PRO: Intraoperative Doppler Tissue Imaging Is a Valuable Addition to Cardiac Anesthesiologists’ Armamentarium

Echocardiography was first introduced as a clinical diagnostic tool in 1953. Edler, a physician at Lund University, Sweden, and Herz, a physicist, borrowed a device, which used ultrasound to detect flaws in metals, and tried it on their own hearts.1 Without changing the ultrasound frequency, they were able to record M mode images of moving structures, later proven to be the mitral valve and the heart walls.2 They noted that patients who suffered from mitral stenosis had a different motion pattern.3 The next five decades saw a dramatic improvement in image quality and the ability to evaluate the heart and the blood vessels: This includes two dimensional, real-time images and Doppler echocardiography (which demonstrates the blood flow and can assess hemodynamics, including pressure gradients, flow velocity, volumes, and resistance). In many instances, this harmless, noninvasive technique replaced cardiac catheterization as the technique of choice and the “gold standard” in the evaluation of heart disease. Transesophageal echocardiography, which was first introduced in 1976 by Frazin et al.,4 further expanded the diagnostic capabilities. At that time, cardiac anesthesiologists joined the ever-expanding users’ circle; with the probe in the esophagus, they were able to monitor anatomy, the hemodynamics and complications during various stages of anesthesia and surgery. This powerful tool improved the quality and the results of these operations. Engineering ingenuity usually goes hand-in-hand with clinical demands. In some cases, however, technical solutions come before a clinical problem in a need of solution is recognized, and thus, physicians had answers available to questions they did not know to ask. Doppler tissue imaging (DTI, also known as myocardial Doppler imaging) is such an example. When DTI was invented, it was not clear what it was needed for. The Japanese group who first described the technology in the mid 1980s used it for the detection of rigid atherosclerotic plaque motion in the experimental laboratory. At that point, it did not interest clinicians and was therefore abandoned. Several years later, in 1989, a French group led by Isaaz reintroduced the concept and used pulsed wave Doppler to evaluate wall motion in healthy and abnormal myocardium.5 It took years of experimentation and experience to realize that we had, not only a fancy gadget, but also an extremely important and useful modality that can provide information that is essential for the assessment and quantification of most cardiac conditions.6 For the last 15 years or so, DTI has been an essential part of the routine echocardiographic examination. Every modern commercial echocardiograph has the ability to perform DTI and/or its derivatives. A test that does not include the measurement of tissue velocities should be considered incomplete. In clinical echocardiography, there are two types of movement-reflecting interfaces resulting in the Doppler effect of sound wave frequency shift: the blood (where the interfaces are the red blood cells) and the solid structures (such as myocardium, aortic walls, and valves, where the interfaces are the tissues of these structures). The velocity of blood is much faster (at least 10 times) than that of the solid tissues. This results in a larger Doppler frequency shift. However, the signal strength of the reflected ultrasound is much stronger in solid tissues than in blood. With the use of simple filters, the blood signals can be suppressed and the tissue signal can be easily presented and recorded. The high signal intensity allows the recording of Doppler tissue signals even when the two dimensional echocardiographic images are technically difficult and of poor quality. Thus, DTI can provide crucial information about left ventricular (LV) systolic and diastolic function even when endocardial resolution is poor, and quantification of chamber size, wall motion, and ejection fraction is difficult, inaccurate or even impossible, as is frequently observed in the intensive care unit or the recovery room where echocardiography is often suboptimal. In the comprehensive timely review of Skubas7 in this issue of the journal, the theory, technology, technique, and utility of DTI are discussed in detail. This editorial will stress some of the most important clinical uses of DTI in the daily practice of the anesthesiologist. Many of these require only simple pulsed Doppler spectral tracing of the velocity of the mitral annulus motion in the longitudinal axis (Fig. 1).Figure 1.: The most valuable single frame: Tissue Doppler with the sample volume at the mitral annulus. A’ peak tissue velocity during atrial contraction, D’ peak diastolic tissue velocity, S’ peak systolic tissue velocity.Easy Estimation and Monitoring of Global LV Systolic Function With the sample volume at the mitral annulus, one can simply measure the height of the systolic DTI wave (S’). An S’ wave of 7 cm/s or more measured in the longitudinal axis usually indicates an ejection fraction of >50%.8 Easy Demonstration, Quantification and Monitoring of Segmental Wall Motion Abnormalities With the use of color DTI, segmental wall motion can easily be evaluated even by the less experienced. Pulsed Doppler, with the sample volume at different sites, can further define and quantify the abnormality.9 Assessment of LV Diastolic Function With the sample volume at the mitral annulus, the simple measurement of the E’ wave in the longitudinal axis is probably the most important single variable for the evaluation of global LV diastolic function. By itself, a low E’ indicates abnormally low relaxation velocity. Segmental diastolic abnormalities are less recognized and less frequently used; however, segmental diastolic abnormalities precede systolic abnormalities in segmental or global ischemia.10 Abnormal diastolic function usually precedes systolic abnormalities in the early stages of cardiac transplant rejection.11 Assessment and Monitoring of LV Filling Pressure This extremely important hemodynamic information can now be obtained in most patients without the need for invasive measurement of the pulmonary capillary wedge pressure. Noninvasive assessment requires the evaluation of the peak early diastolic wave of transmitral inflow (E), and the DTI E’ wave. An E/E’ ratio of 15 or more indicates abnormally increased filling pressures (>14 mm Hg). A ratio of 8 or less suggests normal LV filling pressure. Simple formulas demonstrated a direct correlation between the E/E’ ratio and the filling pressure.12 Evaluation of LV Synchrony and Dyssynchrony Normally, all the walls of the LV reach their peak systolic velocity at the same time. When there is dyssynchrony, the peak DTI is reached at different times. Thus measuring the time interval from the onset of the QRS complex of the electrocardiogram to the peak velocity measured from DTI of the various walls is considered by many as the best technique to assess synchrony or dyssynchrony. A delay of more than 65 ms between 2 opposite walls (or a standard deviation of more than 30 ms among 12 sites of time to peak systolic velocity) is considered dyssynchrony. The value of this determination in the selection of patients with heart failure for biventricular pacing is the reason for an ongoing debate in the current medical literature.13 Differential Diagnosis Between Constrictive Pericarditis and Restrictive Cardiomyopathy14 The two disorders have many similar clinical, hemodynamic, and echocardiographic features and are considered a difficult differential diagnosis. DTI can easily solve this mystery; in constrictive pericarditis, the E’ is normal or higher than normal, whereas in restrictive cardiomyopathy it is usually low. Thus, in constrictive pericarditis the E/E’ ratio is normal, an unusual finding in the setting of increased filling pressure and clinical heart failure. This unusual combination (high LV filling pressure and normal E’) is known as “Annulus Paradoxus.” Differential Diagnosis Between Pathologic and Physiologic LV Hypertrophy In physiologic hypertrophy, as sometimes seen in athletes, the DTI is normal. Patients with hypertrophic cardiomyopathy have an abnormal DTI with a smaller than normal E’.15 Interestingly, in relatives of patients with hypertrophic cardiomyopathy, E’ may be abnormally low in spite of the absence of evidence of hypertrophy. The above noncomprehensive list covers a large percentage of patients referred for cardiac evaluation by echocardiography. With the use of peak velocities of the DTI waves (S’, E’, and to a lesser degree A’), which are easy to recognize and measure, one can find answers to some of the most important clinical issues regarding cardiac patients. A cardiology fellow at our division once asked me, “What is the most valuable single frame in the entire echocardiographic examination?” To me, the answer is unequivocal: a spectral tracing of pulse Doppler DTI with the sample volume at the mitral annulus, which provides simultaneous information about LV systolic and diastolic function. The science of DTI is developing rapidly. The last decade has seen the emergence of the derivatives of velocity measurements, which include strain and strain rate imaging. These variables describe the rate of deformation (strain rate) and its time integral (strain).16 These functions are now commercially available on most modern echocardiography units. Strain rate and strain imaging overcome some of the inherent limitations of DTI, namely rotation, translation and traction by adjacent tissue (In other words, strain and strain rate imaging may eliminate the artifact produced by the effect of motion of a segment which is not the result of its contraction. For example, DTI may record motion of a small scar because of the traction of adjacent, normally contracting myocardium. Strain imaging will show that the segment is indeed scarred.) Another new development allows the evaluation of tissue velocity, strain and strain rate by tracking the acoustic patterns (speckles) seen on gray scale (standard) two dimensional echocardiographic images. This non-Doppler, two dimensional strain imaging method is angle-independent and requires minimal user input (Automatic Function Imaging).17 At present, it is commercially available in units produced by some manufacturers but is not yet in general use. It clearly has great potential and may, in the future, replace DTI. It definitely merits a separate review in this journal. At present, however, DTI remains the “bread and butter” of tissue velocity evaluation. It is hoped that the timely review by Skubas7 will expose the anesthesiology community to this important echocardiographic feature and will encourage its use in daily practice.