Biomedical subjects

K A Wear

Publications and source records attributed to K A Wear.

25 records · Page 2Linked to original sources

PubMed1990

Myocardial ultrasonic backscatter for characterization of ischemia and reperfusion: relationship to wall motion.

We have previously shown that cardiac cycle-dependent variation of integrated backscatter occurs in normal myocardium. To determine whether myocardial ischemia and reperfusion can be distinguished by real-time integrated backscatter imaging we performed 10 min balloon occlusion of the Left Anterior Descending (LAD) coronary artery followed by reperfusion in 10 closed-chest anesthetized dogs. Images were obtained at baseline, during occlusion, and up to 120 min after reperfusion. We measured the magnitude and delay of cyclic variation of integrated backscatter in segments with and without asynergy. Radiolabeled microspheres were used to verify both ischemia and reperfusion. Ischemic segments exhibited decreased magnitude and increased normalized delay of cyclic variation of integrated backscatter (from 3.3 +/- 0.3 dB to 1.4 +/- 0.2 dB, mean +/- SE; and from 0.95 +/- 0.03 to 1.67 +/- 0.15, respectively, all p less than or equal to 0.001). Reperfusion promptly restored the magnitude of cyclic variation toward normal. However, the delay of the cyclic variation was restored only partially. Wall motion analysis of the ischemic sites revealed persistent abnormalities throughout the reperfusion interval despite return to normal of the magnitude and delay of cyclic variation. Thus, real-time integrated backscatter imaging permits detection and differentiation of changes in myocardial acoustic properties indicative of ischemia and of subsequent reperfusion.

Animals↗

PubMed1989

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

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.

Algorithms↗

PubMed1989

Ultrasound integrated backscatter tissue characterization of remote myocardial infarction in human subjects.

To determine whether quantitative ultrasound tissue characterization differentiates normal myocardial regions from segments of remote infarction, 32 consecutive patients with a diagnosis of previous myocardial infarction were evaluated. Images were obtained in real time with a modified two-dimensional ultrasound system capable of providing continuous signals in proportion to the logarithm of integrated backscatter along each A line. In 15 patients, adequate parasternal long-axis images that delineated both normal and infarct segments were obtained with standard time-gain compensation. Image data were analyzed to yield both magnitude and delay (electrocardiographic R wave to nadir normalized for the QT interval) of the cyclic variation of backscatter. Cyclic variation was present in 55 of 56 normal myocardial sites, averaging (mean +/- SEM) 3.2 +/- 0.2 dB in magnitude and exhibiting a mean normalized delay of 0.87 +/- 0.03. The magnitude of cyclic variation in infarct segments was significantly reduced to 1.1 +/- 0.2 dB (42 sites), and the delay was markedly increased to 1.47 +/- 0.12 (21 sites) (p less than 0.0001 for both). In 20 of 42 infarct sites, no cyclic variation was detectable. Thus, ultrasound tissue characterization quantitatively differentiated infarct segments from normal myocardium in patients with remote myocardial infarction.

Adult↗

PubMed1989

Differentiation between acutely ischemic myocardium and zones of completed infarction in dogs on the basis of frequency-dependent backscatter.

The goal of this work was to determine whether the frequency dependence of apparent backscatter coefficient (not corrected for attenuation within the myocardium) could differentiate completed, remote infarction from acute myocardial injury in vivo. Myocardial infarcts were produced in six dogs by coronary artery occlusion. One to 12 months later, acute ischemic injury was induced in each dog by ligation of a coronary artery that supplied a region of myocardium adjacent to the established infarct. Infarct, ischemic, and normal regions were interrogated with a 5-MHz, circular, 0.5-in. diam, broadband, focused, piezoelectric transducer mounted in a water-filled stand-off device placed against the exposed, beating heart. Apparent backscatter coefficients were measured over the range of frequencies from 3-7 MHz. The frequency dependence was obtained from the slope of log apparent backscatter coefficient versus log frequency. No significant difference in frequency dependence was found between normal and acutely ischemic myocardium for periods of up to 2 h of ischemia. In contrast, frequency dependence in regions of remote infarct (1.8 +/- 0.1, mean +/- standard error) was significantly lower than that in acutely ischemic or nonischemic regions (2.3 +/- 0.1) (p less than 0.01). These results suggest that remote myocardial infarction can be differentiated from acutely injured but still potentially salvageable myocardium in vivo on the basis of the frequency dependence of backscatter.

Animals↗

PubMed1989

Contraction-related variation in frequency dependence of acoustic properties of canine myocardium.

Results of experiments performed in several laboratories indicate that contracting myocardium exhibits a cyclic variation of the magnitude of ultrasonic backscatter, with maxima occurring at end-diastole and minima at end-systole. The mechanisms responsible for this variation are not well understood. The purpose of the present study was to determine whether the frequency dependence of backscatter exhibits systematic variation throughout the cardiac cycle, analysis of which may facilitate improved understanding of biologic factors responsible for the cyclic variation of the magnitude of backscatter. In this study, the myocardial backscatter coefficient, as a function of frequency, was measured throughout the cardiac cycle in nine open-chest dogs. The frequency dependence of the backscatter coefficient was computed from a least-squares linear fit to log backscatter coefficient versus log frequency data. A cyclic variation of frequency dependence of backscatter was found with maximum near end-diastole (f2.6 +/- 0.1) and minimum near end-systole (f2.2 +/- 0.1), a significant variation (p less than 0.01). These results suggest that mechanisms responsible for the cyclic variation of backscatter may include changes in the effective size of the dominant scatterers throughout the cardiac cycle. An alternative explanation for the observed variation is an increase in the myocardial attenuation coefficient during systole followed by a decrease in diastole.

Animals↗

PubMed1989

Ultrasonic tissue characterization with integrated backscatter. Acute myocardial ischemia, reperfusion, and stunned myocardium in patients.

We have previously shown in studies of experimental animals that myocardium exhibits a cardiac cycle-dependent variation of integrated backscatter that reflects regional myocardial contractile performance and that is blunted promptly after arterial occlusion and recovers after reperfusion. To define the clinical utility of ultrasonic tissue characterization with integrated backscatter for detection of acute myocardial infarction and reperfusion, 21 patients (14 men and seven women) were studied in the cardiac care unit within the first 24 hours (mean time, 11.3 hours; range, 3.5-23.8 hours) after the onset of symptoms indicative of acute myocardial infarction with conventional two-dimensional and M-mode echocardiography and with analysis of integrated backscatter. The magnitude of cyclic variation of integrated backscatter was measured from several sites within acute infarct regions and normal regions remote from the infarct zone for each patient. The average magnitude of cyclic variation among all patients (n = 21) was 4.8 +/- 0.5 dB in normal regions compared with 0.8 +/- 0.3 dB in infarct regions (p less than 0.05) within the first 24 hours after the onset of symptoms. Among the patients who had two studies, 15 (mean, 7.1 days; range, 2-31 days for second study) underwent coronary arteriography to define vessel patency. In patients with vessels with documented patency (n = 10), the magnitude of cyclic variation in infarct regions increased over time from 1.3 +/- 0.6 to 2.5 +/- 0.5 dB from the initial to final study (p less than 0.05). Patients with occluded infarct-related arteries (n = 5) exhibited no significant recovery of cyclic variation (0.3 +/- 0.3-0.6 +/- 0.3 dB). A blinded analysis of standard two-dimensional echocardiographic images revealed no significant recovery of wall thickening in either group over the same time intervals. Ultrasonic tissue characterization promptly detects acute myocardial infarction and may delineate potential beneficial effects of coronary artery reperfusion manifest by restoration of cyclic variation of integrated backscatter in the presence of severe wall motion abnormalities.

Aged↗

PubMedDate not supplied

Ultrasonic characterization of acute cardiac rejection from temporal evolution of echocardiograms.

Two ultrasonic parameters that characterize cardiac contractile performance were tested for their ability to detect the early stages of acute rejection. Five dogs received heart transplants with a heterotopic abdominal model and were given echographic examinations 1, 4, and 8 days after surgery. The ultrasonic measurements were extracted from pairs of cross-sectional echocardiograms, separated in time by approximately half of a cardiac cycle. The contraction-related changes in the portions of the images corresponding to myocardium were characterized by ratio of mean video amplitude and image-pair correlation. Full-thickness biopsies were taken from the left and right ventricles after each examination so that the correlations between the ultrasonic measurements and the histologic state of the tissue could be determined. Each biopsy was ranked according to the following scale of increasing levels of rejection: 0 (normal), 1 (lymphocyte infiltration), 2 (focal necrosis), 3 (diffuse necrosis), 4 (presence of hemorrhages). The average histologic state for the left ventricle increased from 0.2 +/- 0.4 for the first examination, to 2.0 +/- 0.7 for the second, and 3.6 +/- 0.5 for the third. Similar results were obtained for the right ventricle. The progressions of mean amplitude ratio and correlation were 1.55 +/- 0.14, 1.49 +/- 0.17, 1.19 +/- 0.15, and 0.22 +/- 0.10, 0.28 +/- 0.10, 0.74 +/- 0.25, respectively. Thus in this experiment, these parameters were useful for distinguishing advanced stages of rejection from the normal state and from mild rejection.

Animals↗