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Biomedical subjects

M H Harrison

Publications and source records attributed to M H Harrison.

At least 55 records · Page 3Linked to original sources
PubMed1978

Intravascular volume and tonicity as factors in the regulation of body temperature.

The effect of alterations in intravascular volume and tonicity on thermoregulatory and cardiovascular responses to heat and exercise have been compared in four subjects. Core temperatures were found to be significantly higher during dehydration, and when dehydration was prevented by administration of 1% saline, than when dehydration was prevented by water administration. These higher temperatures were associated with elevated levels of plasma [Na] and osmolarity, but no consistent relationship between temperature and changes in intravascular volume could be demonstrated. Relationships observed between core temperature and plasma tonicity were consistent with the hypothesis that the adverse effects of dehydration on thermoregulation can be attributed to an inhibition of sweating mediated by an increase in either plasma osmotic pressure or plasma [Na]. In separate experiments the heart rate response to exercise was shown to be reduced by saline, compared with water and dehydration, and this may be explained by the smaller reduction in intravascular volume which occurs during exercise following administration of hypertonic saline. It is concluded that the effects of reduced intravascular volume, and increased intravascular tonicity on physical work capacity may be distinguished by the adverse effect on the cardiovascular system of the former, and on the thermoregulatory system of the latter.

Adult↗
PubMed1978

Relationships between ambient, cockpit, and pilot temperatures during routine air operations.

Thermal data obtained from aircraft flying routine sorties from RAF Germany in summer have been reduced to a form suitable for statistical analysis by describing thermal stress in terms of a modified wet bulb globe temperature (WBGT) index, and thermal strain in terms of mean body temperature (Tb). Ambient temperature could be related to cockpit temperature, and cockpit temperature to pilot Tb, by linear equations of positive slope. Relationships between Tb and sortie time could be represented by exponential equations. The relationships between cockpit temperature and sortie time could also, in fixed-wing aircraft, be described by exponential equations, although in helicopters the relationships were better described by linear equations of negative slope. Models capable of predicting cockpit thermal stress and aircrew thermal strain given ambient temperature and sortie time have been constructed. These provide a description of the temperature relationships within aircraft during flight.

Aerospace Medicine↗
PubMed1978

Operational characteristics of liquid-conditioned suits.

The data from several studies of liquid-conditioned suits carried out over the last 12 years at the RAF Institute of Aviation Medicine have been collated, collectively reanalysed, and used to describe the characteristics of personal liquid-conditioning systems in terms of interactions between the conditioning and the conditioned system. Heat exchange across a liquid-conditioned suit (LCS) is shown to be proportional to the inlet temperature (Tin) of the conditioning liquid. Choice of Tin is determined, for resting subjects, primarily by the environmental temperature, by the length of the heat exchange tubing, and by the insulation value of the clothing worn over the LCS. Mean skin temperature (Tsk), but not core temperature (Tc) is directly related to Tin. However, certainly in exercising subjects, steady-state values for Tc can be increased by selecting very low values for Tin, which reduce Tsk below 29 degrees C. It is suggested that mean Tsk should not fall below 30 degrees C when the LCS is used for personal cooling. When the LCS is used for personal heating, sweating should be minimised, or prevented, by not allowing Tsk to rise to levels sufficient to cause an increase in Tc.

Hot Temperature↗
PubMed1978

Core temperature measurement in man.

Transient changes in core temperature were induced in 12 subjects--passively by immersion in a hot bath, and actively by light intermittent exercise. Measurements of core temperature were made at four sites--the auditory canal, mouth, rectum, and oesophagus--and, by using the measurements from the first three of these sites, a mathematical model was derived which permits the calculation of a predicted value for oesophageal temperature (Toe). This model--which takes the general form Toe = ATx + BdTx/dt + C where Tx = amplitude of the temperature change at a specific site, dTx/dt = rate of change of temperature at that site, and A, B, and C = site constants (different for passive and active heating)--provides an accurate prediction of Toe (to within 0.1 degrees C) from both auditory canal and mouth temperature. For prediction of Toe from rectal temperature, however, two models appear to be necessary--one to predict Toe when core temperature is rising, and another to predict Toe during the return to equilibrium when core temperature is declining.

Adult↗
PubMed1977

Heat stress in an aircraft cockpit during ground standby.

Measurements have been made of cockpit temperatures in a Buccaneer aircraft exposed to high air temperatures and radiation loads. With the canopy open 8 cm, and with the wind direction unfavourable for convective cooling, air temperatures inside the cockpit exceeded those outside by approximately 20 degrees C. This reduced to 10 degrees C with a favourable wind direction. An assessment of the likely heating effect of cockpit avionic equipment indicated that the addition of 1 kW and 2 kW of heat would raise cockpit temperatures by 20 degrees C and 30 degrees C respectively. Prediction of the combined effect of solar and avionic heat suggests that, in hot weather conditions, cockpit temperatures will be considerably in excess of the upper limit for effective physiological temperature regulation. Therefore, if aircrews are to be required to remain on ground standby within their aircraft under such conditions, maximum use must be made of convective cooling of the cockpit by the prevailing wind, and of sun shades to eliminate the greenhouse effect completely.

Aerospace Medicine↗
PubMed1976

Effect of heat acclimatization on intravascular responses to acute heat stress in man.

The effects of a 185-min exposure to 48 degrees C db/33 degrees C wb, on intravascular volume and osmolarity and on intravascular electrolyte, aldosterone, and cortisol concentrations have been studied in five male subjects before and after acclimatization to heat. Changes in the hematocrit and plasma protein concentration indicated that a hemodilution occurred during the first 35 min of the heat exposures, and that this was followed by a hemoconcentration. Although these changes in intravascular volume were not affected by acclimatization, the plasma volume after heat acclimatization was 6.7% greater than before. This increase in plasma volume was associated with an elevation in the ratio [Na]/[K]. However, since plasma osmolarity decreased the intravascular expansion could not be explained in terms of elevated electrolyte levels. Plasma aldosterone and cortisol levels were not affected by heat acclimatization, although both were elevated following exercise in the heat. It is concluded that the adrenal cortex is not an important factor in maintaining a state of heat acclimatization once a salt balance has been achieved.

Acclimatization↗
PubMed1976

Skeletal immaturity in Perthes' disease.

Skeletal age was estimated by examination of radiographs of the carpus in 182 children suffering from Perthes' disease after the reliability of the Greulich and Pyle Atlas had been checked for a control group of British children. A striking tendency to delayed skeletal maturation was shown in the children with Perthes' disease. This trait was also found in ninety-three unaffected siblings of the patients. The velocity of skeletal ageing as the disease progressed was estimated. In some patients the carpal skeleton failed to mature at all for periods of up to three years and the term "skeletal standstill" is applied to this phenomenon. The significance of these findings is discussed and it is suggested that the maturation defect may have aetiological significance.

Adolescent↗
PubMed1976

Effects of acute heat stress on tissue carbohydrate in fasted rats.

Exposure of starved female rats to 45 degrees C db, 26 degrees C wb for 0.25-2 h tended to lower blood glucose levels, and increase blood lactate levels, compared with controls at 24 degrees C db, 14 degrees C wb. There was a sustained liver glycogenolysis, but no detectable change in muscle glycogen level. Despite the severity of the stress, the effect of a high environmental temperature on carbohydrate metabolism cannot be satisfactorily interpreted in terms of increased adrenal medullary activity. It is suggested that increased respiratory muscle activity may explain the changes observed.

Adrenal Medulla↗
PubMed1976

Identification of an apprehension effect on physiological indices of thermal strain.

A laboratory assessment of the thermal strain produced by wearing a protective clothing assembly, in addition to a normal flying clothing assembly, in a hot environment is described. In addition to the demonstration of an increase in thermal strain when wearing the protective clothing, two points of importance emerged. First, condition replication revealed a significant apprehension effect which confounded the physiological variables used to evaluate the thermal strain, leading to an over-estimation of the severity of that strain. Second, there was a large between-subject variation amongst the eight subjects used, which made the arithmetic mean a potentially misleading statistic for evaluating the increased physiological strain caused by the additional clothing assembly during the heat exposure. The practical implications of these observations are discussed....

Adult↗
PubMed1976

Measurement of change in plasma volume during heat exposure and exercise.

The application of radio-iodinated human serum albumin (RISA) to the measurement of a continuously changing plasma volume, such as that occurring during heat exposure and exercise, has been considered in terms of the exchange dynamics of albumin between the intravascular and extravascular compartments. In six male subjects resting supine for 2 h in a hot environment, or exercising for 50 min in a thermoneutral or hot environment, there was no statistically significant alteration in the rate of protein efflux from the intravascular space. However, following exercise, protein was added to the circulation at a greater rate than it was lost through the capillary walls. A technique for calculating plasma volume from a single measurement of plasma RISA activity is described. This may be used in conjunction with measurements of changes in haemoglobin concentration of determining plasma volume in situations where alterations in protein exchange dynamics do occur.

Blood Circulation↗
PubMed1975

Nitrogen exchange across the lungs in resting man.

The relationship between the partial pressure of N2 in the inspired air (PIN2), and the nitrogen transfer across the lungs (VN2), has been examined in 8 resting subjects, and a study made of the effect of diet upon this relationship. Alterations in PIN2, over the range 60 mm Hg below, to 20 mm Hg above ambient PIN2, caused changes in VN2 which could be described by a linear equation. These changes agreed both in magnitude and direction with those predicted by a four-compartment electrical analogue of the body N2 stores. They were not affected by the protein content of a meal taken 3/4 hr before the start of measurements of VN2. The study has shown that any imbalance in N2 transfer across the lungs is very small and that the open-circuit method of measuring O2 consumption is valid.

Adult↗
PubMed1975

Metabolic effects of a short exposure to a hot environment in man.

Some metabolic effects of a 2h exposure to an environmental temperature of 48 degrees C DB, 35 degrees C WB, have been studied in six male subjects. The results indicate that adrenal cortical activity does not necessarily increase as a result of exposure to moderate heat stress. However, a consistent increase in lipolysis may indicate increased sympatho-adrenal activity. There was a significant correlation between blood glucose and cortisol levels, suggesting that the increased sympathetic activity may be without effect on carbohydrate metabolism.

Adult↗