A 65-year-old patient with ischemic cardiomyopathy presented for elective left ventricular assist device (LVAD) implantation. Written consent was obtained to publish this report. Intraoperative transesophageal echocardiography (TEE) showed a left ventricular ejection fraction of 18%. Pulmonary artery catheter pressures were 52/24 mm Hg. The patient subsequently developed right ventricular (RV) failure, requiring RV assist device implantation.
Acute refractory RV failure is a highly morbid condition, which occurs in 0.1% of patients after routine cardiac surgery, 2%–3% after heart transplantation and 20%–30% of patients during LVAD placement.1 Those with RV dysfunction have increased intensive care unit and hospital length of stay,2 and the perioperative diagnosis of RV dysfunction adds incremental value for predicting mortality and circulatory failure when compared with left ventricle (LV) ejection fraction alone.3
TEE ASSESSMENT OF RIGHT VENTRICULAR FUNCTION
Because of its complex shape, assessment of RV function is challenging (Table 1).
RV speckle tracking strain analysis is a new, potentially useful tool in the operating room because it is independent of geometric assumptions, relatively angle independent, and can be used in real time.
ASSESSMENT OF MYOCARDIAL DEFORMATION
Lagrangian strain is defined as the fractional change in length of an object. When applied to echocardiography, Lagrangian strain represents the fractional change in length of the myocardium and is expressed as a percentage of the original length. Shortening of the myocardium is reported as a negative value, and lengthening or thickening is reported as a positive value.4
Strain = (Length in Systole – Length in Diastole) / Length in Diastole
The original technology for measuring strain in echocardiography was based on tissue Doppler imaging (TDI). TDI strain has been used for preoperative assessment of RV function with transthoracic echocardiography (TTE).5 Measurements are limited by requirement of high frame rates (>100 frames/second) and angle dependency,4,6 therefore its application for intraoperative TEE is limited.
Speckle tracking echocardiography (STE) is a technology developed to measure myocardial deformation. “Speckles” are ultrasonic patterns of constructive and destructive interference within the myocardium and are followed over time by dedicated software to calculate strain. The main advantage of STE is that it is relatively angle independent and can be used to calculate strain in any imaging plane. STE has been validated with sonomicrometry,7,8 and it is available on most current echocardiography machines.
The complex deformation of the LV during systole can be described with 3 components of strain. Longitudinal strain represents shortening from the base to the apex and is expressed as a negative percentage. Circumferential strain represents the circumferential shortening of the myocardium, also expressed as a negative percentage. Radial strain represents myocardial thickening and is expressed as a positive percentage.
SPECKLE TRACKING STRAIN OF THE RIGHT VENTRICLE
STE can be applied to the RV to measure 2D longitudinal strain (Figure 1). RV contraction has been described as “peristaltic” in that the predominant motion is a sequential longitudinal shortening from the inflow portion of the RV to the infundibulum.9 Longitudinal strain is an important measurement of RV deformation. Because systolic contraction leads to myocardial shortening, RV 2D longitudinal strain is reported as a negative percentage. The more negative the value, the more vigorous the myocardial contraction and, therefore, the better the systolic function. RV strain calculations are reproducible and feasible with a cutoff value >20% (absolute value), or <−20% for RV dysfunction.6
Two measures of 2D longitudinal strain of the RV have been described. Figure 2A depicts RV global longitudinal strain (RV GLS) and represents the average 2D longitudinal strain for all RV myocardial segments: from the basal, mid, and apical segments of the lateral wall of the RV to the 3 corresponding segments of the interventricular septum, all of which are measured in a midesophageal (ME) 4-chamber view of the heart. The latter segments are included because of the significant contribution of the interventricular septum to RV ejection. Figure 2B shows RV free wall strain (RV FWS), which is the average longitudinal strain of only the 3 segments of the lateral wall of the RV. Here, the interventricular segments are removed from the calculation. When various echocardiographic measures of RV systolic function were compared, RV FWS had the highest diagnostic accuracy for predicting RV dysfunction (area under the curve 0.92) with a sensitivity and specificity of 96% and 93%, respectively.10
The advantages of using 2D RV longitudinal strain are that STE measurements are less angle dependent and they incorporate more segments of the RV than just the tricuspid annulus. Both RV GLS and RV FWS, however, are still regional measures of RV function because only part of the RV is analyzed. The accuracy of 2D STE is also limited by out-of-plane motion of the myocardium in the elevational dimension. 3D STE software is available for both the LV and RV and is expressed as “strain area” or the percentage change in an area of myocardium rather than just 2 points. Abnormal 3D RV strain is an independent predictor of mortality in patients with pulmonary hypertension.11
As of this time, however, 3D strain software is only available offline. Most studies of RV strain are performed using software developed for the LV (Figure 2) (Supplemental Digital Content 1, Video 1, http://links.lww.com/AA/B663 and Supplemental Digital Content 2, Video 2, http://links.lww.com/AA/B664). The use of LV strain software may cause confusion as the RV and septal segments may be wrongly labeled depending on how the basal region of interest (ROI) markers are set. Inverting the markers using the LV ME long-axis view model for the RV in the ME 4-chamber view corrects the labeling problem, but it is unclear whether it could affect the accuracy of the measurements.
HOW TO MEASURE RIGHT VENTRICLE 2D LONGITUDINAL STRAIN
Because the tricuspid annulus is in a different plane than the mitral valve annulus, RV-oriented ME 4-chamber view should be used when measuring RV strain (Table 2). The apex of the left ventricle should also be visualized to prevent foreshortening. A frame rate of at least 40–50 frames/s is required for adequate tracking as low frame rates may result in “speckles” being lost between frames. Speckle tracking software is launched directly on the echocardiography machine and applied to the RV. In this review, we describe how to measure 2D RV longitudinal strain using 3 different software vendors: Philips Qlab Chamber Motion Quantification, GE Automated Function Imaging, and Siemens Velocity Vector Imaging.
One important step in any strain imaging is to define the timing of systole. Peak strain is not equal to systolic strain as postsystolic shortening may occur after the end of systole, particularly in ventricular dysfunction. All software automatically uses the electrocardiogram R wave to identify the beginning of systole. For GE, the default setting of end systole is the end of the T wave but this can be adjusted. For Philips, the aortic valve closure (AVC) time must be specified by the operator to define end systole. When measuring RV strain, the pulmonic valve closure (PVC) time should ideally be used instead of the AVC, but the standard 4-chamber view does not usually include either the aortic valve or the pulmonic valve. Additional images therefore must be imported into the processing software to define either the AVC or PVC time. One easy workflow is to perform measurements of LV strain immediately before RV strain. As long as these LV and RV images are acquired in temporal proximity and the heart rate remains constant, the same AVC can be used for calculations of LV strain and RV strain (assuming that the AVC and the PVC closing times are similar). When only RV strain is being calculated one can import either a 2D image of the pulmonic valve (RV inflow/outflow view) or a spectral Doppler tracing of RV outflow tract flow for the specific purpose of defining end systole.
Once the strain software is launched, the next step is to apply one of the strain models to the RV. For Philips and GE, this is done using the LV 4-chamber or the long-axis view model, and the landmarks are manually entered to identify the tricuspid annulus and the RV “apex.” The software then automatically generates a sampling ROI over the RV myocardium and divides it into 6 (3 lateral and 3 septal) in Siemens and GE systems or 7 (with an additional apical segment) in Philips Systems. The ROI should then be reviewed and manually adjusted as needed to include most of the thickness of the myocardial wall, while still excluding the pericardium. Adjustments can be made to each individual segment or by adjusting the overall width and location of the ROI. The labels automatically assigned to the RV segments are related to the LV model chosen and may be incorrect. For the Siemens software the specific RV model is selected and the endocardium traced (Supplemental Digital Content 3, Video 3, http://links.lww.com/AA/B665). The software recognizes and divides the RV the walls into 3 lateral and 3 septal segments. Strain is measured using vector velocity imaging, a technique that extracts cardiac motion by tracking myocardial pixels adjacent to the user-defined trace in any direction multiple subsequent frames. This technique differs for the others described by assessing subendocardial instead of full thickness midmyocardial strain. The GE software provides an additional feature that assesses whether or not the tracking of each segment is accurate. Tracking of the myocardium can also be assessed visually. To calculate the RV FWS (lateral wall only) the septal segments are deselected (Figure 2).
RIGHT VENTRICLE 2D LONGITUDINAL STRAIN AND CARDIAC SURGERY
Preoperative studies suggest that 2D RV longitudinal strain is a better predictor of mortality after cardiac surgery than RV fractional area change 12 and that severely abnormal RV strain measurements may predict RV failure after LVAD implantation5,13 (as occurred in our index case). All these preoperative studies used TTE, however, and there are few intraoperative studies of 2D RV longitudinal strain with TEE.
Tousignant et al14 demonstrated that intraoperative TEE measurements of 2D RV longitudinal strain were feasible. When they compared postinduction TTE RV strain with intraoperative TEE measurements, however, they found that although RV GLS values were similar, RV FWS was significantly higher with TEE.14 Duncan et al15 recently compared intraoperative LV and RV strain measurements before and after aortic valve replacement and found that postcardiopulmonary bypass RV strain worsened while LV strain remain unchanged.
RV function is an important predictor of outcome after cardiac surgery.
RV strain measurements are feasible in the operating room,14 have been shown to correlate with more global measurements of RV function,10 and have significant prognostic value12; the next step will be to further validate these methods of measuring RV strain in the OR with TEE.
Clinician’s Key Teaching Points
- The complex crescentic shape of the RV makes any 2D assessment of RV function challenging. Typical TTE assessment of RV function such as tricuspid annular plane systolic excursion and S′ may be invalid for intraoperative assessment with TEE because of poor alignment of the ultrasound beam with tricuspid valve annular motion.
- Speckle tracking technology allows fast, semiautomated and relatively angle-independent measures of myocardial contraction. Strain is defined as the systolic fractional change in length of the myocardium. Negative values represent shortening, and positive values represent lengthening or thickening. Normal 2D RV longitudinal strain is <−20%.
- Preoperative RV dysfunction as measured with 2D RV longitudinal strain correlates with poor postoperative outcome and postoperatively with RV failure after LVAD implantation.
- The major limitation of 2D speckle tracking strain is that measurements are made in a single plane and do not capture translational movement in the elevational plane.
Name: Natalie Silverton, MD.
Contribution: This author helped create the outline, and write and revise the manuscript.
Name: Massimiliano Meineri, MD.
Contribution: This author helped conceive the paper, edit the images and videos, and create the outline, and edit the manuscript.
This manuscript was handled by: Nikolaos J. Skubas, MD, DSc, FACC, FASE.
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