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 post-exercise stress, delaying the ability to gather peak exercise data, which is critical in accurately assessing the LV and seeking out any abnormalities.
To compare the heart wall motion in pre-exercise stress testing with peak exercise rates, it is important to acquire two different views of the left ventricle (LV) in a biplane (BP) mode simultaneously in one acoustic window instead of viewing the entire 3-D volume.
Live x Plane Enables Faster Imaging
The introduction of the matrix-array transducer has made it possible to perform Live xPlane imaging. The transducer displays two high-resolution, transthoracic views of the heart simultaneously in realtime for faster image acquisition, allowing for peak exercise heart rates.
Live imaging provides orthogonal views from the same heartbeat. The left image serves as a baseline reference while the right image can be electronically rotated to any angle between 0├é┼ş and 180├é┼ş in 5├é┼ş increments. Because images are acquired from one heartbeat, image views can be recorded more rapidly and closer to peak stress for more sensitive detection of wall motion abnormalities (WMA). Live xPlane imaging also provides the opportunity to monitor between dobutamine stress stages for earlier detection of WMAs.
Clinical Study Test Results
During two separate stress echo studies performed by the author and colleagues at University of Chicago Hospitals,1 subjects underwent two exercise treadmill tests. Scans were completed using a Philips broadband S3 transducer and matrix-array x4 transducer. Once subjects reached 85% of their age-predicted heart rates, exercise was terminated and images were acquired in traditional 2-D and BP formats. Two-dimensional rest images were obtained from the apical four-, two-, and three-chamber, and parasternal longand short-axis views using an S3 probe. Following exercise, subjects were repositioned in the left lateral decubitus position as quickly as possible to capture the same five views as during rest, and images were digitally stored. For each view, heart rate and duration of time for image acquisition were recorded.
Using the BP probe, it was possible to visualise two imaging planes simultaneously in a split-screen display and capturing each of the five views following completion of exercise. Results of the studies showed the total acquisition time for 2-D stress echo was 38 +/- 8 seconds compared to 29 +/- 8 seconds for BP imaging. Heart rates were acquired closer to age-predicted maximum with BP imaging in the apical 3- and 2-chamber and parasternal long and short axis views.
The study demonstrated that post-stress image acquisition time was faster by 10 +/- 7 seconds using Live xPlane imaging. Assuming a five-second delay for subjects to move from the treadmill to the imaging position, the average 2-D image acquisition time was 43 seconds.As a result of Live xPlane imaging, it was possible to reduce time between peak exercise and image acquisition, (acquire images closer to peak stress) for more sensitive detection of wall motion abnormalities. This is crucial in overcoming the challenges associated with diagnosing myocardial ischaemia that may resolve quickly following exercise.
Figure 4 Time Duration of Image Acquisition for Individual Views
As a result of this technology, the sensitivities of stress echo are reduced, giving clinicians the ability to more accurately diagnose coronary artery disease and the potential to save millions of lives by starting treatment for the disease earlier.
- Biplane Stress Echocardiography Using a Prototype Matrix-array Transducer, Journal of the American Society of Echocardiography, (Sept. 2003).