Myocardial ischaemia is a coronary artery disease typically caused by critical coronary artery obstruction and is the leading cause of death worldwide. Diagnosing myocardial ischaemia prior to heart attack is important for a patient's long-term prognosis. It is anticipated that more than one million Americans will have a heart attack this year as a result of myocardial ischaemia, with approximately 500,000 of those being fatal, making early detection and treatment of coronary artery disease critical.
One of the leading tests used to diagnose myocardial ischaemia is an exercise stress echocardiography to visualise the electrocardiogram of the left ventricle (LV) regional function. For this test to be maximised diagnostically, it is important that echo images be obtained at the highest possible heart rate.
The objective of the test is to increase the patient's heart rate by having the patient walk on a treadmill to increase myocardial oxygen consumption of the heart. This will induce ischaemia in patients with coronary artery stenosis. In patients who develop myocardial ischaemia, an electrocardiogram (ECG) will note development of any wall motion abnormality. If tests are performed correctly by achieving the right heart rate, and image quality is high, exercise stress echo tests have a high accuracy for detection of coronary artery disease in patients suspected of having the disease. Stress echo images are acquired from several acoustic windows. Clinicians compare resting with exercise heart rates, noting any post-exercise abnormalities such as volume of the left ventricle, regional wall motion and wall thickening whereas these areas appeared normal in pre-exercise exams.
In order to detect stress-induced wall abnormalities, peak-stress images must be acquired to obtain an accurate view of the wall motion. Peak stress is the time during which a patient™s heart rate is as close to 85% of maximum age-predicted heart rate as possible, taking into consideration time involved moving the patient and performing the heart scan. Diagnosis of myocardial ischaemia is made by observation of wall motion abnormalities at baseline or developing with exercise or dobutamine stress. The diagnostic accuracy of stress echo requires the development of ischaemia to induce regional wall motion abnormalities, which can only be detected during peak stress mode. However, acquiring images during peak stress is challenging since there is a limited opportunity in which to view wall motion abnormalities.
Challenges Associated with Obtaining Accurate Exercise Stress Echo Test Results
A crucial part of obtaining accurate stress echo results during peak stress is the ability to acquire images as quickly as possible following the cessation of exercise. It is during this peak stress mode that clinicians have the greatest ability to accurately assess any stress-induced abnormalities. This window of opportunity required to capture images during peak stress is limited because myocardial ischaemia may correct itself after a short period of time. If the target heart rate is not achieved and if image acquisition is not captured during peak stress, false-negative results can be obtained. Abnormalities identified during the peak stress stage may point to coronary artery disease, while images obtained during the post-exercise stress stage may not show any abnormalities.
While realtime 3-D echo has been used to acquire post-stress data of the entire LV, image quality and frame rates can be inferior. To visualise the entire LV myocardium, stress echo images are acquired from several acoustic windows. While these multiple views allow clinicians to view all coronary vascular distributions, the additional imaging adds time to postexercise stress, delaying the ability to gather peak exercise data, which is critical in accurately assessing the LV and seeking out any abnormalities.
- ├óÔé¼┼øBiplane Stress Echocardiography Using a Prototype Matrix-array Transducer├óÔé¼┼Ñ Journal of the American Society of Echocardiography, (Sept. 2003).