Ultrafast Ultrasound: Potential Applications in Cardiac Anesthesia and Intensive Care

INTRODUCTION

Advances in ultrasound technology and a substantial increase in computing power have enabled the development of ultrafast ultrasound.¹ Conventional ultrasound uses line-byline scanning beams. The maximum frame rate possible is limited by the speed of sound through the tissues to around 100 frames per second. On the other hand, in ultrafast ultrasound, ultrasound waves scan the tissues by sending plane waves. This enables the increase in frame rate to around 5000–10000 Hz.¹ A simple analysis of plane waves would result in a decreased spatial resolution due to the interference of the returning waves from tissues adjacent to each other. This is overcome by modifications in the transmission of the ultrasound waves or advanced computation of the received signals or both.² The clinical applications of ultrafast ultrasound include assessment of blood flow velocities, flow patterns, tissue motion, and tissue structure (Fig. 1). Being a noninvasive bedside modality, it could offer a tremendous opportunity for the perioperative physician to make informed decisions and improve clinical outcome.

Coronary ultrafast Doppler angiography (CUDA) has been demonstrated in animal models and healthy volunteers by Maresca et al.³ The technique relies on ultrafast ultrasound and power Doppler analysis. The authors have demonstrated visualization of arterial and venous coronary blood flow in the epicardium, myocardium, and endocardium. Moreover, they have also measured the coronary flow variations in various scenarios including reactive hyperemia, adenosine testing, and myocardial infarction. Finally, the authors have also presented their initial results using transthoracic echocardiography in two adults and two children. Building on this work, Nguyen et al. have studied the changes in myocardial blood volume during the various phases of cardiac cycle in neonates undergoing cardiac surgery.⁴ At present, CUDA is limited to linear probes and an imaging depth of 45 mm.³ When available for clinical use and at larger depths, assessing the trend in coronary blood flow noninvasively can have immense utility intraoperatively and postoperatively in patients undergoing coronary artery bypass grafting and transposition of great arteries. A similar technology has also been used to monitor cerebrovascular blood flow in neonates.⁵

Another application of ultrafast ultrasound or high frame rate ultrasound is in the assessment of intracardiac blood flow pattern. By using the techniques of vector flow imaging and blood speckle tracking, angle-independent visualization of blood flow is possible.² Vector flow imaging can demonstrate the nature of blood flow, that is, laminar, turbulent, helical, persistent, and vortices.⁶ Further, the location of vortices, vortex area, and the timing of vortices in relation to cardiac cycle can also be quantified. In a preliminary study, Hansen et al. demonstrated the normal blood flow pattern in 12 neonates and three infants.⁶ Marchese et al. demonstrated a relationship between abnormal left ventricle morphology and vortex characteristics.⁷ Borrelli et al. demonstrated the utility of vector flow imaging in congenital heart disease where it helped define the diagnosis of transposition of great arteries, tetralogy of Fallot, coarctation of aorta, atrioventricular septal defect, and hypertrophic cardiomyopathy.⁸ In adult echocardiography, using conventional ultrasound and contrast echocardiography, correlation has been demonstrated between blood flow vortex characteristics and heart failure.⁹ Therefore, visualization of blood flow pattern can have a huge role in hemodynamic management in the perioperative period. In addition, four-dimensional vector flow imaging could aid in quantification of flow in shunts and valvular diseases.²

The utility of ultrafast ultrasound extends to analysis of tissue motion and tissue structure during the various phases of the cardiac cycle.² In an in vivo study, the increased temporal resolution with ultrafast ultrasound has allowed tracking of ultrasonic speckles to delineate individual myocardial fibers.¹⁰ Ultrafast ultrasound can also quantify the velocity of shear waves to determine tissue stiffness. Shear waves are secondary waves moving perpendicular to the direction of the primary wave. Various studies have demonstrated a correlation between central venous pressure and liver stiffness measured by shear wave elastography, where the primary wave was an acoustic impulse generated by the probe.^11,12 In addition to imaging shear waves generated by an acoustic impulse, imaging of naturally occurring shear waves from valve closure can also be used to assess myocardial stiffness.^13,14 Accurate assessment of tissue stiffness and diastology at thebedside can help guide fluid management in the intensive care unit.

To conclude, ultrafast ultrasound has numerous applications in cardiovascular diagnosis and monitoring in the perioperative period. The current technical limitations of specific probe requirements, limited scanning depth, and absence of incorporation into transesophageal echocardiography need to be resolved for wide clinical applications.