Types of Stress Tests

Stress echocardiography is effective in demonstrating the effects of exercise and regional wall motion in addition to valvular incompetence. Four views are routinely evaluated, parasternal long, parasternal short, apical four-chamber and apical two-chamber with rest and exercise images juxtaposed. New or significant wall-motion abnormalities suggest myocardial ischemia. In this technique, images are compared at rest and within 1-2 minutes of exercise. This can provide helpful information on both the location and the amount of myocardial tissue in jeopardy, as well as evaluating left ventricular and cardiac functioning (16).

Nuclear perfusion testing and radionucleotide imaging assess myocardial perfusion and are performed with a variety of different tracers and various techniques. Most laboratories currently use single-photon emission computed tomography (SPECT), which can reconstruct anatomic slices of myocardial tissue. This technique is invaluable in assessing regional cardiac blood flow. As with echocardiography, images are produced following exercise or pharmacological intervention and compared with resting images. The pharmacological agents used work in different ways.

Adenosine and dipyridamole increase blood flow to nonarteriosclerotic vessels, unmasking stenoses in other arteries with their vasodilatory action. These agents should not be used if an individual has asthma or severe obstructive lung disease, because bronchospasm can be worsened.

Dobutamine is a positive inotropic agent, inducing ischemia by increasing cardiac workload. This agent should be avoided in individuals with ventricular or atrial arrythmias because it can accelerate ventricular and atrial ectopys and heart rate.

Two major types of radionucleotide isotopes are normally used to assess myocardial perfusion and viability: thallium 201 and Tc 99m (sestamibi, teboroxime, or tetrofos-min). Thallium redistributes to areas of ischemia quickly, whereas sestamibi permits imaging several areas after injection. The SPECT technique is superior to planar imaging. Reversible defects are evidence of myocardial ischemia, whereas fixed defects represent previous infarction and areas of old scarring (15).

Occasionally, apparent perfusion defects can be caused by artifact (breast tissue or diaphragmatic attenuation). This defect commonly occurs in only one view in the inferior or apical area because of imaging defects between the heart and the scanning camera.

Exercise and pharmacological stress testing demonstrate equal sensitivities with both nuclear and stress echocardiography for detecting coronary artery disease and ischemia; the echocardiogram evaluates wall motion, whereas the isotopic scan evaluates perfusion.

It is important to understand and determine the patient's tolerance for exercise by these stress-testing techniques before advising a patient on his capabilities. Stress testing can also provide a valuable insight into the patient's hypertensive blood pressure response, heart rate variability, and recovery times, which can be critical in advising the patients on their maximum duration of exercise.

Any abnormalities in perfusion should be treated aggressively in the diabetic patient, who is already at significant risk for a myocardial event. Reversible defects on thallium imaging, significant ST-segment depressions, and hypomotility on stress echocardiography should warrant evaluation that is more aggressive in the patient with diabetes.

In some patients with diabetes, the presence of electrocardiographic abnormalities makes an exercise ECG inappropriate. Radionucleotide or echocardiography imaging technology would be the test of choice when the amount and the location of jeopardized myocardium is an important consideration (18).

In general, imaging is not only superior to and provides more clinical information than exercise ECG alone, but can improve outcomes by identifying patients at risk with comparable cost effectiveness. Stress echocardiography has higher specificity, but radionucleotide imaging has greater sensitivity, with stress echocardiography being more cost-effective.

The Duke treadmill score can provide an additional method of evaluating the patient that has received an exercise ECG. The treadmill score is the exercise time minus (5 x ST elevation in mm) minus (4 x exercise angina); where 0 = no angina, 1 = nonlimiting angina, and 2 = exercise-limiting angina. Low risk is a score of 5 or higher, moderate risk is -10 to +4; and high risk is -11 or lower.

The indications for radionucleotide perfusion or echocardiographic imaging and not exercise electrocardiography are:

1. Complete left-bundle branch block.

2. Paced ventricular rhythm.

3. Wolff-Parkinson-White pre-excitation syndrome or other conduction abnormalities.

4. Patients with greater than 1 mm of ST-segment depression at rest.

5. Patients with angina who have undergone bypass surgery or stenting, in whom ischemic localization, myocardial viability, or severity of obstructive lesions is desired.

The standard echocardiogram can give the clinician invaluable information in managing the diabetic and the hypertensive patient. Quantification of valvular regurgitation and assessment of valvular stenosis and incompetence can be of value in determining appropriate therapy for hypertension (19).

Assessment of diastolic function by measuring the isovolumetric relaxation time, the deceleration time, the E/A ratios, and the transmitral gradients can help determine the degree of diastolic impairment and serve as a baseline for therapeutic endeavors. These are defined as follows:

1. The isovolumetric relaxation time is the time interval between closure of the aortic valve and the opening of the mitral valve.

Table 2

General Guidelines for Exercise Training in Diabetes

1. Warm up and cool down of 5-10 min each.

• Judicious use of stretching, calisthenics, and low-level aerobic exercise, such as walking or cycling.

2. Types of exercise

• Aerobic: walking, cycling, swimming, and rowing.

• Resistance: weight lifting with machines rather than free weights for safety and ease of use.

3. Intensity

• Aerobic exercise at 55-79% of maximum heart rate for those with multiple risk factors and 55-60% of maximum heart rate for those with low level of fitness.

Resistance training: 8-10 exercises at 30-50%, with one repetition maximum. Minimum of one set of 12-15 repetitions, with workload increasing when 15 repetitions can be done without difficulty.

4. Duration

• Aerobic exercise for 30-45 min

(e.g., treadmill walking at the desired target heart rate for 30 min).

• Resistance training: 20 min for each set of exercises.

5. Frequency

• Aerobic exercise should be done three to four times a week.

• Resistance exercise should be done at least twice a week.

N.B. Exercise stress testing should be performed before any patient embarks on a structured program of exercise.

2. The deceleration time represents the interval between the peak of the E-wave and the return of early diastolic flow velocity to base line.

3. The E-wave represents the rapid filling phase of left ventricular diastole. The A-wave represents atrial systole.

4. The transmittal gradient is the gradient across the mitral valve.

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