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Biomedical subjects

I B Mekjavic

Publications and source records attributed to I B Mekjavic.

17 recordsLinked to original sources
PubMed1992

Tear film bubble formation after decompression.

Decompression disorders can occur after a reduction in environmental pressure due to dissolved gases forming bubbles in affected tissues. The objective of this study was to evaluate the use of tear film bubble formation as an indicator of decompression. Eleven subjects were exposed to a simulated dive to 30.48 m (100 ft) (in sea water) for 15 min. There was a significant (p < 0.001) increase in tear film bubble formation post-dive. This noninvasive technique may be of value in both research and clinical environments for monitoring decompression.

Decompression Sickness↗
PubMed1992

Characteristics of the carotid baroreflex in man during normal and flow-restricted exercise.

Eight subjects were studied in the supine position at rest, during normal dynamic leg exercise (control exercise) and with blood-flow restriction in the working legs (flow-restricted exercise). Graded muscle blood-flow restriction was accomplished by applying a supra-atmospheric pressure of 50 mmHg to the working legs. During incremental-load exercise, flow restriction reduced exercise performance and peak heart rate by 36% and 13%, respectively. The function of the cardiac branch of the carotid baroreflex was studied over its full operational range, at rest and during constant-load control and flow-restricted exercise, by measuring R-R intervals during application of pulse-synchronous graded pressures (40 to -65 mmHg) in a neck-chamber device. Heart rate and arterial pressure were higher during flow-restricted than control exercise, indicating that the flow restriction activated the muscle chemoreflex. Raising the carotid transmural pressure (systolic arterial pressure minus neck-chamber pressure) was accompanied by increasing R-R intervals in all conditions. The set point (point of baseline carotid transmural pressure and R-R interval) coincided with the midportion of the pressure-response curve at rest and with the threshold point of the curve during exercise. The maximal rate of change in relative R-R intervals and the corresponding carotid transmural pressure range were higher during control exercise than at rest and highest during flow-restricted exercise, indicating that exercise and especially flow-restricted exercise increased carotid baroflex sensitivity, and shifted the carotid baroreflex optimal buffering range to higher pressures. The results suggest that the carotid baroflex attenuates exercise heart rate increases mediated by the muscle chemoreflex and/or by central command.

Adult↗
PubMed1992

Human temperature regulation during narcosis induced by inhalation of 30% nitrous oxide.

The study investigated the effect of inhalation of 30% nitrous oxide (N2O) on temperature regulation in humans. Seven male subjects were immersed to the neck in 28 degrees C water on two separate occasions. They exercised at a rate equivalent to 50% of their maximum work rate on an underwater cycle ergometer for 20 min and remained immersed for an additional 100 min after the exercise. In one trial (AIR) the subjects inspired compressed air, and in the other trial (N2O) they inspired a gas mixture containing N2O (20.93% O2-30% N2O-49.07% N2). Sweating, measured at the forehead, and shivering thermogenesis, as reflected by O2 uptake, were monitored throughout the 100-min recovery period. The threshold core temperatures at which sweating was extinguished and shivering was initiated were established relative to resting preexercise levels. Neither the magnitude of the sweating response nor the core threshold at which it was extinguished was significantly affected by the inhalation of N2O. In contrast, shivering thermogenesis was both significantly reduced during the N2O condition and initiated at significantly lower core temperatures [change in esophageal temperature (delta T(es)) = -0.98 +/- 0.33 degrees C and change in rectal temperature (delta T(re)) = -1.26 degrees C] during the N2O than during the AIR condition (delta T(es) = -0.36 +/- 0.31 degrees C and delta T(re) = -0.44 +/- 0.22 degrees C).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Exercise breathing pattern during chronic altitude exposure.

Breathing pattern in response to maximal exercise was examined in four subjects during a 7-day acclimatisation to a simulated altitude of 4247 m (barometric pressure, PB = 59.5 kPa). Graded exercise tests to exhaustion were performed during normoxia (day 0), and on days 2 and 7 of hypoxia, respectively. Ventilation was significantly augmented in the hypoxic environment, as were both the mean inspiratory flow (VT/TI) and inspiratory duty cycle (TI/TTOT) components of it. VI/TI was increased due to a significant increase in tidal volume (VT) and a corresponding decrease in inspiratory time duration (TI). Throughout a range of exercise ventilation, TI/TTOT was increased due to an apparently greater decrease in expiratory time duration (TE) with respect to TI. In all cases, the relation between VT and TI displayed a typical range 2 behaviour, with evidence of a range 3 occurring at very high ventilatory rates. There was essentially no difference observed in the VT-TI relation during exercise between the normoxic and hypoxic conditions. No significant changes were observed in the breathing pattern in response to exercise within the exposure period (from day 2 to day 7), although there was a discernible tendency to a higher stage 3 plateau by day 7 of altitude exposure.

Acclimatization↗
PubMed1989

The increased oxygen uptake upon immersion. The raised external pressure could be a causative factor.

The principal cause of the immediate transient elevation in ventilation (VE, L.min-1) and oxygen uptake (VO2, L.min-1), when a human subject is immersed in cold water is considered to be the stimulation of cutaneous cold receptors. The present study demonstrates that the initial VE and VO2 responses are comprised of a thermogenic and a hydrostatic component. The peak values in VE reached (mean +/- SD) 66.8 +/- 22.3, 53.9 +/- 38.1, 32.2 +/- 15.4, 22.5 +/- 3.6, 19.5 +/- 4.6 L.min-1 during the first minute of immersion in 10 degrees, 15 degrees, 20 degrees, 28 degrees and 40 degrees C water, respectively. Similarly, peaks (mean +/- SD) in VO2 of 1.22 +/- 0.25, 1.01 +/- 0.32, 0.98 +/- 0.39, 0.81 +/- 0.09, and 0.78 +/- 0.26 L.O2.min-1, were reached when subjects were immersed in 10 degrees, 15 degrees, 20 degrees, 28 degrees, and 40 degrees C water. It is concluded that the observed increases in VO2 during the first minute of immersion are partly due to the increased hydrostatic pressure causing a shift of venous blood towards the thoracic region, and a transient increase in the uptake of oxygen into the blood.

Body Temperature↗
PubMed1988

Dynamic moisture permeation through clothing.

Dynamic moisture permeation through clothing often occurs during thermal transience, causing an imbalance between evaporative heat loss from the skin (Esk) and that from the clothing surface (Ecl). A device was designed to observe Esk and Ecl simultaneously. It consists of two relative humidity sensors coupled with thermistors so that densities of water vapor at two points within the boundary layer can be calculated. The rate of local evaporation is then estimated from Fick's law of diffusion. Local evaporation rates from the skin and clothing surface at the chest, arm, and thigh were measured during exposure to controlled ambient temperatures varying from 20 degrees-40 degrees C. The subjects wore four different types of helicopter pilot suits: Nomex/Neoprene, Goretex, cotton ventile, and Nomex/Insulite. For the Goretex and cotton ventile suits, consisting of relatively permeable and hygroscopic fabrics, a sudden increase in Esk, exponential decay of Esk, and a gradual increase in Ecl were observed. These appear to be associated with, respectively, the onset of sweat secretion, moisture build-up within the clothing, and water gain in the fabric. Thus, the device may be useful for observing dynamic moisture permeation through clothing.

Adult↗
PubMed1987

The pattern of breathing during hypoxic exercise.

Breathing pattern was studied in six subjects in normoxia (FIO2 = 0.21) and hypoxia (FIO2 = 0.12) at rest and during incremental work-rate exercise. Ventilation (V) as well as mean inspiratory flow (VT/TI) increased with exercise intensity and were augmented in the hypoxic environment, whereas the ratio between inspiratory (TI) and total (Ttot) breath durations increased with exercise intensity but was unaffected by hypoxia. The relationship of tidal volume (VT) and inspiratory time duration (TI) showed linear, coinciding ranges for the normoxic and hypoxic conditions up to VT/TI values of about 2.5 1.s-1. At higher VT/TI values TI continued to decrease, whereas VT tended to level off, an effect which was more evident in the hypoxic condition. The results suggest that the hypoxic augmentation of exercise hyperpnea is primarily brought about by an enhancement of central inspiratory drive, the timing component being largely unaffected by the hypoxic environment, and that at low to moderate levels of exercise hyperpnea inspiratory off-switch mechanisms are essentially unaffected by moderate hypoxia.

Adult↗
PubMed1987

Determining the rate of body heat storage by incorporating body composition.

The rate of body heat storage (S) is often used as an index of thermal stress. The traditional approach for determining S assumes that the specific heat (Cp) of all body tissues may be averaged to 0.83 kcal X kg-1 X degrees C-1, irrespective of the actual proportion of different tissues in the body. The present theoretical analysis primarily partitions the body into N compartments. However, this analytical approach involves the difficulty of making an exact evaluation of the mass and temperature of multiple compartments. Therefore, by adopting a two-compartment (core and shell) model, the specific heat capacities, mass fractions, and changes in the temperatures of the two compartments are combined, and S is defined as a function of adiposity (= mass fraction of peripheral tissues). Evaluation of a newly developed equation showed close agreement with observations during a series of cold-water immersion trials, and indicated that the new approach seems to adequately predict S as related to body composition.

Adult↗
PubMed1987

Clothing surface area as related to body volume and clothing microenvironment volume.

The evaluation of clothing surface area becomes important in analyses of heat exchange between the clothed body and its environment. The evidence accumulated in the studies of radiation area has led to the conclusion that the ratio of clothing surface area to body surface area (fcl) may be defined as a function of clothing insulation. However, the effects of clothing fit have been disregarded. Radiation area factors (Ar/AD) were derived from pictures of five subjects wearing four different helicopter pilot suits. While sitting on a seat suspended in the center of a box-shaped frame, photographs were taken of the subjects with a fish-eye lens at 90 equidistant points on the six sides of the frame. The derived Ar/AD of 0.7 +/- 0.18 for the unclothed subjects closely agreed with that reported by Fanger, while it scattered in a relatively wide range for a given suit. The present study proposes a new approach for predicting fcl as related to body volume and the clothing microenvironment volume. The predictions show a good correlation with the values derived from the photographs. It is, therefore, suggested that improvement in the prediction of fcl has been achieved by incorporating clothing fit.

Anthropometry↗
PubMed1986

Evaluation of predictive formulae for determining metabolic rate during cold water immersion.

Five models predicting shivering thermogenesis on the basis of steady state skin and core temperature were evaluated: Hayward et al., Stolwijk and Hardy,; Nadel et al.,; Timbal et al., and Brown and Brengelmann, using the empirical data derived from a cold water immersion study by Morrison et al. A residual analysis indicated that all models generated substantial errors of prediction. The best overall predictors were expressions suggested by Hayward et al., while the predictive equation of Nadel et al. ranked second. Derivation of personal coefficients significantly improved the prediction of all models and a subsequent modification of the standard models, adding temperature derivative terms, further reduced the magnitude of the error. An analysis of the residuals indicated that peripheral and core temperatures should be weighted according to the characteristics of thermosensitive neural structures in these regions.

Body Temperature Regulation↗
PubMed1985

Inhibition of shivering in man by thermal stimulation of the facial area.

The contribution of the facial thermoreceptors to thermoregulatory shivering was studied. Seven subjects were exposed to -3 degrees C ambient air for 1 h. Radiant heat was applied to the facial area during 30 s periods at 10 min intervals and the effects on the integrated electromyographic activity (IEMG) in the brachial biceps, trapezius and femoral rectus muscles, and on the heart rate (HR) were studied. During cold-air exposure mean skin temperature as measured at four sites decreased by 15.5 +/- 0.6 degrees C (mean +/- SE) while rectal temperature remained virtually unchanged. During the radiant heat exposures IEMG activity decreased by 18.9 +/- 2.3%; HR decreased by 11 +/- 1 beats min-1. The changes in IEMG occurred within or slightly below the range of maximum cold receptor sensitivity (20-30 degrees C) and coincided with the maximum rate of change in chin temperature. The observed inhibition of shivering IEMG during locally applied facial heat stimulation suggest that the trigeminal region contributes an important input to the overall thermoregulatory responses.

Adult↗