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Biomedical subjects

R E Clark

Publications and source records attributed to R E Clark.

At least 289 records · Page 16Linked to original sources
PubMed1978

A simple method for estimating stresses in natural and prosthetic heart valves.

Flexible-leaflet prosthetic heart valves offer certain advantages over the lateral-flow central-occluding devices in current use. However, achieving sufficient strength and fatigue resistance in the flexible leaflets is a difficult design problem. The present paper contributes to the solution of this problem by describing a simple step-by-step method for estimating the stress in a flexible leaflet valve. The method is especially suited for evaluating preliminary proposed designs since it requires only a few simple measurements and calculations. An example is presented in which stresses estimated by the method are compared with those obtained from a finite element analysis. In addition, an error is pointed out in a previously-published method for estimating stresses in leaflet valves.

Heart Valve Prosthesis↗
PubMed1977

Pharmacological preservation of the ischemic heart.

This study was based on the concept that intracellular accumulation of calcium plays a role in mediating ischemic myocardial injury and that inhibition of entry of calcium into cells may have a salutary effect on the ischemic heart. Nifedipine, a potent vasodilator and inhibitor of transmembrane calcium flux, was infused into the aortic root of 6 dogs (5 microgram/kg/hr) during 2 hours of myocardial ischemia while on cardiopulmonary bypass. Seven control animals received normal saline at the same flow rate and temperature (20 degrees C). The results showed that none of the 7 control animals were able to maintain adequate aortic pressure or cardiac output after 30 to 60 minutes of normothermic reperfusion. All had marked left ventricular failure and were unresponsive to large doses of inotropic agents. In contrast, the 6 dogs treated with nifedipine were weaned from bypass either without difficulty or requiring small doses of calcium chloride and norepinephrine. Light microscopy demonstrated more marked ischemic damage in the control group than in the group of drug-treated dogs. We conclude that the concept of inhibition of transmembrane calcium flux offers a new and potent method for myocardial preservation during ischemia.

Animals↗
PubMed1977

Compratative distortion in three-unit fixed prostheses joined by laser welding, conventioanl soldering, or casting in one piece.

This study evaluated three different joint-forming procedures: laser welding, soldering, and one-piece casting. Fifteen three-unit bridges were fabricated. The findings showed post-jointing distrotion to be significantly less for laser-welded and one-piece castings than for soldering; and significantly greater reliability (least error variance) in joining with the laser than either one-piece casting or soldering.

Dental Abutments↗
PubMed1977

Cardiovascular system simulation requirements.

Circulatory system characteristics are considered with respect to specifying model design parameters for simulators and pulse duplicators. The requirements are investigated to determine what characteristics and parameters are important in design and construction. The specific design depends on the functions of the device to be investigated. The single most important quantity is the modeled proximal capacitance or compliance of the large vessels. More detailed study specifications require more detailed models.

Blood↗
PubMed1977

A simple cardiovascular system simulator: design and performance.

A simple simulator has been constructed, evaluated and used for performance studies of prosthetic aortic valves, balloon assist devices and Koroktof sounds and ausculatory cuff blood pressure measurements. A direct drive piston pump is used. Elastic soft rubber tubes with distributed resistances allow modeling of normal and diseased pressure pulse waves. Pressure pulse amplitude amplification is modeled with tube segments of decreasing diameters. Satisfactory proximal pressure pulse shapes are obtained with a soft rubber tube of uniform diameter.

Aortic Valve↗
PubMed1977

Quantification of wear, hemolysis and coagulation deficits in patients with Beall mitral valves.

Fifty-five patients (35 females and 20 males) were studied by noninvasive means 3.5-8.6 years after isolated mitral valve replacement with Models 103 and 104 Beall prostheses. History and physical exams by three physicians, complete hemograms, SMA 18, iron excretion rates, and coagulation profiles were performed. Additionally, electrocardiograms, echocardiograms, phonocardiograms, cardiac series, and high-speed cinefluorography of the prostheses were obtained. Valve wear was assessed by the disc/cage ratio measured from a "three-legged view" with magnification. At a mean duration of 5.85 years after operation, the entire group had a mean disc/cage ratio of operation, the entire group had a mean disc/cage ratio of 0.906 +/- 0.031 vs a normal value of 0.944 +/- 0.014. The group was mildly anemic and had a urinary iron loss that was 40 times normal. The lactic dehydrogenase (LDH) concentration was more than five times normal. The coagulation profiles were abnormal with respect to the bleeding and stypven times, antiheparin activity, fibrin degradation products, and megathrombocyte index. These abnormalities were unrelated to sex, degree of valve wear, and history of thromboembolism. Males were less anemic and had higher urine iron losses than females. Nine patients with severe valve wear (disc/cage ratio less than or equal to 0.87) were significantly more anemic with large urine iron losses and had elevated total bilirubin, serum glutamic oxaloacetic transaminase, and LDH concentrations (ninefold), when compared to nine patients with minimal wear (disc/cage ratio greater than or equal to 0.925). It is emphasized that the findings of a significant anemia, an LDH concentration greater than 1500 mU/ml, and a disc/cage ratio of less than 0.87 in a patient with an isolated Beall mitral valve prosthesis are indicators for the need to replace the prosthesis in the near future. Re-replacement is urged before significant clinical deterioration.

Adult↗
PubMed1976

Late clinical problems with Beall model 103 and 104 mitral valve prostheses: hemolysis and valve wear.

The purpose of this study was to determine the influence on hemolysis of the spatial orientation of the struts in the Beall mitral valve prosthesis, Models 103 and 104. Thirteen pairs of patients were selected to match struts oriented parallel and perpendicular to the left ventricular outflow tract axis. The average time after operation was 3.73 years. Complete blood counts and relative serum chemistry values were determined.

Adult↗
PubMed1976

Filling defects in the renal pelvis and ureter owing to bleeding secondary to acquired circulating anticoagulants.

A case of transient filling defects in the renal pelves and ureters, secondary to spontaneously occurring acquired anticoagulants, is presented. The relationship of this entity to hemophilia, immunologic disorders, such as systemic lupus erythematosis, and normal post partum patients is discussed. The differential diagnosis for filling defects in the renal pelvis is reviewed.

Acute Kidney Injury↗
PubMed1976

The association of increased plasma MB CPK activity and irreversible ischemic myocardial injury in the dog.

To evaluate the concordance between elevated plasma MB CPK and irreversible myocardial ischemic injury, coronary occlusion was induced for 10 minutes to 48 hours in 21 open chest dogs and 13 conscious animals. Results of plasma CPK and MB CPK assayed in samples obtained serially ofr 24 hours were compared to microscopic changes in hearts from the same animals examined 48 hours after occlusion. Twelve of the 34 dogs died within two hours after coronary occlusion. Among the surviving 22 dogs, one failed to exhibit gross of electrocardiographic evidence of ischemia and was therefore excluded. Twelve had coronary occlusion maintained for 30 minutes or longer and in 11 of these peak plasma MB CPK activity exceeded thenormal range (mean +/- 2 SD) and baseline values by at least 100%. Necrosis was present in the hearts from each manifested by nuclear pyknosis, eosinophilia, shrinkage of cytoplasm, and leukocytic infiltration. In the remaining nine dogs with occlusion for less than 30 minutes, peak plasma MB CPK activity was not elevated and necrosis was not detected. The close concordance between plasma MB CPK elevations and myocardial necrosis was significant (chi2 = 14.5, P less than 0.001), and thus, increased plasma MB CPK activity reflected irreversible myocardial ischemic injury.

Animals↗