LABORATORY INVESTIGATION TISSUE CHARACTERIZATION Sensitive detection of the elfects of reperfusion

We have shown recently that tissue characterization of myocardium with ultrasound reflects changes associated with contractile function throughout the cardiac cycle. To determine whether ultrasonic tissue characterization can sensitively detect the impact of ischemic injury and reperfusion on contractile properties of the heart, we studied the time course of change of backscatter after 5, 20, and 60 min of coronary occlusion followed by reperfusion in 15 dogs. The time-averaged integrated backscatter (IB) and the amplitude and phase of cyclic variation of IB (phase relative to the left ventricular pressure waveform) were measured. A novel ultrasonic index of acute injury was identified, the phase-weighted amplitude of cyclic variation, and calculated by weighting the amplitude of cyclic variation of IB with respect to the phase. We hypothesized that backscatter variables would change dramatically after occlusion and that their restitution after reperfusion would sensitively reflect the extent and time course of reversibility of ischemic injury. After coronary occlusion, segmental wall thickening decreased from approximately 55% to 5% regardless of the duration of ischemia. Changes in backscatter associated with this decrease included an increase in time-averaged IB of approximately 5 dB, a 5 dB decrease in cyclic variation, an 80 degree phase shift, and a 7 dB decrease in phaseweighted amplitude. Wall thickening after reperfusion immediately after the 5, 20, or 60 min occlusions recovered to 45%, 27%, and 12% of baseline values, respectively. Within 3 hr it recovered to 53%, 44%, and 22%. Time-averaged IB recovered initially by 89%, 61%, and 44% (all p < .05) and continued to recover subsequently although more slowly. Ultimate recovery was virtually complete. In contrast to the rapid recovery of time-averaged IB, phase-weighted amplitude recovered initially to only 72%, 41%, and 7% of baseline (all p < .05) and manifested slower and incomplete recovery when ischemia had been present for 20 or 60 min. After reperfusion, the time course of both cyclic variation and phase were reflected by changes in the phase-weighted amplitude. The backscatter variables assessed appear to sensitively delineate the duration, time course of recovery, and reversibility of ischemic injury in response to reperfusion. The results suggest that early recovery of timeaveraged IB corresponds in part to the restoration of tissue ultrastructural integrity. Accordingly, ultrasonic characterization of reperfused myocardium should be useful in defining the myocardium's response to reflow and to interventions such as thrombolysis designed to enhance salvage of ischemic myocardium. Circulation 74, No. 2, 389-400, 1986. CHARACTERIZATION of myocardium with ultrasonic integrated backscatter (IB) permits quantitative detection of pathologic alterations in heart muscle in experimental animals.1`3 In principal, the radiofreFrom the Departments of Medicine and Physics, Washington University, St. Louis. Supported by NIH grant HL17646, SCOR in Ischemic Heart Disease, NASA grant NSG-1601, and a Squibb Cardiovascular Research Fellowship (Dr. Perez). Address for correspondence: Julio E. Perez, M.D., Cardiovascular Division, Washington University School of Medicine, Box 8086, 660 S. Euclid Ave., St. Louis, MO 63110. Received Jan. 14, 1986; revision accepted April 24, 1986. *All editorial decisions for this article, including selection of reviewers and the final disposition, were made by a guest editor. This procedure applies to all manuscripts with authors from the Washington University School of Medicine. Vol. 74, No. 2, August 1986 quency signal backscattered from a myocardial region of interest contains energy at each frequency present in a broadband insonifying pulse. A function that describes the efficiency with which ultrasound is backscattered at each frequency is called the backscatter transfer function.4' The average of the backscatter transfer function is referred to as IB. Several authors have reported substantial differences in backscatter between normal and ischemically injured myocardial tissue, suggesting that IB might represent a useful ultrasonic variable for myocardial tissue characterization.--" We have shown that physiologic contraction and relaxation of myocarditim is paralleled by a cardiac cycle-dependent variation of IB that quantitatively 389 by gest on July 6, 2017 http://ciajournals.org/ D ow nladed from