Tissue Doppler and strain-rate imaging in cardiac ultrasound imaging: valuable tools or expensive ornaments?

Over the last 30 years, with the introduction of anatomic two-dimensional mode and Doppler hemodynamics, echocardiography has become a central part of cardiac imaging. The novel technologies tissue Doppler imaging (TDI) and strain-rate imaging (SRI) have recently been brought to the attention of practicing cardiologists. Numerous experimental animal and clinical studies have confirmed the utility of these techniques [1–7]. It is now the moment to verify how this information translates to patient care. Current clinical reports suggest a promising role in quantifying regional and global cardiac function by providing detailed information that is not easily obtainable using other imaging modalities. It remains to be proven how much of this information is really necessary in clinical practice, and whether the associated increased costs can translate into improved patient care.

What is measured with tissue Doppler & strain-rate imaging?

TDI allows the measurement of speed of motion of the interrogated structure in relation to the transducer. However, tissue velocities are confounded by velocities of adjacent structures. To distinguish wall tethering from true contraction, strain and strain rate can be derived from tissue velocities, either offline [8], or in real time [9]. Strain represents the relative deformation, while strain rate represents the speed of deformation. As TDI appears less noisy than SRI, it is generally preferred. However, the assessment of myocardial function is clearly more accurately evaluated by measuring strain and strain rates [1–3,10,11]. On the other hand, tissue velocities could be seen as having an advantage; for instance, longitudinal mitral and tricuspid annulus velocities represent the composite motion of entire myocardial walls; therefore, they may be better indicators of global function.

Advantages & disadvantages of tissue Doppler & strain-rate imaging

Both TDI and SRI provide quantitative results, a clear improvement over semi-quantitative or qualitative methods. Moreover, the measurement of wall motion using TDI/SRI does not require accurate border detection. Minor out-of-plane motion does not pose a problem, in contrast to the measurement of the classic indices of shortening or thickening using grayscale M-mode or two-dimensional echocardiography. The high temporal resolution (>200 frames/s) afforded by TDI and SRI techniques is higher than any existent non-invasive imaging technique (<50 frames/s in conventional two-dimensional echocardiography and <80 frames/s by tagged magnetic resonance imaging or computerized tomography). Considering that cardiac motion is not sinusoidal, but has multiple troughs and peaks, and that the velocity and acceleration of motion represent valuable mathematical terms to describe the efficacy of any motion, the need for higher temporal resolution becomes immediately evident. This is particularly important when looking at short-lived events such as isovolumic periods. Both TDI- and SRI-derived velocity and acceleration are better measures for describing cardiac motion than classical wall thickening [12–14]. Detection of postsystolic shortening or thickening – a highly specific marker of myocardial asynchrony [15,16] – and potentially of viability [1,17], is facilitated by the high temporal resolution and quantitative nature of these tools.

The disadvantages of TDI and SRI include angle dependence, which limits the number of cardiac segments that can be interrogated. The alignment of the Doppler beam with the direction of wall motion is imperative. This limitation not only calls for additional time for analysis (problematic in a busy clinical practice) but could also introduce additional variability in measurements and limit the sensitivity and specificity for the detection of functional abnormalities if the above precautions are not considered and limitations understood; particularly susceptible are hearts with shape distortion. The noise introduced by imaging artifacts is amplified when using derivatives (i.e., SRI). In addition, the three-dimensional component of wall motion and the variable response and adaptation of the heart to different diseases adds more complexity to the interpretation of findings. Some limitations could be overcome in the future by the introduction of non-Doppler-based methods [18,19], but these techniques have not yet been clinically tested.

Role of tissue Doppler & strain-rate imaging in different pathologies

The role of these quantitative techniques has been repeatedly demonstrated in the detection of acute myocardial ischemia and viability [1–3,9–11,20], for predicting increased filling pressures [6,21,22] and outcome of patients [23,24]. These methods are sufficiently sensitive to facilitate the detection of the early signs of cardiac impairment in various conditions, such as in amyloidosis [4,25], diabetes [5], hypertension [26], valvular disease [27–29], transplant rejection [30] and obesity [31]. TDI provides valuable unique information for differentiating restrictive from constrictive cardiomyopathy [32], physiologic from pathologic hypertrophy and hypertensive hypertrophy from hypertrophic cardiomyopathy [33–35]. Both methods are particularly valuable for the assessment of cardiac asynchrony [7] and regional right-ventricular function in acquired and congenital heart disease [1,3], and potentially, for monitoring the effects of various therapeutic strategies [36]. It is anticipated that the utility of TDI and SRI techniques will further expand in the near future.

In each condition, the sensitivities and specificities to detect functional abnormalities vary, according to the parameters and cutoffs used and the degree of cardiac involvement. The heterogeneity in regional function limits the accuracy for the detection of abnormalities. The effects of other influential factors on cardiac function (pericardial disease, pre-existing right heart or pulmonary disease, loading conditions, status of great vessels and neurohumoral responses) must be taken into consideration in order to permit the recognition of true alterations in contractile function. All diseases and factors have nonspecific effects on regional function. Thus, specificity of findings is low if analysis of regional function is used alone (or worse, a single parameter) to establish the exact cause of the observed wall motion abnormalities. The best solution is to integrate all available information (function, perfusion, structure and metabolism) to reach an adequate and more correct diagnosis.

Can tissue Doppler & strain-rate imaging detect myocardial ischemia better than a trained eye?

Dobutamine stress echocardiography has a high sensitivity and specificity for the detection of myocardial ischemia when interpreted by experts [37,38]; however, there is still significant interobserver variability [39,40]. TDI and SRI may help to accelerate the learning curve in interpreting stress echocardiograms and improve the accuracy of readings [41]. By using TDI and SRI, apart from indices of contractile function, distinct patterns of diastolic motion can be depicted during inducible ischemia with high sensitivity and specificity that cannot otherwise be evaluated by the naked eye [1,42,20]. Such findings could introduce novel paradigms in dobutamine stress echocardiography. Nevertheless, whether TDI and SRI can improve the detection of ischemia beyond a trained eye is less clear [40,41,43,45]. More proof is therefore needed to justify the additional time required to analyze quantitative data and not solely rely on conventional imaging and expert interpreters. Perhaps by using TDI and SRI methods, the interinstitutional agreement in interpretation of stress echocardiograms could be improved, but this remains to be proven.

Increasing role of tissue Doppler & strain-rate imaging in cardiac resynchronization therapy: is it only timing that matters?

TDI and SRI are now being tested for the selection of patients who would benefit from cardiac resynchronization therapy (CRT) and for guiding lead location [7,46–48]. By restoring the synchronicity of myocardial motion, CRT may improve global ejection fraction and the outcome of the patients. With the advent of TDI and SRI, which readily allow assessment of inter- and intraventricular asynchrony, further refinements in CRT have recently occurred. It becomes evident that electrocardiographic QRS duration is not a good predictor of the sequence of ventricular activation [49]. It is now believed that mechanical rather than electrical synchronicity is more important for the prediction of the response following CRT.

Owing to the asynchrony in contraction and relaxation between the early and late-activated segments, there is a waste of mechanical energy and thus, a reduced efficiency of contraction. Changing the pathway of activation and contraction in the heart could minimize the time delay in contraction between segments and improve ejection fraction. A factor largely overlooked is the extent of this mechanical energy that is wasted by asynchrony. If the intrinsic contractility in the heart is sufficiently good to create a large amount of wasted mechanical energy, CRT will theoretically have a large beneficial effect. In contrast, in a dyssynchronous but poorly contracting heart, this benefit would be lower following resynchronization. Thus, not only timing is important but also how much mechanical energy is lost by asynchrony, which could potentially be restored to the ejection phase after CRT.

Currently, there is great enthusiasm but still limited experience with the utility of TDI and SRI for predicting the response after CRT and for guiding the selection of lead location. Published studies are very promising but have included only small number of patients. The best parameter(s) for the detection of cardiac asynchrony still remain to be established. The long-term results for TDI-guided lead location are not known.

Can tissue Doppler or strain-rate imaging information change patient management?

In addition to a promising role in the evaluation of candidates for CRT and in guiding pacing lead location, encouraging results have been obtained with TDI/SRI for detection of cardiac involvement in various disease states. It is hoped that early detection of cardiac impairment could offer the basis for prompt treatment or help with the selection of timing of therapeutic intervention. Although very promising, these results are sparse and need to be confirmed by several centers and large clinical trials.

In summary, TDI and SRI are playing an increasing role in cardiac imaging. These methods provide accurate quantification of cardiac function that may assist with diagnosis and prognosis of the patient. For specific applications, more studies are needed to prove their advantages, including accuracy and cost-efficiency, over existing methods before incorporating them into routine clinical practice.