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Tissue Doppler EchocardiographyReviewer: Andrew Maslow, MD
Information garnered from echocardiography and invasive hemodynamic monitors have taught us much about cardiac physiology and pathophysiology. The introduction of intraoperative transesophageal echocardiography (TEE), has allowed us to visualize the heart in motion and make acceptable subjective qualitative assessments of ventricular function during the intraoperative period. However, we have little ability to objectively quantify regional ventricular function. Tissue Doppler Echocardiography (TDE) enables the echocardiographer to quantify regional and global systolic and diastolic function of both ventricles. 1,2 Tissue Doppler echocardiography, a variation of conventional pulse wave Doppler (PW) echocardiography, measures myocardial motion and velocity. During conventional PW Doppler, a filter eliminates the lower velocities generated by cardiac tissue, allowing the system to concentrate on higher velocities scattered from moving red blood cells. This filter is inactive during TDE, allowing measurement of the higher amplitude lower velocity signals generated by tissue motion. By placing a 1 cm long sample volume along the mitral annulus (global assessment), or within the myocardial tissues (regional assessment), the frequency shift of tissue motion can be quantified. Depending on the orientation of the ultrasound beam to the tissue plane TDE can measure either radial or longitudinal ventricular contraction and relaxation. Longitudinal fibers concentrate along the subendo- and subepicardial layers and along the papillary muscles. These muscle fibers may be more sensitive to changes in afterload, or more likely to develop hypertrophy, fibrosis, and/or ischemia. Currently, most of the literature is generated from transthoracic echocardiography (TTE) using apical cardiac views (analogous to TEE mid esophageal four, two, long axis windows), where analysis of longitudinal cardiac motion is possible. Other imaging planes may include parasternal long axis and short axis views (analogous to transgastric long and short axis views) where radial fibers can be assessed. Tissue Doppler echocardiographic analysis can be displayed alone and on-line from one site, or in conjunction with color M-Mode or color two-dimensional (2DE) echocardiography. The final graphic display includes one positive systolic (S) deflection, and two negative diastolic (Em; Am) waveforms. The systolic waveform is preceded by regional isovolumic contraction time (RIVCT) and the diastolic waves are preceded by regional isovolumic relaxation time (RIVRT). The first diastolic deflection represents the early rapid filling phase (E), which is followed by a period of diastasis, and a second late active filling phase (A) due to atrial contraction. Although similar to conventional Doppler these measures represent regional functions when the sample volume is located within the myocardium and global function when located within the mitral annulus. While the pulse wave tissue Doppler (PW-TDE) imaging is like that of conventional PW Doppler, the color 2D (2D-TDE) and M-Mode TDE displays are more complex. Color displays are based on assignments of colors to represent velocities. Lower velocity colors tend to be darker colors while higher velocities are represented by brighter colors. The accompanying color bar on the display associates the colors with tissue velocities. Image processing for these two displays requires digital storage, detection of low velocity artifacts, and a pixel-by-pixel decoding of the color display against a reference table, to obtain tissue velocities. These are displayed graphically to represent regional or global myocardial velocities, with a simple two-dimensional display or three-dimensional display, the latter including velocity, time, and distance. Additional TDE data includes tissue velocity gradients, myocardial strain and strain rate. The velocity gradient is the difference between two tissue velocities. Data shows that the subendocardium moves faster than the subepicardium, which visually appears as wall thickening and thinning. From measured velocity gradients, the myocardial strain, or the deformation of tissue over time compared to its initial length or thickness, can be measured. The stain rate is the rate of deformation over time and is a strong indicator of contractility. Depending on the angle of interrogation, the strain rate can be measured either circumferentially or longitudinally. Three articles describe the use of tissue Doppler echocardiography and how it can increase our understanding of heart failure in patients with 'isolated diastolic dysfunction' or 'diastolic heart failure (DHF)'. 3-5 Nikitin et al establishes a reference base for normal TDE across four age groups (20-39; 40-59; 60-79; ≥ 80 years) without clinical evidence of heart disease.3 Patients were studied with transthoracic imaging using standard two-dimensional echocardiography, conventional pulse wave (PW) Doppler analysis of transmitral and transtricuspidflows, and tissue Doppler analysis of mitral and tricuspid annuli, the base, mid, and apical left ventricular walls, and the right ventricular free wall. No significant differences were found among the four groups for left ventricular ejection fraction (LVEF) or indexed intracavitary volumes. Patients ≥ 60 years tended to have greater LV mass and septal wall thickness. Normal data from the youngest group included an Em > 8cm/s, and a Em/Am > 1.0. Decreased mitral annular Em (< 7 cm/s by the fifth decade) and Em/Am (< 1.0 by the seventh decade) were consistent with reductions in diastolic function suggested by conventional Doppler exam. However, relatively greater increases in annular Am in the fifth decade imply earlier compensatory augmentation of atrial function than suggested by conventional transmitral PW Doppler. Tissue velocities are higher at the base of the heart and lower toward the apex.3 Apical velocities were low enough to make distinction from noise difficult such that the authors did not analyze apical velocities; a limitation of TDE.3 Analysis of the basal and mid left ventricular segments revealed an age related decline in longitudinal systolic dysfunction directly related to the decline in diastolic function. While no change in right ventricular systolic function was suggested by TDE of the trisuspid annulus or the RV free wall, there were reductions in RV diastolic function with increasing age.3 These data from normal patients demonstrate that decline of left ventricular function is associated with a reduction of LV longitudinal systolic dysfunction despite a normal radial systolic function measured by LVEF.3 The other two manuscripts also show a direct and consistent relationship between reduced ventricular compliance and longitudinal LV systolic dysfunction.4,5 Baruch et al compared clinically normal patients to patients with 'isolated LV diastolic heart failure (DHF)' (clinical CHF with an LVEF > 45%), and patients with systolic heart failure (SHF).4 Measured LVEF for the control group and DHF patients were similar (64 vs. 63%) and significantly greater than the SHF group (32%). Indexed end systolic and diastolic ventricular volumes were similar between control and DHF patients and were significantly less than the SHF group. Left ventricular mass indexes were similar between the patients with DHF and SHF and significantly greater than control patients. Conventional and tissue Doppler analyses similarly showed reductions in diastolic function for the DHF and SHF groups, the latter being more severe. Despite a normal ejection fraction the DHF patients had a significantly lower tissue Doppler mitral annular systolic velocity consistent with global systolic dysfunction, supporting the notion that 'isolated diastolic dysfunction' does not exist. The third manuscript, by Poulsen et al, compared three groups: 1) normotensive patients (control group), 2) hypertensive patients without evidence of diastolic dysfunction (group A) according to conventional Doppler echocardiography, and 3) hypertensive patients with diastolic dysfunction (group B) according to conventional echocardiography.5 All patients recorded an LVEF > 55% and a fractional shortening > 28%. Of the 20 group B patients, 18 were diagnosed with an abnormal relaxation pattern, and two with pseudonormal patterns. The mean LVEF for the control group, group A, and group B, was 62, 62, and 61% respectively. Group B had a significantly greater left atrial size, ventricular wall thickness, left ventricular mass, and lower tissue systolic velocities. A significant correlation was seen between increased left ventricular mass and a reduction in both diastolic and longitudinal systolic dysfunction regardless of the radial systolic function represented by the LVEF. For hypertensive patients, diastolic and systolic dysfunctions occur together, despite a normal LVEF, and are related to the presence of ventricular hypertrophy. Conclusions: Tissue Doppler echocardiography provides a quantitative evaluation of both regional systolic and diastolic myocardial functions. Since it's initial application TDE has applied mostly to non-surgical cardiac patients with a variety of cardiac pathologies. Not only does TDE provide evidence against the existence of 'isolated diastolic dysfunction', but has been useful in differentiating different cardiac pathologies and severities, contributing to therapeutic decisions and outcome predictions. This latest ultrasound technology reminds us that heart muscle fibers are oriented both circumferentially and longitudinally. Conclusive statements regarding systolic heart function based on the LVEF, a measure of circumferential contraction, is only partly correct. Similarly, basing hemodynamic management assuming the LVEF represents all systolic function would be inaccurate. These data are consistent with an increased need for inotropic support to meet perioperative hemodynamic goals, for patients with previously diagnosed 'isolated diastolic dysfunction' and 'normal systolic function'. Future studies and applications seem limitless including assessing whether TDE is a better predictor of cardiac events during both cardiac and non-cardiac surgeries. TDE may become a regular component of stress echocardiography to better identify myocardium at risk. Evaluation of regional function before and after coronary revascularization procedures, in and out of the operating room, could evaluate the immediate effects and successes. TDE may provide the best reproducible quantitative measure of right heart function. The differential effects of anesthetics on the different components of heart function could help develop safer anesthetic regimens. Whether or not tissue Doppler echocardiography becomes widely accepted in the operating room, the information and education that is garnered from it should improve our understanding of the heart and how age, disease, and drugs affect it. This would be expected to improve our assessment and management of patients in and out of the operating room. References
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