Tracking Speckles: Overcoming Conventions to Evaluate Right Ventricular Function

The right ventricle (RV) remains a stubbornly misunderstood companion to the left ventricle (LV). It has a unique shape, which defies practical conventional modeling, and myocardial architecture and contraction pattern that are distinct from that of the LV and may change depending on the underlying pathology.1,2 That is why the conventional echocardiographic techniques for evaluating RV function are limited and newer echocardiographic modalities are welcome. In this issue, Drs Silverton and Meineri3 present a short review outlining the application of speckle tracking echocardiography for the assessment of perioperative RV function. The workload of the RV is about 1/5th to 1/6th that of the LV as it ejects into a highly compliant vascular bed.4 The RV is comprised of an inlet (or sinus) portion, which is responsible for about 80% of the RV output, an outflow tract (or infundibulum), and the apex.1 The fibrous architecture of the inlet portion is in large part comprised of subendocardial longitudinal fibers, accounting for the predominantly tricuspid annular plane (RV base) to RV apex systolic displacement.1 However, circumferential fibers, present in the subepicardial layer, are responsible for the inward (radial) motion of the RV free wall toward the septum. As the authors point out, RV contraction is peristaltic. The base to apex annular motion is accompanied by a radial motion toward the septum, followed by infundibular longitudinal contraction, thus maintaining proper intracavitary motion of blood toward the pulmonary vessels.5 As the sinus portion of the RV is in large part composed of longitudinal fibers, most echocardiographic assessment modalities have concentrated on examining the longitudinal motion of the RV annulus, as well as the motion of the RV free wall. There are various transesophageal echocardiography (TEE) techniques to evaluate RV function such as RV fractional area of change, tricuspid annular plane systolic excursion (TAPSE), and systolic velocity (S′) of the lateral tricuspid annulus. Despite their practicality, these measurements are far from ideal. All depend on pristine 2-dimensional (2D) imaging and tracing of endocardium by the operator, while TAPSE and S′ are only regional measurements. TAPSE is a widely-adopted measurement for RV function that is predicated on the fact that the annular motion represents the sum of the longitudinal myocardial function of the RV free wall. TAPSE by transthoracic echocardiography (TTE) has correlated well with RV ejection fraction by various methods. However, it has not correlated well with TEE measurement, thus putting this convenient assessment method into question.6 Drs Silverton and Meineri3 introduce an additional method for the analysis of RV function, the analysis of RV myocardial deformation through strain analysis. The echocardiographic assessment of myocardial deformation can be performed using Doppler (tissue velocity) or speckle tracking (brightness markers). Doppler-based deformation may be assessed despite relatively poor 2D images; however, it can only measure deformation along the Doppler plane. Brightness (B mode) speckles are natural ultrasound reflectors within the myocardium that can be tracked throughout the cardiac cycle, independent of insonation angle. Global and regional deformation (strain) can be calculated by dedicated software from a single image over 1 heartbeat, with a signal to noise ratio that is dependent upon 2D image quality and requiring a minimum frame rate. In speckle tracking, strain is calculated as the distance separating 2 points (over a unit of time) compared to their original separation distance. The process is quick to perform, usually <30 seconds, and can be repeated as often as needed. The operator needs to place 3 digital calipers and strain is calculated by dedicated software, which is available in all modern echocardiographic platforms, as shown in the provided videos. In this issue, Drs Silverton and Meineri3 detail the steps required to arrive at reliable deformation analyses for the RV ventricle using speckle tracking technology and describe how strain can be a potentially new and useful tool in the operating room. As mentioned by the authors, RV strain is an improvement over other measures of ventricular function; it is independent of geometric assumptions, less angle dependent, and can be performed in real time using both TTE and TEE. Nevertheless, there are still unanswered question with regard to the application and utility of strain in the perioperative period. How is RV strain affected in the dynamic operating room environment with frequent volume shifts and rapidly changing hemodynamics? What is the significance of strain in the perioperative period and what prognostic value does it hold for patients? We know that RV strain is influenced by preload.7 However, the studies examining the change in strain under various preload conditions have quite mixed conclusions.8 For example, in some endurance athletes, RV systolic performance is maintained despite a decreased RV strain because RV size is increased.9 On the other hand, a study using epicardial crystals in an open chest dog model showed that absolute shortening (strain) was increased as preload increased.10 Under normal circumstances, the RV is easily distensible and capable of accommodating changes in volume without any appreciable change in end diastolic stress. The RV curvature changes little and pressure does not rise significantly making Starling recruitment negligible.4 Therefore, it is expected that RV strain should remain relatively stable under normal preload conditions. However, under excessive preload conditions, the increase in diastolic wall stress will negatively affect strain. At the same time, we should be aware that the RV is poorly equipped to generate pressure. Increases in afterload may negatively affect strain depending on the extent and rapidity of the changes. The differences seen in studies investigating preload and myocardial deformation may be due to the large and variable populations examined, as well as the technology used to assess deformation, and further studies are needed to better understand the relationship between RV strain and loading conditions. In addition to global and regional measures of strain, speckle tracking can be used to calculate strain rate, S′, and TAPSE. Strain rate, or the rate at which deformation occurs, is a load-independent measure of contractility that may be affected by heart rate, inotropic drug administration, or disease state. TEE-derived TAPSE by speckle tracking has correlated better than TEE M mode-derived TAPSE when compared to TTE-derived TAPSE.6 Additionally, strain imaging contains “granular” information above and beyond peak systolic deformation. The strain curve can be examined for changes that are not appreciated by the naked eye alone using 2D echocardiography, such as subtle depression of shortening, early stretching (dyskinesis), delayed peak strain, or postsystolic shortening (ischemia). Strain analysis is a modality that when used with complementary markers including RV size, S′, and TAPSE provide a more complete understanding and accurate determination of RV function. In summary, strain will reflect the state of the RV myocardium under the prevailing load conditions and contractility state. What is the significance of strain in the perioperative period? Depressed RV function is a predictor of poor outcome in cardiac surgery.8,11 In the postoperative period, there may be a significant and persistent reduction in echocardiographic markers of RV function that may last for up to 1 year, despite recovery of exercise function.12–14 The measure of isovolumic acceleration is a better predictor of RV recovery but it is difficult to perform using TEE. Therefore, there may be some pitfalls in the traditional perioperative assessment of RV function. Additionally, RV contraction patterns can change significantly in some disease states. In pulmonary hypertension, the RV contraction pattern changes from the predominantly longitudinal to that of a transverse or circumferential pattern. Also, in patients with Eisenmenger’s syndrome, increased transverse strain was found to be a predictor of survival.2 In these disease states, markers of longitudinal function, measured in the free wall alone, do not provide a complete picture of the RV contractile state. The use of speckle tracking into other portions of the RV such as the infundibulum and in different planes (transverse or circumferential) may enhance the understanding or RV function in a variety of disease states and be possibly incorporated into the perioperative assessment. In summary, Drs Silverton and Meineri3 provide us with an excellent introduction into the measurement of RV deformation analysis through speckle tracking technology. Analysis of strain and strain rate in concert with other traditional echocardiographic markers may be a valuable addition in the interpretation of RV function in the perioperative period. Stain can be quickly analyzed as often as needed and at any time in the perioperative period. The only limitation is the availability of dedicated speckle tracking software and the knowledge basis of the operator. DISCLOSURES Name: Claude P. Tousignant, MD, FRCPC. Contribution: This author helped collect the data and prepare the manuscript. Name: Kimberly Howard-Quijano, MD. Contribution: This author helped collect the data and prepare the manuscript. Name: Nikolaos J. Skubas, MD, DSc, FACC, FASE. Contribution: This author helped collect the data and prepare the manuscript. This manuscript was handled by: Thomas R. Vetter, MD, MPH.