Transesophageal echocardiographic assessment of coronary flow reserve and coronary artery stenosis using echo enhancement

Abstract

Echocardiography has been used to visualize proximal portions of coronary arteries since the advent of the technique [1, 2]. Transesophageal echocardiography (TEE) produces high quality images due to the proximity of the probe to the heart and the use of high frequency transducers, and has reawakened interest in the coronary echocardiographic field. Using transesophageal Doppler echocardiography, evaluation of the left main coronary artery (LMCA) has improved, and this is now a useful tool for detecting left main coronary artery stenosis [3–5]. In addition, transesophageal Doppler echocardiography has been used to record blood flow velocity in proximal left anterior descending coronary artery (LAD) in resting conditions and after pharmacological vasodilatation to explore the maximal amount of flow that the coronary vascular bed can accommodate above normal (coronary flow reserve) [6–8]. The importance of coronary flow reserve assessment lies in the fact that it is considered a better indicator of the functional significance of a coronary stenosis [9] — visual inspection by angiography of the anatomical severity of a coronary stenosis does not accurately reflect its physiological importance [10]. However, several limitations have impeded the widespread clinical application of TEE to coronary artery visualization and, consequently, stenosis detection and coronary blood flow reserve assessment. First, apart from the LMCA, the rest of the proximal left coronary tree is poorly visualized by TEE. In particular, the success rate in imaging an adequate portion of the proximal left anterior descending coronary artery, whose pathology carries important clinical and prognostic implications for the patient [11, 12], is low, with a consequent relatively poor diagnostic impact of TEE on the evaluation of proximal LAD coronary stenosis. On the other hand, TEE coronary flow reserve evaluation has a rather long learning-curve and, even after that, the ability to attain a good quality time—velocity curve (especially the systolic curve) in LMA bifurcation is suboptimal. For these reasons the attention of some research groups has been directed towards a lung-crossing contrast agent which, when injected i.v., could enhance the intensity of Doppler signal and thus the signal-to-noise ratio in coronary arteries, improving the quality and feasibility of coronary blood flow velocity Doppler recording. In this chapter we will discuss the clinical usefulness of the lung-crossing contrast agent, SHU 508A in improving assessment of coronary flow reserve and detection of proximal left anterior descending coronary artery stenosis during transesophageal Doppler echocardiography.