Charles W. Miller

Nontraumatic aortic blood flow sensing by use of an ultrasonic esophageal probe

The measurement of blood velocity fields, volume flow, and arterial wall motion in the descending thoracic aorta provides essential hemodynamic information for both research and clinical diagnosis. The close proximity of the esophagus to the aorta in the dog makes it possible to obtain such data nonsurgically using an ultrasonic esophageal probe; however, the accuracy of such a probe is limited if the angle between the sound beam and the flow axis, known as the Doppler angle, is not precisely known. By use of a pulsed Doppler velocity meter (PUDVM) and a triangulation procedure, accurate empirical measurement of the Doppler angle has been obtained, allowing quantification of blood velocity scans across the aorta. Volume flow is obtained by integration of blood velocity profiles and arterial wall motion is measured with an ultrasonic echo tracking device. Accuracy of the probe was substantiated by comparison with ultrasonic and electromagnetic implanted flow cuff measurements. Use of the probe in measurement of blood velocity, volume flow and arterial wall motion at various locations along the 8- and 10-cm length of the descending thoracic aorta in adult beagle dogs is detailed. The simplicity, accuracy, and nontraumatic aspect of the technique should allow increasing use of such a probe in numerous research and clinical applications.

Late radiation response of canine mediastinal tissues

The mediastinal tissues which included heart, lung, trachea and esophagus of 70 adult beagle dogs were irradiated to a range of total radiation doses between 24 and 68 Gy given in 2, 3 and 4 Gy fractions. The purpose of the study was the calculation of alpha/beta ratios for morphologic and functional changes of the mediastinal tissues. Functional assays including echocardiography, electrocardiography, right heart hemodynamics and cardiac output were performed. Histomorphometric analyses of all tissues included in the field were done 2 years after treatment. Euthanasia was performed on 7 of 70 dogs prior to 2 years due to congestive heart failure and seven other dogs had signs of heart failure 2 years after treatment. Heart failure was thought to be caused by either pericardial effusions or constrictive pericarditis in these dogs. Heart failure occurred at doses of 62 and 68 Gy given in 2 Gy fractions, 60 Gy given in 3 Gy fractions and 52 Gy given in 4 Gy fractions. The ED50 values for pericardial fibrosis for 2, 3 and 4 Gy fractions were 46.1, 43.9 and 26.6 Gy, respectively. An alpha/beta ratio of 2.5 Gy was calculated by direct quantal response analysis. Small foci of myocytolytic lesions were detected in 11 dogs. Calculated ED50 values for myocytolysis were 70.4 Gy given in 2 Gy fractions and 50.8 Gy given in 4 Gy fractions. The estimated alpha/beta ratio was 3.2 Gy. Heart rates determined from physical examination and frequency of S-T segment changes increased with increasing dose. No other dose related changes were found in any of the other functional parameters. Functional changes were detected in the 14 dogs with clinical signs of heart failure. Focal consolidation and subpleural fibrosis were present in the irradiated lung volume. These late changes had no detectable physiologic effect in these dogs because of the small volume of lung irradiated. The ED50 values for lung consolidation were 54.3, 45.8 and 26.6 Gy after 2, 3 or 4 Gy fractions, respectively. The estimated alpha/beta ratio was 3.4 Gy. No dose-related changes could be detected in the trachea or esophagus at 2 years after treatment. These results demonstrate that lung and pericardium are the most responsive tissues in the mediastinum within the first 2 years after treatment. Myocardial lesions were present with high ED50 values, but were not found to be functionally significant at 2 years after irradiation. Human clinical data indicate that longer observation periods are needed for development of these lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

Thrombolysis with a snake venom protease in a rat model of venous thrombosis

A fibrin(ogen)olytic protease isolated from the venom of Crotalus atrox (the western diamondback rattlesnake) was tested for thrombolytic activity. The protease, called atroxase, solubilized fibrin when tested on fibrin plates and hydrolyzed fibrinogen rendering it incoagulable with a specific fibrinogenolytic activity of 42 mg fibrinogen/min/mg protein. Atroxase was unable to activate plasminogen. In vivo, fibrinolytic activity was tested on artificial thrombi induced in the posterior vena cava of Sprague-Dawley rats. Thrombolysis was then characterized by angiographic techniques over a period of three hours. Intravenous administration of the protease, at a dosage of 6.0 mg/kg, resulted in thrombolysis within one hour followed by recanalization of the originally occluded vein within two hours. Fibrinogenolytic activity resulted in a 60% decrease in the rats plasma fibrinogen level. Histological examination of kidney, liver, heart and lung tissue showed no necrosis nor hemorrhage. These results are the first step in evaluating the thrombolytic potential of anticoagulant proteases within C. atrox venom using laboratory animals.

The resolution of the ultrasound pulsed Doppler for blood velocity measurements

Abstract Pulsed ultrasound Doppler velocity meters (PUDVM) permit noninvasive blood velocity measurements. The emitted ultrasound beam characteristics primarily determine the resolution of the instrument when recording velocity profiles. The sample volume, the small region over which velocity information data are detected, was found to be > 2·3 mm3 depending on the transducer disk dia., distance in front of the disk, sampling time increment, and pulse length. The shape of the sample volume approximates a cylinder in the near field and a frustrum of a cone in the far field. The end surfaces of the sample volume were affected by the emitted pulse shape. Ultrasonic beam cross-sections were found to be smaller than predicted by theory due to the finite threshold levels of the PUDVM. The variation of the sample volume with range was illustrated by steady laminar flow velocity profile measurements in rigid tubes. The accuracy of velocity measurements was within 5 per cent with slightly larger deviations occurring near the walls due to the finite sample volume.

Inverse effect of chronically elevated blood flow on atherogenesis in miniature swine

The effect of chronically elevated blood flow on the development of atherosclerosis in miniature swine was studied. Fistulas connecting the right external iliac artery and vein were surgically created in four swine, while three were not fistulated. Pulsed Doppler velocity detection cuffs placed around the abdominal aorta and both iliac arteries of all pigs permitted chronic measurements of blood velocity, blood velocity distributions, and blood flow. All swine were fed an atherogenic diet consisting of 20% beef tallow, 3% cholesterol, and 5% cholic acid for 6 months. This diet elevated the serum cholesterol to values exceeding 500 mg/100 ml. Creation of the arteriovenous fistula (AVF) markedly elevated blood velocity and flow in the abdominal aorta and in the shunted iliac artery. In the shunted animals the aortic blood flow was 42.1 +/- 2.0 ml/sec compared with 17.3 +/- 1.4 ml/sec in the unshunted swine. The velocity distribution pattern across the vessel was also indicative of an elevated wall shear stress. After 6 months, the animals were killed and the arterial vessels examined macroscopically and microscopically for the presence of atherosclerotic lesions. In the shunted pigs, 17 +/- 15% of the lumenal surface was occupied by sudanophilic lesions, whereas 80 +/- 8% of the surface was covered by lesions in the unshunted (control) pigs. From these studies, it is apparent that mechanical factors related to blood flow rates can influence the development of atherosclerotic lesions in swine.

Low-dose aspirin does not attenuate platelet aggregation or atherosclerosis in miniature swine but decreases production of aortic wall prostacyclin

The objectives of this study were to determine if, and at what dose, aspirin could attenuate atherosclerosis in hypercholesterolemic Yucatan miniature swine, and to determine the influence of aspirin on aortic wall prostacyclin production and platelet aggregation. 30 Yucatan miniature swine (age 3 months) were fed either regular diet (RD), atherogenic diet (AD), or AD plus one of four aspirin dosages (2,4,8, or 16 mg/kg/d) for 6 months. The extent of atherosclerotic lesions in the abdominal aorta and coronary arteries was evaluated by sudanophilic staining and histological grading using Starys classification, respectively. Aortic wall production of prostacyclin (PGI2) and platelet aggregation were assessed. Lesions were similar among the AD groups (45.3 +/- 4.3%) and significantly higher than RD (1.4 +/- 0.4%). PGI2 production was significantly lower (p < 0.05) in all aspirin-treated groups. Platelet aggregation was not affected by treatment. It is concluded that the range of aspirin dosages (2-16 mg/kg/d) does not attenuate the development of atherosclerosis.

Prostacyclin, thromboxane A2, and atherosclerosis in young hypercholesterolemic swine

Plasma 6-keto-prostaglandin F1 alpha and thromboxane B2 levels were determined to evaluate their role as predictive indicators for the development and progression of coronary atherosclerosis in young hypercholesterolemic swine. 32 young swine were randomly assigned to the control or atherogenic diet group for 10, 30, 90, or 180 days. Lipid profiles were obtained at the onset and repeated throughout the study. Radioimmunoassays of plasma 6-keto-prostaglandin F1 alpha and thromboxane B2 were recorded at 10 day intervals in the 10 and 30 day subjects and at 30 day intervals in the 90 and 180 day subjects. Sections from the proximal left anterior descending coronary artery were classified based on their histological evidence of atherosclerosis by light microscopy. Hypercholesterolemia was positively correlated with development of coronary atherosclerosis (r = 0.704). However, plasma 6-keto-prostaglandin F1 alpha, thromboxane B2, and the thromboxane B2:6-keto-prostaglandin F1 alpha ratio were not found to be predictive indicators (p > 0.05) for the development or early progression of coronary atherosclerosis.

Thoracic aorta prostacyclin production is not altered during early atherosclerosis development in young swine

Decreased prostacyclin synthesis by atherosclerotic vascular tissue has been demonstrated in both adult animal models and man. The purpose of this study was to determine whether the decrease in prostacyclin synthesis by the vessel wall was evident during the initial stages of atherosclerosis development and thus a potential contributor to atherogenesis. Thirty-two Yucatan miniature swine entered the study at 3 months of age and were placed on either an atherogenic or regular diet for 10, 30, 90, or 180 days. At that time, the thoracic and abdominal aorta were harvested. Pairs of discs were punched out of the distal thoracic aorta and placed in Krebs Henseleit HEPES buffer. The buffer was then exchanged for either fresh buffer or stimulated with calcium ionophore A23187 (10 −4 m ). Aliquots were collected at 10 min for analysis of prostacyclin production by radioimmunoassay of its stable metabolite 6-keto-prostaglandin F 1α . Thoracic aorta sections were prepared for atherosclerosis assessment by light microscopy, and the abdominal aorta was stained with Sudan IV. No significant differences were noted between the diet groups at 10 and 30 days. After 90 days, a significant difference was noted ( P P > 0.05) were noted between the diet groups with regard to the basal and stimulated rates of prostacyclin production. Our findings showed that prostacyclin synthesis was not altered during the initial development or early progression of aortic atherosclerosis induced by hypercholesterolemia.

Nontraumatic Measurement of Aortic Blood Velocity Fields, Flow, and Wall Properties

Previously, in formation on aortic velocity fields in dogs has been obtained using heated film probes, which require complicated surgical procedures. The pulsed ultrasound Doppler velocity meter allows such measurements to be made nontraumatically, from a probe inserted into the esophagus. The same probe can also be used in ultrasonic echo measurements of arterial wall motion. For accurate blood velocity measurements using ultrasound, the angle between the sound beam and the flow axis must be precisely known, and this angle is determined using a triangulation procedure. Descending aorta blood velocity measurements in normal Beagles indicate an increase in peak forward velocity with increasing distance from the heart. A unique flow Dattern has also been detected which results in a higher peak shear rate at the dorsal wall. Calculations of the pressure-strain elastic modulus from aortic wall motion shows a definite increase in arterial stiffness with increasing distance from the heart.

Velocity Maps in the Canine Aorta

Velocity distributions (maps) have been recorded at six locations along the aorta in awake and anesthetized beagle dogs using implanted flow cuffs and a pulsed Doppler. The recordings of the velocity maps have been made in 60° intervals around the circumference of the aorta in order to document the influence of individual arterial outflow tracts and curvatures upon the local velocity characteristics.