Continuing Education Activity

Myocardial perfusion scans are a widely used procedure to diagnose ischemic heart disease. This activity reviews the importance of these scans and highlights the role of the interprofessional team in performing them.

Objectives:

• Identify the indications for a myocardial perfusion scan

• Assess the technique of performing a myocardial perfusion scan.

• Evaluate the clinical significance of myocardial perfusion scan.

• Communicate how an interprofessional team approach can improve patient outcomes when using myocardial perfusion scanning on appropriate patient populations.

Introduction

Myocardial perfusion single-photon emission computed tomography (MPS) is an important, cost-effective, and widely used non-invasive imaging modality. With the advancement in imaging techniques and for early detection of coronary artery disease, it is estimated that 9.1 million tests are performed annually in the United States.[1] Myocardial perfusion scan plays a crucial role in diagnosing ischemic coronary artery disease, predicting prognosis, and assessing myocardial viability and effectiveness of medical therapy.[2] The sensitivity of this test is improving continuously due to better technologies, new software, and radiotracers.[3] During a myocardial perfusion scan,  a gamma camera detects radiotracer injected intravenously to measure the distribution of coronary blood flow in the myocardium. Multiple planar projections are collected by rotating the gamma camera at the regular angular interval in a 180-degree arc. Myocardial uptake of the tracer for the scan is dependent on 2 things. The first transportation of the radiotracer to the cell surface is flow-mediated. Second, the intact cell membrane is required to extract the radiotracer. The extraction of radiotracer is directly dependent on blood flow. The same phenomena that cause a reduction in oxygen delivery and ischemic ECG changes on a treadmill can be assessed by perfusion defect on MPS. However, reduced tracer uptake to specific regions helps diagnose the culprit artery responsible for patient symptoms. The difference in radiotracer uptake at rest and stress helps differentiate fixed and reversible defects.[4]

Anatomy and Physiology

The myocardium is divided into 17 segments based on a short axis and a long axis slice. The short-axis view has a septal, anterior,  lateral, and inferior wall. On the long horizontal axis, there are septal walls and lateral walls. On a vertical long-axis view, there is an anterior and inferior wall.[4] The left anterior descending artery supplies the anterior and septal wall, the left circumflex artery supplies the lateral wall, and the right coronary artery supplies the inferior wall. Perfusion is graded from 0-normal uptake, 1-mildly reduced uptake, 2-moderately reduced uptake, 3-severely reduced uptake, and 4-no uptake. If there is no defect in stress and rest images, it is a normal myocardium (see Image. Normal Myocardial Perfusion Scan Showing Location of Segments). If there is a defect in stress images and normal rest images, it is ischemic myocardium. If there is a defect both on rest and stress, it is either a scar or hibernating myocardium. Stress-induced cavity dilation on scan indicates severe and extensive coronary artery disease, either multivessel or left main involved.

Indications

Diagnosis of Ischemic Heart Disease With Intermediate-risk for CAD and or Risk Stratification of Patients with an Intermediate or High Likelihood of CAD

• For identification of the extent, severity, and location of myocardial ischemia
• Assess the functional significance of intermediate  coronary artery stenosis
• Intermediate Duke Treadmill score (score between 4 and -11) on exercise stress test (ETT)
• Repeat testing  in patients with a change in their ischemic symptoms
• Repeat testing in 1 to 3 years in patients with a high likelihood of coronary artery disease
• Severe coronary calcification on CT angiogram with unpredictable ECG
• Asymptomatic patients but with high-risk occupations ( such as firefighters and pilots)

MPS is contraindicated for the screening of  asymptomatic patients with a low likelihood of ischemic coronary artery disease

Assessment of Interventions and Therapy in Ischemic Heart Disease 

• To assess patients 3 to 5 years after revascularization (either CABG or Coronary angioplasty) in high-risk asymptomatic patients
• Repeat testing to evaluate the therapeutic efficacy in patients with ischemic heart disease

MPS is contraindicated for routine assessment of patients after coronary angioplasty or CABG

Before Non-cardiac Surgery

• Intermediate-risk surgery or vascular surgery and risk factors with poor functional capacity (<4 METS)[5]

MPS is contraindicated in low-risk, intermediate-risk, or vascular surgery and has a good functional class.

Contraindications

MPS is contraindicated in pregnant or breastfeeding patients, acute myocardial infarction (<2 days), acute myocarditis, acute pulmonary embolism, acute aortic dissection, acute symptomatic medical illness (such as metastatic cancer), severe symptomatic aortic stenosis, severe pulmonary artery hypertension, high-risk unstable angina, congestive heart failure, severe hypertension (>200/110 mmHg) and sustained cardiac arrhythmias.[6] MPS is relatively contraindicated in patients with significant left main coronary artery disease, asymptomatic severe aortic stenosis (mean gradient > 40 mmHg, aortic valve area < 1cmsq, aortic jet velocity > 4 m/sec), hypertrophic obstructive cardiomyopathy, or forms of severe left ventricular outflow obstruction, complete heart block.

Equipment

The equipment required for a myocardial perfusion scan includes:

• Treadmill machine for exercise
• Pharmacologic agents, vasodilators and Ionotropes
• The antidote to reverse the effects of vasodilator and inotropes, if required
• Radiotracer
• Myocardial perfusion gamma scan machine

Personnel

Personnel involved in conducting a myocardial perfusion scan include:

• A nuclear cardiologist or radiologist
• Nuclear technologist
• Staff nurse

Preparation

Patient preparation: Before the test, fast for 3 hours and avoid caffeine-containing foods and drinks for at least 12 hours. Blood pressure medications with antianginal properties (beta-blocker, calcium channel blocker, and nitrates) should be held 12 hours before the procedure.

Technique or Treatment

Stress

There are 2 ways to perform stress testing:

1. Exercise
2. Pharmacological 

Exercise

This mode is preferable, as it also informs the functional capacity of the patient. The Bruce protocol is applied for exercise, and the patient walks on a treadmill until 85% of MPHR (Maximum predicted heart rate) is achieved. The achievement of the target MPHR (maximum predicted heart rate) does not indicate that the test should end earlier. The radiotracer was injected as close to peak exercise as possible. Patients should be encouraged to exercise for at least 1 minute after the radiotracer injection. However, exercise should be terminated early if patients start having moderate to severe chest pain, marked shortness of breath, extreme fatigue, ataxia, dizziness, or near-syncope, signs of decreased peripheral perfusion such as cyanosis and pallor, patients are not able to exercise further, drop in systolic blood pressure of greater than 10 mmHg from baseline, hypertensive response (systolic blood pressure >230 mmHg and/or diastolic pressure>115 mmHg), unable to monitor the electrocardiogram or systolic blood pressure and in patients with implantable cardioverter defibrillators, when the heart rate achieved is within 20 beats per minute of the lowest heart rate at which therapy (anti-tachycardia pacing or shock) is programmed to be delivered. The recommendation to terminate exercise tests early on an electrocardiogram basis is excessive ST-segment depression (>2 mm from baseline), ST elevation (>1 mm), sustained supraventricular tachycardia, or ventricular tachycardia, development of left bundle branch block or intraventricular conduction delay that cannot be differentiated from ventricular tachycardia.[7]

Pharmacologic Agents

Vasodilators: dipyridamole, adenosine, and regadenoson are 3 agents used as vasodilators for MPS. Vasodilator stress is recommended in all patients who are unable to exercise or who are unable to tolerate submaximal exercise. If the patient has a baseline left bundle branch block, a stress test with a vasodilator is preferable to exercise. Vasodilator testing is contraindicated in high-grade AV block patients, patients with a history of severe reactive airway disease, and patients who have consumed caffeinated drinks. The doses for myocardial perfusion scan of vasodilators are dipyridamole 0.56 mg/kg, adenosine 140 mcg/kg/min, and regadenoson 0.4 mg. Intravenous adenosine continuous infusion starts at 140 mcg/kg/min. A radiotracer injection should be given at 3 minutes, and then adenosine infusion continues for the next 3 minutes. A single intravenous dose of 0.4 mg is given for regadenoson, followed by a 5 ml normal saline flush, and radiotracer is given 10 to 20 seconds after the normal saline flush. For dipyridamole, continuous infusion is given at 0.56 mg/kg for 4 minutes, then stops, and radiotracer should be given after 3 to 5 minutes of stoppage of dipyridamole infusion. To reverse the effects of vasodilators, aminophylline can be given 50 to 250 mg intravenously.[8]

Inotropes are indicated in patients who have contraindications to exercise and vasodilators. Dobutamine is a commonly used inotrope for MPS. Dobutamine infusion starts at 5 to 10 mcg /kg/min and increases every 2 mins until MPHR(Maximum predicted heart rate) is achieved. Once MPHR (maximum predicted heart rate) is achieved, a radiotracer is injected, and then dobutamine should be continued for the next minute. Dobutamine is contraindicated in patients with severe hypertension, unstable arrhythmia, and severe left main disease.

Radiotracer

There are 2 commonly used radiotracers for MPS.

1. Thallium-201
2. Technetium–99

Thallium-201 is a monovalent cation, a property similar to potassium. It has a half-life of 73 hours and emits 80 keV photons. Its peak myocardial concentration is achieved in 5 minutes and has the property of rapid clearance from the intravascular compartment. The differential washout-clearance of thallium 201 is more rapid than normal myocardium. As hyperinsulinemia stated, fasting is essential to reduce blood concentration and slow the distribution of thallium 201. Thallium should be injected at peak stress, and images should be taken 10 to 15 mins of injection. Rest images after 4 hours of stress image.[9]

Advantages of thallium-201:

1. Its redistribution property allows single-injection studies and viability assessment.
2. It is less expensive.
3. It has less abdominal and hepatic uptake during exercise

Technetium–99 has a half-life of 6 hrs and emits 140 keV photons. Limited washout occurs with technetium–99 tracers bound by mitochondria, so imaging can commence later and be repeated. Rest images are obtained after 15 to 45 minutes of stress.[10]

Advantages of technetium–99:

1. It has a shorter half-life, which allows for a higher dose, resulting in better count statistics
2. It has less radiation exposure and attenuation effects
3. It has higher energy levels, which allow improved resolution.

Complications

Complications from a myocardial perfusion scan are rare. A few possible complications are:

• Radiation exposure (low radiation exposure)[11][12]
• Myocardial infarction (rare)
• Arrhythmia (rare)
• Non-ischemic chest pain, especially from dipyridamole and adenosine
• Wheeze

Clinical Significance

Myocardial perfusion scan plays a vital role in diagnosing ischemic heart disease and assessing the effectiveness of medical management in patients with known coronary artery disease. It is a useful non-invasive test to assess the blood supply of the myocardium. It also helps in preoperative cardiovascular risk assessment of patients with poor functional class and intermediate-risk surgery. The risk of perioperative cardiovascular risk is directly proportional to the severity of perfusion defect on myocardial perfusion scan.

Enhancing Healthcare Team Outcomes

A myocardial perfusion scan requires an interprofessional team that includes a nuclear cardiologist or radiologist, a staff nurse, and a nuclear technologist. Proper history taking and a physical examination before the procedure are essential. Which stress mode should be selected depends upon the patient's functional class and comorbidities. Giving a radiotracer and performing a scan at the appropriate time is important to get the best results.