As previously described in greater detail (in chapter 2), auto-regulation is “the ability of an organ to maintain a constant blood flow despite changes in perfusion pressure”. The ratio between resting and maximal possible coronary blood flow is the coronary flow reserve (CFR). CFR can be thought of as the capacity of the coronary circulation to dilate and thus increase flow following an increase in myocardial metabolic demands.
Myocardial blood flow can be quantitated using off-line software in which the components of myocardial blood flow (namely myocardial blood volume and mircobubble velocity) can be calculated. Using the equation given above in the section on assessment of ischaemia, the coronary blood flow can be calculated at rest and following exercise or pharmacological stress.
The ratio of CFR at stress: CFR at rest is the coronary flow reserve.
Wei et al first established this technique in an animal model  and it has since been replicated by other authors. Vogel et al proved that MCE-derived CFR is very similar to that derived from Positron Emission Tomography (PET) . Changes in CFR can assess both the presence  and severity  of coronary artery disease and can also be used in various other conditions (e.g. to determine if a patient presenting for the first time with heart failure has an ischaemic or non-ischaemic aetiology  or to determine prognosis in heart failure patients ).
The use of contrast agents in echocardiography for assessment of LV structure and function – both at rest and during stress – has a large and robust evidence base, including large prospective clinical trials. Myocardial contrast echocardiography (MCE) for assessment of myocardial perfusion and viability has considerable evidence to support its incorporation into a daily clinical service. However, MCE requires considerable technical expertise. Contrast agents can add significantly to the diagnostic information and overall accuracy of the workflow through a clinical echocardiography department.
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