symptoms and normal coronary arteries. In 13 such patients, both coronary vasomotion and flow reserve were studied. The luminal area of the proximal and distal third of the left anterior descending and left circumflex artery were determined by biplane quantitative coronary arteriography using a computer-assisted system. Patients were studied at rest, during submaximal supine bicycle exercise (4.0 minutes, 116 W), and 5 minutes after sublingual administration of 1.6 mg nitroglycerin. Heart rate, mean pulmonary pressure, and mean aortic pressure as well as the percent change of both proximal and distal luminal area were determined. In 10 of the 13 patients, coronary sinus blood flow was measured by coronary sinus thermodilution technique at rest and after dipyridamole infusion (0.5 mg/kg in 15 minutes) 10 +/- 5 days after quantitative coronary arteriography. Coronary flow ratio (dipyridamole/rest) and coronary resistance ratio (rest/dipyridamole) were determined in these patients. Patients were divided into two groups according to the behavior of the coronary vessels during exercise (vasodilation, group 1; vasoconstriction, group 2). Coronary vasodilation of the proximal (luminal area +26%, p less than 0.001) and distal (+45%, p less than 0.001) artery was observed in seven patients (group 1) during exercise and after sublingual nitroglycerin (+46%, p less than 0.001; and +99%, p less than 0.001, respectively). In group 2 (n = 6), however, there was coronary vasoconstriction of the distal vessel segments (-24%, p less than 0.001) during exercise, whereas the proximal coronary artery showed vasodilation (+26%, p less than 0.001) during exercise. After sublingual nitroglycerin, both vessel segments elicited vasodilation (distal coronary, +44%, p less than 0.001; proximal coronary artery, +47%, p less than 0.001). Coronary flow ratio amounted to 2.5 in group 1 and 1.2 in group 2 (p less than 0.05) and coronary resistance ratio to 2.7 in group 1 and to 1.2 in group 2 (p less than 0.05), respectively. Thus, among patients with ischemialike symptoms and normal coronary arteries, there is a group of patients (group 2) with an abnormal dilator response of the distal coronary arteries to the physiologic dilator stimulus of exercise and a reduced dilator capacity of the resistance vessels after dipyridamole (abnormal coronary vasodilator syndrome). The nature of this exercise-induced distal coronary vasoconstriction is not clear but might be due to an abnormal neurohumoral tone that may cause or contribute to the blunted vascular response during exercise.

Coronary hemodynamics in hypertensive heart disease. Basic concepts, clinical consequences, and experimental analysis of regression of hypertensive microangiopathy

Myocardial hypertrophy may influence coronary hemodynamics variably. Therefore, coronary sinus blood flow (gas chromatic argon technique) was determined in patients with left ventricular hypertrophy, with or without dilatation, associated with entirely normal coronary arteriographic results: 12 patients with hypertrophic obstructive cardiomyopathy (left ventricular mass-to-volume ratio, 3.66 +/- 0.52 g/ml), 22 patients with hypertensive heart disease due to essential hypertension (left ventricular mass-to-volume ratio, 2.12 +/- 0.26 g/ml), 18 patients with hypertensive dilatation (left ventricular mass-to-volume ratio, 1.6 +/- 0.48 g/ml), six patients with aortic stenosis (left ventricular mass-to-volume ratio, 1.99 +/- 0.41 g/ml), 12 patients with aortic incompetence, and 20 patients with normal heart function. Coronary sinus blood flow was determined as a control value and as the value following intravenous injection of dipyridamole (0.5 mg/kg of body weight). Coronary reserve was calculated as the ratio of coronary resistance before and after dipyridamole. Normal coronary reserve averaged 4.89 +/- 0.11. Similar values, despite marked left ventricular hypertrophy, were present for both hypertrophic obstructive cardiomyopathy (4.4 +/- 0.19) and aortic stenosis (4.66 +/- 0.12), whereas coronary reserve was considerably reduced in the concentrically hypertrophied hypertensive hearts (3.22 +/- 0.19) (p less than 0.001). Moderate decrease in coronary reserve was found in aortic incompetence and in dilated essential hypertension. These results indicate that patients with nonhypertensive hypertrophy, despite left ventricular mass augmentation, may have normal coronary reserve, whereas at a comparable degree of left ventricular hypertrophy, patients with hypertensive hypertrophy have a specific reduction in coronary reserve. Independent from vascular effects, ventricular dilatation may result in deterioration of coronary reserve because of an abnormal component of coronary vascular resistance. These results were also verified in experimental hypertension. Moreover, prevention and/or regression of the impaired coronary circulation in experimental hypertensive heart disease, most probably due to the reduction of smooth muscle layers of the media of coronary resistance vessels, could be achieved by long-term vasodilator therapy.

Myocardial uptake and clearance of thallium-201 in normal subjects: comparison of dipyridamole-induced hyperemia with exercise stress

Thallium-201 uptake and clearance after dipyridamole infusion may differ from that after exercise stress because the hemodynamic effects of these two interventions are different. In this study of normal volunteers, thallium kinetics after dipyridamole (n = 13) were determined from three serial image sets (early, intermediate and delayed) and from serial blood samples and compared with thallium kinetics after exercise (n = 15). Absolute myocardial thallium uptake was greater after dipyridamole compared with exercise (p less than 0.0001), although the relative myocardial distribution was similar. The myocardial clearance (%/h) of thallium was slower after dipyridamole than it was after exercise. Comparing dipyridamole and exercise, the differences in clearance were large from the early to the intermediate image (anterior, -11 +/- 17 versus 24 +/- 5, p = 0.0005; 50 degrees left anterior oblique, -7 +/- 11 versus 15 +/- 8, p = 0.004; 70 degrees left anterior oblique, 3 +/- 9 versus 21 +/- 6, p = 0.001). In contrast, the differences in clearance were small from the intermediate to the delayed image (anterior, 15 +/- 4 versus 20 +/- 2, p = 0.025; 50 degrees left anterior oblique, 15 +/- 4 versus 19 +/- 3, p = 0.13; 70 degrees left anterior oblique, 15 +/- 3 versus 18 +/- 2, p = 0.047). Thallium uptake and clearance in the liver, splanchnic region and spleen were greater after dipyridamole (p less than 0.001). Blood thallium levels were greater after dipyridamole (p less than 0.05) and cleared more slowly (p = 0.07). Thus, myocardial thallium-201 uptake and clearance after dipyridamole infusion differ from thallium kinetics after exercise. This difference is, in part, related to associated differences in extracardiac and blood kinetics. Diagnostic criteria for the detection of abnormal thallium-201 clearance must be specific for the type of intervention.

Pharmacologic intervention as an alternative to exercise stress

Although thallium exercise imaging has served an important role in clinical cardiology, it is significantly limited by suboptimal sensitivity and specificity, particularly in asymptomatic man. The increasing recognition of silent myocardial ischemia, the significant prevalence of coronary artery disease in asymptomatic middle age men, and the frequent occurrence of myocardial infarction without preceding symptoms in 60% of cases emphasizes the need for a more definitive, noninvasive diagnostic test for the presence of coronary artery disease suitable for screening in asymptomatic or symptomatic patients. Intravenous dipyridamole combined with handgrip stress provides a potent stimulus for purposes of diagnostic perfusion imaging. Although planar and single photon emission computed tomography (SPECT) imaging also have played an important role, these techniques are seriously hindered by their inability to quantitate radiotracer uptake or image modest differences in maximum relative flow caused by coronary artery stenosis. Accordingly, the combination of dipyridamole-handgrip stress with positron imaging of myocardial perfusion has become a powerful diagnostic tool suitable for routine clinical use. With the availability of generator-produced rubidium-82, dedicated clinically oriented positron cameras, the routine application of positron imaging to clinical cardiology has become feasible. Based on published literature, the current clinical indications for positron imaging that may be carried out economically on a routine clinical basis include assessment of myocardial perfusion utilizing rubidium-82 or N-13 ammonia for purposes of reliable, accurate, noninvasive screening for coronary artery disease in symptomatic or asymptomatic patients; assessing noninvasively the physiologic severity of coronary stenoses; myocardial infarct imaging; assessing myocardial viability of reversibly injured or ischemic cells using N-13 ammonia combined with fluorine-18-deoxy-glucose or Rubidium-82 alone in experimental animals; assessing regional or global left ventricle (LV) function by 3-dimensional gated blood pool imaging and/or wall thickening by ECG gating; and assessing the functional significance of collaterals in man.

Thallium myocardial imaging: recent experience using a coronary vasodilator

Thallium-201 myocardial imaging is an important test in the assessment of patients with suspected coronary artery disease. Techniques differ in detail, reliability and in patient acceptability. Three techniques have been compared. Thirty-two-patients were studied in three groups. In the first group (15 patients) exercise thallium scans were compared with scans following an intravenous vasodilator (dipyridamole). In the second group (12 patients) intravenous dipyridamole and oral dipyridamole thallium scans were compared. In the third group (five patients) combined oral dipyridamole and exercise scans were assessed. There were no major differences in the first two groups but the combined test showed a marked increase in image quality and diagnostic yield. Thallium scanning is simplified considerably by the use of oral dipyridamole, without loss of diagnostic quality of safety. It promises to be the method of choice for stress scanning, and is ideal in patients unable to tolerate maximum exercise. The combined exercise--dipyridamole scan helps to evaluate complex problems, particularly those with less severe coronary insufficiency, and can be done without the use of a treadmill.

Intravenous dipyridamole combined with isometric handgrip for near maximal acute increase in coronary flow in patients with coronary artery disease

Twenty-four patients with coronary artery disease were studied during cardiac catheterization to determine the effects of sustained isometric handgrip exercise and intravenous dipyridamole and their combination on coronary and systemic hemodynamics and measured coronary luminal caliber. During 4 to 5 minutes of 25 percent maximal handgrip, blood pressure and heart rate increased 24 and 19 percent, respectively, coronary sinus flow increased to 1.7 x baseline value, and epicardial coronary arteries constricted to increase predicted flow resistance by 40 percent in 36 diseased arterial segments. After a 4 minute intravenous infusion of dipyridamole (0.56 mg/kg body weight), systemic pressure decreased 8 percent, heart rate increased 23 percent, coronary sinus flow increased to 2.4 x baseline value and coronary luminal caliber was unchanged. During isometric handgrip initiated 6 minutes after the infusion of dipyridamole, systemic pressure and heart rate increased to 14 and 31 percent, respectively, above control values, coronary sinus flow increased to 3.3 x baseline value (3.8 x baseline value in patients with normal anterior perfusion) and stenotic flow resistance increased by 36 percent. The response of coronary flow to the combined stresses was 68 percent greater than the response to dipyridamole alone (p less than 0.02); these flow levels exceed values previously reported for the human coronary circulation. Aminophylline plus nitroglycerin appears to assure patient safety.

Hemodilution -- new clothes for an anemic emperor

This review deals with the rationale for the use of hemodilution in patients not subjected to open heart surgery. The claim for an optimum of circulatory oxygen transport at 30% hematocrit has been disproved; hemodilution thus simply means acute normovolemic anemia. Accordingly, it generates a cardiovascular strain and particularly jeopardizes cerebral and myocardial oxygen supply. Potentially serious clinical side effects have been reported. Hemodilution should therefore not be carried beyong the lower normal range for the hemoglobin or hematocrit level, i.e. 12--12.5 g% or 35--36%.

Assessment of coronary stenoses with myocardial perfusion imaging during pharmacologic coronary vasodilatation. IV. Limits of detection of stenosis with idealized experimental cross-sectional myocardial imaging

Because atherosclerosis may be reversible, a routine noninvasive screening test for the reliable diagnosis of mild coronary arterial lesions would allow potential prevention of coronary events in specific patients through intensive dietary management, drug therapy and physical training. To determine the minimal coronary stenosis detectable with myocardial perfusion imaging techniques, standardized stenoses ranging from 31.4 to 72.5 percent diameter narrowing were applied to the left circumflex coronary artery of 12 open chest dogs. Indium-113m-labeled human albumin microspheres were injected into the left atrium under control conditions and technetium-99m human albumin microspheres during maximal coronary vasodilatation induced with intravenous dipyridamole. Hearts were removed, sliced into 1 cm thick cross sections and imaged under a gamma camera. The results demonstrate that 40 percent diameter coronary stenoses can be identified by imaging relative subendocardial underperfusion during pharmacologic coronary vasodilatation. An imaging technique sensitive enough to identify mild coronary lesions for diagnostic screening purposes requires (1) a potent stimulus for coronary vasodilatation, such as intravenous dipyridamole; (2) an imaging agent taken up by the myocardium in proportion to coronary flow at flow rates up to four or more times resting coronary flow so that differences in regional maximal flows caused by mild stenoses can be identified; and (3) cross-sectional tomographic myocardial imaging to visualize relative endocardial-epicardial perfusion, the most sensitive indicator of the hemodynamic effects of coronary stenoses, and to exclude from the imaging field the interfering activity of lung, background and overlying heart structures.

Tolerance to ischemia of hypertrophied human hearts during valve replacement

This study evaluates the tolerance to ischemia during induced cardiac arrest in patients undergoing aortic valve replacement. In all patients cardiac standstill was of 45 minutes duration. Biopsies for electron microscopic study were taken from the left ventricle before induction of arrest, at the end of the ischemic period and 20 minutes after coronary perfusion had been reestablished. Structural ischemic damage was more pronounced in patients with severe hypertrophy and structural reconstitution was delayed. Degenerative changes of the myocardial cells, although observed frequency, apparently did not influence the tolerance to ischemia. It is concluded from this study that patients with severe hypertrophy represent a high-risk group in cardiac surgery because of their reduced tolerance to induced myocardial ischemia during cardiopulmonary bypass.

The influence of the size of acute ischaemic myocardial lesions on coronary reserve and left ventricular function in the dog

Acute ischaemic lesions of 5 and 15% of the left ventricular mass were produced in 10 dogs by ligations of branches of the left anterior descending coronary artery. Under maximal coronary dilatation by Dipyridamole and at constant aortic pressure, an ischaemic lesion of 15% caused reductions of blood flow in the left anterior descending coronary artery of 50% (p less than 0.05), , the R. circumflexus of 36% and of total myocardial blood flow (Argon method) by 44% (p less than 0.02). Parameters of left ventricular function, dp/dtmax and Vmax fell by 26 and 27% respectively (p less than 0.02). Ischaemic lesions of 5% LV mass were detected by 133 Xenon clearance estimations, but had no effect on any of the other variables measured. There was good agreement between estimations of total myocardial blood flow (Argon) and direct measurement of coronary flow with electromagnetic flowmeters (r = 0.957).

Lupus cardiomyopathy: cardiac mechanics, hemodynamics, and coronary blood flow in uncomplicated systemic lupus erythematosus

Right and left heart pressures, left ventricular volumes, indices of contractility, myocardial wall stiffness, and coronary blood flow were determined in five young women with systemic lupus erythematosus (SLE) during diagnostic right and left heart catheterization. Examinations revealed (1) increases of right and left ventricular enddiastolic pressures; (2) decreases of cardiac output, stroke volume, ejection fraction, contractility indices, diastolic left ventricular volume inflow; (3) decreases of pharmacologically induced coronary vasodilation in SLE. The results demonstrate impaired pump function, reduced contractility, increased myocardial wall stiffness, and decreased coronary vascular reserve in SLE. It is concluded that lupus cardiomyopathy associated with an impairment of left ventricular function may be apparent in young women with SLE who have no clinical signs of cardiac dysfunction.

Studies on the regulation of myocardial blood flow in man. I.: Training effects on blood flow and metabolism of the healthy heart at rest and during standardized heavy exercise

In a comparative study 11 athletes and 11 untrained students were investigated at rest, of these 6 trained and 5 untrained individuals during exercise as well. Myocardial blood flow was measured by the argon method. Myocardial oxygen consumption, myocardial substrate uptake of glucose, lactate, and free fatty acids and cardiac output were determined by the direct Fick principle. Exercise was standardized according to 65% of an individual's maximal oxygen uptake (delta VO2 max). Coronary flow reserve was determined by dipyridamole injections. All measurements were made during hemodynamic and respiratory steady-state conditions with the subject in a supine position. At rest, myocardial blood flow and myocardial oxygen consumption were significantly lower in trained subjects compared to the untrained ones. These differences were more pronounced during heavy exercise. They cannot be explained completely by hemodynamic parameters. - During exercise, myocardial substrate uptake shifted to a predominant lactate uptake of almost 90% of total substrate uptake. Total substrate uptake as well as lactate uptake correlated significantly with myocardial oxygen. - Coronary flow reserve was lower in the trained group. It is concluded that the heart muscle of a trained individual requires less energy at a given work load than in the untrained state.

[Value and limitations of coronary blood flow measurement in man (author's transl)]

Attempts to measure coronary blood flow in man have made considerable progress during the last 25 years. The major techniques are based on the direct or indirect Fick principle; coronary flow is calculated from the arterio-coronary venous difference of inert gases or from the precordial recorded disappearance curve of radioactive gases or substances. The accuracy of the techniques depends upon the properties or the indicators used and the precision of their determination. All techniques applied hitherto are intricate and unsuitable for general use. -A lot of information is obtained about coronary circulation in health and disease by coronary flow measurements in man. Further studies in this field may influence pathophysiological and clinical concepts especially concerning coronary heart disease.