Richard M. Steingart

Pulsed doppler echocardiographic measurement of beat-to-beat changes in stroke volume in dogs.

Measurement of stroke volume by pulsed Doppler echocardiography has not been validated against a reference method in vivo. We compared Doppler systolic frequency shift integrals with electromagnetic flowmeter stroke volume in seven open-chest dogs. A pulsed Doppler echocardiographic transducer was held on the aortic arch with the sample volume in the ascending aorta. Stroke volume was varied by epinephrine or pentobarbital infusions, fluid administration or inferior vena caval constriction. Linear regression analysis of stroke volume vs Doppler systolic frequency shift integrals revealed strong correlations and intercepts close to zero (r = 0.74–0.096, p < 0.001). Minor changes in transducer position did not influence Doppler frequency shift integrals substantially. Therefore, pulsed Doppler echocardiography served as an excellent measurement of stroke volume changes in this model. However, serious limitations are presented that may limit its clinical application.

Radionuclide ventriculographic responses to graded supine and upright exercise: critical role of the Frank-Starling mechanism at submaximal exercise

To assess the influence of work load and posture on the response to exercise, 25 patients with coronary artery disease (CAD) and 17 normal subjects underwent graded supine and upright exercise radionuclide ventriculography. In both groups, end-diastolic counts increased with supine exercise (p less than 0.001). The ejection fraction and peak systolic pressure-end-systolic volume relation increased in normal subjects (p less than 0.02), but not in patients with CAD. At upright rest, end-diastolic counts decreased in both groups (p less than 0.001) and then increased with exercise (p less than 0.001). The increase in end-diastolic counts was most pronounced on the transition from upright rest to the 150-kpm work load and resulted in a significant increase in stroke counts (p less than 0.05) for both patients with CAD and normal subjects, without a measurable change in the ejection fraction or the peak systolic pressure-end-systolic volume relation. Later in exercise, end-diastolic counts plateaued, and the ejection fraction and the peak systolic pressure, end-systolic volume relation increased only among normal subjects. Thus, low-level upright exercise is highly dependent on the Starling mechanism in both normal subjects and patients with CAD, with enhanced contractility apparent only during more vigorous exercise in normal subjects.

Radionuclide Ventriculographic Study of Adaptations To Exercise in Aortic Regurgitation

Exercise-gated radionuclide ventriculography has been proposed as a method to evaluate cardiac reserve in patients with aortic regurgitation (AR). Characterization of ventricular function, however, in AR is complicated by the dynamic nature of the leak in individual patients and by variations in severity among patients. Twenty patients with isolated AR were studied to assess the effects of exercise on the regurgitant index. The regurgitant index (left ventricular divided by right ventricular stroke counts) estimates the severity of the leak. The regurgitant index at rest was significantly higher in patients with AR than in patients without AR (3.46 +/- 1.25 versus 1.08 +/- 0.16, p less than 0.001). In patients with AR, the regurgitant index decreased during exercise to 2.6 +/- 0.8 (p less than 0.001), whereas it did not change in the control group (1.16 +/- 0.21, difference not significant). Further, in patients with AR, the greater the regurgitant index at rest, the greater the decrease during exercise (y = 0.56x -- 1.08, r = 0.78, p less than 0.001). End-diastolic counts and stroke count responses from rest to exercise were highly variable, but were explained in part by the decreasing regurgitant index. These data support previous catheterization studies and confirm gated radionuclide ventriculography as a useful tool for monitoring adaptations to exercise in AR.

Myocardial Perfusion Imaging for Preoperative Risk Stratification

This review considers the changing nature of surgical risk assessment and the definition of risk; discusses the pathophysiology of perioperative myocardial infarction in relation to tests of coronary flow reserve; surveys the extensive literature on preoperative myocardial perfusion imaging (MPI) and outlines key trends; presents practical points on image interpretation; addresses the needs of special populations; compares MPI with other modalities; and integrates recommendations from practice guidelines on the effective use of MPI in the preoperative patient.

Exercise thallium-201 scintigraphy in dogs: effects of long-term coronary occlusion and collateral development on early and late scintigraphic images.

To examine the effects of coronary collateral development on thallium-201 (201Tl) distribution the left circumflex coronary artery was ligated in eight dogs. Three days later these animals ran on a treadmill, and 201-thallous chloride was injected into the right atrium at peak exercise. Scintigraphic scanning was begun within 10 min and continued for 3 hr. Scanning was repeated weekly for 6 weeks. In the last week radioactive microspheres were injected into the left atrium at peak exercise to measure regional myocardial blood flow. The scintigraphically determined disparity between perfusion of the ischemic and normal myocardium was most marked at 3 days after ligation. This difference gradually lessened over the first 4 weeks until there was no difference in 201Tl distribution to normally perfused myocardium and tissue distal to the ligation. Concomitant with the improvement in the scintigrams, exercise hemodynamics also improved over this 4 week period with significant increases in cardiac output and decreases in left atrial pressure. Serial coronary angiographic studies in two animals demonstrated the appearance of collaterals in the initial weeks after coronary occlusion, and by 4 weeks the left circumflex artery distal to the obstruction was completely opacified by collateral flow. The ratio of directly measured exercise blood flow to the left circumflex and normally perfused tissues was 0.89 +/- 0.08 at 6 weeks after ligation. Scintigraphic 201Tl redistribution after 3 hr also changed over the weeks after ligation. Three days after ligation washout from the ischemic area was significantly slower than that from the normal myocardium. By 6 weeks loss of 201Tl from the two regions occurred at nearly equal rates. Thus myocardial perfusion and function during exercise after coronary occlusion are dynamic events that change with time. It is likely that coronary collateral development is responsible for these phenomena. Therefore coronary collaterals do have salutary effects in the dog.

Pharmacologic intervention in cardiovascular nuclear medicine procedures

Relevant questions in ischemic heart disease are (1) what is the ischemic threat? (2) What is the extent of ventricular dysfunction? (3) Is the observed dysfunction reversible? Exercise testing can help to identify the ischemic threat. Catheterization studies have shown that resting ventricular dysfunction can be reversed in some patients through pharmacologic or surgical intervention. However, improved ventricular performance in ischemic heart disease may be achieved through a variety of mechanisms. Insight into all components of cardiac performance (regional and global contractillity, preload, afterload, and heart rate) and myocardial perfusion may be required to adequately describe the influence of intervention. Exercise radionuclide ventriculographic studies have demonstrated that stress-induced ventricular dysfunction can be reversed through surgical and pharmacologic intervention. Studies at rest have demonstrated that radionuclide techniques can detect drug-induced changes in ventricular performance in groups of patients. The challenge to cardiovascular nuclear medicine is the prospective identification of patients who would benefit most from aggressive intervention aimed at preventing or reversing ischemic ventricular dysfunction.

Physiologic Intervention in Cardiovascular Nuclear Medicine.

Homeostasis of cardiac output is maintained by a complex intergration of many physiologic responses, both central and peripheral, including pulse,the contractile state of the ventricle, and pre-and after load. In the abnormal ventricle at rest, any or all of the measurable parameters that define left ventricular function may be normal. However, in disease states, exercise can provoke abnormalities in these parameters indicating a reduction in myocardial reserve. Regional asynergy occurs in patients with significant ischemic heart disease during exercise reflecting a local supply-demand mismatch. Recently, radionuclide ventriculography has been combined with exercise techniques in attempts to elucidate details of the normal and abnormal ventricular response to stress noninvasively. The majority of data describe the response of the ventricle during graded supine bicycle exercise. The normal response is to increase myocardial contractility, reducing end-systolic volume, while end-diastolic volume remains constant. This results in an increase in ejection fraction. In ischemic ventricles, regional asynergy develops and ejection fractions may either fall or fail to rise. This appears to be accomplished by an increase or no change in the end-diastolic volume, with an increase in end-systolic volume, particularly in patients with angina during exercise. Radionuclide methods can be applied to large mumbers of patients providing informaton that is unobtainable by more invasive procedures. This review traces the development of knowledge of the ventricular response to exercise, emphasizing the role of radionuclide ventriculography. Although radionuclide angiography has the capacity for advancing the understanding of the ventricular response to exercise, the technical limitations of this technique in specific diagnostic conditions has yet to be defined adequately.

Some unanswered problems with probability analysis: Possible impact on exercise thallium test use

Abstract Probability analysis has been advanced as a means of selecting patient-test combinations that most efficiently lead to a diagnosis in patients with suspected coronary artery disease (CAD). 1 Results of exercise cardiovascular nuclear medicine tests have yielded the greatest shifts in disease likelihood among patients with intermediate pretest likelihoods for disease. Furthermore, the differences in posttest probability produced by nuclear and electrocardiographic exercise tests are greatest among patients with intermediate pretest likelihoods for disease. However, when we examined the actual use of exercise thallium scintigraphy, most patients referred by both house staff and attending physicians had either very high or low pretest likelihoods for disease. 2 One explanation for these ordering practices is a lack of familiarity with probability analysis. 3 Alternatively, the clinical efficacy of a test may not be valued in proportion to its ability to alter disease likelihood. For example, exercise thallium scintigraphy has been advocated as a valuable means of excluding significant coronary disease, particularly when the pretest likelihood for coronary disease is relatively low. 4 In this study we asked house staff to calculate the predictive value of an exercise thallium test result. We then examined the relation between likelihood estimates and stated preferences for testing in an attempt to discover motivations for test ordering in actual practice.

Cardiovascular Nuclear Medicine

The development of radionuclide imaging systems, low energy and easily available isotopes, and small dedicated minicomputers for the acquisition and processing of nuclear medicine data were necessary conditions for the new field of cardiovascular nuclear medicine. This new field combines the imaging of radiology with the quantitative capacities of nuclear medicine to provide a noninvasive means for serial evaluation of the anatomy and physiology of the cardiovascular system. This chapter will describe the field of cardiovascular nuclear medicine, detailing both the methodology and applications.