Tissue harmonic imaging is indicated in stress echocardiography. It improves resolution, reduces artifacts, improves signal-to-noise ratio, greater depth of penetration and improves myocardial signals. Tissue harmonic imaging increases the sensitivity of stress echocardiography as it improves endocardial delineation due to enhanced resolution.[7][11][12][13]

Intravenous contrast agents in combination with harmonic imaging increase the accuracy of the procedure. Increased number of wall segment motion abnormalities can be studied when contrast agents are used.

Stress testing methods:

Exercise stress testing: This is indicated in patients who can exercise. Either treadmill or bicycle can be used for exercise. 

• Treadmill stress echocardiography: Bruce protocol is most commonly used for treadmill exercise echocardiography. Exercise capacity is reported in terms of estimated metabolic equivalents of task (METs). Imaging studies are performed at rest and immediately after completion of exercise and compared for evaluation of wall motion abnormalities.  
• Bicycle stress echocardiography: Less often used in North America than treadmill stress test. Can be performed using supine or upright bicycle ergometry. Usually, bicycle protocol starts at a workload of 25 watts and increases by 25-watt increments every three minutes until an endpoint is achieved. It is quieter, permits sensitive precordial measurements without much motion artifact. Bicycle exercise offers Doppler information in addition to assessment of regional wall motion abnormalities.

Pharmacologic stress testing: This is indicated in patients who cannot exercise. Dobutamine and vasodilators are used for assessing regional wall motion abnormalities. [10], [14]

• Dobutamine echocardiography: A graded dobutamine infusion starting at 5 mcg/kg/min and increasing at three-minute intervals to 10, 20, 30, and 40 ug/kg/min is the standard for dobutamine stress testing. [8] During dobutamine echocardiography, images are obtained before the start of the infusion, at the end of each stage, and during the recovery. The importance of low dose stages plays a role in recognition of viability and ischemia in segments with abnormal function at rest, even if viability assessment is not the main objective of the test. Endpoints are the achievement of target heart rate (defined as 85% of the age-predicted maximum heart rate), new or worsening wall motion abnormalities of moderate degree, significant arrhythmias, and intolerable symptoms. To achieve the target heart rate, Atropine can be used in divided doses of 0.25 mg to 0.5 mg to a total of 2 mg. Atropine increases the sensitivity of dobutamine echocardiography in patients receiving beta-blockers and in those with single-vessel disease.
• Vasodilator stress testing: Adenosine and Dipyridamole can be used in stress echocardiography. [15] Dipyridamole is administered at up to 0.84 mg/kg in two separate infusions: 0.56 mg/kg over four minutes, followed by four minutes of no dose and, if an adequate physiologic response is not achieved, then an additional 0.28 mg/kg is given over two minutes. [16] Atropine is used to enhance test sensitivity. Also, the addition of a handgrip at peak infusion enhances sensitivity. The duration of action of adenosine is shorter than dipyridamole. Adenosine stress is used to assess myocardial perfusion with contrast echocardiography but has not been used widely as a clinical tool.
• Pacing stress testing: This is indicated in patients with a permanent pacemaker. The target heart rate can be achieved by increasing the pacing. This is indicated in patients who are unable to exercise. The pacing protocol consists of two-minute stages, and an increasing paced heart rate to levels of 85% and 100%, respectively, for pre-peak and peak stress information. [10] Images are obtained at rest, the first stage, and pre-peak and peak heart rate. Termination of stress test occurs with the achievement of age-predicted maximal heart rate, new or worsening moderate regional wall motion abnormalities, greater than 2 mm horizontal or downsloping S-T depression, or presence of intolerable symptoms. [10]

Interpretation:

Imaging obtained at resting and stress phase are compared for interpretation of left ventricular (LV) size, shape, and function. A normal response during stress involves LV size becoming smaller compared to rest, while the shape is maintained, and there is increased endocardial excursion along with systolic wall thickening.

In a patient with multiple vessel diseases, exercise echocardiography demonstrates a dilated LV cavity with changes in the shape and reduction of systolic wall thickening of the septum, anterior, and inferior walls. Prolonged systolic wall thickening may also indicate severe coronary artery disease.

The coronary arteriographic cut off of luminal diameter stenosis at which wall thickening abnormalities occur is 54% for exercise, 58% for dobutamine, and 60% for dipyridamole. The sensitivities for the detection of coronary artery disease (CAD) are 85%, 80%, and 78%, with specificities of 77%, 86%, and 91% for exercise, dobutamine, and dipyridamole stress results. [9] However, diagnostic accuracy varies according to the pretest likelihood of CAD in the patient tested.

Dobutamine stress echocardiography: The most common cardiovascular side effects associated with dobutamine are angina, hypotension, and cardiac arrhythmias. [17] Atrial fibrillation and nonsustained ventricular arrhythmias occur in about 3% of patients. [18], [19] Sustained ventricular tachycardia is not common. Dobutamine can also induce left ventricular mid-cavity and outflow tract obstruction. Frequent premature atrial or ventricular contractions occur in about 10%. [20]

Vasodilator stress echocardiography: Major adverse reactions include myocardial infarction, asystole, and ventricular tachycardia. [21] Hypotension and bradycardia may occur but can be treated with aminophylline.

Pacing stress echocardiography: Wenckebach's second-degree heart block may occur, requiring atropine administration. [22]

In terms of clinical significance and diagnostic accuracy, stress echocardiography has an advantage in terms of specificity over standard exercise electrocardiography. When compared to nuclear perfusion imaging studies, stress echocardiography has similar accuracy, with a moderate sensitivity gap that is well balanced by a higher specificity. Both dipyridamole and dobutamine have overall good tolerance and feasibility. The choice of one test over the other depends on patients' clinical characteristics and the physician's preference. Diagnostic accuracy can significantly be affected by antianginal medical therapy, particularly beta-blocking agents, and therefore, it is recommended to withhold medical therapy at the time of testing to avoid a false-negative result.

Healthcare workers, including nurse practitioners, will frequently encounter patients with heart disease. One of the ways to investigate these patients is with stress echocardiography. Stress echocardiography is the combination of two-dimensional echocardiography with a physical, pharmacological, or less commonly, electrical stress with atrial pacing. Stress-induced ischemia generates new or worsening wall motion abnormalities in the segment supplied by the stenosed coronary artery. Stress echocardiography plays an important role in identifying these wall motion abnormalities in the assessment of ischemic heart disease, and also plays a vital role in the evaluation of systolic or diastolic heart failure, valvular pathologies, nonischemic cardiomyopathy, pulmonary hypertension, and congenital heart disease.