Visual assessment of regional myocardial perfusion utilizing radioactive xenon and scintillation photography

Radionuclide-Based Insights into the Pathophysiology of Ischemic Heart Disease: Beyond Diagnosis

This review article discusses the historical origin of cardiac radionuclide-based methods, the physiologic background that justifies their existence, as well as the basic pathophysiologic concepts of coronary artery disease and their connection with the technologic design and application of these methods. Most importantly, this review discusses the important insights that these methods have provided to the understanding of the mechanisms of ischemia, risk stratification, and both treatment choice and treatment efficacy in ischemic heart disease. Nuclear cardiology originated as an attempt to provide complementary physiologic information to the anatomic information provided by coronary angiography. To comprehend the design and applications of nuclear cardiology methods, one must have a basic understanding of coronary artery disease as an inflammatory process that may manifest as acute or chronic states. Basic concepts on myocyte metabolic pathways, coronary blood flow, ischemic cascade, ventricular remodeling, and ejection fraction become critical for this purpose. Insights into risk stratification may permit patient-tailored therapy approaches. Insights into prognosis have made nuclear cardiology a robust tool for outcome predictions, with an exceptionally high negative predictive value. Evaluation of prognosis in special patient populations such as diabetics has originated important pathophysiologic concepts. Most insights into phenomena such as myocardial hibernation, myocardial stunning, and viability have been generated by nuclear cardiology techniques. Finally, new applications of radionuclide-based methods such as molecular identification of "vulnerable" atherosclerotic plaques, "ischemic memory" using fatty acid imaging, and myocardial innervation imaging provide new avenues for insightful research.

Evaluation of myocardial perfusion abnormality by profile analysis for digital subtraction angiogram

For quantitative estimation of ischemia, ECG-synchronized digital subtraction angiography was performed for selective coronary arteriography. The authors obtained sequential myocardial perfusion images at the arterial, capillary, and venous phases. Profile densitometry was performed along the cross section perpendicular to the long axis of the left ventricle to assess regional myocardial perfusion at the capillary phase quantitatively. By this densitometry, the volumes of vascular bed perfused by the left coronary artery could be estimated, and further, nontransmural myocardial infarction could be differentiated from transmural myocardial infarction through the profile of its density curve. This method appears to be useful for the analysis of myocardial perfusion of ischemic heart disease.

Inotropic drugs in acute circulatory failure

The use of inotropic drugs in patients requiring acute circulatory support is reviewed. A knowledge of their various peripheral effects is essential if the appropriate drug is to be used. The place or pressor amines, digitalis, salbutamol and glucagon in the treatment of patients with poor tissue perfusion is limited. Of the catecholamines, adrenaline causes excessive renal vasoconstriction and peripheral gangrene, noradrenaline increase myocardial work and diminishes peripheral perfusion and isoprenaline distributes blood away from the vital organs, namely: brain, kidneys, heart and mesentery. Dopamine is a useful agent as it enhances renal blood flow in low doses and is not excessively chronotropic. Dobutamine has not yet been shown to have significant advantages over other inotropes and requires further examination.

Particulate myocardial perfusion scintigraphy: its clinical usefulness in evaluation of coronary artery disease

The determination of myocardial viability is a crucial parameter in the evaluation of a patient with coronary artery disease. Particulate myocardial perfusion scintigraphy (PMPS) is one of several methods available for this determination. A brief comparison between PMPS and two other commonly used methods is described in this review. The technique and analysis are described and applied to 194 patients with 366 surgically bypassed regions. The patients are analyzed for postoperative regional wall function and for total ventricular function as expressed by ejection fraction. PMPS is a highly accurate and unique study for clinically predicting myocardial viability.

Studies of regional myocardial blood flow: results in patients with left anterior descending coronary artery disease

Measurements of regional myocardial blood flow have been performed rapidly and safely in man at the time of coronary arteriography. Xenon-133 was injected into the coronary artery and estimates of myocardial capillary perfusion were made by computer analysis of the multiple precordial tracer washout curves recorded with a scintillation camera. Blood flow rates/100 g/min in different areas of the heart were calculated by the Schmidt-Kety formula, using an assumed partition coefficient, and were related to the coronary vascular lesions by landmarks provided by the arteriograms. The flow significance of lesions was assessed by making measurements at rest and during interventions that increase coronary blood flow. Perfusion was also estimated in areas supplied by collateral vessels, and regions of scar were detected from the initial distribution of peak tracer activity. In patients with less than 50% obstructions apparent on coronary arteriograms and in those with greater than 50% isolated left anterior descending lesions average mean left ventricular (LV) myocardial perfusion values were not reduced below values found in patients with normal coronary arteries and normal cardiac function. However, in patients with significant two-vessel disease (LAD + right, LAD + circ) mean LV perfusion was significantly reduced. Average regional myocardial perfusion distal to significant lesions was not selectively reduced below the remainder of the ventricle unless the lesions were 100% obstructions. However, both total and distal regional myocardial perfusion increased less in 12 patients with greater than 50% lesions than in 12 others with normal arteriograms or less than 50% lesions. Subnormal flow responses in all of the former group were associated with angina pectoris compared with no angina in the latter. The studies indicate the potential utility of the measurements of regional myocardial blood flow in the evaluation of patients with ischemic heart disease who are potential candidates for myocardial revascularization procedures.

Evaluation of perfusion with the 81Rb-81mKr generator

The development of the 81Rb-81mKr generator-delivery system has made available an ultrashort (13-sec) half-life radionuclide for continuous, constant-rate delivery as a gas, or as an isotonic, nonionic solution in 5% dextrose and water. This system has been evaluated and applied to the scintigraphic imaging of various organs and anatomic regions by intravenous and intraarterial infusion and by inhalation and deglutition. This diffusible, noble gas is inert chemically and biologically. When the gas is administered intraarterially, the volume of tissue being perfused may be defined with the scintillation camera. The 13-sec half-life is of a similar magnitude to the rate of capillary exchange, producing a heterogeneous image with steady-state administration in which the intensity of detectable radioactivity is proprotional to the time of diffusion into the tissue. This system appears capable of defining the rate of capillary exchange and the moment-to-moment changes brought about by vasoactive stimuli in specific organs.

Measurement of regional myocardial blood flow in man: description and critique of the method using xenon-133 and a scintillation camera

A method has been devised to measure regional myocardial blood flow in man. The approach consists of selective injection of xenon-133 into a coronary artery and the external monitoring of radioisotope washout curves from multiple areas of the myocardium with a multiple crystal scintillation camera. Rate constants of isotope washout are calculated using a monoexponential model, and the capillary blood flow rates in multiple regions of the heart are calculated by the Kety formula. The pattern of perfusion rates is related to the coronary arteriogram obtained in the same study. Myocardial perfusion patterns obtained in patients with and without coronary artery disease and during atrial pacing are given, as well as examples of results obtained in myocardial aneurysms, in regions of ischemic myocardium supplied by collateral vessels and after tracer injection into an aortocoronary bypass graft. Advantages of the method are discussed along with limitations related to the tracer, the scintillation camera, the form of mathematical analysis and the three dimensional nature of the heart.

[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.

Measurement of regional myocardial perfusion in man with 133 xenon and a scintillation camera

A method was devised to quantitate regional capillary perfusion in the human heart by measuring the clearance constants (k) of Xenon-133 washout from multiple areas of the myocardium with a multiple-crystal scintillation camera. In 17 subjects, (133)Xe was injected into the right or left coronary artery or both and counts per second (cps) were recorded simultaneously on magnetic tape from each of 294 scintillation crystals viewing the precordium through a multichannel collimator. Data were processed by a digital computer. Crystals detecting the myocardial washout of (133)Xe were distinguished from those monitoring pulmonary excretion by positioning radioactive markers at the cardiac margins, and by a computer printout of the peak cps recorded by each crystal and its time after isotope injection into the coronary artery. The slopes of the initial segment of the multiple (133)Xe curves obtained in each study were calculated by the method of least squares using a monoexponential model. Myocardial blood flow rates in the cardiac regions viewed by the individual crystals were calculated (assuming a blood to myocardium partition coefficient of 0.72) along with the SD of every flow measurement. The pattern of myocardial perfusion rates so obtained was superimposed over a tracing of the subject's coronary arteriogram. Scintiphotographs showing the arrival and washout of isotope from various regions of myocardium and the area of tissue perfused by each coronary artery were obtained by replaying the data tape on an oscilloscope. Significant regional variations in local myocardial perfusion rates were observed in hearts with normal coronary arteries. When capillary flow measurements from crystals overlying the various cardiac chambers were averaged in each subject, the mean myocardial blood flow rate of the left ventricle in 17 patients, 64.1 +/-13.9 (SD) ml/100 g.min, significantly exceeded that of the right ventricle, 47.8 +/-10.9 ml/100 g.min, and of the right atrial region, 33.6 +/-10.3 ml/100 g.min. The approach may facilitate more objective assessment of: myocardial capillary perfusion in patients with angina pactoris, the pharmacology of antianginal drugs, and the efficacy of surgical procedures to revascularize ischemic myocardium.

Left ventricular function in acute myocardial infarction

Left ventricular catheterization was carried out in 40 patients with acute myocardial infarction. Left ventricular end-diastolic pressure (LVEDP) was elevated in 85% of the patients studied. In 14 patients with apparently uncomplicated infarcts, LVEDP averaged 15 mm Hg, and cardiac index (2.98 liter/min/m(2)), stroke volume (38.3 ml/m(2)), and stroke work (49.2 g-m/m(2)) were within normal limits. In 12 patients with clinical signs of left ventricular failure, LVEDP averaged 29.9 mm Hg, cardiac index was at the lower limit of normal (2.79 liter/min/m(2)), but stroke volume (31.6 ml/m(2)) and stroke work (37.3 g-m/m(2)) were reduced. In 14 patients with clinical signs of shock, LVEDP averaged significantly lower than in the heart failure group (21.1 mm Hg), but cardiac index (1.59 liter/min/m(2)), stroke volume (16.5 ml/m(2)), and stroke work (11.1 g-m/m(2)) were markedly reduced. A large presystolic atrial "kick" (average amplitude 9.5 mm Hg) was an important factor in the high LVEDP in the patients with heart failure but not in those with shock. The first derivative of left ventricular pressure was significantly lower in shock than in the nonshock group. Although right atrial pressure (RAP) and LVEDP were significantly correlated (r = 0.49), wide discrepancies in individual patients rendered the RAP an unreliable indicator of the magnitude of left ventricular filling pressure. THESE DATA SHOW THE FOLLOWING: (a) LVEDP is usually elevated in acute myocardial infarction, even in absence of clinical heart failure; (b) cardiac output apparently is supported by increased LVEDP and compensatory tachycardia; (c) in patients with shock, left ventricular function usually is markedly impaired, but inadequate compensatory cardiac dilatation or tachycardia could contribute to the reduced cardiac output in some individuals; (d) lower LVEDP in shock than in heart failure may represent differences in left ventricular compliance.