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Noninvasive Measurement of Coronary
Flow Reserve by Transthoracic
Paolo Voci, MD, PhD; Francesco Pizzuto, MD
Section of Cardiology II, "La Sapienza" University of Rome, Italy
Transthoracic Doppler echocardiographic imaging of coronary flow provides a revolutionary approach in the noninvasive diagnosis of coronary artery disease since alteration of flow, rather than the consequences of flow reduction on heart function and metabolism, are directly evaluated. By this technique, coronary flow velocity reserve (CFR) can be measured in the left anterior descending (LAD) coronary artery during adenosine or dipyridamole infusion.
Transthoracic Doppler echocardiography can be used to screen patients for coronary artery disease and may be particularly useful to follow patients with LAD stent implantation, since symptoms are often atypical and other noninvasive diagnostic tests may be inconclusive.
Coronary Doppler echocardiography is an easy, bedside technique best performed by a small multiHertz transducer allowing independent change of frequency between two-dimensional (3.5-7.0 MHz) and color Doppler (3.5 to 6.0 MHz). To image the middle-distal LAD, the transducer is placed either at the cardiac apex or one intercostal space above, along the interventricular groove, and focused on the proximal field. Once an optimal two-dimensional image is obtained, the transducer is rotated and tilted until one coronary segment is visualized in color Doppler. The Nyquist limit is reduced to 110-170 mm/s for 3.5 MHz, 120-190 mm/s for 5 MHz, and 130-200 mm/s for 6 MHz.
Coronary flow velocity is measured by pulsed Doppler under color coding guide at baseline and during short adenosine infusion (140 mcg/kg/min/90s) and CFR is measured as the ratio between hyperemic and baseline peak or mean flow velocity.
FEASIBILITY OF CORONARY FLOW IMAGING
Technically adequate Doppler flow velocity tracings of the distal LAD are obtained in 98% of the cases. The time required to perform the test is 12±5 min. and large body habits does not necessarily affect the feasibility of the study. Adenosine-induced hyperventilation is the main factor affecting flow velocity recordings, but it never prevents completion of the test.
Maximal increase in coronary flow velocity is obtained within 60s of drug infusion, and flow returns to baseline within 30s of discontinuing the drug. No major adverse reactions occur during or after adenosine infusion. Flushing, bradycardia, nausea, chest pain, headache are rare, mild and transient. However, most patients have some degree of hyperventilation (which may be marked in 10%) but rapidly disappear at the end of the infusion. Adenosine is an expensive, but very flexible drug allowing to repeat the test several times in few minutes. Dipyridamole is an alternative drug. It is cheaper, but the infusion lasts longer and can be performed only once, since aminophilline should be administered to reverse the side effects of the drug at the end of the infusion.
The technique is suited to study the physiologic impact of any disease in the LAD territory. Restenosis after stenting is an ideal model to study flow changes in this artery. Restenosis still remains a significant problem in patients with proximal-middle LAD stent. Angiography is the method of choice to detect restenosis, but should be limited to patients with proven evidence of ischemia. There have been conflicting results in the literature about the relative merit of different diagnostic techniques to detect restenosis, and a recent meta-analysis showed that exercise electrocardiography is poorly diagnostic, while scintigraphy and stress echocardiography perform better.
Recently, contrast-enhanced electron beam computed tomography and phase-contrast MRI have been used to noninvasively image the coronary arteries, but they are expensive, have been applied only to limited series of patients, and have a suboptimal feasibility. CT and MRI measure luminal narrowing, which is a rough estimator of stent dysfunction, and provide little information about functional changes in flow.
Coronary Doppler echocardiography has the advantage of directly measuring flow in the target vessel (stented LAD), overcoming the interpretation problems of other diagnostic techniques as scintigraphy and stress echocardiography, and may also differentiate restenosis subgroups, including subocclusion. A CFR value of >2.5, measured by transthoracic Doppler echocardiography, virtually excludes a flow-limiting stenosis. This is in keeping with the results of the DEBATE study, which identified a cut-off value of 2.5 to predict coronary events after angioplasty. A CFR <2 is found in patients with significant LAD stenosis or in-stent restenosis. This is in agreement with other invasive and noninvasive studies, mainly with SPECT, which established a cut-off value of 2.0 for "normal" flow reserve. Although SPECT is an imperfect method to assess the hemodynamic significance of coronary artery stenosis, 2.0 is a clear-cut value for decision making, and has been adopted as "the gold number" to detect significant stenosis in daily clinical practice.
Despite the LAD is the most important artery of the heart (widow maker), the assessment of flow reserve in other coronary arteries is desirable. A recent preliminary report from our group shows that imaging of the posterior descending artery is feasible in around 50% of the patients. However, technical improvements are needed to obtain the same success rate of LAD imaging.
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