Abdulmassih S. Iskandrian

Dipyridamole cardiac imaging

Dipyridamole cardiac imaging is a useful alternative technique to exercise stress testing in the evaluation of patients with ischemic heart disease. Intravenous dipyridamole is still in the investigational phase, while oral dipyridamole is widely available. The hemodynamic effects of dipyridamole include an increase in coronary blood flow (due to coronary vasodilation) which is in excess of the increase in myocardial oxygen consumption and cardiac output. The disparity in the increase in coronary blood flow relative to the cardiac output results in an increase in myocardial thallium activity and an increase in the myocardial/background activity ratio. The quality of the thallium images is better or similar to that of exercise thallium images. The optimal dose of intravenous dipyridamole is 0.56 mg/kg, and of the oral dose it is 300 to 400 mg, although higher doses may be necessary in some patients. Analysis of the thallium images has been to a large extent based on visual inspection of the planar images. Delayed images are helpful to establish the nature of the perfusion abnormalities (transient or fixed). The process of redistribution is based on disparate rates of washout from the normal and abnormal zones. The sensitivity and specificity of dipyridamole thallium imaging, whether intravenous or oral, have been shown in a number of studies to be quite adequate and comparable to that achieved during exercise thallium imaging. Dipyridamole two-dimensional echocardiography has also been used in the detection of coronary artery disease; transient (new or worsening of preexisting) wall motion abnormalities have been found to be a specific marker of coronary artery disease. Transmural as well as regional coronary steal phenomena have been postulated as the mechanism for dipyridamole-induced regional wall motion abnormalities. Compared to exercise two-dimensional echocardiography, dipyridamole echocardiography provides high-quality studies and in higher proportions of patients. The results of dipyridamole thallium imaging have also been extremely important in identifying high-risk patients after acute myocardial infarction or patients with peripheral vascular disease undergoing elective vascular surgery; the presence of a dipyridamole-induced perfusion abnormality identifies patients at high risk for future cardiac events. Thus, dipyridamole cardiac imaging is helpful in the diagnosis of coronary artery disease and in risk stratification.

Thallium imaging with single photon emission computed tomography

Evaluation of myocardial perfusion with thallium-201 SPECT has advantages over planar images. These advantages are related to better contrast of the images, lack of superimposition of normal and abnormal areas, and a three-dimensional representation of the site and extent of perfusion abnormalities (ischemia, scar, or both). For this reason, rotational tomography is superior to planar imaging in assessing the extent of coronary artery disease, in the detection of small infarcts, and for quantitative measurements. Several techniques have provided accurate quantitative data for infarct sizing both in animals and men. The ability to quantitate infarct size (or ischemia) will be extremely important in studies of myocardial salvage, risk stratification, and longitudinal studies to evaluate the effects of medical and surgical interventions.

New myocardial perfusion imaging agents: Description and applications

Myocardial perfusion imaging is the most commonly performed procedure in nuclear cardiology. Thallium-201 has been the agent of choice due to its favorable physiologic kinetics: the initial myocardial uptake corresponds to the regional blood flow distribution because of high extraction efficiency during the first transit and the ability to redistribute over time is useful to distinguish scar from ischemia. The physical characteristics of thallium, however, are not ideal for nuclear imaging because of low energy photopeaks, long half-life, and the need for a cyclotron for its production. On the other hand, technetium-99m has very favorable physical characteristics such as a photopeak that has an optimal energy for imaging, a short half-life, and in addition the agent is generator-produced. Thus it is natural to search for technetium-99m-labelled agents for myocardial perfusion imaging to overcome the poor physical characteristics of thallium-201. In this paper, we review the background, technical aspects, clinical application, and future direction.

Relation between myocardial thallium-201 kinetics during exercise and quantitative coronary angiography in patients with one vessel coronary artery disease

This study examined the relation between the kinetics of thallium-201 and coronary stenosis in 30 patients with one vessel coronary artery disease; 25 patients had no visible collateral vessels. The myocardial thallium concentration in the postexercise images and percent washout were determined in the distribution of the diseased vessel and a normal vessel, and each was expressed as a ratio. Coronary stenosis was assessed as minimal diameter stenosis, minimal area stenosis and percent diameter stenosis. The correlations between the myocardial concentration ratio or washout ratio and the descriptors of coronary stenosis improved when the patients with collateral vessels were excluded. There were significant correlations between the myocardial thallium concentration ratio and minimal diameter stenosis (r = 0.73, p less than 0.001), minimal area stenosis (r = 0.72, p less than 0.001) and, to a lesser degree, percent diameter stenosis (r = -0.51, p less than 0.01). Similarly, there were significant correlations between washout ratio and minimal diameter stenosis (r = 0.50, p less than 0.01) and minimal area stenosis (r = 0.45, p less than 0.02) but not percent diameter stenosis (r = 0.37, p = 0.06). Thus, variation in thallium kinetics in relation to the severity of coronary stenosis can be demonstrated with conventional imaging in patients with one vessel disease. The myocardial thallium concentration and washout are physiologic expressions of the severity of perfusion deficit and are dependent on collateral flow. The myocardial thallium concentration ratio and washout ratio correlate better with minimal diameter and area stenosis than with percent diameter stenosis.

The role of nitrates, beta blockers, and calcium antagonists in stable angina pectoris

Numerous controlled studies have shown that nitrates, beta blockers, and calcium antagonists are effective in the treatment of stable angina pectoris. The pharmacokinetics, pharmacodynamics, and hemodynamic effects of these agents are different, and thus combination therapy offers additive improvement and also counterbalancing of the undesirable side effects of each drug. The choice of therapy depends on the severity of symptoms, associated diseases, compliance, side effects, and status of left ventricular function. The main mechanism of improvement is a decrease in myocardial oxygen consumption, though an increase in coronary blood flow is another potential reason for the use of calcium blockers. This review considers the properties of these drugs, their mechanism of action, and the results of randomized studies.

Merits and limitations of quantitative treadmill exercise score

This study examined the ability of the treadmill exercise score (TES) in determining the presence and extent of coronary artery disease (CAD). The score was derived from the integrated area of ST segment depression and ST slope in two leads (V5 and a VF), corrected for R wave amplitude, exercise time, and percent of maximum predicted heart rate. The ST segment depression was measured at 80 msec after the J point. There were 34 patients with no significant CAD, 38 patients with one-vessel CAD (greater than or equal to 50% diameter stenosis), and 58 patients with multivessel CAD. The TES showed a considerable scatter in patients with and without CAD. A receiver operating characteristic curve showed different levels of sensitivity and specificity, depending on the cut-point. The TES was similar to ST segment depression in detecting CAD (predictive accuracy, 77% vs 78%, p = NS). A markedly abnormal score (less than -1.0) was seen in 41 patients, of whom 32 (78%) had multivessel CAD. On the other hand, a score greater than 0 was seen in 49 patients, of whom 40 (82%) had no or one-vessel CAD. In 40 patients with TES between -1.0 and 0, 17 (43%) had multivessel CAD and 23 (57%) had no or one-vessel CAD. In 51 patients with nondiagnostic ST changes, the TES correctly classified the extent of CAD in 20 patients (40%). Thus, the TES has a similar accuracy to the ST segment depression criteria in detecting CAD. The extent of CAD can, however, be ascertained in 80% of the patients with very high or very low TES.