Adjusting Therapy in Pulmonary Arterial Hypertension – Novel Echocardiographic Techniques
Doppler echocardiography is an angle-dependent Doppler technique. Tethering by adjacent segments and in particular of the mitral valve annulus (annular calcification or prosthetic valves) may cause errors in assessment. These limitations also apply to tissue tracking. Complex fibre architecture, translation and rotation of the heart in the chest are common limitations to all cardiac imaging modalities. Deformation imaging can be obtained with Doppler and non-Doppler techniques.
The shape of a structure deforms during motion when different elements of the structure move with different velocities. Myocardial strain is the percentage of shortening or lengthening of a myocardial segment. Strain rate is the rate at which the myocardium shortens or lengthens.9
to a higher density of scan lines on conventional echocardiography. STE also has better reproducibility as it does not require using an ROI. Limitations include poor image quality, which induces poor wall tracking, and the technique may not be reliable when the frame rate is 120 per minute have been described.14
Other limitations include out-of-plane
motion of speckles, drop-outs and insufficient temporal resolution. Although speckle-derived strain has been validated in various circumstances, this recent technique requires further supporting data. 3D speckle tracking is a step forward to achieve this goal.
These modalities partially compensate for
tethering of abnormal myocardial segments by adjacent normal segments and passive motion of the heart within the chest. Doppler strain is calculated as the spatial gradient between two velocities separated by a short distance (sample distance) within the myocardial wall. Systolic strain rate correlates more closely with invasive parameters, including peak elastance, than systolic tissue velocity in experiments.10
Vector velocity imaging combines speckle tracking with border and annulus tracking while correcting for changes in RR periodicity. It could overcome some of the limitations associated with speckle tracking by utilising other features to maintain focus on the ROI. While some reduction in global function has been shown using this technique in one study of children with pulmonary hypertension, its true potential remains unexplored.15
Doppler systolic strain rate and isovolumic myocardial acceleration are indices of global LV systolic performance that are less dependent on loading conditions. Myocardial acceleration during isovolumic contraction has been validated as a sensitive, non-invasive method of assessing left and RV performance.
The main limitations of Doppler strain and strain rate relate to reproducibility. Increasing region of interest (ROI) size, changing sample distance and very high frame rate >200 per second do not totally solve these problems. In theory, peak systolic strain rate is the best available parameter for measuring segmental function. However, Doppler strain rate curves are noisy. Doppler strain is obtained through integration of the strain rate signal; therefore, noisy strain rate signals may co-exist with a less noisy strain signal of questionable value. Doppler strain and strain rate are angle- dependent Doppler techniques and loading variations and myocardial stiffness are important determinants of myocardial deformation.
Speckle Tracking and Vector Velocity Imaging Non-Doppler deformation imaging includes speckle tracking and vector velocity imaging. Speckle-tracking echocardiography (STE) enables an objective assessment of three myocardial deformation components: longitudinal, radial and circumferential, or torsion.11
Speckles are
unique natural acoustic markers generated by the reflected ultrasound beam. The speckle pattern is unique for each myocardial region and is relatively stable throughout the cardiac cycle. The geometric position of speckles changes from frame to frame, and represents local tissue movement. These markers are tracked by calculating frame-to-frame changes using a sum of absolute difference algorithm. The displacement between speckles represents myocardial deformation (strain). Tracking a defined region of speckles allows the calculation of displacement velocity, strain and strain rate. An image frame rate recorded in conventional 2D mode may not be adequate for strain-rate calculations. Longitudinal parameters are obtained in apical views, while radial and circumferential parameters and torsion are obtained in short-axis views. Radial-strain measurements did not correlate well with sonomicrometry, demonstrated larger variability and have not been as extensively studied.11–13
STE is an angle-independent technique, as the movement of speckles can be followed in any direction.11,12
EUROPEAN CARDIOLOGY STE has better lateral resolution due 3D Echocardiography
3D echocardiography includes full-volume, realtime 3D and 3D zoom. Acquisition is usually carried out over four to six cardiac cycles. Despite technical improvements, this technique remains mainly useful in patients with good images, which might explain why 3D echocardiography has not been widely used in clinical practice.16,17
LV
ejection fraction (LVEF) and volume magnetic resonance imaging (MRI) data increase with 3D.18
Although these imaging modalities have
been widely available for more than 10 years, they are not routinely used in many echo laboratories due to cumbersome image acquisition and to the expertise required for post-processing analysis and interpretation.
The Right Ventricle
The right ventricle is a difficult chamber to functionally assess because of its complex shape, thin walls, trabeculations, extreme load dependency and variable position within the chest during respiration and changes in body position and in response to dilation. The importance of the right ventricle is increasingly recognised, although our tools for assessing RV function remain crude. In patients with LV dysfunction, the presence of associated RV dysfunction is an independent predictor of mortality.19
In pulmonary hypertension,
We appear to have reached the limit of current medical therapies in our ability to selectively reduce pulmonary pressures; however, we have evidence that not all therapies are equal in terms of the impact on RV function. Sildenafil appears to improve RV function, despite a similar impact on PAPs compared with bosentan.21
mortality is due to RV failure, and those in whom the right ventricle is well adapted to pressure work (Eisenmenger’s) survive much longer with similar or higher pressures than those with a normal right ventricle (idiopathic PAH [iPAH]), who in turn have a significantly better prognosis at higher pressures than those with RV replacement fibrosis (scleroderma).20
Assessing the RV contractile function and response over time requires not just the ability to measure volumes and rate of contraction, but also the ability to resolve the changing environment of the right ventricle. Concerns such as accurate determination of pre-load and afterload, including not just pulmonary vascular resistance but also pulmonary artery compliance relating to dynamic work required to distend proximal pulmonary arteries, and impedance (inertia of stored blood and vessel tone) arise. Given that
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