10 Evaluation of Coronary Blood Flow by Echo Doppler

Nozomi Watanabe

Background

Anatomy (Fig. 10-1)

The coronary arteries provide arterial blood to the myocardium. Transthoracic Doppler echocardiography allows us to detect coronary blood flow in the epicardial coronary arteries, septal branches, and perforator branches.

• The left main coronary artery originates from the left coronary sinus of the aortic root, and then bifurcates into the left anterior descending (LAD) and the left circumflex (LCx) arteries.

• The LAD runs over the anterior interventricular sulcus and extends apically, ending distal to the apex.

• The LCx is covered by the left atrial appendage in its proximal portion, and then takes position in the left atrioventricular sulcus.

• The right coronary artery (RCA) originates from the right coronary sinus of the aortic root, and goes into the right atrioventricular sulcus. The posterior descending artery (PDA) runs into the posterior interventricular sulcus.

• Commonly, Doppler echocardiography can detect:

• the left main coronary and its bifurcation (proximal LAD and LCx),

• middle to distal segments of the LAD,

• the proximal segment of the RCA,

• the PDA, and

• the distal segment of the LCx.

Figure 10-1 Anatomy of coronary arteries, shown in an anterior (left) and posterior (center) view of the heart and in a cross-sectional view of the papillary muscle level (right). The left main coronary artery bifurcates into the LAD, which runs over the anterior interventricular sulcus, and the LCx, which is related to the left atrioventricular sulcus. The LAD extends down, ending distal to the apex, often extending up into the posterior interventricular sulcus. The septal branches arise in an acute angle from the LAD, coursing close to the endocardium on the right side of the interventricular septum. The LCx is covered by the left atrial appendage in its proximal portion and then takes position in the left atrioventricular sulcus. The RCA has its origin at the right coronary aortic sinus and goes into the right atrioventricular sulcus. The posterior descending (PD) artery runs in the posterior interventricular sulcus.

Pathophysiology

• Normal coronary flow is characterized by a biphasic flow profile, consisting of small systolic and large diastolic flow (Fig. 10-2).

• The color Doppler signal is detected mainly in diastole.

• Doppler spectral tracings of coronary flow provide some useful clinical information such as coronary flow direction, flow velocity, and flow pattern.

• Coronary flow reserve (CFR) is expressed as a ratio of maximum flow to resting flow, with the normal response being an increase in flow to four times baseline (Fig. 10-3).

• CFR begins to decrease at 40% to 50% diameter stenosis for a vasodilatory stimulus.¹

• CFR decreases to only twice baseline at approximately 75% diameter stenosis, which indicates significant myocardial stenosis.

Figure 10-2 Normal coronary flow is characterized by a biphasic flow profile, which consists of low velocity systolic and diastolic flow, with higher flow velocities in diastole compared to systole.

Figure 10-3 Coronary flow reserve (CFR) expressed as a ratio of maximum flow to resting flow. With progressive coronary stenosis, baseline flow remains normal until the coronary artery is narrowed by 80% to 85% diameter stenosis. However, with a vasodilatory stimulus, CFR begins to decrease at 40% to 50% diameter stenosis compared to the normal response of a fourfold increase in flow.

Overview of the Transthoracic Approach in Coronary Detection

Strengths

• Noninvasive

• Portable

• Cost effective

• Physiologic assessment of coronary flow

• CFR measurement

Weaknesses

• Limited sites of coronary detection

• Low success rate in the RCA and LCx

• No histologic assessment

• Requires technical training

Anatomic Imaging and Physiologic Data Acquisition

Acquisition

Before Scanning

• A high-resolution and high-sensitivity ultrasound system is required.

• Choose a high-frequency transducer (∼ to 12 MHz) to detect the mid- to distal LAD.

• A relatively low-frequency transducer (∼ to 5 MHz) is used for the proximal LAD, RCA, and LCx.

• The Doppler velocity range is set in the range of 10 to 30 cm/s (Fig. 10-4).

• The Doppler wall filter is set to the lowest level so that low-velocity signals are recorded close to the baseline (Fig. 10-5).

• The patient is examined in the left lateral decubitus position.

• Intravenous injection of a contrast agent enhances Doppler signal intensity.

Figure 10-4 Detection of the coronary flow signal. The Doppler velocity range is set in the range of 10 to 30 cm/s because coronary flow velocities are low compared with intracardiac flow. In this example, as the color Doppler velocity scale is decreased sequentially from 50 to 30 to 19 cm/s, the color flow signal in the LAD is better visualized.

Figure 10-5 The Doppler filter should be set to “low” or “minimum“ to obtain the full coronary flow spectrum. Use of a medium (med) filter results in a black band of missing signals on both sides of the baseline (top panel). With the filter on the low setting (bottom panel), the complete Doppler signal is recorded.

Detection of Coronary Flow Signal by Color Doppler

Left Main Trunk, Proximal Left Anterior Descending Artery, and Left Circumflex Artery (Use Low-Frequency Transducer)

• The basic steps for detection of coronary blood flow are shown in Figure 10-6.

• The left main trunk (LMT) and its bifurcation can be detected at the level of the left coronary sinus, in a short axis view (Fig. 10-7).

• The proximal segment of the LAD and LCx can be detected occasionally.

• The proximal RCA can be identified at the level of the right coronary sinus in a short axis or long axis view.

Figure 10-7 To visualize the LMT and proximal LAD, position the transducer at the level of the aortic root in the parasternal short axis view. Often these images can be acquired using a relatively low-frequency transducer. The LMT is identified as a tubular structure that originates from the left coronary sinus. By adjusting the orientation of the ultrasound beam from the distal side of the LMT, the proximal LAD can be visualized. Right upper panel, Tubular structure of the LMT and proximal LAD are clearly visualized in a patient with Marfan syndrome. Right lower panel, Severe stenosis in the left main coronary artery showing the thickened arterial wall and narrowed lumen (white arrows). Ao, aorta; LA, left atrium; RA, right atrium; RVOT, right ventricular outflow tract.

Figure 10-6 Step-by-step approach for coronary flow by color and PW Doppler.

Middle to Distal Portion of the Left Anterior Descending Artery

• The middle portion of the LAD can be identified in short axis images of the left ventricle (LV), located in the anterior interventricular sulcus (Fig. 10-8).

• Visualization is optimized with a high-frequency transducer.

• The LAD appears in a circular cross section containing the color Doppler flow signal.

• Rotate the transducer counterclockwise to image the LV in a long axis view along the intraventricular sulcus under the guidance of color Doppler flow mapping.

• The distal portion of the LAD can be found in the apical long axis view at the intraventricular sulcus, again as a circular cross section containing the color Doppler flow signal.

• Rotate the transducer clockwise (toward the two-chamber view position) to detect the flow signal in the long axis view.

• Coronary flow detection in the distal portion of the LAD is important because CFR should be measured distal to a stenosis.

Figure 10-8 Detection of LAD flow signal by color Doppler. To visualize the distal portion of the LAD, image the LV in long axis section from the apex and search the coronary flow signal around the intraventricular sulcus. Choose a high-frequency transducer to detect the LAD signal and use color Doppler for flow detection. Once the LAD is found in cross section (left panel), rotate the transducer clockwise (toward the two-chamber view position) as shown to detect the flow signal in the long axis view.

Right Posterior Descending Artery

• Obtain an apical two-chamber image of the LV and angle the ultrasound beam superiorly to image the posterior interventricular sulcus using a low-frequency transducer (Fig. 10-9).

• Examine this area using color Doppler flow mapping to locate the right posterior descending coronary artery.

• Contrast-enhanced Doppler technique improves the success rate.

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