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Biomedical subjects

M Allen

Publications and source records attributed to M Allen.

At least 217 records · Page 12Linked to original sources
PubMed1989

El Chichon volcanic aerosols: impact of radiative, thermal, and chemical perturbations.

We examine the consequences of the eruption of the El Chichon volcano on the Earth's stratospheric chemistry. Formed after the eruption, the volcanic aerosol cloud, with a peak particle density at 27 km, was very efficient at altering the radiation field. The results of a one-dimensional radiative transfer model show that the total radiation increased by 8% within the aerosol layer longward of 3000 angstroms. At certain altitudes and wavelengths below 3000 angstroms, the total radiation decreased by 15%. Consequently, there are changes in the photolysis rates obtained with a one-dimensional photochemical model: for example, O2 photodissociation rate constants decrease by 10%, while O3 photodissociation rate constants increase by a comparable amount. A combination of this radiation change and the effect of a temperature variation of a few degrees causes the abundance of O3 to decrease by 7% at 24 km, in good agreement with the Solar Backscattered Ultraviolet experiment (SBUV) measurements of a 5-10% decrease. The combined radiative and thermal perturbations on the concentrations of O, O(1 D), OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO3, HO2NO2, Cl, ClO, ClO2, HOCl, ClNO3, and HCl are computed and presented in detail. However, these changes as calculated are insufficient to explain the observations of significant decreases in NO and NO2 and increases in HCl. A heterogeneous reaction catalyzed by aerosol surfaces which transforms ClNO3 into HCl provides a pathway for sequestering NOx, and at the same time reduces ClNO3 in favor of HCL. The inclusion of this reaction in the model leads to a satisfactory single-step explanation of the otherwise puzzling observations of NO, NO2, and HCl. The observed lack of change in HNO3 cannot be explained by this hypothesis. The effects of a number of heterogeneous reactions, some believed to be important for the Antarctic stratosphere, have been assessed with our model. We also examine the hypothesis of direct injection of gases from the volcano into the stratosphere. Only an unrealistically large injection (60% column increase above 12 km) results in an HCl increase in agreement with observations. An equally large water injection decreases HCl, and decreases the NO and NO2 by as much as 20%, but still does not simulate the observed NOx decrease.

Aerosols↗
PubMed1989

Sensitivity study of advection and diffusion coefficients in a two-dimensional stratospheric model using excess carbon 14 data.

Using the California Institute of Technology/Jet Propulsion Laboratory two-dimensional transport model, with transport coefficients taken from Yang and Tung (1989), we study the time evolution of excess carbon 14 in the stratosphere and the troposphere from October, 1963 to December, 1966. The model provides a satisfactory simulation of the observed data. Due to the impulsive nature of its source, initial distributions of excess carbon 14 exhibit large spatial gradients. This permits important constraints on the range of transport coefficients in the lower stratosphere to be derived. The standard model uses the circulation and eddy diffusivity of the year 1980. Large deviations (by factor of 2) from this standard transport are ruled out by our model. A self-consistently derived Kyy which is small (approximately 10(9) cm2 s-1) in tropical regions, but is larger (approximately 10(10) cm2 s-1) at higher latitudes is preferred. A Kzz as large as 1 x 10(4) cm2 s-1 would be inconsistent with the data. Excess carbon 14 is removed from the atmosphere with surface deposition velocities vS = 3 x 10(-3) cm s-1 and vN = 5 x 10(-3) cm s-1 in the southern and northern hemispheres, respectively. The last result is contrary to the current understanding that the oceans are the dominant sink for excess 14C.

Atmosphere↗
PubMed1989

Hydrogen and deuterium loss from the terrestrial atmosphere: a quantitative assessment of nonthermal escape fluxes.

A comprehensive one-dimensional photochemical model extending from the middle atmosphere (50 km) to the exobase (432 km) has been used to study the escape of hydrogen and deuterium from the Earth's atmosphere. The model incorporates recent advances in chemical kinetics as well as atmospheric observations by satellites, especially the Atmosphere Explorer C satellite. The results suggest: (1) the escape fluxes of both H and D are limited by the upward transport of total hydrogen and total deuterium at the homopause (this result is known as Hunten's limiting flux theorem); (2) about one fourth of total hydrogen escape is thermal, the rest being nonthermal; (3) escape of D is nonthermal; and (4) charge exchange and polar wind are important mechanisms for the nonthermal escape of H and D, but other nonthermal mechanisms may be required. The efficiency to escape from the terrestrial atmosphere for D is 0.74 of the efficiency for H. If the difference between the D/H ratio measured in deep-sea tholeiite glass and that of standard sea water, delta D = -77%, were caused by the escape of H and D, we estimate that as much water as the equivalent of 36% of the present ocean might have been lost in the past.

Atmosphere↗
PubMed1989

A prospective, randomized evaluation of effect of ventricular fibrillation duration on defibrillation thresholds in humans.

The effect of ventricular fibrillation duration in humans on defibrillation efficacy as it pertains to the time of intervention of an automatic implantable defibrillator is unknown. If a difference in defibrillation efficacy exists in the early period after ventricular fibrillation onset, it may affect algorithms used by antiarrhythmic devices for arrhythmia detection and therapy. Therefore, a prospective, randomized evaluation was performed of the effect of ventricular fibrillation durations of 10 s and 20 s on defibrillation thresholds in 10 survivors of sudden cardiac arrest undergoing implantation of an automatic cardioverter defibrillator. The initial duration of ventricular fibrillation was chosen randomly. Subsequently, each patient served as his or her own control for the alternate duration of ventricular fibrillation to that chosen initially. The mean leading edge defibrillation threshold voltage was 411 +/- 114 V when ventricular fibrillation persisted for 10 s and 419 +/- 125 V when it persisted for 20 s (p = 0.73). The mean defibrillation threshold current was 11.4 +/- 2.8 A when ventricular fibrillation persisted for 10 s and 11.4 +/- 3.2 A when it persisted for 20 s (p = 0.97). The delivered energy defibrillation threshold was 11.5 +/- 5.9 J when ventricular fibrillation persisted for 10 s and 12.0 +/- 6.9 J when it persisted for 20 s (p = 0.67). These findings show that the defibrillation threshold does not change between 10 and 20 s of ventricular fibrillation in out-of-hospital survivors of cardiac arrest at the time of surgical implantation of an automatic defibrillator. The data may have influence on the programming of defibrillator detection algorithms.

Algorithms↗
PubMed1989

Reminiscence groups with confused nursing center residents: an experimental study.

Reminiscence groups have been used successfully in several types of elder-care settings. This article is a study of the use of reminiscence groups with confused elderly residents of nursing centers in an attempt to improve the quality of subjects' social behaviors. Results partially supported the hypotheses. Social and demographic data were correlated with improvement or lack of it. The study suggests that the skills of the therapist are an important variable in reminiscence therapy as is true with other types of professional intervention.

Aged↗
PubMed1988

The coagulation factor VII regulator is located on 8p23.1.

Cytogenetic and coagulation studies have been performed on two patients with different abnormalities of chromosome 8, i.e. del(8p23.1----pter) and dup(8q23.1----qter). Results confirm the existence of a regulatory mechanism for clotting factor VII on chromosome 8 and define its location to the p23.1----p23.2 region.

Blood Coagulation↗
PubMed1988

A study of intellectual abilities in high-functioning people with autism.

This research extends previous research regarding the intellectual functioning of autistic individuals on standardized measures of intelligence (Wechsler Adult Intelligence Scale-Revised and the Wechsler Intelligence Scale for Children-Revised). In Study I 33 individuals with autism who closely fit the DSM-III criteria were studied. Clear evidence was found that differentiates these individuals' verbal intellectual processes from their visual-motor intellectual abilities. Principal components analysis was used to examine the interrelationship among the various intellectual abilities which such tests of intelligence measure. In Study II the intellectual abilities of a group of autistic 8- to 12-year-olds were compared to age-matched groups of children with receptive developmental language disorder, dysthymic disorder, or oppositional disorder. The intellectual abilities of autistic children were significantly different from the other groups of children.

Adolescent↗
PubMed1988

HDO in the Martian atmosphere: implications for the abundance of crustal water.

The physical and chemical processes that lead to the preferential escape of hydrogen over deuterium in the Martian atmosphere are studied in detail using a one-dimensional photochemical model. Comparison of our theory with recent observations of HDO suggests that, averaged over the planet, Mars contains 0.2 m of crustal water that is exchangeable with the atmosphere. Our estimate is considerably lower than recent estimates of subsurface water on Mars based on geomorphological analysis of Viking images. The estimate can be reconciled if only a small fraction of crustal water can exchange with the atmosphere.

Atmosphere↗
PubMed1988

The timing of evening meal and ranitidine administration--effects on patterns of 24 hour intragastric acidity.

Intragastric pH-metry was utilized to assess the effect of the time of meal ingestion and ranitidine administration on 24-h intragastric acidity. Twelve volunteers with a documented history of duodenal ulcer were studied in a four-way crossover design. Subjects randomly received ranitidine at 18.00 and 22.00 hours, with and without food. Serial blood samples were collected and analysed for ranitidine by high pressure liquid chromatography. Over the interval of 18.00-0.700 hours, the mean hydrogen-ion activity was significantly lower with the 18.00 hour dose than with the 22.00 hour dose (P less than or equal to 0.05). There were no differences between the four treatments in median pH or mean hydrogen-ion activity over the 23-h study interval. There were no differences between treatments in peak ranitidine concentrations, time to peak concentration, area under the serum-concentration time curve or elimination half-life.

Adult↗
PubMed1987

Salivary dermal analogue tumors arising in lymph nodes.

Dermal analogue tumor, an unusual type of monomorphic salivary adenoma, occurs in the parotid gland and rarely in other salivary tissues. This report describes three patients with dermal analogue tumors arising from ectopic salivary tissue in lymph nodes. Two tumors appeared in the periparotid lymph nodes and one in the lateral upper cervical region. All of the patients were men, aged 50 to 60 years, who all had a painless neck mass for 1 year or longer. Currently, the patients are free of disease 14, 3, and 2 years, respectively, after surgical excision. Dermal analogue monomorphic adenomas join several other salivary tumors in possible intranodal origin and should not be confused with metastases.

Adenoma↗
PubMed1987

Evidence for methane and ammonia in the coma of comet P/Halley.

Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH3, and uniquely NH3, in coma gases in order to explain the data. A ratio of production rates Q(NH3)/Q(H2O) = 0.01-0.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH4)/Q(H2O) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material.

Ammonia↗
PubMed1987

Comparison of calcium channel blocking agents and an anticholinergic agent on oesophageal function.

The effects of oral doses of three calcium blockers and an anticholinergic drug on oesophageal function were compared. Nifedipine (20 mg) and hyoscyamine (0.25 mg) significantly reduced lower oesophageal sphincter (LES) pressure and oesophageal contractile pressure. Verapamil (120 mg) and diltiazem (60 mg) had no significant effect on any of the oesophageal variables measured. Oesophageal transit time and oesophageal contractile duration were not affected significantly by any of the agents. Only hyoscyamine significantly prolonged acid clearance time. The combination of nifedipine and hyoscyamine was no more effective in decreasing LES pressure or oesophageal contractile pressure than either agent alone. Either nifedipine or hyoscyamine would appear to be potentially effective for the treatment of oesophago-spastic and other hypertensive motor disorders, but hyoscyamine may lead to prolongation of acid clearance from the oesophagus.

Adult↗
PubMed1987

Cognitive evoked potentials and disorders of recent memory.

P3 potentials evoked by spoken words were recorded from seven patients with disorders of recent memory and seven age-matched controls. Inclusion criteria included a clearly identifiable P3 to pure tone stimuli. Absence or marked decrement of P3 to word stimuli was seen in the patients even though they could perform the word task. The results are consistent with an encoding disorder and failure in elaboration of information processing.

Adult↗