Contrast echocardiography displays increased subendocardial perfusion after nitroglycerin administration

Nitroglycerin-induced heterogeneous subendocardial myocardial blood flow observed by cardioscopy in patients with coronary artery disease

It is controversial as to whether or not nitroglycerin (NTG) increases subendocardial myocardial blood flow (SMBF), and if it does, whether arterial or venous blood flow is increased in patients with coronary artery disease. This study was performed to examine NTG-induced changes in SMBF.Changes in SMBF induced by NTG (200 µg, i.v.) were examined by cardioscopy in 58 left ventricular wall segments of 58 patients with coronary artery disease. NTG-induced red and purple endocardial colors were defined as increased arterial and venous SMBF, respectively. Endocardial color before NTG administration was classified into brown, light brown, pale and white. Endomyocardial biopsy of the observed portion and (201)Tl scintigraphy were performed in 40 of these patients immediately after cardioscopy and several days after cardioscopy, respectively.Upon administration of NTG, SMBF increased in 48 of 58 wall segments; arterial SMBF in 34 and venous SMBF in 12 wall segments; arterial SMBF in all 24 brown to light brown segments; venous SMBF, arterial SMBF and no change in 12, 10 and 5 of pale segments, respectively; and no change in all 10 white wall segments. (201)Tl-scintigraphy and endomyocardial biopsy revealed that brown, light brown, pale and white endocardial color represented no ischemia, mild ischemia, severe ischemia and fibrosis, respectively.NTG caused an increase in either arterial or venous SMBF depending on control endocardial color, wall motion and severity of coronary stenosis.

Transmural variation of myocardial attenuation measured with a clinical imager

The objective of this study was to quantify the transmural variation in attenuation for the septal and lateral walls of the heart. Our approach was to utilize a commercially available ultrasonic imaging system to acquire images of excised sections of eight sheep hearts with an orientation similar to that encountered in the apical four-chamber view. The measured values (mean +/-SE) of the slope of attenuation for the transmural regions of the septum are: 1.40 +/-0.11, 0.99 +/-0.09, and 1.85 +/-0.16 (dB/cm/MHz) for the left subendocardial, midmyocardial, and right subendocardial zones, respectively. The analogous data from the lateral wall are: 1.42 +/-0.11, 0.83 +/-0.07, and 1.20 +/- 0.16 (dB/cm/MHz) for the subendocardial, midmyocardial, and subepicardial zones, respectively. These data demonstrate that ultrasonic attenuation associated with the septum and the lateral wall, when imaged in a manner similar to that of the apical four-chamber view, is anisotropic.

Transmural variation of myocardial attenuation and its potential effect on contrast-mediated estimates of regional myocardial perfusion

Promising technical developments suggest that it may be feasible to use contrast echocardiography to estimate regional myocardial perfusion. Although the optimal approach has not yet been determined, the use of a nonlinear (harmonic) response of the contrast agent is common to several recent advances. The purpose of this article is to delineate the relation between the anisotropic (angle-dependent) ultrasonic attenuation of the myocardium through which the sound wave has propagated and the regional, nonlinear response of the contrast agent. Apparent perfusion will be modulated by this regionally varying, path-dependent attenuation, which is determined by the local angle between the propagating sound wave and the myofiber orientation. We illustrate the potential magnitude of the effect of myocardial anisotropy for the apical 4-chamber view by examining propagation along the septum and the lateral wall. We present experimentally measured values of the attenuation of excised sheep myocardium, showing statistically significant differences in the attenuation in the mid wall compared with that in symmetrical zones to the left and right of the mid wall, reflecting the well-known myofiber orientations in these 3 regions. The nonlinear (harmonic) response of a contrast agent depends on the local pressure amplitude, which for a given mechanical index is determined by the attenuation accumulated along the path to the point where the regional perfusion is estimated.

Clinical applications of contrast echocardiography

BACKGROUND

Contrast media, used in conjunction with newly developed echocardiographic techniques, can currently be used in several clinical settings: (1) the study of myocardial perfusion, (2) delineation of the endocardial border in technically difficult echocardiographic examinations, and (3) enhancement of low-intensity blood flow, especially coronary blood flow, to study coronary flow reserve.

METHODS

Published studies were reviewed to identify the advantages of associating contrast perfusion with classic or new echocardiographic and ultrasonographic imaging techniques in the study of myocardial perfusion and coronary artery flow.

RESULTS

Several studies demonstrated the usefulness of contrast echocardiography, even in patients with a bad acoustic window, in evaluating opacification of the left ventricle or in enhancing echocardiographic color Doppler studies of coronary flow and coronary flow reserve. Preliminary results of transthoracic echocardiographic studies of myocardial perfusion are described.

CONCLUSIONS

The clinical applications of contrast echocardiography are effective in exploiting examinations that provide poor diagnostic information (ventricular cavity opacification) or in obtaining new physiopathologic data (microvascular opacification/perfusion and coronary flow reserve). The evaluation of coronary flow reserve by contrast-enhanced transthoracic Doppler ultrasonography is an attractive new diagnostic modality that points the way toward important new clinical applications of contrast echocardiography. This technique is useful in evaluating the severity of coronary artery disease of the left anterior descending coronary artery and in all clinical conditions in which the effects of therapeutic interventions aimed at improving coronary flow reserve need to be monitored.

Effects of tissue anisotropy and contrast acoustic properties on myocardial scattering in contrast echocardiography

In this study we explored the potential effects that tissue anisotropy, in conjunction with the acoustic properties of contrast, may have on quantitative measurements of myocardial perfusion with the use of ultrasonic contrast agents. We used a computer simulation of the parasternal short-axis view, based on previously measured values for the anisotropy of backscatter and attenuation of myocardium, to predict the backscattered energy from 18 specific regions within the heart before and after myocardial contrast perfusion. Results demonstrated a regional variation of contrast enhancement in the short-axis view and variations caused by incremental increases in contrast level for specific myocardial regions. Thus quantitative assessment of myocardial perfusion with contrast echocardiography is influenced by the anisotropic properties of the myocardium, and the resulting postcontrast image will depend on the interaction between tissue properties and contrast acoustic properties. The degree of myocardial enhancement caused by the presence of contrast may depend on the spatial position of the specific region investigated with respect to the transducer and the amount of contrast in the myocardium.