Early identification with ultrasonic integrated backscatter of viable but stunned myocardium in dogs

Abstract

It has been shown that canine and human hearts exhibit a cardiac cycle-dependent variation of integrated backscatter (cyclic variation) that reflects intrinsic regional contractile performance. To determine whether ultrasound tissue characterization can identify viable though stunned myocardium before recovery of regional wall thickening, transient ischemic injury was produced in eight open chest dogs for 15 min followed by reperfusion for 2 h. Cyclic variation and wall thickening were measured before ischemia, at 15 min after the onset of ischemia and at selected intervals after the onset of reperfusion from multiple sites within the ischemic zone with a novel combined two-dimensional and M-mode acquisition system. Cyclic variation and wall thickening were computed from digitized M-mode integrated backscatter images with an algorithm developed and validated for this purpose. Magnitude and "delay" of cyclic variation and wall thickening were compared. Delay represents the degree of synchrony of regional cyclic variation or wall thickening with global ventricular mechanical systole. Baseline cyclic variation and wall thickening magnitudes were 3.8 +/- 0.2 dB and 37 +/- 1.4%, respectively. With ischemia, cyclic variation and wall thickening decreased to 1.7 +/- 0.2 dB and 17 +/- 2%, respectively (p less than 0.05, compared with baseline). Cyclic variation recovered to baseline levels within 20 min after reperfusion (3.3 +/- 0.4 dB, p = NS). Wall thickening remained depressed for 2 h after the onset of reperfusion (23 +/- 2%, p less than 0.05 compared with baseline). Delay of cyclic variation in a unitless ratio expressed as delay (in milliseconds) divided by the QT interval (in milliseconds) increased from 0.87 +/- 0.03 at baseline to 1.10 +/- 0.12 with ischemia, a change consistent with mild asynchrony, and returned to baseline (0.95 +/- 0.07, p = NS compared with baseline) within 20 min after reperfusion. Delay of wall thickening was 0.88 +/- 0.02 at baseline, increased to 1.23 +/- 0.09 with ischemia and remained significantly increased 2 h after reperfusion (1.07 +/- 0.05, p less than 0.05 compared with baseline). Recovery time constants for cyclic variation and wall thickening with reperfusion reflected earlier restoration of cyclic variation (8.1 min) than of wall thickening (420.5 min). Thus, cyclic variation recovers before wall thickening with reperfusion. Its analysis appears to provide a useful index of the presence of viable and potentially salvageable tissue in regions of stunned myocardium that is independent of wall thickening.

Three-dimensional characterization of human ventricular myofiber architecture by ultrasonic backscatter.

“…We have shown that quantitative indexes of ultrasonic integrated backscatter and attenuation derived from analysis ofradio frequency (rf)' data can differentiate normal myocardial tissue from cardiomyopathic, infarcted, and stunned tissue in experimental animals and in patients (16)(17)(18)(19). Recently, we have demonstrated that the magnitudes of backscatter and attenuation depend on the angle of insonification of the left ventricular wall with respect to the predominant orientation of myofibers in canine heart (20,21).…”

Ultrasonic Tissue Characterization in Acute Myocarditis. A Case Report.

“…8,9 In contrast, experimental and clinical studies have shown that improvement of the blunted CV-IB preceded that of myocardial systolic dysfunction in acute myocardial infarction treated with reperfusion therapy. 2,5 Milunski et al reported that after a brief coronary occlusion in dogs, recovery of regional wall motion was significantly delayed in comparison with that of CV-IB, 5 and Takiuchi et al reported that in patients with acute myocardial infarction the improvement of phase-corrected IB index in the infarct zone preceded that of wall motion recognized with conventional echocardiogram, and that this index at the early stage after reperfusion seemed to be a useful prognostic indicator of functional improvement. 2 Although the exact mechanisms underlying the delay in LV wall motion recovery still remain uncertain, possible explanations are: (1) heterogeneous recovery of myocardial contraction, (2) constrained improvement of injured myocardium by adjacent normal segments, which increase the afterload, and (3) injury to the intracellular structure that mediates between sarcomere shortening and myocardial contraction.…”

<b>Dipyridamole Stress Echocardiography and Ultrasonic Myocardial Tissue Characterization in Predicting Myocardial Ischemia, in Comparison With Dipyridamole Stress Tc-99m MIBI SPECT Myocardial Imaging</b>

“…CVIBS is blunted promptly by ischemia and is augmented after reperfusion in animals and humans. 25,[29][30][31] Lin, et al 32) found that the cut-off value was 2.0 db in predicting reversible myocardial dyssynergy after revascularization. Vitale, et al 33) showed that the maximum-minimum difference of the integrated backscatter (CVIBS) was reduced significantly by stress in the septum (4.6 ± 0.9 versus 2.7 ± 0.8) (difference, 1.9 db).…”

“…Recovery of CVIBS after reperfusion occurs more quickly than the recovery of regional systolic wall motion not only in dogs but also in patients undergoing thrombolytic therapy for acute myocardial infarction. 30,34) However, Lin, et al showed that ultrasonic tissue characterization measures intramural contractility that may reveal subtle structural alterations in normokinetic myocardium corresponding to the ischemic response provoked by stress tests. 32) They have found that the normokinetic myocardium with ischemic burden, verified by dobutamine stress echocardiography and thallium scintigraphy, showed diminished CVIBS.…”

“…In animals, the cyclic variation of IBS (CVIBS) is blunted promptly by ischemia and is augmented after reperfusion. 7,8) However, the cyclic variation of integrated backscatter measured during the cardiac cycle has been shown in adults to be decreased when the myocardium is ischemic even before myocardial contraction has decreased. 9,10) The aim of this study was to validate whether dipyridamole stress ultrasonic tissue characterization compared with dipyridamole stress echocardiography and dipyridamole stress technetium-99m methoxyisobutyl isonitrile (Tc99m-MIBI) single photon emission computerized tomography (SPECT) myocardial perfusion scintigraphy could predict myocardial ischemia in patients with chronic coronary artery disease.…”