James M. Griffith

Assessment of normal and atherosclerotic arterial wall thickness with an intravascular ultrasound imaging catheter

A prototype intravascular ultrasound imaging catheter with a 20 MHz transducer was used to obtain 59 cross-sectional images in 14 segments of human atherosclerotic arteries. Three distinct components of the arterial wall were visualized on the ultrasound images: a highly reflective intima, an echolucent media, and a moderately reflective adventitia. Images were obtained at 1 mm increments in vitro and were compared with histologic sections at the same levels. Measurements of the arterial layers showed a close correlation between ultrasound images and histologic sections for the thickness of the intimal plaque (r = 0.91), the media (r = 0.83), and the total wall thickness (r = 0.85). The ultrasound images overestimated the mean intimal and total wall thickness by 0.3 mm and 0.7 mm compared to measurements in histologic sections (p less than 0.001). Intravascular imaging with high-frequency ultrasound is an accurate method for measuring microanatomic arterial dimensions and the extent of atheromatous involvement of the arterial wall. This method could represent an important adjunct to traditional angiographic techniques for assessing the severity of atherosclerosis.

Mechanism of Left Ventricular Outflow Obstruction in Patients with Obstructive Asymmetric Septal Hypertrophy (Idiopathic Hypertrophic Subaortic Stenosis)

Left ventricular outflow obstruction in patients with idiopathic hypertrophic subaortic stenosis or obstructive asymmetric septal hypertrophy is due to abnormal forward motion during systole of the anterior mitral leaflet. To determine why some patients with this disease hav left ventricular outflow obstruction whereas others do not, we studied a large number of patiens with assymetric septal hypertrophy using both one- and two-dimensional echocardiography. In 100 patients with asymmetric septal hypertrophy and 22 normal subjects, mitral valve position at the onset of systole was quantitated by measuring the distance from the ventricular septum to the mitral valve and the distance from the mitral valve to the posterior left ventricular wall. None of the normal subjects and only 3 (6 percent) of 51 patients with nonobstructive asymmetric septal hypertrophy had a septal-mitral valve distance of less than 20 mm compared with 23 (66 percent) of 35 patients with obstructive asymmetric septal hypertrophy. Moreover, the mitral valve at the onset of systole was actually positioned forward in the left ventricular activity. Two-dimensional studies in 11 patients with obstructive asymmetric septal hypertrophy revealed that contraction of the malaligned papillary muscles did not cause the abnormal forward mitral valve motion. We propose that the left ventricular outflow obstruction in patients with obstructive asymmetric septal hypertrophy occurs as a result of two factors: (1) narrowing of the left ventricular outflow tract at the onset of systole, and (2) hydrodynamic forces generated by contraction on the left ventricle.

A Sector Scanner for Real Time Two-Dimensional Echocardiography

During the past several years one-dimensional pulse-echo ultrasound techniques have proven extremely useful in cardiac diagnosis. A one-dimensional system, however, only visualizes structures lying along a single straight line. The spatial relationships of the various cardiac structures are therefore not so easily defined as with two-dimensional systems which display the heart by constructing a plane image composed of many straight lines. We have developed a sector scanning system for obtaining two-dimensional echocardiograms in real time using ultrasonic pulse-echo techniques. Images are produced by angling rapidly a single transducer through a 30-degree sector from a fixed spot (between ribs) on the patients chest. Thirty complete sectors (or frames) are produced per second. The use of a large diameter transducer ensures that signal strength is good and cardiac structures, including endocardium, can be visualized. Other advantages include high transducer sensitivity, real time imaging and easy visualization of various regions of the heart. Experience with more than 100 patients indicates that diagnostic quality two-dimensional echocardiograms can be readily obtained in essentially the same patients from whom one-dimensional echocardiograms are recorded and can usually be performed in less time.

Regional contractility. Selective depression of ischemic myocardium by verapamil.

SUMMARY The effects of verapamil (0.02−0.2 mg/kg) on contractility in normal and partially ischemic myocardium were compared with the changes following propranolol (0.01−1.0 mg/kg). Regional contractile function was studied in open-chest dogs with ultrasonic crystals and ischemia was controlled by graded occlusion of a carotid-to-coronary artery shunt. Reduction in shunt perfusion pressure (40−55 mm Hg) resulted in hypokinesia. Verapamil depressed contractility in ischemic myocardium in 5/5 dogs, but did not alter the maximum velocity of shortening (max V) or end- diastolic segment length in normal myocardium. Propranolol in doses sufficient to depress ischemic myocardium also depressed contractile function in normal myocardium. In two dogs without coronary occlusion, verapamil (up to 1.0 mg/kg) increased end-diastolic segment length but did not reduce max V.We conclude that verapamil selectively depresses ischemic myocardium, a finding that may have clinical implication since ischemic injury can be decreased by reducing contractility (and thereby MVO2).

Cross-sectional echocardiography in the diagnosis of congenital heart disease. Identification of the relation of the ventricles and great arteries.

Using a mechanical sector-scanner, two-dimensional echocardiograms were obtained from 28 normal subjects, 15 patients with tetralogy of Fallot, 11 patients with complete transposition of the great arteries and six patients with double outlet right ventricle. The image obtained perpendicular to the long axis of the left ventricle at the base of the ventricular septum was superimposed on the image obtained perpendicular to the long axis at the origin of the great arteries. In normal subjects, these superimposed images demonstrated that the aorta originated posterior and to the left of the ventricular septum. In patients with tetralogy of Fallot, the aorta was displaced anterior and to the right resulting in the aorta overriding the ventricular septum. In patients with double outlet right ventricle both great arteries originated anterior to the ventricular septum (i.e., from the right ventricle). In patients with complete transposition, the aorta originated anterior and the pulmonary artery posterior to the ventricular septum. Thus, cross-sectional echocardiography permits noninvasive identification of the relation of the ventricles and great arteries and, therefore, provides important information for the diagnosis of patients with congenital heart disease.

Switched Gain - A Technique for Simplifying Ultrasonic Measurement of Cardiac Wall Thickness

Pulse-echo ultrasound, a valuable tool for noninvasive cardiac examination, has been used extensively to determine left ventricular volume and wall thickness. It is often difficult, however, to visualize simultaneously the endocardial and epicardial surfaces of the left ventricular posterior wall because of dynamic range and grey scale limitations. Although the signal reflected at the epicardial-lung interface is much stronger than the signal from surrounding myocardium and lung, it is often obliterated when the receiver gain is increased sufficiently to record the endocardial echo. We have developed a switched gain technique which allows both wall surfaces to be visualized in real time. An oscillator rapidly switches the receiver gain between two levels that are independently set to display the two wall surfaces; thus, echograms are generated with high and low gain portions closely mixed. The endocardial surface is seen best at high gain; the epicardial surface at lower amplification.

Video scanner-analog computer system for semiautomatic analysis of routine echocardiograms

Echocardiography is being used increasingly in cardiac diagnosis because it permits rapidly moving cardiac structures to be visualized with ease and safety. Echocardiographic studies previously have been limited to qualitative observations because detailed quantitative measurements of cardiac structural motion have required time-consuming manual information processing. A video scanner-analog computer system has been developed that greatly simplifies the analysis of routine echocardiograms. Up to eight nonoverlapping cardiac structures are first traced manually on transparent paper from selected portions of echocardiographic records, and then Converted by a television camera into a picture consisting of 525 video lines. These lines are processed sequentially by eight signal detectors, each generating a digital value proportional to the distance of the traced signals from the beginning of the video line. The data are converted to analog form by analog computer techniques and recorded on a strip chart. Evaluation of the system has demonstrated that signals of up to 12 Hz can be accurately processed without attenuation. In addition, a comparison of the scanner-derived instantaneous left ventricular transverse dimension with that measured directly from the original echocardiograms revealed excellent correlation ( r = 0.98). The use of this system to analyze cardiac structures may have wide applicability in cardiac research and diagnosis because it allows complex quantitative data to be obtained easily from routine echocardiograms.

Identification of congenital malformations of the great arteries in infants by real-time two-dimensional echocardiography.

Real-time, two-dimensional echocardiography was used to identify great artery relations in 23 infants and small children, including 16 patients with angiographically documented transposition of the great arteries, tetralogy of Fallot, or pulmonary atresia. Using this technique, the heart was scanned perpendicular to its long axis at the origin of the great arteries. Great arteries cross-sectioned perpendicular to their long axes appear as circles; when sectioned longitudinally, these arteries appeared as elongated, sausage-shaped structures. In patients with normally related great arteries, a circular structure (aorta) always was positioned posterior to an elongated, sausage-shaped structure (distal right ventricular outflow tract and proximal main pulmonary artery). In transposition of the great arteries, two adjacent circular structures were observed; the anterior circle (aorta) was located to the right, left or directly anterior to the posterior circle (pulmonary artery). In pulmonary atresia or hypoplasia, a large posterior circle (aorta) was associated with an anteriorly positioned structure that was either short and small (atretic right ventricular outflow tract) or elongated with an area of severe narrowing (hypoplastic right ventricular outflow tract). Thus, real-time two-dimensional echocardiography provides a rapid, noninvasive means of accurately identifying congenital malformations of the great arteries in infants and small children and may be a useful adjunct to cardiac catheterization in the diagnosis of cyanotic congenital heart disease.

Effect of prolonged space flight on cardiac function and dimensions

By taking advantage of the capabilities of echocardiography to measure noninvasively left ventricular volume, stroke volume, and ejection fraction, and of the fact that the astronauts were routinely subjected to lower body negative pressure (whereby cardiac filling is progressively decreased), it was possible to construct classic ventricular function curves noninvasively, thereby obviating the difficulties encountered in comparing cardiac function at different end-diastolic volumes preflight and postflight. In this manner, the effect of an 84-day period of weightlessness on cardiac structure and function was evaluated in the Skylab 4 astronauts.

Intravascular Ultrasound Imaging: A New Method for Guiding Interventional Vascular Procedures

A method to assess the degree to which an atheroma plaque has been disrupted by percutaneous interventional methods could be of considerable benefit. An intravascular ultrasound catheter could provide quantitative information about the distribution and quality of the atheroma prior to and following a balloon dilatation, laser, or atherectomy procedure. Additionally, the ultrasound transducer could be configured within an angioplasty balloon to visualize the arterial wall in cross section during the dilatation. Visualization of the atheroma and arterial wall also might be of benefit to help characterize the type of tissue within the plaque, which may potentially help suggest which of several alternative therapies may be most effective. The intravascular imaging catheter would provide a feasible method of identifying normal and diseased arterial wall structures during diagnostic and interventional angiographic procedures. This distinction is critical during laser therapy of eccentric plaques to prevent exposure of the uninvolved wall (Fig. 7). These high quality ultrasound images may allow quantitative assessment of the extent of atheromatous involvement of artery walls as well as the character of the atheroma tissue. Such an approach, performed percutaneously in the catheterization lab, could represent a fundamental departure from traditional angiographic methods for assessing the severity of coronary, carotid, or peripheral arterial disease.