Lewis J. Thomas

Risk factors for sudden death after acute myocardial infarction: Two-year follow-up☆

The risk of sudden coronary death after myocardial infarction (MI) was assessed in 533 patients who survived 10 days after MI and were followed for up to 24 months (mean 18) in the Multicenter Investigation of the Limitation of Infarct Size. Analysis of multiple clinical and laboratory variables determined before hospital discharge revealed that frequent ventricular premature beats (VPBs) (greater than or equal to 10/hour) on ambulatory electrocardiographic monitoring and left ventricular (LV) dysfunction (radionuclide LV ejection fraction less than or equal to 0.40) were independently significant markers of risk for subsequent sudden death believed to be the result of a primary ventricular arrhythmia. The incidence of sudden death was 18% in patients with both LV dysfunction and frequent VPBs (11 times that of patients with neither of these findings). Seventy-nine percent of all sudden deaths occurred within 7 months after the index MI. In 280 survivors reclassified 6 months after MI with regard to the presence or absence of frequent VPBs and LV dysfunction, these risk factors could not be associated with sudden coronary death over a further follow-up period of up to 18 months; the overall incidence of sudden cardiac death was low (1.4%) after 6 months. Thus, the presence of frequent VPBs in association with LV dysfunction early after MI identifies patients at high risk for sudden death over the next 7 months.

Sensitive detection of the effects of reperfusion on myocardium by ultrasonic tissue characterization with integrated backscatter.

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 phase-weighted 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 less than .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 less than .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 time-averaged IB corresponds in part to the restoration of tissue ultrastructural integrity.(ABSTRACT TRUNCATED AT 400 WORDS)

Sparse array imaging with spatially-encoded transmits

The frame rate in medical ultrasound imaging may be increased significantly by reducing the number of transmit firings per image frame. Cooley et al. (1994) and Lockwood et al. (1995) have described synthetic aperture imaging systems where each frame is imaged using data obtained from a small number of transmit elements fired in succession. These synthetic transmit aperture systems have potential for very high frame rates, but they also suffer from low SNR. Here, the authors present a method for increasing the SNR of such systems by using spatially-encoded transmits. The transmitted power is increased by having multiple active transmitters in each firing. The active transmitters are encoded in a spatial code which allows the received data to be subsequently sorted by each transmitter for synthetic aperture beamforming. The authors present the spatial coding and decoding theory and show experimental results to demonstrate its application.

Sudden death and its relation to QT-interval prolongation after acute myocardial infarction: Two-year follow-up

Risk of sudden death was assessed in 533 patients who survived 10 days after acute myocardial infarction (AMI) and were followed for up to 24 months (mean 18) in the Multicenter Investigation of the Limitation of Infarct Size. Analysis of clinical and laboratory variables measured before hospital discharge revealed that the QT interval, either corrected (QTc) or uncorrected for heart rate, did not contribute significantly to prediction of subsequent sudden death or total mortality. In this population, frequent ventricular premature complexes (more than 10 per hour) on ambulatory electrocardiographic monitoring and left ventricular (LV) dysfunction (radionuclide LV ejection fraction of 0.40 or less) identify patients at high risk of sudden death. In patients with these adverse clinical findings, the QTc was 0.468 +/- 0.044 second among those who died suddenly and 0.446 +/- 0.032 second in survivors, and was not statistically significant as an additional predictor of sudden death. Consideration of the use of type I antiarrhythmic agents, digoxin, presence of U waves and correction for intraventricular conduction delay did not alter these findings. Although QT-interval prolongation occurs in some patients after acute myocardial infarction, reduced LV ejection fraction and frequent ventricular premature complexes are the most important factors for predicting subsequent sudden death in this patient population.

Analytic evaluation of sampled aperture ultrasonic imaging techniques for NDE

This paper presents a theoretical comparison of three generic sampled aperture ultrasonic imaging systems for nondestructive evaluation. The imaging systems are categorized according to their source-receiver combination for data acquisition: common-source, back-scatter, and full-array imaging. First, forward modeling is performed for a point source and a point receiver. This is then used to model the received data set for each of the imaging categories. Subsequently, the inversion algorithm for each category is derived, and their performance is evaluated in terms of resolution, noise, and computation. We show that in terms of resolution, back-scatter imaging is the best, followed by full-array and common-source imaging. However, in terms of material noise, full-array imaging is the best, with back-scatter and common-source imaging having the same material noise response. Full-array imaging is the only system with inherent redundancy to reduce electronic noise, but at the expense of significantly more computation. The physical transducer is in the full-array category, allowing mechanical scanning to be traded for dynamic focusing and computational power.<<ETX>>

Synthetic transmit aperture imaging using orthogonal Golay coded excitation

The frame rate in medical ultrasound imaging may be increased significantly by reducing the number of transmits per image frame. Cooley et al. (1994) and Lockwood et al. (1995) have described synthetic transmit aperture (STA) systems where each frame is imaged using data obtained from a small number of point sources fired in succession. These systems have potential for very high frame rates, but they also suffer from low SNR. In this paper we present a computationally efficient method to increase the SNR of STA systems by using spatio-temporal encoding which increases SNR by 101og(ML) dB, where M is the number of active phase centers or transmits and L is the temporal code length. By using an orthogonal Golay set for the spatio-temporal encoding, the received data can be sorted by each transmit phase center and pulse-compressed for subsequent synthetic aperture beamforming. Computer simulations are used to demonstrate the method.

A Real-Time Integrated Backscatter Measurement System for Quantitative Cardiac Tissue Characterization

Abstmct-To quantify integrated backscatter of ultrasound for realtime tissue characterization of myocardium in vivo an analog system that is used in conjunction with a two-dimensionaliM-mode echocardingraphic imager was developed and evaluated. The system measures the energy contained in a portion of the received rf signal from a selected myocardial segment with a CdS acoustoelectric detector. A standard substitution technique is used to calibrate the system relative to a stainless steel reflector. Integrated backscatter is sampled at 5 ms intervals, its time-variation recorded and simultaneously integrated to yield the time-averaged integrated backscatter. Both time-varying and time-averaged integrated backscatter are displayed in real-time. Timeaveraged integrated backscatter in five dogs was -54 f 2.4 dB (SD). Cyclic variations during the cardiac cycle ranged from 5-10 dB. Both the time-averaged and time-varying features of integrated backscatter are consistent with values obtained with earlier measurement systems that required off-line analysis of data. The real-time data acquisition system developed should facilitate clinical tissue characterization with ultrasonic backscatter.

Aperture formation on reduced-channel arrays using the transmit-receive apodization matrix

We present the transmit-receive apodization matrix (T/R matrix) as a tool for aperture synthesis on reduced-channel arrays. Reduced-channel arrays have a small number of transmit/receive channels multiplexed to a conventional dense array. For a 1D array of N elements, the T/R matrix is the N/spl times/N matrix of apodization values, where the rows correspond to transmit element positions and the columns correspond to receive element positions. We show that the round-trip beam pattern may be obtained from this matrix simply as the Fourier transform of its cross-diagonal sum. The aperture synthesis process consists of choosing the T/R matrix under certain constraints. If the T/R matrix has rank one, then a single transmit with parallel receive forms the beam (conventional case), and the round-trip beam can be separated into the product of the transmit beam and the receive beam. As the rank of the T/R matrix increases, greater beamforming flexibility is achieved, allowing for tradeoffs in SNR, frame-rate, and system complexity.

Abrupt withdrawal of beta-blockade therapy in patients with myocardial infarction: effects on infarct size, left ventricular function, and hospital course.

The effects of abrupt withdrawal or continuation of beta-blockade therapy during acute myocardial infarction were evaluated in 326 patients participating in the Multicenter Investigation of the Limitation of Infarct Size (MILIS). Thirty-nine patients previously receiving a beta-blocker and randomly selected for withdrawal of beta-blockers and placebo treatment during infarction (group 1) were compared with 272 patients previously untreated with beta-blockers who were also randomly assigned to placebo therapy (group 2). There were no significant differences between the two groups in MB creatine kinase isoenzyme (15.8 +/- 10.9 vs 18.2 +/- 14.4 g-eq/m2, respectively) estimates of infarct size, radionuclide-determined left ventricular ejection fractions within 18 hr of infarction (0.44 +/- 0.15 vs 0.47 +/- 0.16) or 10 days later (0.42 +/- 0.14 vs 0.47 +/- 0.16), creatine kinase-determined incidence of infarct extension (13% vs 6%), congestive heart failure (43% vs 37%), nonfatal ventricular fibrillation (5% vs 7%), or in-hospital mortality (13% vs 9%). Patients in group 1 had more recurrent ischemic chest pain (p = .002) within the first 24 hr after infarction, but not thereafter. However, this did not appear to be related to a rebound increase in systolic blood pressure, heart rate, or double product. In a separate analysis, 20 propranolol-eligible group 1 patients randomly selected for withdrawal of beta-blockade (group 3) were compared with 15 patients randomly selected for continuation of prior beta-blockade therapy (group 4). This comparison yielded similar results. These data indicate that the beta-blockade withdrawal phenomenon is not a major clinical problem in patients with acute myocardial infarction. beta-Blockade therapy can be discontinued abruptly during acute myocardial infarction if clinically indicated.

Comparison of left ventricular function and infarct size in patients with and without persistently positive technetium-99m pyrophosphate myocardial scintigrams after myocardial infarction: Analysis of 357 patients☆

One hundred nine patients with persistently positive technetium-99m pyrophosphate (Tc-99m-PPi) myocardial scintigrams 6 months after acute myocardial infarction (MI) (Group A) and 185 patients without such persistently positive scintigrams (Group B) were compared with regard to enzymatically determined infarct size, early and late measurements of left ventricular (LV) function determined by radionuclide ventriculography, and preceding clinical course during the 6 months after MI. The CK-MB-determined infarct size index in Group A (17.4 +/- 10.6 g-Eq/m2) did not differ significantly from that in Group B (16.0 +/- 14.6 g-Eq/m2). Similarly, myocardial infarct areas in the 2 groups, determined by planimetry of acute Tc-99m-PPi scintigrams in those patients with well-localized 3+ or 4+ anterior pyrophosphate uptake, were not significantly different (35.7 +/- 13.4 vs 34.4 +/- 13.1 cm2, respectively). However, patients in Group A had significantly lower LV ejection fractions than those in Group B, both within 18 hours of the onset of MI (0.42 +/- 0.14 vs 0.49 +/- 0.14, p less than 0.01) and at 3 months after MI, both at rest (0.42 +/- 0.14 vs 0.51 +/- 0.14, p less than 0.01) and at maximal symptom-limited supine bicycle exercise (0.44 +/- 0.17 vs 0.51 +/- 0.17, p less than 0.01). Peak exercise levels achieved in the 2 groups were not significantly different. Furthermore, patients in Group A demonstrated a greater incidence of congestive heart failure during the initial hospital admission (41 vs 24%; p less than 0.01) and a greater requirement for digoxin (p less than 0.05) and furosemide (p less than 0.01) after discharge.(ABSTRACT TRUNCATED AT 250 WORDS)