Heart Views

EDITORIAL
Year
: 2002  |  Volume : 3  |  Issue : 1  |  Page : 2-

A question of time tissue doppler imaging in coronary artery disease


Riccardo Rambaldi, Jeroen J Bax, Don Poldermans 
 Erasmus Medical Centre, Rotterdam, The Netherlands

Correspondence Address:
Don Poldermans
Dr. Molewaterplein 403015 GD Rotterdam
The Netherlands




How to cite this article:
Rambaldi R, Bax JJ, Poldermans D. A question of time tissue doppler imaging in coronary artery disease.Heart Views 2002;3:2-2


How to cite this URL:
Rambaldi R, Bax JJ, Poldermans D. A question of time tissue doppler imaging in coronary artery disease. Heart Views [serial online] 2002 [cited 2021 Jun 13 ];3:2-2
Available from: https://www.heartviews.org/text.asp?2002/3/1/2/64419


Full Text

The review by Yip and Ommen [1] addresses the recent developments of tissue Doppler imaging to assess myocardial viability and ischemia objectively. It may be a difficult topic for a reader not daily involved in (stress) echocardiography. Therefore, the intention of this editorial is to ease the reading of the study by providing keys of interpretation.

The features of this review are:



Objective assessment of myocardial viability and ischemia during stress echocardiography by Doppler imaging techniques.Discussion of new Doppler based techniques like, tissue Doppler imaging (TDI), myocardial velocity gradient (MVG) and strain rate imaging (SRI).From these investigations, new parameters are expected, in particular, quantitative parameters, which should be extracted at any time of cardiac systole or diastole in order to objectively recognize myocardial viability and ischemia. So far, myocardial viability and ischemia are recognized only subjectively by wall motion.

The search for objective echocardiographic markers of myocardial ischemia and viability has been under way almost since the introduction of echocardiography. Considering the huge amount of research on this topic and the few solid results attained, it appears evident that this is one of the most demanding tasks of echocardiography. Also the use of these techniques during stress echocardiography (pharmacological or exercise) made this even more difficult to accomplish. This last difficulty explains the gap that emerged since the introduction of TDI to evaluate various non-coronary cardiac diseases at rest and during stress, the later for the detection of coronary artery disease. Since coronary artery disease is the number one cause of death in developed countries, research efforts focus on its early detection.

The excellent review paper by Yip and Ommen focused on the demanding and intriguing task of recognizing and quantifying myocardial ischemia and viability by TDI, addressing the limitations of the technique and future directions.

We will examine (1) TDI, (2) MVG, (3) strain (rate) and (4) the most recent isovolumic parameters.

For the reader, we repeat some definitions:



TDIMyocardial velocity gradient (MVG)Strainthe most recent isovolumic parameters.

 1) TDI



(1) TDI is the color Doppler technique modified to colorize the myocardium, in order to measure the velocities of myocardial contraction and relaxation.

Pulsed wave Doppler

is the sampling of a myocardial point or region of interest from which to display a velocity profile over time.

Myocardial motion, which includes both contraction and relaxation, may be studied either radially (endo- epicardial direction of motion in parasternal imaging) or longitudinally (cardiac base to apex direction of motion in apical imaging).

Since the apical imaging allows measurement of higher velocity values than the parasternal imaging, it appears more sensitive to detect velocity changes. The most elusive remains the cardiac apex because of extremely low velocity values and its direction of motion, which is eccentric from the direction of the Doppler beam.

Global and regional assessment of left ventricular function to detect myocardial ischemia and coronary artery disease:

The authors describe peak mitral annular velocities in apical views as indexes of global left ventricular function correlating to stroke volume and ejection fraction, and simultaneously indexes of individual ventricular wall motion, reflecting the longitudinal contraction of each sampled wall.

Each ventricular wall roughly represents a coronary field, with the exception of the basal posterior septum, which depends on the right coronary artery and the mid and apical segments, which are supplied by the left descending coronary artery.

A correlation between Doppler sampling at the level of the mitral ring in apical images and each coronary field is therefore possible.

The information, which is lost by this sampling, concerns distinguishing proximal versus distal coronary vessels.

For this purpose, a more segmental approach is obtained by placing additional pulsed Doppler samplings between the mitral ring and the cardiac apex, or by using color TDI.

The regional asynchrony, followed by systolic velocity reduction induced by dobutamine on TDI, can precede other signs of myocardial ischemia, thus rendering TDI more sensitive than dobutamine stress echocardiography alone.

Time-to-peak systolic velocity and time-velocity integral are additional promising parameters for detection of myocardial ischemia. Implementation of objective parameters into the stress echo package would allow higher reproducibility and higher accuracy to detect myocardial ischemia and coronary artery disease [2].

Regional assessment of left ventricular function to detect myocardial viability:

Viability can be studied using the same approaches as in myocardial ischemia. The most promising among the various ischemia parameters are systolic velocity increase and post-systolic shortening induced by dobutamine. TDI appears more sensitive and equally specific than dobutamine stress echocardiography. However, TDI remains influenced by global cardiac translation and by tethering from adjacent myocardial segments. MVG and SRI were both developed to eliminate the bias of global cardiac translation and tethering.

Both these techniques need short clarifications:

 (2) Myocardial velocity gradient (MVG)



It represents the differential velocity of two points adjusted for the distance between them. Since the introduction of TDI, a radial velocity gradient was calculated (endocardial minus epicardial velocity divided by wall thickness). The velocity gradient was calculated off-line and was limited to radial direction.

 (3) Strain



Strain is tissue deformation as a function of applied force (stress), normalized to tissue original length.

It was first studied by ultasonomicrometry and magnetic resonance imaging.

Strain reflects the functional properties of the tissue. It is a dimensionless measure of regional lengthening and shortening of the myocardium.

It is obtained with an algorithm similar to MVG, but based on direct processing of Doppler signals, rather than on post-processing.

The algorithm calculates spatial differences in tissue velocities between 2 adjacent myocardial points, implying either compression or lengthening of the myocardium in the between.

Strain was reported having a better correction for the whole heart motion and a higher spatial resolution than MVG. Myocardial ischemia may be recognized as a dishomogeneous distribution of regional strain.

Strain rate

is the time derivative of strain, measuring the rate of deformation.

It represents the shortening velocity per unit of fiber length. As an example, strain is equivalent to the distance covered by a car, while strain rate corresponds to the car speed.

Strain rate (and strain) throughout the cardiac cycle

These are represented as a cyclic curve of a single point over time or as a color coded imaging (SRI) over M-mode imaging.

M-mode is applied both in short axis imaging (radial direction) and apical imaging (longitudinal direction).

The superiority of strain rate over TDI to eliminate heart translation bias is under investigation. Moreover, SRI changes during dobutamine appear preceding TDI changes induced by myocardial ischemia, therefore rendering SRI more sensitive for detecting myocardial ischemia.

From all these techniques it appears evident that most of the past limitations of TDI have been overcome. However, angle-dependency remains the major limitation of any Doppler-derived technique (TDI, MVG, SRI) for evaluating ischemia.

 (4) The main focus of the review is on the recently detectable isovolumic parameters



Both the isovolumic contraction (IVC) and relaxation (IVR) phases recently became possible to investigate due to high temporal resolution of echocardiographic imaging.

This was possible due to important software implementations allowing a high frame rate imaging.

TDI, MVG and SRI are all incorporated into the study of isovolumic phases.

Both IVC and IVR phases represent situations in which the afterload to left ventricular contraction and relaxation is very low.

This is a privileged situation for myocardial contraction to express itself, especially in cases of stunning or hibernation, in which the low or absent myocardial thickening during ejection phase, i.e.against a very high afterload, becomes possible during the isovolumic relaxation phase. Myocardial velocity during IVC decreases to zero or inverts direction of motion in cases of myocardial ischemia.

This aspect seems peculiar with the delayed onset of contraction in response to ischemia.

Myocardial velocity during IVR appears increased, late-peaking, and prolonged by ischemia.

This aspect, called post-systolic thickening (if radial) and shortening (if longitudinal) seems also peculiar with the delayed onset of contraction in response to ischemia. Both the onset of post-systolic shortening and its late-peaking precede systolic velocity reduction, thus they are possibly more accurate in detecting ischemia.

A growing corpus of investigation is demonstrating that post-systolic shortening is active delayed contraction, manifested by sudden decrease of afterload during IVR [3],[4].

In hibernating and ischemic myocardium, dobutamine reduces systolic velocities and increases post-systolic shortening, while in stunned and normal myocardium dobutamine increases systolic velocities and normalizes post-systolic shortening.

The pathophysiologic substrate of this dichotomy is an exciting area to investigate. During ischemia, post-systolic shortening are reported to precede the systolic changes. Post-systolic shortening also appears less load-dependent than the early diastolic relaxation, and thus it should reflect only left ventricular relaxation.

This could be a further step forward in the assessment of the ischemic cascade and possibly also into the viability cascade [5].

From the review by Yip and Ommen, it is too early to predict which of these parameters will be implemented into the dobutamine stress echocardiography package.

But from the large amount of fascinating data, it seems just a question of time.

References

1Yip GW and Ommen SR. Tissue Doppler imaging in coronary artery disease. Heart Views 2002;3(1):p?
2Grocott-Mason R, Payne N, Wilkenshoff U, et al. Can off-line tissue Doppler echocardiography make dobutamine stress echocardiography objective? Eur Heart J 1999;20(suppl):687.
3de Zeeuw S, Trines SAIP, Verdow PD, Dunker DJ. Cardiovascular profile of the calcium sensitizer EMD 57033 in open-chest anaesthetized pigs with regionally stunned myocardium. Br J Pharmacol 2000;129:1413-1422.
4Soei LK, de Zeeuw S, Krams R, Duncker DJ, Verdouw PD. Ca(2+) sensitisation and diastolic function of normal and stunned porcine myocardium. Eur J Pharmacol 1999;386:55-67.
5Rambaldi R, Poldermans D, Bax JJ, Boersma E, Valkema R, Roelandt JRTC. Post-systolic shortening during dobutamine stress echocardiography: delayed contraction of viable dyssynergic myocardium. Submitted.