Biomedical subjects

David S Martin

Publications and source records attributed to David S Martin.

11 recordsLinked to original sources

PubMed2006

Computer systems analysis of spaceflight induced changes in left ventricular mass.

Circulatory adaptations resulting in postflight orthostasis have frequently been observed in response to space travel. It has been postulated that a decrement in left ventricular mass (LVM) found after microgravity exposure may be the central component in this cardiovascular deconditioning. However, a physiologic mechanism responsible for these changes in the myocardium has not been determined. In this study, we examined the sequential alterations in echocardiographic measured LVM from preflight to landing day and 3 days into the postflight recovery period. In a previous study in returning astronauts we found a comparative 9.1% reduction in postflight LVM that returned to preflight values by the third day of recovery. This data was further evaluated in a systems analysis approach using a well-established advanced computer model of circulatory functioning. The computer model incorporates the physiologic responses to changes in pressures, flows and hydraulics within the circulatory system as affected by gravitational forces. Myocardial muscle progression to atrophy or hypertrophy in reaction to the circulatory load conditions is also included in the model. The integrative computer analysis suggests that these variations in LVM could be explained by simple fluid shifts known to occur during spaceflight and can reverse within a few days after reentry into earth's gravity. According to model predictions, the reductions in LVM found upon exposure to microgravity are a result of a contraction of the myocardial interstitial fluid space secondary to a loss in the plasma volume. This hypothesis was additionally supported by the published ground-based study in which we followed the alterations in LVM and plasma volume in normal subjects in which hypovolemia was induced by simple dehydration. In the hypovolemic state, plasma volume was reduced in these subjects and was significantly correlated with echocardiographic measurements of LVM. Based on these experimental findings and the performance of the computer systems analysis it appears that reductions in LVM observed after spaceflight may be secondary to fluid exchanges produced by common physiologic mechanisms. Reductions in LVM observed after microgravity exposure have been previously postulated to be a central component of spaceflight-induced cardiovascular deconditioning. However, a recent study has demonstrated a return of astronauts' LVM to preflight values by the third day after landing through uncertain mechanisms. A systems analysis approach using computer simulation techniques allows for a dissection of the complex physiologic control processes and a more detailed examination of the phenomena. From the simulation studies and computer analysis it appears that microgravity induced reductions in LVM may be explained by considering physiologic fluid exchanges rather than cardiac muscle atrophy.

Adult↗

PubMed2006

Reflection anisotropy spectroscopy study of the adsorption of sulfur-containing amino acids at the Au(110)/electrolyte interface.

Protein interactions with surfaces are key to understanding the behavior of implantable medical devices. The optical technique of reflection anisotropy spectroscopy (RAS) has considerable potential for the study of interactions between important biological molecules and surfaces. This study used RAS to investigate the adsorption of S amino acids onto Au(110) in a liquid environment under different conditions of potential and pH. Certain spectral features can be associated with the Au(110), as reported previously, while other features are assigned to bonds between the amino acids and the Au surface. The RA spectra are shown to be influenced by the structure of the amino acid, the solution pH, and the applied electrode potential. This work has assigned the negative feature at 2.5 eV to the Au-thiolate, bond while the positive feature at 2.5 eV is assigned to the disulfide bond. The broad spectral feature at 3.5 eV is attributed to the Au-amino interaction, while the sharper feature at slightly higher energy is associated with the Au-carboxylate interaction. Sulfur-containing amino acids are frequently found on the outside of protein molecules and could be used to anchor the protein to the surface.

Amino Acids, Sulfur↗

PubMed2006

Teacher quality: a comparison of National Board-certified and non-Board-certified teachers of deaf students.

THE STUDY was designed to identify specific components of teacher excellence, focusing initially on the characteristics of the small number of teachers of the deaf who are certified by the National Board for Professional Teaching Standards (NBPTS), then comparing those with the characteristics of other teachers identified as master teachers by university faculty in teacher preparation in deafness. Classroom observation, written lesson plans, teacher questionnaires on beliefs, and content analysis of interactive electronic focus groups were used to compare the two groups of teachers. Results indicated similarities between Board-certified and non-Board-certified master teachers in regard to teacher behaviors and commitment to well-founded pedagogical principles. Differences were found in classroom priorities and in the greater level of interconnectivity expressed by Board-certified teachers as the result of becoming Board certified. Recommendations are made for preparing teachers of deaf students.

Achievement↗

PubMed2005

Mechanism of spaceflight-induced changes in left ventricular mass.

Decrements in left ventricular (LV) mass observed after microgravity exposure have been previously postulated to be a central component of spaceflight-induced cardiovascular deconditioning. In this study, echocardiographic measurements of LV mass in astronauts demonstrated a comparative 9.1% reduction in postflight LV mass that returned to preflight values by the third day of recovery. A ground-based study in normal subjects determined that these pre- to postflight LV mass changes could be reproduced by simple dehydration. Reductions in LV mass observed immediately after spaceflight may be secondary to simple physiologic fluid exchanges.

Adult↗

PubMed2005

The identification of mandible fractures by helical computed tomography and panorex tomography.

The introduction of computed tomography (CT) in 1972 revolutionized the radiographic evaluation of patients who have experienced trauma. However, panoramic tomography (PT) continued to be superior in sensitivity to CT in the identification of mandible fractures and has been considered the gold standard for the past 3 decades. In 1989, a faster, higher-resolution spiral or helical CT (HCT) became widely available, and its efficacy in multiplanar evaluation and diagnosis of fractures of the upper two thirds of the face has been well established. The sensitivity of this new-generation HCT in comparison to PT in the detection of mandible fractures has not been determined. The purpose of this study was to compare the sensitivity, physician interpretation error, and interphysician agreement of HCT and PT in the identification of mandible fractures. The number and anatomical location of mandible fractures identified by HCT and PT was not significantly different. However, the number and location of 96% of fractures identified by HCT was agreed on by neuroradiologists compared with only 91% of fractures identified by PT. Furthermore, the interphysician agreement when no fracture was identified was 96% by HCT versus only 81% by PT. In conclusion, HCT has enhanced imaging quality, equivalent sensitivity in identification of fractures, decreased interpretation error, and greater interphysician agreement in the identification of mandible fractures. HCT has surpassed PT as the current gold standard for the radiographic evaluation and diagnosis of mandible fractures.

Humans↗

PubMed2004

Cardiovascular risk factors and sequelae in hypertensive children identified by referral versus school-based screening.

To determine whether systematic differences exist between hypertensive children referred for evaluation by primary care providers and children identified through community-based screening, cardiovascular risk factors and surrogate markers of hypertensive injury were compared based on subject source (referral versus screening). Children referred to a hypertension clinic for persistently elevated blood pressure were compared with children identified as hypertensive during school screening of 5102 students in Houston public schools. M-mode echocardiography of the left ventricle was performed and subsequently reviewed by 2 independent sonographers blinded to identifying subject information. Subsets of subjects also underwent carotid artery ultrasound for measurement of intimal-medial thickness, overnight urine collections for microalbuminuria, and fasting serum cholesterol, triglycerides, and glucose. Ninety-seven total subjects (54 screening and 43 referral) met inclusion criteria and had technically adequate echocardiography performed. The prevalence of left ventricular hypertrophy (LVH) was 37%. Referral subjects demonstrated significantly greater left ventricular mass index (38.8 versus 34.2 g/m(2.7); P<0.01) and a higher prevalence of LVH (49% versus 28%; P<0.05). Among subjects who underwent carotid ultrasound (n=75), carotid intimal-medial thickness was significantly higher in referral subjects (0.61 versus 0.57, P<0.05). When controlling for BMI z score, which was significantly higher in referral subjects, systematic differences by subject source did not persist. These findings suggest that hypertensive children who are predominantly overweight, independent of the manner in which patients come to medical attention, will manifest evidence of more severe cardiovascular disease assessed by surrogate markers such as left ventricular mass index or carotid artery intimal medial thickness.

Adolescent↗

PubMed2004

Presyncopal/non-presyncopal outcomes of post spaceflight stand tests are consistent from flight to flight.

INTRODUCTION: The overall prevalence of orthostatic hypotension after short duration (6-18 d) spaceflight is 20% with existing countermeasures. However, it is not known if the outcomes of stand tests for orthostatic tolerance are consistent within individuals on subsequent flights, or if first time fliers are more (or less) likely to experience orthostatic hypotension and presyncope than are veteran astronauts. METHODS: There were 50 astronauts who were studied retrospectively. Stand test data, which had been collected before and after spaceflight, were compared from at least two flights for each astronaut. For 25 of these astronauts, their first flight in this database was also their first spaceflight. For the remaining 25, their first flight in this database was their second, third, or fourth flight, as data were available. RESULTS: No subject became presyncopal during preflight testing. Of the 50 subjects, 45 (90%) had the same outcome on their first and second flights of this study. Of 14 subjects on whom we had data from a third mission, 12 had the same stand test outcome on all 3 flights (86% same outcome across 3 flights). There was no correlation between flight duration and orthostatic tolerance (r = 0.39). DISCUSSION: These data support the idea that astronauts are predisposed to orthostatic tolerance/intolerance after spaceflight and that this predisposition is not altered by subsequent flights. Flight durations within this data set did not alter the likelihood of orthostatic intolerance and rookie fliers were no more likely to experience orthostatic intolerance than were veteran astronauts.

Adaptation, Physiological↗

PubMed2003

Ultrasound in space.

Physiology of the human body in space has been a major concern for space-faring nations since the beginning of the space era. Ultrasound (US) is one of the most cost effective and versatile forms of medical imaging. As such, its use in characterizing microgravity-induced changes in physiology is being realized. In addition to the use of US in related ground-based studies, equipment has also been modified to fly in space. This involves alteration to handle the stresses of launch and different power and cooling requirements. Study protocols also have been altered to accommodate the microgravity environment. Ultrasound studies to date have shown a pattern of adaptation to microgravity that includes changes in cardiac chamber sizes and vertebral spacing. Ultrasound has been and will continue to be an important component in the investigation of physiological and, possibly, pathologic changes occurring in space or as a result of spaceflight.

Aerospace Medicine↗

PubMed2003

Pressor response to intravenous tyramine is a marker of cardiac, but not vascular, adrenergic function.

Intravenous injections of the indirect sympathetic amine, tyramine, are used as a test of peripheral adrenergic function. The authors measured the time course of increases in ejection fraction, heart rate, systolic and diastolic pressure, popliteal artery flow, and greater saphenous vein diameter before and after an injection of 4.0 mg/m(2) body surface area of tyramine in normal human subjects. The tyramine caused moderate, significant increases in systolic pressure and significant decreases in total peripheral resistance. The earliest changes were a 30% increase in ejection fraction and a 16% increase in systolic pressure, followed by a 60% increase in popliteal artery flow and a later 11% increase in greater saphenous vein diameter. There were no changes in diastolic pressure or heart rate. These results suggest that pressor responses during tyramine injections are primarily due to an inotropic response that increases cardiac output and pressure and causes a reflex decrease in vascular resistance. Thus, tyramine pressor tests are a measure of cardiac, but not vascular, sympathetic function.

Adult↗

PubMed2002

Infantile Alexander's disease: serial neuroradiologic findings.

Serial neuroimaging studies in Alexander's disease were obtained on an African-American girl who died at 4z\x years of age. She presented with macrocephaly, psychomotor retardation, spasticity, a seizure disorder, and hydrocephalus. A thorough metabolic evaluation of defined leukodystrophies, including Krabbe's disease, adrenoleukodystrophy, metachromatic leukodystrophy, Canavan's disease, and Leigh disease, was negative. A diagnosis of Alexander's disease was made based on the clinical features and ruling out all other possible causes. It was confirmed by pathologic findings of numerous subpial, subependymal, and perivascular Rosenthal fibers throughout the entire cerebrum. Interestingly, autopsy also identified the stenotic sylvian aqueduct owing to Rosenthal fiber accumulation, explaining the origin of hydrocephalus. The evolution of magnetic resonance imaging findings appears to be unique in this disease.

Alexander Disease↗

PubMed2002

Comparison of echocardiographic changes after short- and long-duration spaceflight.

BACKGROUND: Previous echocardiographic studies of astronauts before and after short-duration (4-17 d) missions have demonstrated a decrease in resting left ventricular stroke volume, but maintained ejection fraction (EF) and cardiac output. Similar studies before and after long-duration (129-144 d) spaceflight have been rare and their overall results equivocal. METHODS: Echocardiographic measurements (M-mode, 2-D, and Doppler) were obtained from short-duration (n = 13) and long-duration (n = 4) crewmembers to evaluate cardiac chamber sizes and function. RESULTS: Compared with short-duration astronauts, long-duration crewmembers had decreases in EF (+6+/-0.02 vs. -10.5+/-0.03%, p = 0.005) and percent fractional shortening (+7+/-0.03 vs. -11+/-0.07%, p = 0.015), and an increase in left ventricular end systolic volume (-12+/-0.06 vs. +39+/-0.24%, p = 0.011). CONCLUSIONS: These data suggest a reduction in cardiac function that relates to mission duration. As the changes in BP and circulating blood volume are reported to be similar after short- and long-duration flights, the smaller EF after longer spaceflights may be due to a decrease in cardiac function rather than altered blood volume.

Adult↗