Cardiac Stress Testing: Advantages of Stress Echocardiography Over Nuclear Stress Testing
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Muhamed Saric, MD, PhD
Director, UH & DOC Echocardiography Labs
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All forms of stress testing have two fundamental parts: (1) stress induction; and (2) visualization of possible stress-induced coronary supply-demand mismatch.

Stress Induction

Stress induction can be achieved by either physical exercise or infusion of pharmacologic agents.

Exercise

In the United States, treadmill exercise is preferred over other exercise protocols while in other parts of the world bicycle ergomentry is often performed instead.

Dobutamine

Infusion of dobutamine, a b₁ adrenergic agonist, over an approximately 15-min interval (with addition of atropine, if necessary), closely mimics the cardiovascular effects of exercise since it accelerates the heart rate and increases myocardial contractility.

Coronary Vasodilators

Infusion of coronary vasodilators such as dipyridamole (Persantine^®) or adenosine induces coronary supply-demand mismatch through alterations in coronary blood supply rather that through an increase in cardiac demand, as is the case with exercise and dobutamine stress testing.

Stress Visualization

After obtaining resting images, stress induction is performed and then stress images are acquired. Stress-induced mismatch can be visualized by electrocardiography (EKG), echocardiography or some form of nuclear imaging (also known as perfusion scintigraphy).

Electrocardiograph (EKG)

During all forms of stress testing, EKG is recorded continuously. Stress-induced ST segment depressions represent electrocardiographic definition of a coronary supply-demand mismatch.

However, EKG monitoring alone is often inadequate in correctly identifying individuals with coronary artery disease, especially if the underlying prevalence of the disease is low in the population under study (e.g. young and middle-aged women).

When 1 mm of horizontal or downsloping ST depression was used as a criterion for positive stress test in studies [1] in which patients agreed to undergo both plain exercise stress test and coronary angiography, the approximate sensitivity and specificity were as shown in

Figure 1.

Figure 1: Sensitivity & Specificity of Plain Exercise Stress Test

Source: AHA/ACC Guidelines for Exercise Stress Testing¹

The true diagnostic value of the exercise EKG lies in its ability to exclude the presence of significant coronary artery disease in a large proportion of subjects because of the test’s relatively high specificity.

However, the test’s low sensitivity often necessitates inclusion of either echocardiographic or nuclear imaging in order to increase the chance of detecting coronary disease.

Stress Echocardiography

When inadequate coronary perfusion is present due to coronary artery stenosis, the myocardial region subtended by the stenotic vessel develops a stress-induced decrease in contractility. This transient regional wall motion abnormality is what is visualized by echocardiography during stress testing.

Nuclear Imaging (Perfusion Scintigraphy; SPECT)

Nuclear imaging with radioactive isotopes of either thallium (²⁰¹Tl) or technetium (⁹⁹Tc), ideally detects perfusion abnormalities directly. However, this is often hampered by either breast or diaphragm artifacts which may result in false positive diagnosis of coronary stenosis (see below).

Forms of Stress Testing

Since any form of stress induction can theoretically be paired with any form of stress visualization, numerous stress-testing protocols exist (see Table 1).

Table 1: Forms of Stress Testing

Shaded rectangles in Table 1 represent the common stress protocols in the United States. For historical reasons, dobutamine stress testing is almost always paired with echocardiography while vasodilator stress testing almost always uses nuclear imaging in this country. In contrast, dipyridamole stress echocardiography is very popular in Europe but rarely performed in the United States.

Stress Echocardiography vs. Nuclear Stress Testing

To date, no well-designed head-to-head comparison between stress echocardiography and perfusion scintigraphy has been published. The best evidence about the relative clinical utility of the two forms of stress testing comes from two meta-analyses (neither of which was adjusted for possible referral bias).[2]^,[3]

In these studies, there was no statistically significant difference with respect to sensitivity (except for single-vessel coronary artery disease which favored nuclear imaging). In contrast, there was a significant difference in specificity in favor of stress echocardiography in Fleischmann’s analysis (Table 2).

Table 2: Meta-Analyses of Stress Echo vs. Nuclear Stress Testing

The authors of the larger of the two meta-analyses, state in their conclusions:

“Exercise ECHO and exercise SPECT have similar sensitivities for the detection of coronary artery disease, but exercise ECHO has better specificity and, therefore, higher overall discriminatory capabilities as used in contemporary practice.”³

American Heart Association (AHA) and American College of Cardiology (ACC) guidelines — the most authoritative source on cardiology practice guidelines — express the same sentiment regarding Fleischmann’s analysis:

“[E]xercise echocardiography had significantly better discriminatory power than exercise myocardial perfusion imaging.”[4]

Lower specificity of nuclear studies results in a higher rate of false positive nuclear stress tests when compared to stress echocardiography. False positive results occur either in anteroapical segment of left ventricle (due to interference from breast tissue) or in the inferior left ventricular segment (due to interference from the diaphragm).

Which Stress Imaging Protocol Should One Order?

One may argue that the choice of a stress imaging protocol that is best suited for a particular patient depends on whether a referring physician is more interested in ruling out or ruling in coronary artery disease.

Ruling Out Coronary Artery Disease

Due to its higher specificity, stress echocardiography is more likely than nuclear stress imaging to exclude the presence of coronary artery disease in normal individuals. If ruling out coronary artery disease is the primary objective, then stress echocardiography, in my opinion, should be the preferred test.

Ruling In Coronary Artery Disease

Since sensitivity to detect coronary artery disease does not significantly differ between stress echocardiography and nuclear stress imaging (except for single-vessel disease which favors nuclear imaging), one may choose a stress protocol based on personal preference, lab experience, convenience to the patient, study population characteristics and the possible need to obtain anatomic and physiologic data beyond the left ventricular myocardium.

Lab Experience

Our echo labs at University Hospital and the DOC are staffed with highly experienced and dedicated personnel. Since I assumed directorship of the Echo Labs, my team and I have performed more than a thousand stress echocardiographic studies at University Hospital (see Table 3).

In addition, several hundred stress echocardiograms were performed at the DOC in the same time period.

Table 3: Stress Echos Performed at UH (1999-2001)

I am the principal reader of the vast majority of stress echocardiographic studies performed at either location.

My interpretive skills are certified by the American Board of Echocardiography. After passing a special competency exam, I was awarded a comprehensive certification in all forms of modern cardiac ultrasound imaging including stress echocardiography.
(See: http://www.echoboards.org/ascexam/testamurs01.html for verification of certification status). This is the most rigorous echocardiographic certification in the United States.

Patient Convenience

Our patients and their physicians may prefer stress echocardiography for the following reasons:

· Stress echocardiography uses no radioactive materials and thus poses no radiation exposure risk.

· The entire stress echocardiographic study can be performed within 30 minutes. In contrast, there is significant time delay between acquisition of rest and stress images during nuclear stress testing. The delay is at least a few hours; sometimes the patients may even have to return to the nuclear lab the next day.

· Stress echocardiography results are available immediately after the completion of the test. I strictly enforce the policy that no hospitalized patient should leave the Echo Lab without a stress test report in the chart.

· Immediate availably of stress results is crucial for discharge planning and thus shortens patients’ length of hospital stay.

· Within 24 hours of test performance, results of all cardiac echo studies (including stress echocardiograms) appear in electronic format in Logician.

Particularities of Study Population

The following comorbid conditions may influence a referring physician’s choice of stress imaging: (1) bronchospastic disorders such as asthma and COPD; (2) end-stage liver disease; and (3) obesity; (4) hypertension; and (5) left bundle branch block.

Bronchospastic Disorders

Both asthma and COPD are prevalent in our referral population. If a patient with bronchospastic disorders cannot exercise then dobutamine stress echocardiography is a safer option than dipyridamole (Persantine^®) or adenosine nuclear stress testing.

ACC/AHA guidelines state that ”both dipyridamole and adenosine may cause severe bronchospasm in patients with asthma and or chronic obstructive lung disease; therefore, they should be used with extreme caution — if at all — in these patients.”⁴

End-stage Liver Disease

All patients with end-stage liver disease undergo cardiac stress testing when being evaluated for possible orthotopic liver transplant (OLT). Because most of them cannot exercise, they are referred for pharmacologic stress testing instead.

Nuclear imaging combined with either dipyridamole or adenosine is reported to have a poor predictive value for coronary artery disease in OLD candidates. It is believed that vasodilators such as dipyridamole and adenosine may not be as effective in OLT patients as they are in general population because chronic liver disease is associated with decreased arteriolar resistance at baseline.[5]

Data on dobutamine stress echocardiography (DSE) in OLT patients are very limited. Based on a very small study, DSE appears to be more sensitive but less specific for coronary artery detection[6] than dipyridamole or adenosine nuclear imaging in this population (Table 4).

Table 4: Pharmacologic Stress Testing
in Patients with End-Stage Liver Disease

Obesity

Morbid obesity is unfortunately prevalent in our referral population. Imaging the heart in obese patients is difficult because excess fat often hampers both echocardiographic and nuclear scanning.

Very obese patients are disqualified from nuclear stress testing if their weight exceeds the weight-bearing limit of nuclear imaging tables (usually 300 lbs). For such patients, stress echocardiography is the only feasible form of stress imaging.

Hypertension

Individuals with uncontrolled systolic blood pressure at rest are at risk for severe hypertension during either exercise or dobutamine stress testing. For such individuals referring physicians should consider stress induction with dipyridamole or adenosine followed by either nuclear or echocardiographic imaging.

Dipyridamole (or adenosine) stress echocardiography is the preferred form of pharmacologic stress testing in Europe. Our Echo Lab at UH is one of the few in the United States that offer this stress modality.

We employ a state of the art dipyridamole stress echo protocol, which includes atropine if needed for stress induction. Such a protocol has been shown to provide the same degree of sensitivity and specificity for detection of coronary artery disease as the more common dobutamine stress echo protocols.[7]

Left Bundle Branch Block

In patients with native or ventricular pacemaker-induced left bundle branch (LBBB), a false positive stress test for ischemia in LAD distribution may occur irrespective of imaging technique if stress induction is performed with exercise or dobutamine.

Therefore, in patients with LBBB, exercise nuclear, exercise echo and dobutamine echo may all lead to false positive stress test results.

This may be avoided if a coronary vasodilator (dipyridamole or adenosine) is used for stress induction. It is generally agreed that dipyridamole or adenosine nuclear stress testing is the preferred stress test modality in patient with LBBB.⁴

For patient with LBBB who are ineligible for dipyridamole or adenosine nuclear stress testing (such as morbidly obese individuals) our Echo Lab at UH provides an alternative: dipyridamole stress echo (see section on Hypertension).

Additional Diagnostic Data

In principle, nuclear stress testing provides only information about left ventricular perfusion and systolic function. Stress echocardiography, on the other hand, provides a wealth of additional diagnostic information including:

· Assessment of left ventricular diastolic function

· Assessment of valvular function. This is especially important in determining hemodynamic significance of aortic or mitral valve stenosis under stress conditions.

· Assessment of filling pressures. Echocardiographic methods have been developed for estimation of pulmonary artery systolic and diastolic pressures; mean left and right atrial pressures, etc.

· Detection of complications of myocardial infarction such as ventricular septal defect or ruptured papillary muscle.

· Assessment of pericardium.

Conclusions

This document summarizes the inherent advantages of stress echocardiography over nuclear stress imaging. I believe that because of such advantages the number of stress echocardiograms will continue to increase and that stress echocardiography will remain the preferred choice of stress imaging for our referring physicians.

Cardiac Stress Testing: Advantages of Stress Echocardiography Over Nuclear Stress Testing ± Muhamed Saric, MD, PhD Director, UH & DOC Echocardiography Labs ±