Indications for stress echocardiography

‘Stress echocardiography’ refers to a transthoracic echo examination performed during or immediately after exercise or pharmacological stress. The most common application of stress echocardiography is in detecting myocardial ischaemia.

 

Patients with coronary artery disease who have not suffered a myocardial infarction will exhibit normal contractile function at rest. With stress, however, regional wall motion abnormalities will develop as the stenosed artery is unable to maintain adequate blood perfusion (or, if the region is being fed by collateral flow, the donor artery is no longer able to feed the collateral vessels). The patient may also exhibit symptoms of ischaemia.

 

Stress echo can also be used to assess for myocardial viability after an infarction. Low dose dobutamine or gentle exercise stress can augment blood flow and therefore contractility of viable segments which may be abnormal at rest (which will once again exhibit wall motion abnormalities at higher levels of stress – this is termed the biphasic response). These segments are termed ‘hibernating myocardium,’ and contractile function can be restored with revascularisation. If contractile function fails to improve with low stress, revascularisation is unlikely to deliver any benefit to the patient.

 

Stress echo is more sensitive to ischaemia than ECG or stress ECG. This is because regional wall motion abnormalities occur earlier in the ‘ischaemic cascade’ than electrocardiogram changes or even chest pain.

 

Whilst exercise is the most appropriate test for achieving peak stress, it presents additional imaging challenges (such as rapid breathing), which can result in poor image quality and interobserver variability. Treadmill exercise is even more challenging than supine bicycle exercise, because of the danger that transient ischaemia will resolve in the time it takes for the patient to cease exercising, come to the bed and the echocardiographer to obtain the images. In addition, many patients are unable to exercise or can only exercise suboptimally. For these reasons, pharmacological stress – particularly dobutamine in Europe (often with the addition of atropine or hand grip exercises to achieve target heart rate) – is usually the favoured methodology, invariably in conjunction with tissue harmonic imaging and a microbubble contrast agent to improve endocardial border definition. It is important, however, for a variety of types of stress echo to be available, particularly given that different methods have different contraindications (dobutamine versus dipyridamole versus exercise). High-end ultrasound systems will have a contrast imaging mode, utilising a lower mechanical index to avoid bubble destruction.

 

An even more sensitive test than the detection of wall motion abnormalities is the detection of perfusion defects. This can also be performed with echocardiography, termed ‘myocardial contrast echocardiography’ (MCE). An absence of perfusion will show as a dark region in the myocardium. Delayed perfusion can also be observed using high power (high MI) burst of ultrasound, followed by much lower output powers, in order to observe the time taken for reperfusion to occur.

 

Other recognised uses of stress echocardiography are for provoking a gradient across a stenotic aortic valve in low flow states. If dobutamine results in an increased gradient across the aortic valve, the patient is likely to benefit from valve surgery. Patients without contractile reserve and no increase in gradient are unlikely to benefit (Sicari et al., 2008).

 

Another application of stress echocardiography is in establishing the severity of mitral stenosis, particularly where symptoms do not match the observed severity of the stenosis. For example, a patient with a moderate-severe stenosis may claim to be unsymptomatic because they are (perhaps inadvertently) making changes to their lifestyle to accommodate their condition. In such an instance, a significant increase in the gradient across the mitral valve with exercise or dobutamine stress, as well as an elevation of pulmonary pressures (measured from the tricuspid regurgitant jet), would indicate a more severe level of stenosis which may require valve repair or replacement.

 

It is sometimes appropriate to assess contractile reserve in patients with regurgitant lesions, for similar reasons to those discussed above (a mismatch between apparent severity and symptoms). A lack of contractile reserve and a rise in pulmonary artery systolic pressure >60mmHg during exercise would indicate the presence of LV dysfunction (Sicari et al., 2008).

 

Dynamic mitral regurgitation is another example. In this instance, exercise stress is the most appropriate. In functional ischaemic mitral regurgitation, MR severity may actually reduce with exercise, due to an increase in closing forces. A failure to improve or a worsening of severity is a poor prognostic sign.

 

Stress echocardiography may also be indicated in high risk patients undergoing non-cardiac surgery.