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Biomedical subjects

J A Levenson

Publications and source records attributed to J A Levenson.

At least 19 recordsLinked to original sources
PubMed2006

Fast thermo-optical excitability in a two-dimensional photonic crystal.

We experimentally demonstrate excitability in a semiconductor two-dimensional photonic crystal. Excitability is a nonlinear dynamical mechanism underlying pulselike responses to small perturbations in systems possessing one stable state. We show that a band-edge photonic crystal resonator exhibits class II excitability, resulting from the nonlinear coupling between the high-Q optical mode, the charge-carrier density, and the fast (sub-micros) thermal dynamics. In this context, the critical slowing down of the electro-optical dynamics close to the excitable threshold can delay the optical response by an amount comparable to the duration of the output pulse (5 ns). The latter results from a short thermal dynamical excursion along a high local intensity manifold of the phase space.

Arsenicals↗
PubMed2006

Nonadiabatic dynamics of the electromagnetic field and charge carriers in high-Q photonic crystal resonators.

We address, both experimentally and theoretically, phase and amplitude dynamics of the electromagnetic field in a two-dimensional photonic crystal when femtosecond pulses are injected. We demonstrate that the usual adiabatic approximation underlying the dynamics of field and carriers in a semiconductor resonator is no longer valid, since in general the photon lifetime cannot be neglected with respect to the carrier recombination lifetime. Parameter regions where adiabaticity is broken are shown, and the ubiquity of the observed dynamical scenario in the new generation of active photonic microresonators is predicted.

Journal Article↗
PubMed2005

Ultraslow light propagation in an inhomogeneously broadened rare-earth ion-doped crystal.

We show that coherent population oscillations effect allows us to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits us to achieve a group velocity as low as 2.7 m/s with a transmission of 40%. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40% to 90%, and the light group velocity from 2.7 m/s to 100 m/s.

Journal Article↗
PubMed2001

Photonic band edge effects in finite structures and applications to chi 2 interactions.

Using the concept of an effective medium, we derive coupled mode equations for nonlinear quadratic interactions in photonic band gap structures of finite length. The resulting equations reveal the essential roles played by the density of modes and effective phase matching conditions necessary for the strong enhancement of the nonlinear response. Our predictions find confirmation in an experimental demonstration of significant enhancement of second harmonic generation near the photonic band edge. The measured conversion efficiency is in good agreement with the conversion efficiency predicted by the effective-medium model.

Journal Article↗
PubMed1994

Integrating cognitive therapy and pharmacotherapy in the treatment and prophylaxis of depression: a novel approach.

Psychiatric clinicians frequently prescribe biologic treatments such as antidepressant medication in combination with psychologic treatments such as psychotherapy. In the present article we propose a model integrating antidepressant treatment with Beck's cognitive therapy, a form of psychotherapy with established efficacy in the acute treatment of depression. We argue for adding cognitive therapy following successful pharmacological treatment, i.e., for spending cognitive therapy resources in the continuation phase of treatment, where they are most likely to make a unique and separate contribution to patient well-being, particularly in the areas of relapse prevention and treating residual symptoms. We encourage researchers to compare this treatment strategy to other approaches in terms of its ability to (1) prevent relapses and recurrences of depressive episodes, and (2) to impact positively on the overall quality of life in recovered depressed patients.

Antidepressive Agents↗
PubMed1994

Single beat evaluation of circumferential aortic elastin elastic modulus in conscious dogs. Potential application in non-invasive measurements.

A description of the arterial wall elastic properties comprehends both collagen and elastin, clearly shown in a biphasic stress-strain relationship. From chronically instrumented conscious dogs, aortic pressure-diameter curves can be obtained in a single beat, which is impossible to perform in human beings. In control conditions, the collagen fibers are almost not distended and the resistance to stretch is mainly supported by the elastin fibers. Therefore, the mechanical properties of the aorta are almost purely elastic in the basal beat to beat conditions. In this study we propose and test five indexes, which include as variables: systolic, diastolic and mean arterial pressure and diameter; besides, arterial compliance and pressure-strain elastic modulus as suggested to evaluate the elastic behaviour of the elastic fibers. This data can be easily obtained by non-invasive methods, such as Doppler-ultrasound techniques and auscultative esphygmomanometrical measurements, while the indexes evaluated can be retrieved from a single beat evaluation. Of three measurements performed in chronically instrumented conscious dogs on different days, one of these indexes, the ME5 = [formula: see text] x Rdias proved to be an accurate and reliable parameter to evaluate the mechanical behaviour of arteries. This kind of parameter may be useful for research and evaluation of several diseases that markedly alter the arterial wall compliance.

Algorithms↗
PubMed1990

Imipramine and weight gain during the treatment of recurrent depression.

Weight change was examined in 128 depressed outpatients treated successfully with an average dose of 215 mg imipramine and then continued on that treatment regimen for 5 additional months, an average duration of over 33 weeks. More than half the patients experienced a change in weight of 5% or less and only 13.3% experienced a gain of greater than 10%. While males and females did not differ in their pattern of change during acute treatment, females experienced a greater percent change (P less than 0.06) and change in body mass index (P = 0.05) during continuation treatment. The clinical implications of these findings are discussed, especially with respect to the extreme group.

Adult↗
PubMed1990

Psychiatric aspects of adult leukemia.

This report has discussed the normal psychological responses associated with diagnosis and treatment of leukemia, specific psychological disorders that are encountered in the context of treatment, and psychological issues that develop after definitive treatment as patients become survivors from leukemia. Psychopharmacologic and psychotherapeutic strategies to treat the specific disorders of anxiety, depression, and delirium have been outlined. Similarly, survival issues have been considered as an ever-growing number of patients are cured of leukemia.

Adult↗
PubMed1987

Brachial artery cross-sectional area and distensibility before and after arteriolar vasodilatation in men with sustained essential hypertension.

Blood pressure, volume distensibility (VD), and cross-sectional area (CSA) of the brachial artery were studied using pulsed Doppler systems in 51 patients with sustained essential hypertension in comparison with 21 normotensive controls of the same age. In hypertensive patients, in baseline conditions, CSA was significantly increased and VD decreased--the two parameters strongly and negatively correlated independent of the blood pressure level. Arteriolar vasodilatation was produced by three pharmacological agents--cadralazine, a dihydralazine-like compound; nicorandil, a nicotinamide derivative; and nitrendipine, a calcium entry blocker. For the same blood pressure reduction, cadralazine significantly reduced CSA, while nicorandil and nitrendipine increased it. Nitrendipine significantly increased VD, which was not modified by cadralazine and nicorandil. For cadralazine and nicorandil, a significant negative correlation was observed between VD and CSA. The relationship was the same as in baseline conditions. With nitrendipine, no significant correlation was observed between the two parameters. At any given CSA, distensibility was higher with nitrendipine than with cadralazine or nicorandil. The study provided evidence that, in men with essential hypertension, in basal conditions, the negative relationship between VD and CSA reflected intrinsic alterations of the arterial wall, while cadralazine, nicorandil, and nitrendipine caused a similar degree of arteriolar dilatation, nicorandil and nitrendipine caused active arterial dilatation as well, and changes in distensibility after drug administration were not directly related to blood pressure level and were mediated either by predominant geometrical modifications (cadralazine, nicorandil) or by the predominant relaxing effect of the drug on arterial smooth muscle tone (nitrendipine), or both.

Adult↗
PubMed1987

Arterial dynamics, cardiac hypertrophy, and antihypertensive treatment.

The relationship between cardiac hypertrophy, ventricular function, and aortic volume distensibility was studied in men with sustained essential hypertension by echocardiography and determination of pulse-wave velocity. The more reduced the aortic distensibility, the higher was the blood pressure response to exercise and the greater were the degree of cardiac hypertrophy and the modifications in systolic time intervals. Changes in cardiac performance as determined by vasodilators equipotent in relation to blood pressure reduction were found to have markedly different effects on aortic distensibility; volume distensibility was unchanged by cadralazine but was improved by calcium entry blockers and isosorbide dinitrate. Our findings suggest that alterations in the buffering function of the large arteries, as indicated by decreased volume distensibility, participate in the increased afterload in patients with essential hypertension and thus influence the degree of cardiac hypertrophy and changes in cardiac function and selective use of antihypertensive drugs may influence cardiovascular morbidity and mortality by their differential effect on large-vessel distensibility.

Antihypertensive Agents↗
PubMed1986

Effect of cadralazine on brachial artery hemodynamics and forearm venous tone in essential hypertension.

Forearm venous tone and brachial artery hemodynamics, including determinations of the arterial diameter and compliance by the use of pulsed Doppler systems, were measured in 16 patients with sustained essential hypertension before and after acute oral cadralazine dosing. Systolic and diastolic blood pressures significantly decreased, whereas heart rate increased. Brachial artery diameter and vascular resistance decreased, respectively, from 0.501 +/- 0.015 to 0.485 +/- 0.015 cm (P less than 0.001) and from 124.8 +/- 13.8 to 99.3 +/- 11.9 mm Hg/ml . sec (P less than 0.01). Blood flow velocity increased (P less than 0.05) but volumic flow, pulse wave velocity, and brachial artery compliance did not change. Forearm venous tone increased but the increase was inversely related to the degree of arteriolar vasodilatation. Our results indicate that, with cadralazine, forearm vascular resistance decreased while forearm blood flow was unchanged, the dilatation of small arteries contrasted with a significant reduction in the diameter of the large brachial artery, and the decrease in blood pressure was associated with a lack of increase in arterial compliance and changes in venous tone. This suggests an overriding influence of the activation of the autonomic nervous system on the action of cadralazine on large arteries and veins.

Administration, Oral↗
PubMed1986

Comparative effects of propranolol and pindolol on small and large arteries and veins of the forearm circulation in hypertensive man.

Brachial artery diameter (pulsed Doppler method), forearm vascular resistance, and venous tone (plethysmographic method) were studied in 18 patients with sustained essential hypertension. Hemodynamic parameters were reevaluated after 3 months of treatment by propranolol (9 patients) or pindolol (9 patients). For the same decrease in pressure, propranolol decreased heart rate significantly while pindolol did not, indicating the role of intrinsic sympathomimetic activity. After pindolol, forearm vascular resistance and venous tone significantly decreased while brachial artery cross-sectional area significantly increased. After propranolol, forearm vascular resistance and brachial artery cross-sectional area did not change significantly, while forearm venous tone increased markedly. The study shows that, in the long term, pindolol dilates small and large arteries and veins of the forearm circulation whereas Propranolol apparently does not.

Adult↗
PubMed1986

Haemodynamic effects of vasodilating drugs on the common carotid and brachial circulations of patients with essential hypertension.

The haemodynamic pattern of the common carotid artery was studied in men with sustained essential hypertension using pulsed Doppler methods before and after administration of vasodilating drugs. Captopril produced both a fall in vascular resistance and an increase in arterial diameter of the common carotid artery. Isosorbide dinitrate increased markedly the arterial diameter but did not change vascular resistance. Nitrendipine decreased vascular resistance with no change in the arterial diameter. In the common carotid circulation of patients with essential hypertension, vasodilating drugs may either dilate small arteries (nitrendipine), large arteries (isosorbide dinitrate), or both (captopril).

Administration, Oral↗
PubMed1986

Effects of beta-adrenergic blockade on the arterial vasculature in essential hypertension.

The effect of beta-blockade was studied in 3 different kinds of human hypertension: borderline, sustained and isolated systolic hypertension. Young patients with borderline hypertension had a similar decrease in cardiac output with both nonselective and selective beta-blockade. Only nonselective beta-blockade decreased brachial artery blood flow and increased forearm vascular resistance. In patients with sustained essential hypertension, chronic administration of 2 nonselective beta-blockers, propranolol and pindolol, caused a similar significant decrease in blood pressure with different effects on forearm circulation. Pindolol produced a significant vasodilation of both large and small arteries of the forearm while propranolol did not. In patients with isolated systolic hypertension, short-term beta-adrenergic blockade with propranolol had different effects according to age. In younger patients, propranolol significantly decreased systolic pressure with a concomitant increase in rapid ventricular ejection. In older patients, a lack of systolic pressure reduction was observed with an increase in total peripheral resistance and a decrease in systemic arterial compliance. The results suggested that beta-adrenergic blockade in hypertension may affect blood vessels with different effects, according to age, to the characteristics of hypertension and to the specific properties of the beta-blocking agent. The vascular effects involve not only resistive vessels but also large arteries.

Adrenergic beta-Antagonists↗
PubMed1986

Isosorbide dinitrate: relationship between pharmacokinetics and brachial artery hemodynamics in essential hypertension.

Pharmacokinetics of isosorbide dinitrate (ISDN) and brachial arterial hemodynamics have been studied in 15 patients with sustained essential hypertension. The hemodynamic study was performed by using a pulsed Doppler device enabling evaluation of the diameter of the brachial artery with an error of less than 10%. After intravenous administration until plateau concentrations were reached, the ISDN infusion was stopped in order to study the disappearance curve of the drug and the pharmacokinetic parameters. ISDN caused a significant decrease in systolic pressure, a significant increase in arterial diameter, and no change in heart rate. Brachial hemodynamics were not correlated with the plasma concentration in the steady state or the area under the disappearance curve. In contrast, the changes in arterial diameter during perfusion were significantly correlated with the apparent distribution volume, a finding that might indirectly reflect the affinity of ISDN for vascular tissues.

Adult↗
PubMed1986

Vaso-dilating drugs and the large arteries in essential hypertension.

Vasodilating drugs are commonly used in the treatment of hypertension. These antihypertensive agents are considered as acting on small arteries which are preferentially damaged in the hypertensive vascular disease. Recent studies have emphasized that the hemodynamic abnormalities in hypertension were not restricted to arterioles but involved also large arteries, resulting in a decrease in arterial compliance. This finding is potentially important since cardio-vascular morbidity and mortality in patients treated for hypertension is related to ischemic accidents related to alterations of large vessels, as observed in the coronary and the carotid-cerebral circulation. From this observation, it ensues that the study of the effects of vasodilating drugs on the large arteries of hypertensive patients should be extremely relevant to improve cardiovascular morbidity and mortality. On the basis of experimental studies, several cases of pharmacological dissociation in the dilatation of small and large arteries have been observed in animals, mainly in the coronary circulation. However, little research has been done on the effect of antihypertensive drugs on large arteries in men with essential hypertension. In the present review, the hemodynamic concepts and the methodologic basis for the study of large arteries in man are presented. Thereafter, the different aspect of the dilatation of small and large arteries in hypertension are analyzed, with the study of consequences on the conduit and the buffering functions of large arteries and on the development of cardiac hypertrophy. Finally, large arteries have also an important baroreflex function involved in the neuro-humoral consequences of vasodilatation. This point will be particularly described and interpreted.

Arteries↗
PubMed1985

Effects of acute and chronic angiotensin-converting enzyme inhibition on large arteries in human hypertension.

The effects of angiotensin-converting enzyme inhibition on large arteries have been examined in uncomplicated essential hypertensive patients (grade 1-2 WHO). These effects were determined from (a) changes in arterial compliance as measured from the slope of the decline in arterial pressure during diastole and (b) alterations in diameter of the brachial artery and blood flow velocity within its lumen, as assessed by pulsed Doppler velocimetry. Both acute and chronic ACE inhibition were accompanied by a significant increase in arterial compliance and a dilation of the brachial artery. This response might be related to changes in plasma and/or intraarterial angiotensin and/or to changes in plasma potassium. Whatever their mechanism, the arterial dilatation and increase in compliance would improve the buffering and the conducting functions of the large arteries, and these in turn may, if persistent, prove beneficial in possibly preventing arterial complications of hypertension.

Angiotensin-Converting Enzyme Inhibitors↗