Biomedical subjects

G W Mack

Publications and source records attributed to G W Mack.

58 records · Page 4Linked to original sources

PubMed1986

Influence of beta-adrenergic blockade on the control of sweating in humans.

To evaluate the role of beta-adrenergic receptors in the control of human sweating, we studied six subjects during 40 min of cycle-ergometer exercise (60% maximal O2 consumption) at 22 degrees C 2 h after oral administration of placebo or nonselective beta-blockade (BB, 80 mg propranolol). Internal temperature (esophageal temperature, Tes), mean skin temperature (Tsk), local chest temperature (Tch), and local chest sweat rate (msw) were continuously recorded. The control of sweating was best described by the slope of the linear relationship between msw and Tes and the threshold Tes for the onset of sweating. The slope of the msw-Tes relationship decreased 27% (P less than 0.01), from 1.80 to 1.30 mg X cm-2 X min-1 X degree C-1 during BB. The Tes threshold for sweating (36.8 degrees C) was not altered as the result of BB. These data suggest that BB modified the control of sweating via some peripheral interaction. Since Tsk was significantly (P less than 0.05) reduced during BB exercise, from a control value of 32.8 to 32.2 degrees C, we evaluated the influence of the reduction in local skin temperature (Tsk) in the altered control of sweating. Reductions in Tch accounted for only 45% of the decrease in the slope of the msw-Tes relationship during BB. Since evaporative heat loss requirement during exercise with BB, as estimated from the energy balance equation, was also reduced 18%, compared with control exercise, we concluded that during BB the reduction in sweating at any Tes is the consequence of both a decrease in local Tsk and a direct effect on sweat gland.

Adult↗

PubMed1986

Hypothermia impairs but hyperthermia does not promote inert gas elimination in the rat.

The volume of nitrogen eliminated (VN2) and washout rate constant (k) during a 2-h period of O2 breathing in the rat was altered in response to changes in body temperature. VN2 and cardiac output (Q) were determined simultaneously on 3 groups of unanesthetized but restrained male rats at 3 different body temperatures: normothermic (36 degrees C), hyperthermic (39 degrees C), and hypothermic (33 degrees C). Under normothermic conditions, the rats eliminated 12.8 ml N2/kg in 2 h of O2 breathing while Q, determined by a thermodilution technique, was 430 ml X min-1 X kg-1. A significant reduction in VN2 (33%) and k (44%) was seen during hypothermic exposures in which rectal temperature dropped 3 degrees C and Q decreased to 395 ml X min-1 X kg-1. During hyperthermic exposures, VN2, 11.7 ml N2/kg, and k decreased while Q increased to 468 ml X min-1 X kg-1, however, these changes were not significant when compared to the normothermic conditions. The overall relationship between VN2 and Q is described by: VN2 = 4.44 + 0.019 Q (r = 0.536, P less than 0.01). Under conditions of altered body temperature, Q accounts for only 29% of the VN2 response. Characteristic changes in regional blood flow, such as a decreased adipose tissue blood flow, may account for the observed reduction in VN2 and k during hypo- and hyperthermic conditions. The results of this study indicate that hypothermia prolongs the duration required for desaturation of nitrogen.

Animals↗

PubMed1984

Influence of blood flow on cutaneous permeability to inert gas.

A thermally regulated Plexiglas chamber was designed for investigation of transcutaneous diffusion of N2 and helium (He) in the human hand. Influence of cutaneous blood flow in this process was studied simultaneously with gas diffusion measurements. Changes in cutaneous blood flow (Q, in ml X min-1 X 100 ml tissue-1) were effected by altering ambient temperature (T) from 20 to 40 degrees C (Q = 0.08 X 100.07T). We found that the rate of inert gas diffusion through human skin, expressed as conductance (G, in ml STPD X h-1 X m-2 X atm-1), increases exponentially as a function of blood flow, and was indistinguishable between He and N2 (G = 21.19 X 100.0124Q). The permeability, diffusion coefficient per unit diffusion distance (D/h, in cm/h), also rose exponentially as a function of blood flow. But permeability for He (D/h = 0.1748 X 100.0203Q) was greater than that for N2 (D/h = 0.1678 X 100.0114Q). As cutaneous blood flow rises, because of increased temperature, the apparent diffusion distance falls linearly for both N2 and He. The change is more prominent for He than for N2 diffusion. Estimated replacement time for the body stores of N2 by transcutaneous diffusion alone was shortened from 26.8 h at 31 degrees C to 15.1 h at 37 degrees C. It is suggested from this study that beneficial results may be derived during decompression procedure 1) by maintaining an appropriate transcutaneous pressure gradient of inert gases, and 2) by elevating ambient temperature.

Capillary Permeability↗

PubMed1984

Isoproterenol infusion promotes nitrogen washout in rats under normobaric conditions.

Cardiac output and the rate of N2 elimination were measured simultaneously in unanesthetized rats during isobaric desaturation with 100% O2. Whole-body N2-washout curves for the rat are characterized by three compartment half times derived by exponential stripping, representing the slow, intermediate, and fast components of the system. During saline infusion (control) the respective half times for these compartments were 120, 15, and 2 min. Isoproterenol infusion increased cardiac output by 40% and the volume of N2 eliminated by 10% over a 2-h washout. More importantly, the half time of the slowest compartment decreased from 120 to 70 min. The intermediate compartment half time shortened from 15 to 13 min, while the fast compartment was unaffected. The decrease in slow compartment half time will contribute significantly to the shortening of the duration of stay during decompression. A plot of slow compartment rate constants vs. measured cardiac output (range 280-690 ml X min-1 X kg-1) demonstrated a linear relationship between perfusion and the tissue-blood gas exchange rate. The results indicate that enhanced cardiac output and generalized vasodilation shorten the time required to desaturate the slower body tissues. Various maneuvers or drugs that demonstrate similar cardiovascular responses should be beneficial during prolonged decompression procedures involved in saturation diving.

Animals↗