Applied Radiology

Dr. Watanabe and Dr. Hozumi are Assistant Professors and Dr. Yoshikawa is a Professor and Chairman of the Department of Internal Medicine and Cardiology, Osaka City University Medical School, Osaka, Japan.

Coronary Artery Disease; Increasing Risk

Despite many efforts to prevent coronary artery disease, the number of patients with coronary artery disease (CAD) is increasing. According to the World Health Organization, there are now 7.1 million deaths from coronary heart disease globally, and it is predicted that this figure will have risen to 11.1 million by 2020. ¹ Thus, continuous efforts to identify coronary narrowing and to evaluate coronary circulation in patients without coronary narrowing are a critical mission for us. In this article, we provide a guide to a new approach for coronary circulation using transthoracic Doppler echocardiography (TTDE) (Figure 1).

Echocardiographic Approach to CAD

Coronary flow velocity reserve (CFVR) has been accepted as one of the most reliable indices in detecting functional coronary stenosis. At the same time, it has been accepted as an index for evaluating coronary circulation in patients without coronary narrowing. However, this index has been measured by a Doppler guide wire or Doppler catheter, which are invasive; or positron emission tomography, which is expensive. Although these techniques successfully introduced the utility of this index for assessment of CAD or coronary risk factors in vivo, their use is limited in patients with known CAD.

Recently, technologic advancements in echocardiography have permitted us to evaluate coronary flow dynamics noninvasively, using transthoracic Doppler echocardiography. ^2­4 The success rate of CFVR measurement is > 90%, which is feasible for daily clinical use. Thus, we can approach coronary flow not only in the cath lab, but also in the echo lab, or even at the patient's bedside.

How to Measure CFVR using TTDE

CFVR measurement by TTDE consists of three components: 1) visualization of coronary flow by color flow mapping; 2) coronary flow velocity recordings by pulsed Doppler method; and 3) off-line measurement of coronary flow velocities and CFVR. CFVR is calculated as a ratio of hyperemic to basal coronary flow velocity.

Step 1: Visualization of coronary flow by color flow mapping

Echocardiographic images can be obtained from the acoustic window around the apex; usually on midclavicular line in the fourth and fifth intercostal spaces in the left lateral decubitus position. After the lower portion of the interventricular sulcus is located in the long-axis cross-section, the transducer should be rotated in a clockwise manner to search flow signals on the left anterior descending artery (LAD) under the guidance of color flow mapping. The characteristic of coronary blood flow signals is linear signals that persist during entire diastole (Figure 2).

Step 2: Coronary flow velocity recordings

Coronary blood flow velocity can be recorded by pulsed-wave Doppler (3.5 MHz) using a sample volume (1.5 to 2.0 mm) placed on the color signal in the mid to distal LAD. Adenosine is administered by intravenous infusion (0.14 mg/kg per minute) for 2 minutes to record spectral Doppler signals during hyperemia. Electrocardiogram, heart rates, and blood pressure should be monitored continuously during examination in all patients. During the recording, the examiner should try to hold the transducer on the same position as much as possible.

Step 3: Measurement of coronary flow velocity and CFVR

Coronary blood flow velocity can be measured off-line by contouring the spectral Doppler signals using the integrated evaluation program in the ultrasound system (Figure 3). Both peak and mean diastolic flow velocities at baseline and peak hyperemia should be measured as parameters necessary for CFVR assessment. CFVR is calculated as the ratio of hyperemic to basal peak (peak CFR) and mean (mean CFR) flow velocity. Usually, diastolic flow velocity is alternatively used. Normal CFR is defined as >2.0 on the basis of the results of previous studies that had evaluated flow velocities in the distal LAD.

Tips for Coronary Visualization

First, recognition of the anatomy is needed. A cross-section of coronary imaging cannot be obtained in
standard two-dimensional images. Because the LAD exists in anterior interventricular sulcus, the landmarks for coronary imaging are the left ventricular wall, right ventricular wall, and interventricular septum.

Next, specific machine settings for coronary flow are critical. The most important setting is modification of velocity range in color flow mapping. When using velocity range for routine color flow mapping (approximately 60 cm/s), it is difficult to visualize coronary flow, which is diminished by wall filtering in most cases. Thus, velocity range should be set around 20 cm/s to visualize coronary flow.

A final important point is the use of the appropriate transducer for coronary flow visualization. We recommend using a high-frequency (5 MHz) transducer because the distal LAD is in the near field for most patients. However, selection of the transducer must be made in specific cases. When the distal LAD exists in the far field, a lower frequency transducer may be better.

When flow visualization or velocity recordings are not sufficient for analysis, a contrast agent is useful as a Doppler enhancer. ^5,6 Although the number of the patients who need a contrast agent differs between study populations, the proper use of a contrast agent increases the success rate of flow velocity recordings (Figure 4).

Conclusion

Coronary flow assessment by TTDE, which is completely noninvasive, opens a new window for physiological assessment of coronary circulation. *