|Year : 2017 | Volume
| Issue : 1 | Page : 27-31
Evaluation of coronary arteries by transthoracic echocardiography
Cardiac Clinic, Focus Diagnostics, Hyderabad, Telangana, India
|Date of Web Publication||7-Apr-2017|
A V Anjaneyulu
Focus Diagnostics, Hyderabad, Telangana
Source of Support: None, Conflict of Interest: None
Though several new cardiac imaging techniques have emerged, invasive coronary angiography remains the gold standard in evaluating coronary arteries for measuring anatomic severity of the stenotic lesion and assisting in intracoronary interventions. Several workers have attempted to image the coronary arteries by echocardiography. With improvements in ultrasound machines and introduction of harmonic imaging and high frequency transducers, direct visualization of long segments of all three coronary arteries is now possible. Though the entire artery cannot be reconstructed, multiple segments can be visualized, flow velocity can be measured, thus enabling us to obtain useful anatomic and physiologic information. Careful evaluation of coronary flows can translate into a wide variety of clinical applications.
Keywords: Coronary arteries, Doppler flow, transthoracic echocardiography
|How to cite this article:|
Anjaneyulu A V. Evaluation of coronary arteries by transthoracic echocardiography. J Indian Acad Echocardiogr Cardiovasc Imaging 2017;1:27-31
|How to cite this URL:|
Anjaneyulu A V. Evaluation of coronary arteries by transthoracic echocardiography. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2017 [cited 2017 Aug 19];1:27-31. Available from: http://www.jiaecho.org/text.asp?2017/1/1/27/204059
| Introduction|| |
Although several new cardiac imaging techniques have emerged, invasive coronary angiography remains the gold standard in evaluating coronary arteries for measuring anatomic severity of the stenotic lesion and assisting in intracoronary interventions. Several workers ,, have attempted to image the coronary arteries by echocardiography. With improvements in ultrasound machines and introduction of harmonic imaging and high-frequency transducers, direct visualization of long segments of all three coronary arteries is now possible. Although the entire artery cannot be reconstructed, multiple segments can be visualized and flow velocity can be measured, thus enabling us to obtain useful anatomic and physiologic information. Careful evaluation of coronary flows can translate into a wide variety of clinical applications [Table 1].
| Technical Considerations|| |
A high-quality ultrasound machine gives good quality imaging for coronaries. The 5.0 or 3.5 MHz narrow-band sector transducers in second harmonic mode are used for B-mode examination, and color Doppler mapping and spectral Doppler coronary flow velocity assessment are performed at 2.5 or 2 MHz. After obtaining good quality B-mode image, coronary segments should be searched, using color flow with or without the use of harmonics. Sample size of color Doppler should be at minimum, velocity range should be set with a low Nyquist limit (15–20 cm/s), and filters should be decreased. Some ultrasound systems provide special color maps for coronary artery evaluation including second or third harmonics of B-mode and color, low Nyquist limit, special gain for B-mode, and Doppler mode, etc.
| Segmental Evaluation of Coronary Arteries|| |
Standard parasternal short and long axis views from second or third intercostal space or low parasternal long or short axis views from fourth or fifth intercostal space should be used. Coronary arteries appear as linear intramyocardial color segments of approximately 0.5–2.5 cm in length and 2–4 mm in diameter. Initially, after obtaining a short segment, step-by-step movement of the transducer along the course of the vessel will bring longer segments into view.
| Left Main and Left Anterior Descending Artery|| |
With the patient in left lateral decubitus position, parasternal short axis view of the great vessels is obtained. In the plane immediately below the pulmonary trunk, left main (LM) and proximal left anterior descending (LAD) can be visualized by B-mode. The LM has approximately 1–3 cm length and is usually visualized along its entire course [Figure 1]. More distal parts of LAD can be visualized using color Doppler. The LM and proximal LAD can also be viewed from a modified apical five-chamber view within the area lateral to the sinus of Valsalva. The middle and distal LAD may be assessed from low parasternal position. The interventricular groove is located in short axis view, and then the transducer is tilted toward the base of the heart or rotated to obtain modified long axis view aligning parallel to the groove. The apical view is used for visualizing the distal LAD and its flow velocity measurements.
|Figure 1: Normal coronary flows. (a) Short left main coronary artery dividing into left anterior descending, Cx – Both of them showing laminar flows. (b) Laminar flow in left main continuing into proximal left anterior descending. (c) Normal Doppler flow velocity in distal left anterior descending (0.3 m/s)|
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| Left Circumflex Artery|| |
The circumflex (Cx) is visualized in parasternal short axis view in a plane immediately below the pulmonary trunk just below the left atrial appendage. The middle Cx is best visualized in the plane of mitral annulus. Apical modified five-chamber view also can be used to image the origin of Cx from LM. Flow velocity can be measured in the proximal part of the Cx.
| Right Coronary Artery|| |
Proximal right coronary artery (RCA) can be easily visualized in parasternal short axis plane. Middle and distal RCA segments need color Doppler assistance for their visualization. Distal RCA can be assessed using apical window. Apical four-chamber view is obtained, and then the transducer is tilted more perpendicular to the chest so that the imaging plane traverses the inferior wall of the heart and the posterior interventricular groove. Posterior descending artery (PDA) flow can be seen in this view.
Normal flow velocities
Coronary flow is biphasic, predominantly diastolic. Normal flow velocity spectrum consists of two forward flow components: smaller, dome-like systolic and bigger diastolic [Figure 1]c. The maximal flow velocities in nonstenosed proximal parts of LAD, Cx, and RCA is in the range of 0.7 m/s. Controls with normal coronary angiograms and normal LV function had a peak diastolic velocity in the distal LAD in the range of 21.2 ± 7.9 cm/s. The proportion of maximal flow in diastole velocities to systole change distal to a significant stenosis. The velocity increases at the site of stenosis. Flow velocities vary considerably depending on many factors: Heart rate, blood pressure, thickness of myocardium, hyperdynamic circulatory states - hyperthyroidism, anemia, renal failure, etc.
The size of the area where color Doppler examination is carried out should be kept at minimum. Usually, a small part of the artery is seen at first. Then, by moving step-by-step up and down the course of the vessel, the entire artery (or its long part) may be assessed. A 5 mm gate for pulsed Doppler evaluation and the manufacturer default filter setting is used. The Doppler angle should not exceed 60°. The color Doppler pattern in the normal coronary artery is uniformly consistent with laminar flow. At the site of stenosis, focal flow acceleration and turbulence may be detected as aliasing zone. The Doppler velocity should be measured at the point of focal aliasing. Krzanowski et al. and Saraste et al. showed that a local peak diastolic flow velocity >2.0 m/s could be used as a sign of diameter reduction of >50% for all three main coronary arteries. Since LM and proximal LAD segment are almost horizontal and exceed this angle, we rely on mosaic flow to determine significant stenosis in these segments. A local maximal flow velocity >1.5 m/s is also a reasonably accurate sign of stenosis. Hozumi et al. have observed localized aliasing by color flow mapping in 100% of patients with LAD restenosis of >50% after coronary angioplasty and 56% of patients without restenosis. Color flow aliasing is dependent on several hemodynamic factors (perfusion pressure, heart rate, blood pressure, myocardial mass, etc.) and technical setting (depth of scanning, quality of Doppler wave pattern, angle of incidence between Doppler beam and flow direction, etc.). The ratio of stenotic to prestenotic velocities of >2.0 has been used by most workers, as this allows exclusion or minimization of hemodynamic influence.
Artifacts and other sources of errors
Most of the artifacts are due to the heart movement and make the examination difficult, especially for RCA and Cx. Flow in the adjacent pericardial sac can create a strong signal which may be confused with flow within a coronary artery. This can be resolved by timing pericardial flow signal is systolic whereas coronary flow is predominantly diastolic. Some large branches of coronary arteries - the ramus, diagonal, and marginal branches can be confused with principal coronary arteries, mostly the LAD and Cx.
| Feasibility of Visualizing Coronary Arteries by Transthoracic Echocardiography|| |
Feasibility of visualizing coronary arteries with the addition of harmonics, newer transducers, and contrast agents has been reported to be as high as 100% for distal LAD and 33%–97% for PDA. Imaging of entire Cx and mid-RCA has been possible with a low rate [Table 2]. Only skilled operators can be expected to achieve a 90% success rate in visualizing the coronary arteries. Transthoracic echocardiography (TTE) with both the use of peak diastolic flow velocity >2.0 m/s and stenotic to prestenotic peak velocity ratio >2.0 revealed 48% of all LAD stenoses, 30% of all Cx stenoses, and 14% of all RCA stenoses [Table 3].
|Table 2: Success rate of the detection of main coronary arteries by transthoracic echocardiography (Boshenko et al. 2008)|
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|Table 3: Diagnostic accuracy of Doppler transthoracic echocardiography in the detection of >50% stenosis in main coronary arteries (Boshchenko et al. 2008)|
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| Left Main Coronary Artery Stenosis|| |
We studied a total of 1456 patients with coronary artery disease (CAD), of which 801 patients (55%) had adequate coronary flow assessment by TTE. Using the criteria of either color aliasing [Figure 2] or a peak diastolic flow velocity of >1.5 m/s, LM stenosis of >50% could be diagnosed with a sensitivity of 85% and specificity of 88%. LM flows should be specifically assessed by TTE in patients with rest angina, familial hypercholesterolemia, Takayasu disease (aortoarteritis), and also in patients who undergo LM stenting to assess stent patency. Prior knowledge of LM stenosis will enable the angiographer to take precautions while engaging the LM ostium during coronary angiography.
|Figure 2: Mosaic flow in left main in a patient who had flash pulmonary edema with chest pain|
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| Detection of Total Coronary Occlusion-Coronary Collaterals|| |
The absence of coronary flow on color Doppler map is not a reliable sign of total occlusion. At present, TTE is not a suitable method to diagnose acute coronary occlusions. However, chronic total occlusions (CTO) can be picked up by identifying coronary collaterals. Watanabe et al. proposed the reversal of coronary flow in epicardial collateral vessels to be the main ultrasound sign of CTO. It has been established that retrograde flow in distal LAD is a good marker of total LAD occlusion with 88% sensitivity and 100% specificity  and retrograde flow in PDA is a good marker of RCA occlusion with 67% sensitivity and 100% specificity. Identifying intramyocardial collaterals in the interventricular septum [Figure 3] has increased the sensitivity in the CTO detection from 88% to 96% for the LAD and from 67% to 80% for the RCA., Identifying collateral flow in interventricular septum is a vital clue to diagnose anomalous left coronary artery origin from pulmonary artery.
|Figure 3: Coronary collaterals by transthoracic echocardiography. (a) Mosaic collateral flow in distal interventricular septum directed from base to apex of left ventricle (right coronary artery → left anterior descending collaterals). (b) Right coronary artery injection in the same patient showing left anterior descending opacification through collaterals at the apex|
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| Detection of Recanalized Left Anterior Descending After Thrombolysis by Transthoracic Echocardiography|| |
We have attempted to show reperfusion after thrombolysis in LAD in 12 patients of acute anterior wall myocardial infarction. In seven patients, flow could be demonstrated in LAD within 15 min to 6 h after initiating thrombolysis. Furthermore, a focal area of color turbulence could be shown in these patients in LAD after the reappearance of flow [Figure 4], which correlated with the site of significant stenosis on coronary angiography.
|Figure 4: Recanalized left anterior descending flow 60 min after thrombolysis. (a) Focal area of mosaic flow in mid left anterior descending. (b) Coronary angiogram in the same patient showing severe stenosis (90%) in mid left anterior descending|
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| In-Stent Restenosis|| |
Direct assessment of stent flow is possible in LM [Figure 5]a and proximal LAD segments. Any turbulence with increased flow velocities >1.5 M/s would suggest restenosis [Figure 5]b. Indirect assessment of LAD and RCA stents can be done by evaluating coronary flow reserve (CFR). A CFR value of <2 after intravenous adenosine in distal LAD would suggest significant in-stent restenosis in proximal or mid-LAD. A similar finding in PDA would suggest in-stent restenosis in RCA.
|Figure 5: Stent flow evaluation by transthoracic echocardiography. (a) Stent in left main showing normal laminar flow. (b) Stent in mid left anterior descending showing mosaic flow|
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| Ischemic Cardiomyopathy|| |
When a diagnosis of dilated cardiomyopathy is made by TTE, the presence of mosaic flow pattern in proximal coronary arteries (LM, LAD, or Cx) is a strong pointer to ischemia as the cause for myocardial dysfunction. It is worthwhile to interrogate the proximal coronary segments by TTE to diagnose ischemic cardiomyopathy, in which myocardial dysfunction can be improved with revascularization.
| Noninvasive Assessment of Coronary Flow Reserve by Transthoracic Echocardiography|| |
CFR is defined as the ability of coronary blood flow volume to increase under maximal coronary hyperemia when compared with flow volume at rest. Intracoronary Doppler with adenosine or papaverine infusion remains to be the reference standard for the assessment of CFR. Hozumi et al. performed the first validation of TTE comparing CFR in the LAD with simultaneous intracoronary Doppler guidewire assessment. TTE reflected the invasive measurement of coronary flow velocity and CFR accurately, and the agreement between the two methods was 0.98 for diastolic peak velocity and 0.97 for CFR.
CFR assessment may be performed at the bedside within a few minutes. Adenosine should be infused intravenously at the rate of 140 mcg/kg/min for 2 min. Coronary flow velocities are measured before and immediately after the cessation of adenosine infusion. Distal LAD is the preferred site for assessment of flow velocities. The cutoff value for CFR is generally accepted to be 2.0 for predicting significant LAD stenosis in patients, for decision-making regarding intervention, after intracoronary intervention, and with in-stent restenosis. In one study, CFR with a cutoff value <2.0 was a significantly better predictor (90% sensitivity and 96% specificity) of LAD stenosis than multidetector computed tomography (80% sensitivity and 93% specificity). A transthoracic CFR in the LAD <2.0 provided data consistent with those obtained by single-photon emission computed tomography for physiologic estimation of stenosis severity with 94% sensitivity and 100% specificity (Hirate).
| Conclusion|| |
In summary, TTE with the help of harmonic imaging, contrast agents, and high-frequency transducers can be used for the diagnosis of coronary stenosis as a noninvasive, inexpensive, non-X-ray, high time resolution technique. Doppler TTE assessment of coronary flows in CAD patients can provide a lot of useful functional information with a wide range of clinical applications [Table 1].
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]