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Biomedical subjects

R Aronson

Publications and source records attributed to R Aronson.

At least 19 recordsLinked to original sources
PubMed1999

Photon diffusion coefficient in an absorbing medium.

A number of investigators have recently claimed, based on both analysis from transport theory and transport-theory-based Monte Carlo calculations, that the diffusion coefficient for photon migration should be taken to be independent of absorption. We show that these analyses are flawed and that the correct way of extracting diffusion theory from transport theory gives an absorption-dependent diffusion coefficient. Experiments by two different sets of investigators give conflicting results concerning whether the diffusion coefficient depends on absorption. The discrepancy between theory and the earlier set of experiments poses an interesting challenge.

Absorption↗
PubMed1998

Nonlinear effects of localized absorption perturbations on the light distribution in a turbid medium.

A theoretical model of photon propagation in a scattering medium is presented, from which algebraic formulas for the detector-reading perturbations (delta R) produced by one or two localized perturbations in the macroscopic absorption cross section (delta mu a) are derived. Examination of these shows that when delta mu a is titrated from very small to large magnitudes in one voxel, the curve traced by the corresponding delta R values is a rectangular hyperbola. Furthermore, while delta Rinfinity identical to lim delta mu a-->infinity delta R is dependent on the location of the detector with respect to the source and the voxel, the ratio delta R/ delta Rinfinity is independent of the detector location. We also find that when delta mu a is varied in two voxels simultaneously, the quantity delta R (delta mu a,1 [symbol: see text] delta mu a,2) is a bilinear rational function of the delta mu aS. These results apply not only in the case of steady-state illumination and detection but to time-harmonic measurements as well. The validity of the theoretical formulas is demonstrated by applying them to the results of selected numerical diffusion computations. Potential applications of the derived expressions to image-reconstruction problems are discussed.

Absorption↗
PubMed1997

Optical imaging of anatomical maps derived from magnetic resonance images using time-independent optical sources.

We present a model suitable for computing images of absorption cross sections of thick tissue structures illuminated at near infrared (NIR) wavelengths from tomographic projection data. Image reconstruction is accomplished by solving a system of linear equations derived from transport theory. Reconstruction results using different algebraic solvers are shown for anatomical maps of the breast, derived from magnetic resonance imaging data, containing two simulated pathologies, in which case qualitatively good reconstructions were obtained. Evaluation of magnetic resonance (MR) data to optimize NIR optical tomographic imaging methods and to assess the feasibility of a combined MR-optical measurement scheme is discussed.

Adipose Tissue↗
PubMed1997

Radiative transfer implies a modified reciprocity relation.

The usual reciprocity relations of radiative transfer do not hold two points located in regions of different index of refraction. Modified reciprocity relations that involve the relative index are derived. The result has computational as well as theoretical consequences.

Light↗
PubMed1996

Triiodothyronine augmentation in the treatment of refractory depression. A meta-analysis.

BACKGROUND: Several trials have addressed the efficacy of liothyronine sodium therapy in euthyroid, nonpsychotic depressed patients refractory to tricyclic antidepressant therapy. We undertook a meta-analysis of these trials. METHODS: The MEDLINE database (1966 to May 1995) and published reference lists were examined for controlled clinical trials of triiodothyronine augmentation in euthyroid patients with refractory depression. Quality assessment and data abstraction were performed independently by two reviewers. Results were aggregated three ways: the relative response rate compared with controls, accepting each trial's definition of clinical response; absolute improvement in response rates; and improvements in depression scores, analyzed as continuous variables without a prespecified threshold for clinical response. RESULTS: Aggregating eight studies with a total of 292 patients, patients treated with triiodothyronine augmentation were twice as likely to respond as controls (relative response, 2.09; 95% confidence interval [CI], 1.31 to 3.32; P = .002). This corresponded to a 23.2% absolute improvement in response rates (95% CI, 4.5% to 41.9%; P = .02). Improvements in depression scores were moderately large (standardized effect size, 0.62; P < .001). However, study quality was uneven, and results were statistically heterogeneous. Among the four randomized double-blind studies, pooled effects were not significant (relative response, 1.53; 95% CI, 0.70 to 3.35; P = .29), but one study with negative results accounted for most of the intertrial heterogeneity in results. CONCLUSIONS: Triiodothyronine augmentation may be an effective empirical method of increasing response rates and decreasing depression severity scores in a subgroup of patients with depression refractory to tricyclic antidepressant therapy, but the total number of patients randomized was small, and additional placebo-controlled data are required for a definitive verdict. Since therapeutic trends now favor other drugs, future trials might usefully examine triiodothyronine augmentation with selective serotonin reuptake inhibitors or compare potentiation strategies, eg, lithium vs triiodothyronine, for managing refractory depression. Such trials would benefit from much larger sample sizes than those reviewed here.

Adult↗
PubMed1995

Boundary conditions for diffusion of light.

In connection with recent work on remote imaging of random media by light, a straightforward generalization of the proper diffusion boundary conditions is presented that takes into account Fresnel reflection. The Milne problem at exterior boundaries is solved for various values of index of refraction, absorption, and scattering anisotropy parameters to yield extrapolated end points and extrapolation distances. A generalized interface condition is derived to replace the usual condition of continuity of intensity. Benchmark-quality numerical results are given for the extrapolation distance and for the new index-dependent parameter in the interface conditions. Difficulties in using the extrapolated end point when the index is sufficiently large are discussed, and a new image procedure suitable for this case is presented.

Diffusion↗
PubMed1994

Mechanism of current-induced early afterdepolarizations in guinea pig ventricular myocytes.

We investigated possible ionic mechanisms that cause early afterdepolarizations (EADs) following the injection of constant inward current in guinea pig ventricular myocytes by several interventions that affect failure of action potential repolarization. The amount of constant current was adjusted to measure the threshold potential (Vth) associated with the minimum inward current required for inducing EADs [threshold current (Ith)] and also the magnitude of EADs at Vth and following adjustment of current to generate takeoff potentials of -30 and -20 mV. Interventions associated with either inhibition of Ca2+ release from the sarcoplasmic reticulum (ryanodine 5 x 10(-6) M) or L-type membrane Ca2+ channel current (verapamil 1.1 x 10(-5) M and nisoldipine 5 x 10(-7) M) reduced or abolished EADs arising from -30 or -20 mV. Cells that generated delayed afterdepolarizations (DADs) in the absence of depolarizing current after 20 stimulations at 5 Hz either in control solution or following interventions associated with Ca2+ loading (reduced extracellular [K+] or increased extracellular [Ca2+]) also developed a marked shift in Vth of current-induced EADs at 1-Hz stimulation to more negative potentials [60.3 +/- 10.7 mV (mean +/- SD, n = 17) vs. -41.7 +/- 6.4 mV in cells without DADs in control solution (n = 25), P < 0.001]. Ca2+ loading also increased the magnitude of EADs arising from Vth and -20 mV. Exposure to quinidine (1.23 x 10(-5) M), which blocks both Na+ and delayed rectifier K+ channels, significantly reduced Ith but had only minimal effect on the magnitude of EADs. Our results suggest that L-type Ca2+ channel current and [Ca2+]-sensitive inward current associated with release of Ca2+ from the sarcoplasmic reticulum are the major currents that cause this form of EADs, and that Ca2+ loading promotes the development of large EADs likely to propagate to normal tissue.

Action Potentials↗
PubMed1993

Perceptions of vaccine efficacy, illness, and health among inner-city parents.

A resurgence of measles in the past decade has focused attention on the limitations of current immunization programs, particularly for inner-city, low-income populations. As part of a larger study of immunization rates, we discussed perceptions of disease severity and vaccine efficacy, as well as the prioritization of the tasks of parenthood, with 40 parents of infants living in inner-city Baltimore to discover their beliefs about immunization. Vaccines were considered only partly successful; susceptibility to chickenpox after vaccination was repeatedly cited as evidence of vaccine failure. Fever was seen as a primary indicator of illness; thus, vaccines were believed to cause, rather than prevent, illness. Immunization was not considered a high-priority parental responsibility. These findings suggest future interventions be aimed at changing parental perceptions of vaccines as ineffective and of fever after immunization as an indicator of illness. Finally, immunizations should be made easily available, even during clinic visits for a child's illness.

Baltimore↗
PubMed1993

Urban adolescent high-risk sexual behavior: corroboration of focus group discussions through pile-sorting. The AIDS Youth Research Team.

Risk activities for acquisition of the human immunodeficiency virus (HIV) remain prevalent among urban adolescents. While interdisciplinary approaches to examine the variables contributing to risk/protective behaviors have been promoted, strategies for such explorations require further formulation. Recently we employed focus group discussions to explore factors placing urban adolescents at risk for engaging in HIV risk behaviors. The focus group format enables substantial interaction on a topic in a limited time period, but does not always provide expression of the full range of behavioral options. In this study we investigated the use of pile-sorts for confirmation of impressions from focus group discussions among 57 urban youths aged 10-14. The pile-sorts revealed some support for most of the views expressed in the group discussions. However, the sorts revealed more variability in views than was expressed in the group discussions. Substantial gender and age-based differences in perceptions were revealed with potentially important intervention implications.

Adolescent↗
PubMed1992

Cardiac contractile proteins in hypertrophied and failing guinea pig heart.

OBJECTIVE: The aim was to study changes in contractile proteins which accompany marked hypertrophy and heart failure in mammalian hearts initially containing predominantly V3 isomyosin. METHODS: Left ventricular myosin and myofibrillar ATPase activity and right ventricular actomyosin ATPase activity were measured in normal guinea pig hearts, in hearts which were hypertrophied as a result of progressive left ventricular systolic overload following ascending aortic banding, and in hypertrophied hearts from animals which showed signs of overt congestive heart failure. Male guinea pigs weighing 225-275 g at the time of aortic banding were used for the studies. RESULTS: Left ventricular myosin and myofibrillar ATPase activity and right ventricular actomyosin ATPase activity were correlated with body weight, left and right ventricular weight, and left ventricular peak systolic pressure during aortic occlusion. Left ventricular myosin ATPase activity and right ventricular actomyosin ATPase activity were markedly depressed in hypertrophied ventricles compared to control ventricles. Cardiac myofibrillar ATPase activity was lower in hypertrophied failing hearts than in control hearts over a wide range of calcium concentrations. In control animals and in those without heart failure, there was a nearly identical inverse relationship between left ventricular mass up to 1600 mg and myosin ATPase activity. Hypertrophied failing hearts were larger but showed little further reduction in cardiac myosin ATPase activity. Representative gel scans of non-dissociating pyrophosphate gels of left ventricular myosin from an 8 d postoperative aortic constricted animal and from its age and weight matched control showed predominantly V3 isomyosin with small amounts of V1 isoenzyme. However, preparations taken from guinea pigs 16 d after aortic constriction showed only the V3 isoform, whereas the V1 isoform was still apparent in control. Hypertrophied failing left ventricles developed less pressure per unit mass during brief aortic occlusion than non-failing left ventricles with comparable myosin ATPase activities. CONCLUSIONS: These observations raise important questions as to the distribution of myosin isoforms in the normal adult guinea pig, and the possibility that myosin ATPase activity might be altered by post-translational modification. Although cardiac myosin ATPase activity correlates with left ventricular performance, it cannot fully explain the depressed performance of failing hearts in this model. Additional immunological studies of cardiac contractile proteins are required as well as studies designed to explore the implications of altered myosin ATPase activity for both contractile function and overall cellular homeostasis.

Animals↗
PubMed1991

Voluntary hyperventilation in obesity hypoventilation.

Arterial blood gas analysis was performed before and after 60 to 90 s of voluntary hyperventilation in 27 consecutive patients with occlusive sleep apnea syndrome (OSA) and daytime hypercapnia. The percentage of fall in PaCO2 from baseline was examined in relationship to age, body mass index, sleep-disordered breathing indices, and pulmonary function variables. In 14 subjects without airflow obstruction, only one individual could not voluntarily hyperventilate into the normal range, whereas 6 of 13 subjects with airflow obstruction could not hyperventilate to eucapnia. The average percentage of fall in PaCO2 was 16 mm Hg (SEM = 1.3 mm Hg). The percentage of fall in PaCO2 correlated significantly with FEV1/FVC ratio (r = 0.47, p = 0.01) and with FEV1 (r = 0.5, p = 0.008). Although the baseline PaCO2 did not correlate with FEV1, the posthyperventilation PaCO2 did (r = 0.54, p = 0.003). Voluntary hyperventilation studies herein suggest a predominant role for impairment of ventilatory control in the maintenance of hypercapnia in OSA since a fall of PaCO2 into the normal range can usually be obtained. The correlation between the percentage of fall in PaCO2 and spirometric measures of respiratory mechanics, as well as the inability of some subjects to normalize the PaCO2 voluntarily suggests an added role for respiratory mechanical impairment in obesity hypoventilation.

Adult↗
PubMed1990

Growth in children with congenital hypothyroidism detected by neonatal screening.

A prospective analysis of physical growth in 56 children with congenital hypothyroidism compared the children's height, weight, and head circumference with population percentiles. Two measures of skeletal maturation and predictions of adult height were also compared with population standards. The mean percentiles for the children's height and head circumference were higher than population means. Although mean bone age scores were slightly higher, predictions of adult height did not differ significantly from midparental height (arithmetic mean of sum of parental heights) and population means. The duration of intrauterine hypothyroidism as measured by gestational bone age and the duration of postnatal hypothyroidism were inversely correlated with heights attained up to 9 years. This association suggests a possible long-term influence of early hypothyroidism on growth. In children with congenital hypothyroidism, maintenance of serum thyroxine levels in the upper half of the normal range results in normal growth patterns.

Age Determination by Skeleton↗
PubMed1989

Compensatory hypertrophy and failure in gradual pressure-overloaded guinea pig heart.

Left ventricular hypertrophy has been produced in the guinea pig by a procedure that gradually increases left ventricular afterload. A mildly constricting band was placed around the ascending aortas of very young guinea pigs (225-275 g). With growth to 500-1,000 g, left ventricular systolic pressure increased and left ventricular hypertrophy developed. In approximately 50% of these animals, the hypertrophy was associated with normal left ventricular function and with no unusual symptoms or evidence of heart failure. The other animals developed dyspnea, which appeared an average of 41 days after banding. Dyspneic animals had normal body weight, markedly increased right ventricular and lung weights, decreased left ventricular norepinephrine content, diminished maximum left ventricular pressure generating capacity, and a significantly higher incidence of left ventricular interstitial and perivascular fibrosis. These findings demonstrate that even when left ventricular overload is imposed gradually by banding the aortas of young animals, myocardial decompensation ultimately ensues in a significant proportion of such animals. The slow imposition of loading, the slow rate of decompensation, and the ability to identify animals in heart failure by clinical dyspnea make this model uniquely valuable for studies on the mechanisms of heart failure.

Adaptation, Physiological↗
PubMed1989

Breath-by-breath interactions between inspiratory and expiratory duration in occlusive sleep apnea.

We examined interactions between inspiratory duration (TI), expiratory duration (TE), and inspiratory (esophageal) pressure (Pes) generation in seven subjects with confirmed occlusive sleep apnea. Breath-by-breath values of TI, TE, and Pes were identified by digital computer during 21 260-s epochs of repetitive occlusive apnea during non-rapid-eye-movement sleep. The control theory of interacting nonlinear oscillators was used to categorize the interaction between TI and TE for each epoch as either 1) synchronization, the strongest possible interaction between biological oscillators; 2) relative entrainment, a moderate interaction between oscillators; or 3) relative coordination, a weak interaction. The latter two interactions were characterized by systemic oscillations in the moving cross-correlation between TI and TE. The relationship between TI and Pes was analyzed in a similar fashion. Significant oscillations were present in all three parameters (P less than 0.0001 for each). We observed significant negative correlations between TI and TE and between TI and Pes (P less than 0.001 for each) when all breaths for all epochs were pooled. In no epoch was there a significant positive correlation between TI and TE or Pes. All three interactions were observed between TI and TE: five epochs of synchronization, nine of relative entrainment, and seven of relative coordination. In contrast, 19 of 21 epochs exhibited synchronization between TI and Pes, with 2 epochs of relative entrainment. The relative frequency of TI vs. Pes synchronization was significantly greater than TI vs. TE synchronization (P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗