David D. McPherson

Delineation of the extent of coronary atherosclerosis by high-frequency epicardial echocardiography.

Postmortem studies suggest that coronary angiography does not always accurately delineate the extent of coronary-artery disease. We examined this problem in living human hearts by performing high-frequency epicardial echocardiography at the time of cardiac surgery. The ratio of the diameter of the lumen of the coronary artery to the thickness of its wall was used to quantify the severity of coronary lesions. In 11 patients with no angiographic evidence of coronary disease anywhere in the coronary tree, the mean (+/- SEM) ratio was 5.9 +/- 0.3. In 21 patients with angiographic disease at the site evaluated by echocardiography, the mean ratio was lower (2.3 +/- 0.2, P less than 0.05), reflecting encroachment into the arterial lumen by atherosclerotic plaque. In 15 patients with arterial segments that were angiographically normal but with arterial stenoses elsewhere in the coronary tree, the mean ratio was 4.1 +/- 0.3, with marked overlap with the values in the patients who had angiographic disease at the site of the echocardiographic evaluation. These results demonstrate, in living human hearts, that diffuse coronary atherosclerosis is often present when coronary angiography reveals only discrete stenoses. This finding suggests that coronary angiography may underestimate the severity and extent of coronary disease.

Coronary arterial remodeling studied by high-frequency epicardial echocardiography: An early compensatory mechanism in patients with obstructive coronary atherosclerosis☆☆☆

Coronary arterial remodeling is a compensatory mechanism that may limit the adverse effects of coronary obstructive lesions by expansion of the entire vascular segment. To determine if this compensatory anatomic change occurs in patients, high-frequency epicardial echocardiography using a 12 MHz transducer was performed during open heart surgery in 33 patients (10 with normal coronary arteries undergoing valvular surgery and 23 with coronary atherosclerosis). From stop-frame videotape high-frequency epicardial echocardiographic images, cross-sectional measurements of luminal area and total arterial area (lumen, intima, media and dense adventitia) were made in the patients with atherosclerosis at the site of arterial lesions and from the most proximal portion of the same artery. Remodeling was defined as enlargement of the total arterial area. In normal arteries measurements were made from proximal and midarterial locations. In the patients with normal coronary arteries, total arterial area, as determined by high-frequency echocardiography, decreased from the proximal site to the midportion of the artery (from 10.4 +/- 0.9 to 8.4 +/- 1.0 mm2, p less than 0.05); luminal area also decreased (from 6.0 +/- 0.6 to 4.5 +/- 0.7 mm2, p less than 0.05). In patients with coronary arterial lesions, luminal area also decreased from the proximal site to the arterial lesion site (from 5.3 +/- 0.6 to 2.3 +/- 0.3 mm2, p less than 0.05), but total arterial area increased (from 11.6 +/- 1.0 to 13.0 +/- 1.0 mm2, p less than 0.05). Of the 25 coronary arteries evaluated, only 4 had angiographic evidence of coronary collateral formation. These data indicate that coronary arterial remodeling is an important compensatory mechanism in obstructive coronary disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessing the Physiologic Significance of Coronary Obstructions in Patients: Importance of Diffuse Undetected Atherosclerosis

T HE HALLMARK OF coronary atherosclerosis is an apparently localized obstruction of a conduit coronary vessel. Until Sones introduced the technique of selective coronary angiography,’ for the most part the anatomy of these localized coronary obstructions could only be defined at postmortem. The ability to anatomically define coronary obstructions with a broadly applicable clinical technique has vastly altered our approach to the diagnosis and treatment of coronary atherosclerosis. Each new step forward, however, immediately brings forth new questions. Thus, coronary angiography has forced physicians to develop approaches of defining the physiologic significance of coronary lesions that can be demonstrated angiographically. Accurately assessing the physiologic significance of coronary obstructive lesions is critical to clinical decision making. The need for coronary bypass surgery, percutaneous transluminal coronary angioplasty, the adequacy of prior bypass surgery and percutaneous transluminal coronary angioplasty, and the validation of noninvasive approaches to the diagnosis of coronary disease all critically depend on assessing the physiological significance of coronary obstructive lesions. For more than two decades, the physiological significance of coronary obstructions detected by coronary angiography has been assessed primarily by visual estimates of percent stenosis. To a lesser extent clinicians have used other aspects of the patients’ clinical presentation including symptoms and results of various noninvasive procedures. Because percent stenosis remains the gold standard today in more than 99% of hospitals that perform coronary angiography, this review will examine the advantages and disadvantages of this approach.

High frequency epicardial echocardiography for coronary artery evaluation: In vitro and in vivo validation of arterial lumen and wall thickness measurements

The purpose of this study was to determine the accuracy of a new high frequency echocardiographic technique for the quantitative assessment of coronary artery luminal and wall dimensions. In 32 open chest animals, high frequency echocardiographic measurements (echo) of luminal diameter correlated well with in vitro histologic measurements (Histo) (r = 0.86; high frequency echo = 0.89 Histo + 0.79) (range 1.7 to 5.8 mm). Similar results were found in the evaluation of five human autopsy hearts studied in vitro. Coronary artery wall thickness measurements in human autopsy hearts showed a good correlation with high frequency echocardiographic measurements (r = 0.86; high frequency echo = 0.65 Histo + 0.24) (range 0.3 to 0.8 mm). In eight open chest calves, high frequency echocardiographic measurements of total vessel diameter correlated well with sonomicrometer measurements (Sono) (r = 0.94; high frequency echo = 1.03 Sono + 0.4) (range 2.1 to 5.3 mm). Inter- and intraobserver variability measurements of high frequency echocardiographic measurements demonstrated excellent reproducibility (r = 0.95, interobserver variability for wall thickness; r = 0.97, interobserver variability for luminal diameter; n = 10 postmortem human coronary arteries). In conclusion, high frequency echocardiography is an accurate and reproducible method of measuring coronary luminal and wall geometry and may be a potentially useful tool for in vivo coronary artery evaluation in patients.

Intraoperative evaluation of coronary artery bypass graft anastomoses with high-frequency epicardial echocardiography: experimental validation and initial patient studies.

There is currently no accepted approach for intraoperative evaluation of the technical adequacy of coronary artery bypass graft anastomoses. High-frequency epicardial echocardiography performed intraoperatively could assess coronary artery bypass graft anastomoses by providing on-line short-axis (cross-sectional) and longitudinal two-dimensional images of the vessels. To validate measurements of anastomoses with high-frequency epicardial echocardiography, luminal diameter determined by high-frequency epicardial echocardiography was compared with that determined histologically after perfusion fixation in 12 dogs studied after coronary artery bypass grafting. Technical errors were deliberately created in some grafts. The results of these animal validation studies showed that maximum luminal diameter of the anastomosis by high-frequency epicardial echocardiography correlated well with histologic measurements (r = .92; high-frequency epicardial echocardiography = 0.8 histology + 0.3). All deliberately created technical errors were detected by an independent observer using high-frequency epicardial echocardiography. After completion of the animal studies, we demonstrated the clinical applicability of this approach in 12 patients. Fifteen coronary artery bypass graft anastomoses were examined intraoperatively with high-frequency epicardial echocardiography. The measured maximum luminal diameter of the anastomosis was greater than the maximum luminal diameter of the native artery, as expected, in all end-to-side anastomoses. However, the maximum luminal diameter of the side-to-side anastomoses was equal to or slightly less than that of the native artery. In this initial patient group, minor technical errors were noted in two of 15 graft anastomoses. In conclusion, high-frequency epicardial echocardiography can accurately measure coronary arterial bypass graft anastomoses and has potential for intraoperative detection of technical errors and inadequacies.

Finite element analysis of myocardial diastolic function using three-dimensional echocardiographic reconstructions: application of a new method for study of acute ischemia in dogs.

The effect of acute myocardial ischemia on the myocardial elastic modulus has been a matter of controversy. To evaluate this question, diastolic elastic modulus was assessed by finite element analysis of left ventricular geometry using three-dimensional echocardiographic reconstructions and right and left ventricular pressure recordings. Elastic properties were estimated before and after coronary occlusion in 6 open-chest dogs. Elastic modulus values were derived by means of a computer program that determined the global elastic modulus that best predicted the diastolic changes in left ventricular geometry. In the finite element analysis after coronary occlusion, two analyses were performed: one utilizing the control elastic modulus for all segments of the left ventricle and one in which ischemic (dyskinetic) segments were assigned a higher elastic modulus. Results showed that the control elastic modulus was a poor predictor of diastolic left ventricular expansion after coronary occlusion. The finite element analysis in which the ischemic segments were assigned a higher elastic modulus better predicted ischemic diastolic wall motion patterns. Error values (difference between predicted and actual left ventricular segmental diastolic motion) were: control, 1.9 ± 0.3 mm (mean ± SD), ischemia, 2.9 ± 0.5 mm, and 2.2 ± 0.4 mm using the stiffer elastic modulus for ischemic segments. Error values were significantly higher (p < 0.05) under ischemic conditions when the control elastic modulus was uniformly applied compared with control and ischemia with dyskinetic segments assigned a higher elastic modulus. From these data, it is concluded that the myocardial diastolic elastic modulus is increased by ischemia and that this approach may allow clinical assessment of intrinsic muscle stiffness.

Automated impedance-based energy adjustment for defibrillation: experimental studies.

In defibrillation, current flow depends on the energy selected and the transthoracic impedance. If transthoracic impedance is high, current flow may be inadequate to defibrillate. We developed a method by which high transthoracic impedance is automatically compensated for by an increase in operator-selected energy when impedance is high. Transthoracic impedance was predicted in advance of the first shock by passing a low-level current between the defibrillator electrodes during the defibrillator charge cycle; a microprocessor monitored current flow and determined impedance. In 28 mongrel dogs we manipulated transthoracic impedance by placing glycerin-soaked gauze pads between the paddle electrodes and the chest. If the predicted impedance exceeded a preset value, the delivered energy was automatically increased by 40% or 100%. Using this impedance-based energy adjustment technique, we found significant improvements in current flow and success rate of shocks when energy was automatically increased to compensate for high transthoracic impedance. The use of transthoracic impedance as a basis for energy adjustment appears a promising technique to minimize the hazards of high electrical energy; it allows low-energy shocks in most patients while avoiding inappropriate low energies in patients with high impedance. Clinical trials are justified.

Ultrasound characterization of acute myocardial ischemia by quantitative texture analysis

In this study we tested the efficacy of quantitative texture analysis in the identification of acute myocardial ischemia using an ultrasound data acquisition system that digitizes and stores echocardiographic data in polar format. In nine closed-chest dogs, data were acquired before and after coronary occlusion using a 2.4 MHz echocardiographic system. Regions of interest were analyzed at end-diastole and end-systole from the ischemic area and normal area at the same depth of field. Ultrasound data were evaluated using previously reported quantitative gray level texture measures. After occlusion, texture changes indicative of ischemia were found in systolic images. The directional component of the data analysis was important; analysis in the azimuthal direction was more accurate than in the axial direction. Six texture measures exhibited significant changes in the ischemic region from control to occlusion when analyzing data in the azimuthal direction. One false positive result occurred (significant texture change in the normal region from control to occlusion) in the azimuthal direction. Several false positive alterations in the normal regions from control to occlusion were found when the texture was evaluated in the axial direction. For accurate assessment of ischemic changes, preocclusion image data were required. We conclude that quantitative echocardiographic texture analysis using polar format data can identify subtle changes in myocardial texture such as that due to acute ischemia, using data acquired through the chest wall.

Exercise electrocardiographic mapping in normal subjects

To investigate the spectrum of change in multiple-lead exercise electrocardiograms, 120-lead body surface potential maps (BSPM) in normal adult subjects during upright, graded, submaximal exercise testing were recorded. Results showed that in the normal group, exercise was associated with substantial electrocardiographic changes on the body surface, many of which persisted during early recovery. The QRS waveform was minimally altered during exercise. Despite, however, no change in QRS duration, there was significant reduction in QRS potential range with consequent reduction QRS integral value. The ST waveforms changed markedly with exercise, showing abbreviated duration and increased slope. This was reflected by significantly increased ST potential range from rest to immediate cessation of exercise, which returned towards resting value during recovery. The effect of the altered ST-segment waveform was also reflected in torso potential distributions at two time instants during the ST-segment. When a spatially-fixed position on the ST-T waveform was evaluated (ST-segment offset), exercise resulted in small potential changes, especially over the torso area occupied by the standard V1 to V6 chest leads. However, when a temporally-fixed point (80 ms after QRS offset) was evaluated, there were large increases in potential over the precordium with exercise. Isointegral ST-segment maps, which reflect both spatial and temporal ST properties, showed that exercise was associated with substantial decreases in values over the precordium and inferior torso, and although diminished, they tended to persist through five minutes of recovery. Thus, electrocardiographic repolarization parameters are particularly affected by physiological exercise and, although the underlying causes of these changes remain undefined, they should be taken into account when evaluating the population at risk.

Asymptomatic aneurysm of the interatrial septum

A case of interatrial septal aneurysm discovered by two-dimensional echocardiography and confirmed by ultrasound contrast technique is described. Previous reports are reviewed, emphasizing the differential diagnosis, prognosis and recommendations for follow-up.