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Biomedical subjects

D A Ludwig

Publications and source records attributed to D A Ludwig.

34 records · Page 2Linked to original sources
PubMed1994

Effect of simulated microgravity on cardiopulmonary baroreflex control of forearm vascular resistance.

The stimulus-response characteristics of cardiopulmonary baroreflex control of forearm vascular resistance (FVR) were studied in 11 healthy men before and after 7 days of 6 degrees head-down bedrest to test the hypothesis that microgravity alters this reflex response. We assessed the relationship between stimulus [changes in central venous pressure (delta CVP)] and reflex response (delta FVR) during unloading of cardiopulmonary baroreceptors with lower body negative pressure (LBNP; 0 to -20 mmHg). delta CVP during bedrest and LBNP was estimated from peripheral vein pressures in the dependent right arm. Compared with prebedrest baseline, plasma volume and estimated CVP were decreased by 13 and 33%, respectively, at 7 days of bedrest. Progressive reflex forearm vasoconstriction occurred in response to graded reductions in estimated CVP during LBNP, and delta FVR per unit delta CVP was doubled after bedrest. The increase in sensitivity of the cardiopulmonary baroreflex control of FVR was related to reduced circulating blood volume, suggesting that enhanced peripheral vasoconstriction in individuals adapted to microgravity can be attributed, in part, to hypovolemia. In addition, microgravity appears to alter the stimulus for cardiopulmonary baroreceptors to a lower operational range of CVP, suggesting the possibility of chronic resetting.

Adult↗
PubMed1994

Predicting orthostatic intolerance: physics or physiology?

To adequately assess the viability of any particular causal theory of orthostatic intolerance, physical and physiological parameters thought to be associated with orthostasis must be evaluated simultaneously within the same individual. Time, cost, and complexity of instrumentation generally limit studies of orthostatic intolerance to a single independent effect for any given sample of subjects. We, therefore, measured 6 key physical and physiological variables associated with various theories of orthostatic intolerance in 14 males to test the hypothesis that physical factors rather than physiological reflex mechanisms were dominant in contributing to orthostatic tolerance. Measurements included height, plasma volume (expressed as percent of total weight), variation in R-R interval, leg compliance, and carotid-cardiac (high pressure), and cardiopulmonary (low pressure) baroreflex sensitivity. Subjects' orthostatic intolerance was quantified by time to syncope during progressive supine lower body decompression. Correlations, regression coefficients, and indices of replicability were calculated using 500 to 1,000 bootstrap resamplings of the original 14 observations. Although all six measurements correlated with time to syncope when evaluated individually, only height (negative), percent plasma volume weight (positive), and, to a lesser extent, carotid-cardiac baroreflex sensitivity yielded consistent (reproducible) results when all measures were tested simultaneously. These results suggest that while orthostatic intolerance may be dependent upon a variety of physiological reflexes, physical factors such as height and plasma volume tend to dominate the prediction of time to syncope during lower body negative pressure (LBNP). Physiological reflexes, which act to maintain adequate profusion to the brain during normal terrestrial posture in humans, may be overwhelmed by the additional orthostatic challenge imposed by progressive lower body decompression.

Adult↗
PubMed1993

Small N experiments.

Explore the source record for details and available documents.

Reproducibility of Results↗
PubMed1991

Carotid baroreceptor influence on forearm vascular resistance during low level lower body negative pressure.

The degree of forearm vasoconstriction induced by low levels of lower body negative pressure (LBNP) provides a measure of the responsiveness of the cardiopulmonary baroreflex. The validity of this measurement is based on the assumption that this vasoconstriction response is not influenced by unloading of carotid baroreceptors. To test the hypothesis that arterial baroreceptor unloading does not alter the degree of forearm vascular resistance during low levels of LBNP, we exposed 12 subjects to -15 and -20 mm Hg LBNP with and without additional artificial (+10 mm Hg neck pressure) unloading of the carotid baroreceptors. There was no measurable influence of carotid unloading on forearm vascular resistance at either level of LBNP. We conclude that forearm vascular resistance measured during cardiopulmonary baroreceptor unloading is unaffected by carotid baroreceptor unloading within the magnitude encountered during low levels of LBNP.

Adult↗
PubMed1991

A statistical note on the redundancy of nine standard baroreflex parameters.

An accepted method for measuring the responsiveness of the carotid-cardiac baroreflex to arterial pressure changes is to artificially stimulate the baroreceptors in the neck with a pressurized neck chamber. Nine physiological responses to this type of stimulation are quantified and used as indicators of the baroreflex response function. Thirty male humans between the ages of 27 and 46 underwent the carotid-cardiac baroreflex test. The data for the nine response parameters were analyzed by principle component factor analysis. The results indicated that 92.5% of the total variance across all nine parameters could be explained in four dimensions. The first two dimensions reflected location points for R-R interval and carotid distending pressure, respectively. The third factor was composed of measures reflecting the gain (responsiveness) of the reflex. The fourth dimension was the ratio of baseline R-R interval to the maximal R-R interval response during simulated hypertension. The data suggest that the analysis of all nine baroreflex parameters is likely to be redundant and researchers should account for these redundancies either in their analyses or conclusions.

Adult↗
PubMed1991

Errors in measurement of +Gz acceleration tolerance.

Most acceleration studies estimate a subject's G-level tolerance by taking only one determination (test) for a given condition. The purpose of this study was to examine the error structure and reliability of an individual's acceleration tolerance and to provide design considerations for future experimentation. A hierarchical (nested) design was used to estimate the sources of variation in measuring G-level tolerance. Six males rode relaxed in the USAF School of Aerospace Medicine human-use centrifuge and were exposed to a 0.1 G/s onset rate profile until greyout. Each subject was tested on three randomly selected days with three repeated determinations within a day. This design allowed for an estimate of both day-to-day and measurement error within a testing session. A single +Gz tolerance determination was found to be moderately unreliable (reliability coefficient = 0.74). Under the best of circumstances a subject's G-level tolerance cannot be estimated with any more accuracy than about +/- 0.3 G with 95% confidence. This degree of accuracy can only be obtained with multiple measurements.

Acceleration↗
PubMed1990

Baroreflex responses to acute changes in blood volume in humans.

To test the hypothesis that acute changes in plasma volume affect the stimulus-response relations of high- and low-pressure baroreflexes, eight men (27-44 yr old) underwent measurements for carotid-cardiac and cardiopulmonary baroreflex responses under the following three volemic conditions: hypovolemic, normovolemic, and hypervolemic. The stimulus-response relation of the carotid-cardiac response curve was generated using a neck cuff device, which delivered pressure changes between +40 and -65 mmHg in continuous steps of 15 mmHg. The stimulus-response relationships of the cardiopulmonary baroreflex were studied by measurements of forearm vascular resistance (FVR) and peripheral venous pressure (PVP) during low levels of lower body negative pressure (0 to -20 mmHg). Altered vascular volume had no effect on response relations of the carotid-cardiac baroreflex but did alter the gain of the cardiopulmonary baroreflex (-7.93 +/- 1.73, -4.36 +/- 1.38, and -2.56 +/- 1.59 peripheral resistance units/mmHg for hypovolemic, normovolemic, and hypervolemic, respectively) independent of shifts in baseline FVR and PVP. These results indicate greater demand for vasoconstriction for equal reductions in venous pressure during progressive hypovolemia; this condition may compromise the capacity to provide adequate peripheral resistance during severe orthostatic stress. Fluid loading before reentry after spaceflight may act to restore vasoconstrictive capacity of the cardiopulmonary baroreflex but may not be an effective countermeasure against potential post-flight impairment of the carotid-cardiac baroreflex.

Adult↗
PubMed1987

Logistic risk model for the unique effects of inherent aerobic capacity on +Gz tolerance before and after simulated weightlessness.

Small sample size (n less than 10) and inappropriate analysis of multivariate data have hindered previous attempts to describe which physiologic and demographic variables are most important in determining how long humans can tolerate acceleration. Data from previous centrifuge studies conducted at NASA/Ames Research Center, utilizing a 7-14 d bed rest protocol to simulate weightlessness, were included in the current investigation. After review, data on 25 women and 22 men were available for analysis. Study variables included gender, age, weight, height, percent body fat, resting heart rate, mean arterial pressure, VO2max, and plasma volume. Since the dependent variable was time to greyout (failure), two contemporary biostatistical modeling procedures (proportional hazard and logistic discriminant function) were used to estimate risk, given a particular subject's profile. After adjusting for pre-bed-rest tolerance time, none of the profile variables remained in the risk equation for post-bed-rest tolerance greyout. However, prior to bed rest, risk of greyout could be predicted with 91% accuracy. All of the profile variables except weight, MAP, and those related to inherent aerobic capacity (VO2max, percent body fat, resting heart rate) entered the risk equation for pre-bed-rest greyout. A cross-validation using 24 new subjects indicated a very stable model for risk prediction, accurate within 5% of the original equation. The result for the inherent fitness variables is significant in that a consensus as to whether an increased aerobic capacity is beneficial or detrimental has not been satisfactorily established. We conclude that tolerance to +Gz acceleration before and after simulated weightlessness is independent of inherent aerobic fitness.

Acceleration↗
PubMed1984

The effect of square wave exposure profiles upon the performance of passive organic vapor monitoring badges.

Three commercially available passive organic vapor monitoring badges were exposed to ethyl benzene vapor in a dynamic test atmosphere characterized by well defined square wave concentration profiles having periods of 2, 6, and 10 minutes. These concentrations fluctuations between zero and 150 ppm caused no significant bias in the TWA concentration indicated by the badges. A slight, but statistically significant, interaction between badge type and exposure profile is attributable to random analytical error in the data provided by one badge type.

Air Pollutants, Occupational↗
PubMed1983

Interwedge variation in the membrane filter method for airborne asbestos fibers.

The poor precision of the current asbestos method (NIOSH P & CAM 139) has been linked to the variation in fiber deposition on the filter, but the cause of this variation had been unknown. A fluidized bed generator was constructed to produce a test atmosphere containing asbestos fibers, and air samples were collected. Fibers were counted at filter positions corresponding to the trench pattern in the cassette directly below the filter. The average fiber counts on wedges taken from above radial trenches were significantly higher (Pr greater than F = 0.0001) than counts on wedges taken from other portions of the filters. The chief cause of variation in asbestos fiber deposition on the filter is the trench pattern in the cassette supporting the filter.

Air↗
PubMed1982

Subchronic toxicity of chlorine dioxide and related compounds in drinking water in the nonhuman primate.

Subchronic toxicities of ClO2, NaClO2, NaClO3 and NH2Cl were studied in the African Green monkeys (Cercopithecus aethiops). The chemicals were administered in drinking water during 30-60 days subchronic rising dose protocols. The only unexpected and significant toxic effect was elicited by ClO2; this chemical inhibited thyroid metabolism in the animals at a dose of ca. 9.0 mg/kg/day. A statistically significant decrease of serum thyroxine occurred after the fourth week of exposure to 100 mg/l.concentration. The extent of thyroid suppression was dose dependent in each individual monkey, and was reversible after cessation of exposure. NaClO2 and NaClO3 failed to elicit similar effects in doses up to ca. 60 mg/kg/day. Also, NaClO4 or NH2Cl did not cause T-4 suppression in doses of 10 mg/kg/day. The selective thyroid effect of ClO2 was unexplained and it appeared to be paradoxical since ClO2 was rapidly reduced by the oral and gastric secretions to nonoxidizing species (presumably Cl-). No evidence of thyroid effects were detected in the serum of human volunteers who ingested approximately 1 mg/l. of ClO2 in drinking water as a result of routine use in the community water treatment process. Sodium chlorite induced dose-dependent oxidative stress on hematopoesis, causing decreased hemoglobin and red cell count and increased methemoglobin content. At the same time, serum transaminase (SGPT) levels showed significant subclinical elevation. The hematologic effects of NaClO2 rebounded during exposure indicating compensatory hemopoietic activity taking effect during oxidative stress. Sodium chlorate and chloramine did not induce detectable hematologic changes in the animals.

Animals↗
PubMed1982

Effects of age and temporal placement of a modeled skill on children's performance on a balance task.

The present investigation explored the effect of age and temporal placement of a modelled skill on performance on a balance task. 60 boys, aged 7 and 9 yr., were randomly assigned to one of three conditions. The model was presented before any trials were attempted, midway, or not at all during 12 trials. A 2 (age) X 3 (model condition) X 12 (trials) repeated-measures design was utilized. Analysis of variance indicated significant effects of age, temporal appearance of the model, and an interaction of model by age for time on-balance. Model affected younger subjects but not older ones. Treatment did not affect off-balance errors. Findings for age and temporal placement are not consistent with some previous research.

Child↗
PubMed1982

EMG changes during acquisition of a motor skill.

Currently there are only a few studies which attempt to explain the neuromuscular changes which accompany skilled motor learning. Conflicting results have created the need for further research concerned with the electromyographical correlates of motor skill acquisition. A shuffleboard task was devised that isolated the triceps as the major agonist and the biceps as the major antagonist. Ten undergraduate males enrolled at Iowa State University participated in the study. Multivariate analysis and single degree of freedom components based on polynomial regression were used to test linear, quadratic and cubic trends for eight EMG characteristics plus movement time obtained from a systematic sampling of practice trials. Results indicated that trial period means for movement time, latency period and time to peak activity for the triceps exhibited significant linear trends (negative) during practice. No changes were seen in the integrated electromyograms for either the triceps or biceps. It was concluded that force parameters as measured by integrated EMG activity could be determined very early in the learning process and the distribution of these forces through various aspects of timing characterized learning.

Adolescent↗
PubMed1981

Down's syndrome: percentage reporting on birth certificates and single year maternal age risk rates for Ohio 1970-79: comparison with upstate New York data.

Estimates of single year maternal age risk rates for Down's Syndrome births to White residents of Ohio over the period 1970-79 are reported. The rates were estimated from birth certificate data, which first necessitated estimating the percentage of reporting of Down's Syndrome on Ohio birth certificates. Using data from cytogenetic laboratories within the state, percentages of Down's Syndrome cases reported were found to be 36.5 (319/875) for White and 33.9 (342/1,010) overall. Final single year maternal age risk rates reported here are corrected for underreporting using these figures. They should be useful for genetic counselors and obstetricians. Comparisons of observed, uncorrected single year maternal incidence rates were made within Ohio 1970-74 vs 1975-79, and between Ohio and upstate New York for various time periods using both a hierarchial log-linear model for multiway cross-tabulations and a weighted least squares solution. All comparisons showed excellent agreement, indicating no evidence for temporal or geographic differences and implying that environmental factors are unimportant in determining single year maternal age incidence rates in Down's Syndrome for the time periods and populations studied.

Adolescent↗