Myocardial contrast echocardiography – Use in viability assessment and acute myocardial infarction

Jiwan Pradhan; Roxy Senior

doi:10.4103/jiae.jiae_36_19

CONTEMPORARY TOPIC

Year : 2019 | Volume : 3 | Issue : 2 | Page : 63-65

Myocardial contrast echocardiography – Use in viability assessment and acute myocardial infarction

Jiwan Pradhan¹, Roxy Senior²
¹ Department of Cardiovascular Research, The Northwick Park Hospital, London, England
² Department of Cardiovascular Research, The Northwick Park Hospital; Department of Cardiology, The Royal Brompton Hospital; National Heart and Lung Institute, Imperial College, London, England

Date of Web Publication

29-Aug-2019

Correspondence Address:
Roxy Senior
The Royal Brompton Hospital, London
England

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jiae.jiae_36_19

Abstract

Myocardial contrast echocardiography (MCE) is now guideline-directed tool to assess myocardial perfusion both at rest and during stress. Its prognostic value has been established in the scenario of stable and unstable coronary artery disease and heart failure through assessment of myocardial ischemia, myocardial viability, coronary flow reserve, and microvascular dysfunction. We will discuss the pathophysiologic basis of MCE and its role in myocardial viability assessment both in the setting of chronic ischemic left ventricular dysfunction and acute myocardial infarction.

Keywords: Acute myocardial infarction, myocardial contrast echocardiography, viability assessment

How to cite this article:
Pradhan J, Senior R. Myocardial contrast echocardiography – Use in viability assessment and acute myocardial infarction. J Indian Acad Echocardiogr Cardiovasc Imaging 2019;3:63-5

How to cite this URL:
Pradhan J, Senior R. Myocardial contrast echocardiography – Use in viability assessment and acute myocardial infarction. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2019 [cited 2023 May 29];3:63-5. Available from: https://jiaecho.org/text.asp?2019/3/2/63/265753

Myocardial Contrast Echocardiography Pathophysiology

Current generation myocardial contrast agent uses highly inert microbubbles which do not dissolve in the blood, hence retaining its backscatter capability. They are osmotically stabilized microbubbles <7 μm containing a central encapsulated gas with outer shell. The characteristic lipid or albumin shell prevents outward diffusion, hence resembling intravascular rheology of erythrocytes to remain entirely intravascular. The signals obtained from the contrast agents depend on various factors such as size, type of shell, and encapsulated gas.

The myocardial tissue has a predictable and linear ultrasound scattering property at low intensity of ultrasound, whereas the microbubbles do not. Myocardial contrast echocardiography (MCE) relies on this nonlinear oscillation capability of the microbubbles.

The intensity of ultrasound is commonly reflected as a nondenominational ratio called mechanical index (MI) which is peak negative pressure of ultrasound divided by square root of center frequency. A typical B-mode tissue echocardiography uses MI of 0.9–1.4. At this MI, there is a marked destruction of the microbubbles, and the tissue also gives off nonlinear signals and prevents optimal visualization of contrast. At very low (<0.2) MI, the tissue does not reflect harmonic signals and reflects linear signals; hence, the tissue appears dark. However, the microbubbles even at this low MI reflect nonlinear signals, and thus, the blood both in the cavity and in the myocardium will be visualized through microbubble signals. However, at this low MI setting, contrast-specific imaging with multipulse cancellation techniques is used to optimally image myocardial function and perfusion.

The volume of blood within the coronary circulation at rest in diastole is around 12 ml/100 g of the left ventricular myocardium.^[1] Ninety percent of this blood volume is contained in the capillaries in the myocardium, and the relative capillary blood volume is represented by ultrasound signal intensity emitted from the microbubble (contrast agent) when a steady state in the circulation is reached.

Once a steady state of contrast is reached in myocardium, the microbubbles maybe destroyed with high intensity ultrasound. After this, the rate of reappearance of contrast agent is the mean myocardial bubble velocity. Capillary blood velocity is 1 mm/sec and the ultrasound beam elevation is 5 mm. Hence during resting stage, it takes 5 seconds (which is within 5 cardiac cycles at heart rate of 60 bpm) for complete replenishments which shortens up to 1-2 seconds (2-3 cardiac cycles at heart rate of 120 bpm) at stress in the myocardium subtended by normal coronaries. The concentration of the contrast agent as observed as intensity of contrast in myocardium when a steady state is reached is the microvascular cross sectional area (CSA) or the capillary blood volume. The product of microvascular CSA and myocardial bubble velocity gives the myocardial blood flow(MBF) or the myocardial perfusion.^[2]

Myocardial Blood Flow Reserve

The ratio myocardial blood flow at rest and maximal hyperemia is myocardial blood flow reserve (MBFR). As described above, myocardial blood flow is the product of peak contrast intensity and myocardial flow velocity. MBFR assessed by MCE has been shown to be reproducible ^[3],[4] and also been validated with positron emission tomography (PET) scan.^[5]

Myocardial Contrast Echocardiography in the Context of Viability

Hibernating heart a condition where the myocardium is metabolically active but mechanically dysfunction is secondary to recurrent and chronic persistent ischemia leading to left ventricle dilatation and dysfunction of systole. A meta-analysis of 24 studies revealed that without revascularization, annual mortality is reduced by 80% in hibernating heart.^[6]

The integrity of microvasculature reflects a viable myocardium.^[7] Hence, myocardial blood flow as assessed by MCE reflects a viable tissue. In a questionable myocardial segment of >6 mm, a preserved perfusion by MCE denotes a viable segment. On the contrary, a perfusion defect detected by MCE, 10–15 s after flash-replenishment imaging, corresponds to nonviable myocardium.^[8],[9] More specifically, the presence of significant signal intensity which reflects capillary volume suggests myocardial viability.^[10]

Viability data obtained from MCE are comparable to other modalities. Although dobutamine stress echocardiography (DSE) has the highest specificity in detecting viability, its sensitivity is lower. When combining MCE with DSE, MCE helps to detect dobutamine unresponsive viable myocardium, hence increasing sensitivity in detecting hibernating myocardium.

Viability Assessment by Coronary Flow Reserve

As coronary flow reserve (CFR) is the ability of microvasculature to respond to stimulus, MBFR assessed by MCE can detect CFR noninvasively. The CFR obtained by MCE is reproducible ^[3],[4] and comparable with PET scan.^[5] It also has incremental predictive value in assessing mortality in ischemic and nonischemic heart failure.^[11]

Myocardial Contrast Echocardiography in Acute Myocardial Infarction

MCE can detect perfusion defect and wall motion abnormality. In patients presenting in the emergency department with chest pain, MCE hence has high diagnostic and prognostic value in detecting acute myocardial infarction, well above routine demographic, clinical, and electrocardiographic variables.^[12] In this setting, due to its ability to detect collateral blood supply, MCE and not wall thickening abnormality can detect the infarct size accurately.^[13]

No reflow which is damage to microvasculature after revascularization of offending arteries in AMI setting is detected by MCE. The phenomenon is secondary to ischemia, reperfusion, endothelial dysfunction, and distal thromboembolism ^[14] resulting in higher mortality, adverse remodeling, and poor healing of infarct.^[15],[16] In a study of 170 patients who have ST-segment elevation myocardial infarction and were treated with emergent percutaneous coronary intervention, MCE performed 24–48 h after index event detected up to fourfold event rate in patients with microvascular obstruction.^[16]

The extent of myocardial necrosis by just assessing the extent of wall motion abnormality on echo could be overestimated. The reason for this is in risk areas, myocardium may remain dysfunctional- “stunned” even after myocardial perfusion is restored. MCE helps to detect the extent of perfusion in dysfunctional segments leading to more accurate prediction of patient outcome.^[17] Its use has been validated by multiple studies.^[17],[18]

Recommendation of Myocardial Contrast Echocardiography for the Detection of Myocardial Viability

The European Association of Cardiovascular Imaging 2017^[19] has the following recommendations:

Class IIA, level B – MCE may be performed to improve the detection of myocardial viability, particularly in dobutamine nonresponsive segments, where wall thickness is preserved
Class I, level A – The flash-replenishment technique should be used for the assessment of myocardial perfusion.

Financial support and sponsorship

Nil.

Conflicts of interest

R. S. obtained speaker fees from Bracco, Milan, Italy, Lantheus Medical Imaging, Boston, USA, and Philips Healthcare, Eindhoven, Holland.

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