What is myocardial contrast echocardiography?

What is myocardial contrast echocardiography?

Myocardial contrast echocardiography (MCE) is used for the assessment of myocardial microcirculation and endocardial demarcation. MCE uses gas-filled microbubbles that are inert and remain wholly within the vascular space. These microbubbles have an intravascular rheology similar to that of red blood cells [1]. A continuous intravenous infusion of microbubbles is given to achieve a steady state. These microbubbles are destroyed by high energy ultrasound. The rate of microbubble replenishment within the ultrasound beam is measured and represents the mean red blood cell velocity [2].

Normally, the ultrasound beam fills within 5 seconds when the resting myocardial blood flow is normal. If if fills slowly, it means that myocardial blood flow is reduced. It will fill faster when there is hyperemic flow. The ultrasound signal received after full replenishment represents the relative blood volume within the myocardium in the region of the beam. Comparing this with the signal from the left ventricular cavity gives a measure of the blood volume fraction. During MCE, left ventricular cavity has increased echogenicity compared to the surrounding myocardium which has a lower blood volume. MCE gives blood flow velocity and volume fraction. The product of blood flow velocity and volume fraction is proportional to the myocardial blood flow.

In acute myocardial infarction, MCE done after one or two days after reperfusion, when the hyperemia has subsided, can give an estimate of the region with no-reflow. This will give an indication of the infarct size and extent of viable myocardium. If assessed immediately after coronary reperfusion, coronary hyperemia may result in underestimation of myocardial necrosis. No reflow is a marker of myocyte necrosis [3]. Collateral flow can also be assessed during coronary occlusion prior to interventional procedures. The extent of collateral perfusion during coronary occlusion estimates the extent of viable myocardium. 

Vasodilator stress imaging is also feasible with MCE. While the ultrasound beam replenishes in 5 seconds with normal resting flow, it can replenish in one second if the maximal hyperemic flow achieved by vasodilator is 5 times normal. So, the 5 second rest images would look like the one second stress image. Regional changes indicate reduced coronary flow reserve due to coronary artery disease. If the finding is global, it indicates reduced flow reserve due to systemic conditions like hypertension, diabetes mellitus or dyslipidemia.

Second generation ultrasound contrast agents have microbubbles with diameter in the range of 1 to 10 microns. They contain a gas surrounded by a phospholipid membrane or protein and are useful in endocardial demarcation [4]. European Society of Cardiology guidelines on MCE recommends it in patients with suboptimal ultrasound images to improve endocardial visualization. This will facilitate more accurate assessment of regional function and ejection fraction. MCE is useful in more accurate diagnosis of apical hypertrophic cardiomyopathy, left ventricular non-compaction, apical thrombi and left ventricular pseudoaneurysms.

References

1. Lepper W, Belcik T, Wei K, Lindner JR, Sklenar J, Kaul S. Myocardial contrast echocardiography. Circulation. 2004 Jun 29;109(25):3132-5. doi: 10.1161/01.CIR.0000132613.53542.E9. PMID: 15226230.
2. Wei K, Jayaweera AR, Firoozan S, Linka A, Skyba DM, Kaul S. Quantification of myocardial blood flow with ultrasound-induced destruction of microbubbles administered as a constant venous infusion. Circulation. 1998 Feb 10;97(5):473-83. doi: 10.1161/01.cir.97.5.473. PMID: 9490243.
3. Senior R. Role of myocardial contrast echocardiography in the clinical evaluation of acute myocardial infarction. Heart. 2003 Dec;89(12):1398-400. doi: 10.1136/heart.89.12.1398. PMID: 14617544; PMCID: PMC1767974.
4. Rotaru L, Nanea T. Assessment of myocardial perfusion using contrast echocardiography – Case report. J Med Life. 2015 Oct-Dec;8(4):471-5. PMID: 26664473; PMCID: PMC4656955.