Biomedical subjects

I B Mekjavić

Publications and source records attributed to I B Mekjavić.

At least 19 recordsLinked to original sources

PubMed1998

Ocular bubble formation as a method of assessing decompression stress.

Tear film bubble formation and ultrasound reflectivity of the lens-vitreous humor compartments were monitored following simulated dives in a hyperbaric chamber. the sensitivity of these methods in determining decompression stress was compared with the results of precordial Doppler ultrasound. In addition, the utility of these diagnostic techniques in testing decompression dive profiles was evaluated. Eleven divers completed two series of chamber dives according to the decompression schedule of the Professional Association of Diving Instructors. The first dive series comprised dives to 70 feet of seawater (fsw) for 15, 29, and 40 min. The second series comprised maximum duration no-stop decompression dives to 40 fsw for 140 min, 70 fsw for 40 min, 90 fsw for 25 min, and 120 fsw for 13 min. Before and immediately after each dive, the following measurements were obtained from each subject: eye surface tear film bubble counts with a slit-lamp microscope, lens and vitreous humor reflectivity using A- and B-mode ophthalmic ultrasonic scan, and precordial Doppler ultrasonic detection of venous gas bubbles. Tear film bubble assessment and ocular scanning ultrasound were observed to be more sensitive in detecting decompression stress than the conventional Doppler ultrasonic surveillance of the precordial region. In contrast to precordial Doppler ultrasonic surveillance, which failed to detect any significant changes in circulating bubbles, tear film bubble formation displayed a dose-response relationship with increasing duration of the 70-fsw dives. Reflectivity changes of the lens-vitreous humor interface were not significant until the no-stop decompression limit was reached. In addition, for each of the no-stop decompression limit dives, increases in the average tear film bubble formation and lens-vitreous humor interface reflectivity were similar. Ocular bubble observations may provide a practical and objective ocular bubble index for analyzing existing decompression schedules and predicting individual susceptibility to decompression sickness.

Adolescent↗

PubMed1996

Effect of hypoglycemia on thermoregulatory responses.

The effects of hypoglycemia on sweating, skin blood perfusion, and shivering responses were investigated in 10 healthy male volunteers. They exercised on an underwater cycle ergometer while immersed to the neck in 28 degrees C water for 20 min at 50% of their maximal work rate. The exercise-induced elevation in esophageal temperature (T(es)) initiated the sweating response (Esw) and increased skin blood perfusion (SkBP) as measured at the forehead. In the 99-min postexercise immersion period, the values of T es relative to resting level (delta T(es)) at which Esw abated, SkBP reached preexercise values, and shivering commenced were defined as the delta T(es) thresholds for cessation of sweating, passive vasodilation, and onset of shivering, respectively. Two trials were conducted 1 wk apart. The subject was hypoglycemic in one trial and euglycemic in the other (plasma glucose was maintained at 2.8 and 5 mM, respectively) with the use of the hyperinsulinemic (insulin infusion rate = 60 mU.m-2.min-1) glucose-clamp technique. Oxygen uptake, Esw, T(es), mean skin temperature, heat flux from the skin, and SkBP were recorded at minute intervals. Although heat flux and SkBP attained significantly higher end-exercise levels during euglycemia, the responses were similar during the postexercise cooling period. Hypoglycemia did not affect the Esw response during the exercise and cooling periods. Whereas the exercise delta T(es) response was unaffected by hypoglycemia, the decrease in T(es) was greater (P < or = 0.005) during the hypoglycemic than during the euglycemic condition. Hypoglycemia did not alter the delta T(es) threshold for cessation of sweating and passive vasodilation but reduced (P < or = 0.001) the delta T(es) threshold for onset of shivering (from -0.09 +/- 0.07 degrees C in the euglycemic condition to -0.65 +/- 0.12 degrees C in the hypoglycemic condition). The present results indicate that hypoglycemia (2.8 mM) does not affect the delta T(es) threshold for cessation of thermoregulatory sweating or the threshold for passive vasodilation during recovery from exercise-induced moderate heat stress but that it decreases the core temperature threshold for shivering during cold exposure.

Adult↗

PubMed1995

Gender differences in physiological reactions to thermal stress.

Following an extensive anthropometric evaluation, thermoregulatory responses were studied in nine men and nine women who performed immersed exercise with post-exercise rest in 28 degrees C water. During the post-exercise period esophageal temperature (Tes), oxygen consumption, heat flux and skin blood perfusion were monitored at 10 s intervals, with average minute values used for calculations. The delta Tes (relative to resting Tes) at which sweating abated and shivering commenced were defined as the delta Tes thresholds for the cessation of sweating and onset of shivering, respectively. No significant gender differences were evident in the sweating and shivering threshold delta Tes values, or the magnitude of the null-zone. Using z-tests for parallelism the rates of core cooling across the null-zone were not found to differ significantly between genders, nor were the slopes of the perfusion: delta Tes responses across the null-zone or the post-threshold shivering responses (ml.kg-1.min-1.degrees C-1). The slope of the sweating response (measured from immersion until sweat cessation; g.m-2.min-1 degree C-1) was, however, significantly lower in the female than in the male samples (z = 3.93; P < 0.01). Despite the gender-related dimorphic distribution of adipose tissue, both men and women lost equal proportions of their total heat flux from central and peripheral measurement sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1995

Nitrogen narcosis attenuates shivering thermogenesis.

Thermoregulatory responses of eight healthy subjects (six men and two women) were compared when they were head-out immersed in 15 degrees C water at both 1 and 6 ATA. Both trials were conducted in a hyperbaric chamber. During the immersions, esophageal temperature (T(es)) and skin temperature at two sites (chest and calf) were recorded at minute intervals. Oxygen uptake was determined at 5-min intervals with the Douglas bag method. The order of the two trials was alternated. The rate of T(es) cooling was greater during the 6-ATA trial [2.1 +/- 0.5 degrees C/h (SE)] than during the 1-ATA trial (1.3 +/- 0.5 degrees C/h; P < 0.01). Despite the greater rate of core cooling, and presumably a greater thermal drive for shivering, the oxygen uptake response for a similar decrement in T(es) was lower during exposure to 6 than to 1 ATA (P < 0.05). Also, for similar displacement in T(es), the subjects perceived the immersions at 6 ATA to be less cold than those at 1 ATA (P < 0.05). It is concluded that the development of hypothermia in compressed-air divers may be due, in large part, to the attenuation of heat production and cold perception. Most likely, the observed effects on the autonomic responses and thermal perception are due to an inhibitory action of hyperbaric nitrogen on central neural structures involved in temperature regulation.

Adult↗

PubMed1995

Human temperature regulation during subanesthetic levels of nitrous oxide-induced narcosis.

The present study investigated whether nitrous oxide (N2O) attenuates shivering thermogenesis during cold water immersion in a dose-dependent manner. Seven male subjects were immersed to the neck for 60 min in 20 degrees C water on five separate occasions while breathing either air (AIR) or a normoxic mixture of 10, 15, 20, or 25% N2O balanced with N2. All N2O concentrations investigated caused a significant (P < 0.02) reduction in shivering thermogenesis compared with AIR. Despite similar heat flux from the skin, the relative changes in esophageal temperature from resting preimmersion levels were significantly greater (P < 0.05) during the N2O trials compared with AIR, with no significant difference among the N2O conditions. A dose-dependent trend in the perception of thermal comfort was observed for the N2O conditions. It is concluded that shivering thermogenesis, and thus thermal balance, is affected to the same degree for the range of inspired N2O concentrations investigated, with no discernable dose-dependent effect.

Adult↗

PubMed1995

Inhalation rewarming from hypothermia: an evaluation in -20 degrees C simulated field conditions.

The present study evaluates the efficacy of inhaling warm moist air as a method of rewarming from hypothermia in -20 degrees C field conditions. The method of inhalation rewarming is compared to two other methods of rewarming: a) passive rewarming; and b) passive rewarming, with a respiratory heat exchanger designed to minimize respiratory heat loss. Eight male subjects were rendered hypothermic by immersion in 15 degrees C water for 1 h. They were withdrawn from the tank earlier, in the event that their rectal temperature (Tre) decreased to 35 degrees C, or by 1.5 degrees C from the pre-immersion value. Upon completion of the immersion, they were placed in a well-insulated sleeping bag assembly and transferred to a cold room maintained at -20 degrees C for a 2 h rewarming period. They participated in 3 trials: Control-passive rewarming; Heat Treat-inhalation rewarming with the Heat Treat; HME-passive rewarming in conjunction with a respiratory heat and moisture exchanger (HME). During the rewarming period, inspired air temperature was -19.4 +/- 1.1 degrees C in the control trial. In the HME and Heat Treat trials subjects breathed via an oro-nasal mask. The inspired air temperature was +20.5 +/- 1.2 degrees C in the HME and +36.2 +/- 2.9 degrees C in the Heat Treat trial. The post-immersion drop in Tre was significant in all conditions. The reduction in the post-exposure drop in Tre observed with the Heat Treat may be attributed to the minimization of respiratory heat loss, since the magnitude of the reduction was similar to that observed with the HME.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1994

Shivering thermogenesis during acute hypercapnia.

The effects of acute hypercapnia on human thermoregulation during cold exposure were investigated by immersion of eight male subjects to the neck in a 15 degrees C water bath until their core temperatures dropped to 35 degrees C or until 1 h of immersion had elapsed. Air was inspired throughout each experiment, with the exception of a 15-min period commencing with the attainment of an esophageal temperature (Tes) of 36.5 degrees C, during which subjects inspired a gas mixture containing 4% CO2, 20% O2, and 76% N2. Oxygen uptake (VO2, L.min-1), inspired minute ventilation (Vi, L.min-1), esophageal temperature (Tes, degrees C), rectal temperature (Tre, degrees C), mean unweighted skin temperature (Tsk, degrees C), mean heat flux (Q, W.m-2), and electromyographic activity (EMG, mV) of the trapezius and masseter muscles were recorded continuously. VO2 and integrated EMG activity (IEMG) were used as the primary indicators of shivering thermogenesis. Shivering EMG was attenuated immediately following the switch of the inhaled gas mixture from air to 4% CO2. For both the masseter and trapezius muscles the IEMG was significantly suppressed (p < 0.05) during the hypercapnic period. The IEMG values preceding the switch to the hypercapnic mixture were 15% greater than those during the CO2 period. Similarly, IEMG values in the post-CO2 period were 55% greater than during the CO2 period. It is concluded that acute periods of hypercapnia during cold exposure may result in transient suppression of shivering tremor, but this does not appear to affect thermal balance, as reflected in the absence of any significant effect on Tes.

Acute Disease↗

PubMed1994

Treatment of mild immersion hypothermia by direct body-to-body contact.

Body-to-body contact is often recommended for rewarming mildly hypothermic victims in the field. This procedure involves a euthermic individual donating heat to the recipient by direct contact in an insulated bag. However, this technique has not been critically evaluated and may not be beneficial because there is limited direct contact between recipient and donor, peripheral vasoconstriction may impair heat transfer to the core, skin warming may blunt the recipient's shivering response, and cold stress to the donor may be excessive. The present study was designed to evaluate whether donation of heat by a donor would be sufficient to enhance rewarming of a hypothermic subject (recipient). Six pairs of recipients (5 men, 1 woman) and donors (2 men, 4 women) participated in the study. Esophageal and skin temperatures, cutaneous heat flux, and oxygen consumption were measured. Recipients were immersed in 8 degrees C water until esophageal temperature decreased to a mean of 34.6 +/- 0.7 degrees C (SD). They then were rewarmed by one of three methods: rewarming by the endogenous heat generated by shivering only (SH), body-to-body rewarming (BB), or rewarming with a constant-heat source manikin (MAN). Mean afterdrop for the three conditions was 0.54 +/- 0.2, 0.54 +/- 0.2, and 0.57 +/- 0.2 degrees C for SH, BB, and MAN, respectively (NS), and the rate of rewarming was 2.40 +/- 0.8, 2.46 +/- 1.1 and 2.55 +/- 0.9 degrees C/h for SH, BB, and MAN, respectively (NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1994

Perception of thermal comfort during narcosis.

We examined the perception of thermal comfort in six male subjects immersed in water at 28 degrees C (study I) and 15 degrees C (study II), breathing either room air (AIR) or a normoxic mixture containing 30% N2O (N2O). Immersions were terminated if esophageal temperature (Tes) decreased by 2 degrees C from resting levels or to 35 degrees C. At regular intervals, subjects rated their perception of thermal comfort on a 21-point scale (thermal comfort vote, TCV; +10 = very, very hot, 0 = neutral, -10 = very, very cold). For similar decreases in Tes from resting preimmersion values (mean +/- SD = -0.90 degrees +/- 0.13 degrees C and -0.92 degrees +/- 0.15 degrees C during the AIR and N2O trials in study I, and -0.90 degree +/- 0.22 degree C and -0.89 degree +/- 0.27 degree C during the AIR and N2O trials in study II), subjects perceived the immersions as less cold during the N2O trials. The median TCVs for the AIR condition of -5 in study I and -7.75 in study II, were significantly lower than those reported by the subjects for the respective N2O conditions (1.75 in study I and -5.5 in study II). It is concluded that behavioral adjustments required for maintaining thermal balance may be diminished during narcosis due to the altered perception of thermal discomfort. Assuming that the effect of inert gas narcosis on thermoregulatory responses is similar to that of N2O, then combined with the significant attenuation of heat gain mechanisms by anesthetic gases, the attenuation of the perception of thermal comfort may represent a significant factor in the etiology of hypothermia observed in compressed air divers.

Body Temperature Regulation↗

PubMed1993

Comparison of core threshold temperatures for forehead sweating based on esophageal and rectal temperatures.

A protocol incorporating successive hot and cold water immersions, causing respective warming and cooling of the body, has been used to determine the core threshold for sweating. Disparate results have been reported for the core threshold of sweating, and these have been attributed to the possible existence of core temperature gradients during such a protocol. Spatial and temporal core temperature (Tc, degree C) gradients during dynamic changes in body temperature may give rise to different values of core temperature thresholds for sweating, depending on the Tc measurement site. In addition, during such an immersion protocol skin temperature transients may influence expression of thresholds using esophageal temperature (Tes). With these considerations, the effects of Tc gradients and skin temperature on Tc thresholds for sweating were examined. Subjects (n = 22) were immersed to the neck in 40 degrees C water until Tes reached 38.5 degrees C (phase 1), followed immediately by cooling in 30.6 degrees C water until extinction of sweating was observed (phase 2). Cooling was continued in the latter bath after the sweating extinction until total immersed time reached 50 min or until shivering was initiated (phase 3). During the trials continuous assessment was made of rectal temperature (Tre) and Tes, mean unweighted skin temperature (Tsk, degree C), forehead sweating rate (Esw, g.m-2.min-1), oxygen consumption (VO2, L.min-1), and surface heat flux (Q, W.m-2). With the current protocol it appeared inappropriate to determine the Tc thresholds for onset of sweating, as sweating was initiated prior to any significant displacement of Tc, but was most likely influenced by Tsk and its rate of change.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

Effects of prolonged CO2 inhalation on shivering thermogenesis during cold-water immersion.

We investigated the effect of prolonged hypercapnia on human thermoregulation during immersion of seven male subjects in a 15 degrees C water bath until their esophageal temperature dropped to 35 degrees C or until 1 h had elapsed. In the control trial, subjects inspired room air, whereas in the other trial the inhaled gas mixture was a 4% CO2:20% O2:76% N2 gas mixture. Oxygen uptake (VO2, liter.min-1), inspired minute ventilation (VI, liter.min-1), esophageal temperature (Tes, degree C), mean unweighted skin temperature (Tsk, degree C), mean heat flux (Q, W.m-2), and electromyographic (EMG, mV) activity of the trapezius muscle were recorded. VO2 and integrated EMG (IEMG) activity were used as the primary indicators of shivering thermogenesis. There was a tendency for elevated VO2, albeit not significant, in the CO2 trial compared to the air trial. We observed no significant differences in the IEMG between the air and CO2 trials. These results suggest that prolonged inhalation of a gas mixture containing 4% CO2 does not have a significant inhibitory effect on shivering thermogenesis and does not enhance the cooling rate of the body core. The absence of any shivering attenuation is most likely due to the small blood PCO2 increase incurred by inhalation of 4% CO2, compensation of hypercapnic-induced respiratory acidosis, and a strong thermal drive from core and peripheral regions. It is unlikely that elevated PICO2 levels contribute significantly to the etiology of hypothermia in divers.

Adult↗

PubMed1992

The effect of 30% nitrous oxide on thermoregulatory responses in humans during hypothermia.

Clinical studies have reported that body core temperature decreases during prolonged surgery and anesthesia. Although this finding has been attributed primarily to increased heat loss resulting from exposure of body cavities and infusion of cold solutions, it is generally recognized that anesthesia interferes with the thermoregulatory system. The present study examined the effects of mild narcosis induced by 30% N2O on shivering thermogenesis and cutaneous thermoregulatory vasoconstriction in humans, during exposure in a much more intense peripheral thermal stimulus than the ones often used in clinical studies. Nine male subjects were immersed in 15 degrees C water on two separate occasions. During one occasion subjects inspired air (control condition), and during the other occasion the inspired gas mixture contained 20% O2, 30% N2O, and 50% N2 (N2O condition). On both occasions, subjects were immersed to the neck for 60 min, or until their core temperature decreased by 2 degrees C from the preimmersion value. Following the cooling phase, subjects rewarmed via endogenous thermogenesis while lying in a well-insulated bed for 48 min. In the N2O condition, subjects continued to inspire the anesthetic gas mixture during the 48-min period of recovery. O2 uptake (VO2), esophageal temperature (Tes), mean skin temperature (Tsk), mean heat flux (Q) and forearm-fingertip temperature gradient (Tsk-gr) were recorded at 1-min intervals. Tsk and Q in both conditions stabilized within 10 and 25 min of immersion, respectively, and were not significantly different between the two conditions. The cooling rate of Tes was greater during the N2O than the control condition. VO2 increased during the immersion in both conditions and was greater in the control than in the N2O condition. In both conditions, VO2 increased linearly with decreasing Tes, but at any given Tes, VO2 was higher in the control than in the N2O condition. No significant difference was observed in cutaneous thermoregulatory vasoconstriction between the two experimental conditions, as indicated by the Tsk-gr values. The estimated Tes threshold for shivering (estimated from the O2 consumption vs. delta Tes regression) was reduced by 0.95 +/- 0.26 (SE) degrees C during the immersion phase and by 0.39 +/- 0.05 (SE) degrees C during the rewarming phase in the N2O condition compared to the control conditions. Although the thermosensitivity (gain) of shivering appeared preserved during the immersion phase, it was reduced during the N2O rewarming phase.(ABSTRACT TRUNCATED AT 400 WORDS)

Administration, Inhalation↗

PubMed1992

Relationship between physique and rectal temperature cooling rate.

Despite many attempts to relate components of physique to core temperature cooling rate, no consistent relationship has emerged. The inconsistencies among the reported findings may arise from unaccounted thermoregulatory responses or incomplete physique assessment or both. A study was designed to examine this relationship in the range of rectal temperatures (Tre) within which shivering and sweating are absent, defined as the null zone, thus minimizing the contribution of these effector responses. Twenty healthy subjects (10 male and 10 female), representing a variety of physiques, participated in the study. The anthropometric protocol included 5 heights, body mass, 8 skinfolds, 10 girths, and 4 breadths. This permitted derivation of a body surface-area-to-mass ratio and estimates of adipose and muscle tissue masses using a cadaver-validated mass fractionation model. Subjects were heated in a 40 degrees C bath followed immediately by cooling in a 30.6 degrees C bath. During the cooling, forehead sweating rate and oxygen uptake were monitored to establish the boundaries of the Tre null zone. In addition, on-line recordings were made of Tre, skin temperature, and surface heat flux at six sites. The rate of cooling of Tre (Tre), in the range of temperatures between thresholds for sweating and shivering, was correlated to the components of physique. Estimates of adipose and skeletal muscle tissue masses did not correlate to Tre in the range of core temperatures investigated. However, total mass exhibited a significant correlation (r = 0.5, P less than or equal to 0.05) with Tre, but gender seemed to distort this relationship, possibly due to differences in adipose tissue distribution.

Adipose Tissue↗

PubMed1992

Temperature and humidity within the clothing microenvironment.

The present study investigates clothing microenvironment conditions that may develop during prolonged exposure of workers to a hot environment. Five subjects were exposed to a linear increase in ambient temperature from 20-40 degrees C over a 90-min period, and then remained at 40 degrees C for an additional 90 min. During the exposures, subjects were clad in four types of helicopter personnel suits (Gore-Tex, Cotton Ventile, Nomex/Insulite, and Nomex/Neoprene), incorporating both dry-suit and wet-suit designs. Continuous assessment was made of skin temperature, rectal temperature, and of microenvironment temperature, relative humidity, and vapor pressure (T mu, RH mu, and VP mu) 8 mm from the surface of the skin. Results indicate that although microenvironment temperatures were similar among suits and slightly lower than that of the environment, the RH mu and VP mu were much greater than those of the ambient air. The Nomex/Insulite and Nomex/Neoprene suits showed the highest VP mu, of which only the Nomex/Insulite resulted in significantly greater increases in rectal temperature, likely due to complete covering of the body with the impermeable insulite component. The present study demonstrates the need to discern between the ambient conditions and the conditions encountered next to the skin when protective clothing is worn.

Adult↗

PubMed1991

Core temperature "null zone".

An experimental protocol was designed to investigate whether human core temperature is regulated at a "set point" or whether there is a neutral zone between the core thresholds for shivering thermogenesis and sweating. Nine male subjects exercised on an underwater cycle ergometer at a work rate equivalent to 50% of their maximum work rate. Throughout an initial 2-min rest period, the 20-min exercise protocol, and the 100-min recovery period, subjects remained immersed to the chin in water maintained at 28 degrees C. On completion of the exercise, the rate of forehead sweating (Esw) decayed from a mean peak value of 7.7 +/- 4.2 (SD) to 0.6 +/- 0.3 g.m-2.min-1, which corresponds to the rate of passive transpiration, at core temperatures of 37.42 +/- 0.29 and 37.39 +/- 0.48 degrees C, as measured in the esophagus (Tes) and rectum (Tre), respectively. Oxygen uptake (VO2) decreased rapidly from an exercising level of 2.11 +/- 0.25 to 0.46 +/- 0.09 l/min within 4 min of the recovery period. Thereafter, VO2 remained stable for approximately 20 min, eventually increased with progressive cooling of the core region, and was elevated above the median resting values determined between 15 and 20 min at Tes = 36.84 +/- 0.38 degrees C and Tre = 36.80 +/- 0.39 degrees C. These results indicate that the core temperatures at which sweating ceases and shivering commences are significantly different (P less than 0.001) regardless of whether core temperature is measured within the esophagus or rectum.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1991

Estimation of regional cutaneous cold sensitivity by analysis of the gasping response.

Regional cutaneous sensitivity to cooling was assessed in males by separately immersing four discrete skin regions in cold water (15 degrees C) during head-out immersion. The response measured was gasping at the onset of immersion; the gasping response appears to be the result of a nonthermoregulatory neurogenic drive from cutaneous cold receptors. Subjects of similar body proportions wore a neoprene "dry" suit modified to allow exposure to the water of either the arms, upper torso, lower torso, or legs, while keeping the unexposed skin regions thermoneutral. Each subject was immersed to the sternal notch in all four conditions of partial exposure plus one condition of whole body exposure. The five cold water conditions were matched by control immersions in lukewarm (34 degrees C) water, and trials were randomized. The magnitude of the gasping response was determined by mouth occlusion pressure (P0.1). For each subject, P0.1 values for the 1st min of immersion were integrated, and control trial values, although minimal, were subtracted from their cold water counterpart to account for any gasping due to the experimental design. Results were averaged and showed that the highest P0.1 values were elicited from whole body exposure, followed in descending order by exposures of the upper torso, legs, lower torso, and arms. Correction of the P0.1 response for differences in exposed surface area (A) and cooling stimulus (delta T) between regions gave a cold sensitivity index [CSI, P0.1/(A.delta T)] for each region and showed that the index for the upper torso was significantly higher than that for the arms or legs; no significant difference was observed between the indexes for the upper and lower torso.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1991

Contribution of core cooling rate to shivering thermogenesis during cold water immersion.

The contribution of core cooling rate to the thermogenic response in humans, for a similar combination of core and peripheral thermal inputs, was studied. Seven male subjects were immersed in 15 degrees C water on two occasions. Trial A was conducted without any intervention. During Trial B extremity blood flow was occluded for 10 min to allow limb blood to cool toward the temperature of the surrounding tissues. Upon release of cuff pressure the cooled, trapped blood returned to the core region instigating a decrease in esophageal temperature (Tes), with a concomitant increase in heat production (H). The slope of the Tes-H relationship during the dynamic post-occlusion phase was defined as central thermosensitivity (beta B). The slope of the Tes-H relationship during Trial A (beta A), evaluated over a similar range of core temperatures but over a longer time period, was compared with beta B to determine the influence of core cooling rate on the thermogenic response. The rate of core cooling (Tes) increased from -0.05 +/- 0.01 degrees C.min-1 in Trial A to -0.23 +/- 0.02 degrees C.min-1 with cuff occlusion-release in Trial B, resulting in a significant increase in beta B when compared to beta A (-2.99 +/- 0.36 vs. -1.90 +/- 0.24 W.kg-1.degrees C-1). Results of this study indicate that during cold water immersion: 1) dynamic core temperature significantly contributes to the magnitude of metabolic heat production; and 2) individual differences exist in central thermosensitivity.

Adult↗

PubMed1990

Determination of esophageal probe insertion length based on standing and sitting height.

The present study derives simple formulas for the prediction of optimal insertion length of an esophageal temperature-sensitive probe from the measurements of either standing or sitting height. The formulas assume that the optimal site for an esophageal temperature probe is in the region of the esophagus bounded by the left ventricle and aorta, corresponding to the level of the eighth and ninth thoracic vertebrae (T8 and T9, respectively). An esophageal probe was constructed of polyethylene tubing containing 1-cm segments of alternating radiopaque and nonradiopaque tubing in the distal 20 cm of the probe. The probe was inserted through a nostril into the esophagus of 20 subjects (12 males and 8 females) of various heights (range 163-194.6 cm) and weights (range 52.2-100.8 kg), and lateral chest radiograms were obtained for determination of the insertion length of the probe (L) required to situate the probe in the retrocardiac esophagus. Analysis of the radiograms demonstrated that, at the level of the intervertebral disc between T8 and T9, the probe was below the tracheal bifurcation and close to the left ventricle. The distance from the nasal flare to this level showed a good correlation with the subject's stretched stature (r2 = 0.71) and sitting height (r2 = 0.86). The following equations were derived to predict the placement of the esophageal probe at the T8/T9 level based on standing height: L (CM) = 0.228 x (standing height) - 0.194, and sitting height: L (cm) = 0.479 x (sitting height) - 4.44.

Adolescent↗