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Biomedical subjects

D B Cassidy

Publications and source records attributed to D B Cassidy.

8 recordsLinked to original sources
PubMed2005

Experiments with a high-density positronium gas.

We have created a high-density gas of interacting positronium (Ps) atoms by irradiating a thin film of nanoporous silica with intense positron bursts and measured the Ps lifetime using a new single-shot technique. When the positrons were compressed to 3.3 x 10(10) cm-2, the apparent intensity of the orthopositronium lifetime component was found to decrease by 33%. We believe this is due to a combination of spin exchange quenching and PS2 molecule formation associated with colliding pairs of oppositely polarized triplet positronium atoms. Our data imply an effective cross section for this process of 2.9 x 10(-14) cm-2.

Journal Article↗
PubMed2001

Doppler broadening of in-flight positron annihilation radiation due to electron momentum.

We report the first observation of electron momentum contributions to the Doppler broadening of radiation produced by in-flight two-photon annihilation in solids. In these experiments an approximately 2.5 MeV positron beam impinged on thin polyethylene, aluminum, and gold targets. Since energetic positrons easily penetrate the nuclear Coulomb potential and do not cause a strong charge polarization, the experimental annihilation line shapes agree well with calculations based on a simple independent-particle model. Moreover, annihilations with the deepest core electrons are greatly enhanced.

Journal Article↗
PubMed1992

Improved regional myocardial blood flow, left ventricular unloading, and infarct salvage using an axial-flow, transvalvular left ventricular assist device. A comparison with intra-aortic balloon counterpulsation and reperfusion alone in a canine infarction model.

BACKGROUND: It has been suggested that left ventricular unloading at the time of reperfusion provides superior infarct salvage over reperfusion alone. The purpose of this study was to show that the Hemopump transvalvular axial-flow left ventricular assist device provides superior left ventricular unloading, ischemic zone collateral blood flow, and infarct size reduction compared with intra-aortic balloon counterpulsation and reperfusion alone. METHODS AND RESULTS: Eighteen dogs were instrumented with regional myocardial function sonomicrometers in the ischemic and control zones. The left anterior descending coronary artery just distal to the first diagonal branch was instrumented with a silk snare and Doppler flow probe. Additionally, pressure catheters were placed in the left atrial appendage, left ventricular apex, and ascending aorta for hemodynamic measurements. Regional myocardial blood flow was determined by using 15-microns radioactive microspheres. Measurements were made in the control state, immediately after coronary occlusion, at 1 and 2 hours after coronary occlusion, with reperfusion, and 1 hour after reperfusion. In treated animals, left ventricular assistance was maintained during the entire period of occlusion and reperfusion. The Hemopump was associated with a significant decrease in left ventricular systolic and diastolic pressure, whereas mean arterial pressure was maintained. Intra-aortic balloon counterpulsation resulted in no significant changes in left ventricular systolic pressure and a modest decrease in left ventricular diastolic pressure. Regional unloading as assessed by sonomicrometers was significant in the Hemopump animals and absent in the balloon pump animals. Absolute regional myocardial blood flow in the ischemic zone increased slightly (p = 0.002) in the Hemopump animals and did not change in the balloon pump animals. Infarct size expressed as percentage of the zone at risk was 62.6% in the control animals, 27.22% in the balloon pump animals, and 21.7% in the Hemopump animals. CONCLUSIONS: Mechanical unloading of the ventricle during ischemia and reperfusion appears to result in significant infarct salvage compared with reperfusion alone. The Hemopump appears to provide superior left ventricular systolic and diastolic unloading compared with intra-aortic counterpulsation in a canine model.

Animals↗
PubMed1991

Effects of rotational atherectomy in normal canine coronary and diseased human cadaveric arteries: potential for plaque removal from distal, tortuous, and diffusely diseased vessels.

To test the hypothesis that tortuous, diseased vessels could be successfully treated with a flexible rotational atherectomy device we evaluated the BARD atherectomy device with quantitative angiography and histology in normal canine coronary arteries and diseased human below-knee amputation specimens. The mid left anterior descending and the circumflex vessels were treated in 4 dogs serially with 1 wk separating treatments. The acute and follow-up anterior descending artery size was unchanged (1.41 mm before, 1.39 mm after, and 1.59 mm at 1 week). Similar findings were obtained in the circumflex vessels. In 4 adult human amputated legs, diseased peroneal or tibial arteries were treated with a significant reduction in the percent luminal diameter stenosis from 62.75 prior to intervention to 36.5 following intervention (p = 0.021). The luminal diameter increased from 0.81 to 1.54 mm (p = 0.06). In 2 canine arteries there was histologic evidence of localized perforation of the arterial wall, but there was no angiographic evidence of perforation or dissection and no significant myocardial necrosis in the distribution of the treated vessels at 1 wk. The majority of the diseased human vessels demonstrated smoothly cut atheromas with sparing of the media. The rotational atherectomy catheter system holds promise for removal of plaque in relatively small, diffusely diseased, tortuous vessels.

Animals↗
PubMed1991

Comparison of technetium-99m teboroxime tomography with automated quantitative coronary arteriography and thallium-201 tomographic imaging.

Technetium-99m (Tc-99m) teboroxime is a new perfusion tracer that is highly extracted and rapidly cleared by the myocardium. To determine the feasibility of Tc-99m teboroxime imaging in the diagnosis of patients with suspected coronary artery disease, 30 patients underwent single photon emission computed tomography imaging with Tc-99m teboroxime (25.2 +/- 1 mCi) at peak exercise and again 60 min later at rest. All patients underwent either a thallium stress test (n = 26) or automated quantitative coronary arteriography (n = 25), or both, without intervening revascularization or infarction. Images were reviewed by two investigators who had no knowledge of clinical data. Coronary lesions with greater than or equal to 50% diameter narrowing by quantitative coronary arteriography were considered significant. Both thallium and Tc-99m teboroxime detected disease in all patients with two or three vessel disease. One vessel disease was detected with Tc-99m teboroxime in 9 of 10 patients and with thallium in 8 of 10 (p = NS). In patients without angiographically significant disease. Tc-99m teboroxime demonstrated normal perfusion in six of eight patients and thallium in three of five (p = NS). Overall, when presence or absence of disease detected by Tc-99m teboroxime or thallium was compared with quantitative coronary arteriography, there was no difference between Tc-99m teboroxime and thallium. These results suggest that Tc-99m teboroxime is comparable to thallium as an imaging agent. The rapid biologic half-life, 5.3 min, allows studies to be completed in 60 to 90 min.

Angiography↗
PubMed1989

Acute MI: role of thrombolysis and intervention.

Several issues confront the physician treating the patient with an acute infarction. If the patient is a thrombolytic candidate, a 20 to 30 per cent failure rate still exists, and it is difficult to predict on clinical grounds who has had a successful trial of thrombolytics or not; therefore, considerable clinical judgment must be applied. If the patient has relief of pain and diminished ST segment elevation, it is likely that thrombolytic therapy has been successful. Limitations concerning the benefits of thrombolytic or angioplasty therapy for the acute infarction also exist with regard to baseline patient characteristics. Older patients, especially women, seem to have more complications and less beneficial results from acute revascularization than do others. Patients with anterior infarctions as compared with posterior or inferior wall infarctions probably have a higher benefit from intracoronary thrombolysis. The acute myocardial infarction patient can be treated in multiple ways. Based on the preceding information and our own clinical experience, some recommendations can be made. Other sources with their own recommendations are available as well. First, because of the uncertainty still present in deciding optimum therapy for any given patient, as many patients as possible should be included in randomized prospective clinical trials that are now ongoing. If the patient or treating physician elects not to take part in such a trial, much of the therapy will be based on available resources. In small hospitals without acute catheterization or angioplasty facilities, intravenous thrombolytic therapy should be instituted as quickly as possible. In patients who are not able to receive thrombolytic therapy, acute catheterization with consideration for either angioplasty or acute bypass surgery should be undertaken if the patients are relatively young and early on in their course. Treatment of older patients, especially women, should be tempered by the knowledge that there are diminishing returns in aggressive approaches to these patients. It would appear that the presence of cardiogenic shock itself, although a predictor of higher cardiac mortality, should not preclude an aggressive approach and indeed this patient may benefit greatly from revascularization as well as pharmacologic and mechanical support of the cardiovascular system. If thrombolytic therapy without catheterization is undertaken, there remains the potential for either nonrevascularization or early closure.(ABSTRACT TRUNCATED AT 400 WORDS)

Angiography↗