07 September 2016

Lecture #7: THE CARDIOVASCULAR SYSTEM




MYOCARDIAL PERFUSION SCINTIGRAPHY

 Indication

1.      Detection of Coronary Artery Disease (CAD)

2.      Documentation of myocardial perfusion abnormalities before and after interventional therapy

3.      Detection of hibernating myocardium

4.      Detection of myocardial perfusion abnormalities secondary to causes other than CAD.

 Patient Preparation

1.      The patient should be fasting for at least 6 hours.

2.      Certain drugs should be held for some time as it may affect exercise response

Drugs                                     Discontinue prior to test

Nitroglycerine                                  1 hour

Long–acting nitrates                        1 day

Tranquilizer / Sedative                   1 day

Anti–arrhythmic agents                  2 days

Beta–blockers                                   2 days

Diuretics                                            4 days

Anti–hypertensives                          4 – 7 days

Digitalis                                             1 – 2 weeks

Myocardial Perfusion Scintigraphy is a comparison of a heart in a resting to that of a heart in a stressed state pictured in different tomographic slices through the aid of radiopharmaceutical. The stress state is being simulated through Treadmill Exercise Test and in patients with limited exercise capacity, through Pharmacologic Stress Inducing Agents. The resting state may be accomplished before or after stress testing procedure depending on department’s protocol.

1.      Stress Testing Procedure

a.      Treadmill Exercise Test

Electrode positions in Treadmill Exercise

ECG patterns of different elctrodes (aVR, aVL and AVF is not included for stress testing)


The purpose of this test is to evaluate the potential CAD patient in a controlled setting by recreating symptoms the patient experiences on exertion. In treadmill testing, a certain level of exercise must be reached for adequate evaluation of the patient. This level is usually 85% of the patient’s maximum heart rate.

Example: Patient is 40 years old.

220 – 40 = 180 bpm x 0.85 = 153 bpm

The 153 bpm can be seen at the monitor of the Treadmill Machine. It is at this point where the radiopharmaceutical will be injected.

Types of Exercise Protocol

(1)   Bruce Protocol

In this protocol, speed and elevation of the treadmill increases every 3 minutes. These stages, as they are called represent an in increase in the patient’s workload, thereby increasing the patient’s heart rate.

(2)   Modified Bruce Protocol

This is a variation of the Bruce Protocol in which the elevation increases slightly every 3 minutes, but the speed remains at a slow pace. This reserved for patients with very limited exercise capacity (obese or senile or female patients).

b.     Pharmacologic Stress Inducing Agent

Several patients may not be able to perform the conventional treadmill exercise because of physical limitations (such as those with amputated legs) or poor motivation. This patient can be “stressed” pharmacologically.

Types of Pharmacologic Stress Inducing Agents

(1)   Persantin (Dipyridamole)

Mode of action:

Persantin increases the level of exogenous adenosine in the patient’s blood, which acts as powerful vasodilator. This vasodilation creates a “myocardial steal” phenomenon that diverts blood away from the myocardium that is served by coronary arteries that may have significant stenosis

Dosage:

0.56 mg/kg body weight diluted to 50 cc saline

Antidote:       Aminophylline

Contraindication:

Patients with history of bronchospasm, pulmonary disease, prior intubation, systemic hypotension (BP systolic <90 mmHg), severe mitral valve disease

(2)   Adenosine

Mode of action:

It activates the A2 receptors on the cell membrane of smooth muscle.

Dosage:

200 µg/kg body weight

Antidote:       Aminophylline

Contraindication:

Patient with advanced (2nd and 3rd degree) atrioventricular block or sick sinus syndrome

(3)   Dobutamine

Mode of action:

It acts as positive inotropic agent (a substance that affects the force of myocardial contraction) that stimulates the myocardium’s beta–1 receptors. This stimulation increases the force and frequency of the contraction.

Dosage:

Initial infusion starts with 5 µg/kg/min for 3 minutes.

Stepped increases follow at 5 minutes intervals to infusion rates of 10, 20, 30 & 40 µg/kg/min.

Antidote:       Propanolol

Contraindication:

Unstable angina, obstructive or hypertrophic myopathy

c.       Combined modified Bruce and Persantin Stress Testing

This is a combination of a low–level exercise and dipyridamole infusion which shows decrease side effects, increase target–to–background ratios and enhanced ECG effects of dipyridamole.

Procedure of the test

(1)   Dipyridamole is prepared in its usual concentration

(2)   Dipyridamole administration begins when the patient begins walking and continues for the full 4 minutes.

(3)   The patient continues walking during the second stage.

(4)   The radiopharmaceutical is injected at the end of the second stage.

(5)   The patient continues walking for 1 minute into the third stage at which time exercise is terminated

(6)   Aminophylline may be administered after dipyridamole infusion without affecting subsequent images.

2.      Types of radiopharmaceutical used

a.      Thallium–201 or Thallous Chloride

Thallium is an analog of the element, potassium, which is used by the myocardium during contractions. Thallium does not collect in the myocardium permanently but is constantly pumped in and out of the myocardial cells.

For coronary blockages, because of the restrictions of blood flow, Thallium cannot enter the myocardium to the same extent as when blood flow is unrestricted. This condition is intensified during exercise, when the myocardium’s demand for oxygen is greater.

Since the amount of Thallium uptake is based on the amount of blood flow to that area of the myocardium, ischemic regions will concentrate less Thallium and appear to as areas of decreased radioactivity on the images.

Click here for poisonous form of Thallium

Dosage:    2 – 4 mCi

Advantages:

(1)   Short waiting period for the patient prior to imaging

(2)   Long half life of Thallium (72 hours) permits longer delayed imaging

Disadvantages:

(1)   High radiation exposure for the patient

(2)   Attenuation defects found in Thallium makes it less reliable for obese patients and woman with large breast.

(3)   24 hours delayed imaging is sometimes inconvenient for the patient.

b.     sestaMIBI or Cardiolite

sestaMIBI, where MIBI stands for “methoxy–isobutyl–isonitrile” and “sesta” stands for six number of  MIBI compound attached to Technetium. When diluted with technetium, Tc99m–sestaMIBI provides a very accurate diagnosis of a potential CAD patient. Unlike Thallium, use of sestaMIBI requires protocol depending on the one utilized by the department.

Types of protocol

(1)   Two–day Stress–Rest Protocol – patient is stressed as in any other stress testing protocol. If a defect is seen on the initial stress image or if ECG clinical changes occur during the stress procedure, rest study will be advised the following day.

Dosage: 20 to 30 mCi (rest and stress)

(2)   One–day Stress–Rest Protocol – stress procedure is done using 10 mCi sestaMIBI, imaging follows after 45–90 minutes. Reinjection of 25–30 mCi sestamIBI is done 3–4 hours later and imaging proceeds after 60– 90 minutes.

(3)   One–day Rest–Stress Protocol – this the same as one day Stress–Rest Protocol except that the sequence is reversed.

Advantages of sestaMIBI

(1)   The availability of Technetium makes it possible to be done on a random basis.

(2)   Less radiation for the patient.

(3)   Suitable for obese patients and women with large breast.

Disadvantage of sestaMIBI

(1)   Long waiting period for the patient prior to imaging. Much of the activity not collected in the myocardium will be collected in the hepatobiliary system. A fatty meal will resolve this problem thus imaging proceeds after 45 – 90 minutes

(2)   The short half–life of Technetium demands for further reinjection of the isotope.

c.       Dual isotope procedure

This is a combination of Thallium and sestaMIBI and has shown a high sensitivity for the detection of coronary stenosis with much shorter time in the department for the patient as compared to that for single tracer studies using Thallium or sestaMIBI.

Procedure of the test

(1)   The patient at rest is injected with 3 mCi Thallium.

(2)   SPECT imaging follows after 10–30 minutes.

(3)   After rest imaging, the patient is prepared for stress testing.

(4)   The patient undergoes stress testing and 20–30 mCi sestaMIBI are administered at the peak of exercise.

(5)   Imaging occur 60–90 minutes post–injection.

d.     Other radiopharmaceuticals used:

(1)   Tc99m Tetrofosmin

(2)   Tc99m Teboroxime

(3)   Rubidium–82 (50 mCi) – for PET scanners
 

INFARCT–AVID IMAGING

1.      Myocardial infarction begins with the denial of blood to the myocardium by a blocked coronary artery or by blunt chest trauma that injures part of the heart.

2.      As myocardial cells begin to die, tissue necrosis begins. During the cell necrosis, the cellular mitochondria take in phosphates while the cell releases large amounts of macromolecules in the form of proteins.

3.      The action of mitochondria and the high binding affinity of polyphosphates to proteins made Tc99m–pyrophosphate one of the first clinically useful myocardial imaging agents.

4.      In most cases, this type of imaging is performed on patients for whom other data such as the ECG or cardiac enzymes are inconclusive.

Procedure of the test

1.      Administer 15–25 mCi Tc99m pyrophosphate intravenously

2.      As with any bone agent, encourage the patient to drink fluids and to void frequently

3.      Image 2–3 hours following tracer administration. Obtain 1 million counts

4.      Increased tracer uptake in the region of the myocardium is abnormal. When interpreting the images, the physician typically compares the activity in the cardiac areas to bone activity to grade the severity of the infarct.

GATED EQUILIBRIUM RADIONUCLIDE VENTRICULOGRAPHY

 Gated equilibrium radionuclide ventriculography (RVG) is a procedure in which the patient’s blood is radiolabeled and ECG–gated cardiac scintigraphy is obtained. An RVG may be acquired at rest, during exercise or following either pharmacologic or mechanical intervention.

Synonyms of RVG:

1.      Gated cardiac–blood pool imaging

2.      Multigated Acquisition (MUGA)

3.      Gated equilibrium Radionuclide Angiography

Parameters obtained from RVG includes:

1.      Global ventricular systolic function

2.      Regional wall motion

3.      Ventricular volume (qualitative and quantitative)

4.      Responses of above parameters to exercise or intervention

5.      Systolic and diastolic indices

6.      Stroke volume ratios

Benefits obtained from RVG:

1.      In determining long–term prognosis

2.      Assessment of short–term risk (pre–operative evaluation)

3.      Monitoring the response to surgery or other therapeutic intervention

Clinical indication of RVG:

1.      Known or suspected Coronary Artery Disease (CAD)

a.      CAD without myocardial infarction

b.      Remote myocardial infarction

c.       Acute myocardial infarction

2.      Known or suspected Congestive Heart Failure

a.      To distinguish ischemic from non–ischemic causes

b.      To distinguish systolic from diastolic causes

3.      Evaluation of cardiac function in patients undergoing chemotherapy

4.      Assessment of ventricular function in patients with valvular stenosis and/or insufficiency

Acquisition parameters

1.      List mode acquisition is useful for making a composite cardiac cycle from a heterogenous population of beats.

2.      16 frames per R–R interval are required for accurate assessment of ventricular wall motion and assessment of ejection fraction.

3.      32–64 frame per R–R interval is necessary for detailed measurement of diastolic filling parameters

4.      Typical acquisition are for a total of 3–7 million

Sources of error:

1.      Heparin to chronic renal will decrease labelling efficiency and reduce the target–to–background ratios.

2.      Non–separation of left ventricle from other cardiac structures will cause inaccuracy in ejection fraction

3.      QRS complex should the triggering signal

4.      Inadequate counts acquisition

5.      Heart rate variability

6.      Inclusion of non–ventricular activity or exclusion of ventricular activity from ventricular ROI may cause underestimation or overestimation of ejection fraction

7.      Inclusion of structures such as the spleen or descending aorta in background may alter left ventricular ejection fraction.






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