Cardiovascular responses with standard and extended bladder coverage G-suits during rapid decompression.

This study compared the cardiovascular responses of subjects exposed to 60,000 ft. rapid decompressions while wearing the Combined Advanced Technology Enhanced Design "G" Ensemble (COMBAT EDGE or CE) and the Tactical Life Support System (TLSS). Eight subjects were rapidly decompressed from 22,500 ft (6,858 m) to 60,000 ft (18,288 m), once wearing the CE ensemble and once wearing the TLSS ensemble. There were significant differences in heart rate, stroke volume, cardiac index, and mean arterial pressure (p < 0.0001), due to garment type, with TLSS providing better cardiovascular support. Oxygen saturation did not decrease to the same degree with CE as with TLSS (p < 0.0001). Both TLSS and CE provided sufficient physiological support to maintain oxygen saturations above 65% during the 3-min exposures to 60,000 ft altitude. Short-term physiological support at higher altitudes with greater PPB levels or longer duration excursions at 60,000 ft may not be possible without the greater G-suit bladder coverage and cardiovascular support provided by TLSS-type garments.

Effect of extending G-suit coverage on cardiovascular responses to positive pressure breathing.

The purpose of this study was to compare cardiovascular responses of subjects exposed to long-duration positive pressure breathing (PPB) while wearing a standard (Combat Edge; CE) vs. extended coverage (Tactical Life Support System; TLSS) G-suit. Twelve experienced subjects, wearing TLSS and CE, were separately exposed to counterbalanced 60, 70, 80, and 88 mm Hg PPB for up to 10 min continuously. Termination resulted if presyncopal symptoms arose. G-suit inflation was 4 x mask/jerkin pressure. Using TLSS, all subjects completed 10 min of PPB at all levels, vs. 7 and 5 subjects completing 10 min at 80 and 88 mm Hg, respectively, using the CE ensemble (p < 0.001). Heart rate was significantly elevated at all PPB levels using CE (p < 0.0001) vs. TLSS. Stroke and Cardiac Indexes were significantly lower with CE at all levels vs. TLSS (p < 0.0001), and mean arterial blood pressure failed to be maintained at the 80 and 88 PPB mm Hg levels using CE (p < 0.0001). Extended G-suits afford superior protection against PPB-induced cardiovascular dysfunction vs. standard ensembles and consequently permit use of higher levels of PPB. This is due to the larger and more uniform application of pressure in the leg G-suit bladders, augmenting venous return and stroke volume.

Cardiovascular responses to positive pressure breathing using the tactical life support system.

The improved protection afforded by the Tactical Life Support System (TLSS) vs. other partial pressure ensembles has not been reported with respect to the cardiovascular effects of positive pressure breathing (PPB). Nine seated subjects wearing TLSS were exposed to 30, 50, and 70 mm Hg PPB (breathing air) with four times this pressure in the G-suit. Experiments were conducted at ground-level in order, separated by 4 min rest and preceded by a 1-min control period. Stroke volume and cardiac output (SV, CO) and indexes (SI, CI) were determined by impedance cardiography. Mean arterial pressure (MAP) was directly related to PPB level, increasing by 23%, 32%, and 47% for each PPB level, respectively (p less than 0.01). HR, SV, and CO were unaffected after 4 min of 30, 50, and 70 mm Hg PPB. The results indicate that cardiovascular function decay is less severe than that reported using other PPB ensembles at similar PPB levels. Improved protection is most likely due to the greater pressurization of the G-suit and the 45% greater bladder volume in the leg bladders, leading to restored venous return and SV.

Heat balance of subjects wearing protective clothing with a liquid- or air-cooled vest.

The goals of this study were, first, to determine the extent of the heat strain induced by wearing the Canadian Forces (CF) aircrew chemical defence individual protection ensemble (CD IPE) under simulated hot cockpit conditions, and second, to determine the effectiveness of a liquid cooled (LC) and an air-cooled (AC) vest in relieving such heat strain. Seven (7) healthy male subjects were subjected to three heat exposures (37 degrees C, 50% r.h., for 150 min, time-weighted metabolic rate of about 240 W, 1 week apart) either with no cooling (NC), LC or AC vests. NC was only tolerated for 95 +/- 5 min, whereas all subjects completed the 150-min tests with AC or LC (p less than 0.01). The large rate of increase in rectal temperature (Tre) during NC (1.00 +/- 0.05 degrees C/h) was attenuated by 51% with LC and by an even greater amount with AC (64%, p less than 0.01). NC entailed a sweat rate of almost 1 kg/h, which was reduced 38% by LC and 51% by AC (p less than 0.01). The combined dry and evaporative heat losses (HEKC of LC and AC vests were significantly greater than that of NC (164 +/- 7 and 181 +/- 9 vs. 124 +/- 9 W, respectively; p less than 0.01). The results demonstrate that subjects wearing CF aircrew IPE under simulated hot cockpit conditions can only tolerate 95 min of the 150-min test, and experience significant heat strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of inert gas narcosis on behavior--a critical review.

The effects of inert gas narcosis on behavior before unconsciousness are reviewed with particular attention to four issues. The first is whether the qualitative behavioral effects of all inert gases are identical. Evidence is limited but does not contradict an affirmative answer. This is consistent with the unitary hypothesis of narcosis at the physicochemical level. The second issue concerns the relative merits of four approaches to narcosis; (a) the descriptive model, (b) the hierarchical organization hypothesis, (c) the operant paradigm, and (d) the slowed processing model. It is concluded that the latter two are showing some promise. In particular, operant techniques allow more sophisticated measures of narcosis in animals than behavioral end points, such as loss of the righting reflex. The slowed processing model claims that the majority of performance deficits in humans are caused by a single fundamental deficit, slowing of information processing due to decreased arousal. This slowing is usually accompanied by alterations in task strategy. These alterations, in combination with cumulative slowing in working memory, are said to account for the various manifestations of narcosis on complex tasks. The third issue concerns adaptation to narcosis. There is some evidence that adaptation can occur but it is unclear whether the cause is learning specific to narcosis or development of a physiological tolerance. However, adaptation has not always been found and the variables controlling its presence or absence have yet to be identified. The fourth issue concerns the modifying effects of various factors, such as carbon dioxide and anxiety, on narcosis. Methodological and conceptual problems hinder interpretation of the evidence in this area but, contrary to some current views, there appears to be no conclusive evidence that any factor other than ethanol potentiates narcosis. Some implications of these conclusions for diving operations are discussed.

Respiratory gas exchange during positive pressure breathing and rapid decompression to simulated altitudes of 18.3 and 24.4 km.

Respiratory gas exchange was studied, using the technique of mass spectrometry, during events of slow and rapid decompression of human subjects to simulated altitudes of 60,000 and 80,000 ft (18.3 and 24.4 km, respectively). Positive breathing pressures and G-suit counterpressures were employed in three series of decompression experiment. Low levels of inspired carbon dioxide and nitrogen reflected the rebreathing of gases throughout the experiments. Application of a positive breathing pressure of 70 torr, accompanied by a jerkin pressure equal to breathing pressure and a G-suit counterpressure of four times the breathing pressure on the trunk and limbs, respectively, maintained alveolar oxygen at physiologically safe levels during decompression to 60,000 ft (18.3 km) altitude. Similarly, 80 torr positive breathing pressure, in combination with four times the breathing pressure in the G suit, adequately satisfied the requirements for oxygen during rapid decompression to 80,000 ft (24.4 km) simulated altitude.

Narcotic effects of nitrous oxide and compressed air on memory and auditory perception.

Three experiments were conducted to examine the effects of 35% N2 O (nitrous oxide) on human memory and auditory perception. In Experiment I, dichotic listening performance was found to be impaired. Experiment II used the same technique but was controlled for attenuation of sound transmission in the middle ear. No impairment was found. The perceptual effect found in Experiment I was peripheral, not central, and N2O did not impair short-term memory (STM). Experiment III used one-trial free recall of a word list. The shapes of the serial position curves were interpreted as indicating that N2O impairs long-term memory (LTM) but not STM. Experiment III provided no evidence, using cued recall, that the LTM deficit was due to impaired retrieval. Comparing these results with those for compressed air led to the conclusion that both N2O and hyperbaric nitrogen display an identical pattern of effects. A reason for the decrement found in some N2O STM studies may have been confounding the measurement of STM with that of LTM.

Induced vestibular dysfunction in squirrel monkeys during rapid decompression.

The symptoms of postural instability and dizziness associated with decompression sickness could be ascribed to either damage of the vestibular apparatus or to central nervous system damage. However, a histological study of monkeys exposed to decompression reveals that these symptoms primarily result from damage to the vestibular apparatus (unless there are accompanying central deficits). Furthermore, the damage is of a type that causes new bone growth to occlude the otic fluid spaces of the semicircular canals. In some instances, there is sufficient bone growth to render the cristae ampullares as non-functional end organs. Such diminished vestibular function would present a serious threat to the diver.

Protection against the physiological effects of positive pressure breathing.

The degree of protection afforded by three jerkin G-suit systems (British, Canadian and Swedish) using different pressures against the adverse physiological effects produced by high levels (50 mm Hg and 70 mm Hg) of positive pressure breathing (PPB) was investigated at ground level in 10 male subjects. The British and Canadian systems were evaluated with G-suit pressure equal to breathing pressure and 3.2 times breathing pressure, whereas the Swedish system was studied only with 3.2 times breathing pressure in the G suit. The experimental design was a 10 X 10 Latin swuare. PPB produced significant increases in heart rate, peripheral resistance, and systolic, diastolic and mean arterial blood pressures, and significant decreases in stroke volume and cardiac output. These effects were directly related to the level of PPB. Our results show that the British system provides the highestdegree of protection against PPB, followed by the Swedish and the Canadian systems. It was also found that a G-suit pressure of 3.2 times breathing pressure significantly increases the degree of protection afforded by the British and Canadian systems.

Survival test of submersible life support systems.

An experiment to validate predictions concerning submersible survivability was performed in December, 1975, by members of the Canadian Forces in the CF Submersible Lockout Vehicle SDL-1 in Halifax Harbour in water of 4 degrees C temperature at a depth of 40 ft. Data was collected relevant to the life support equipment to determine if it would operate for a simulated 6-h mission followed by a 24-h immobility period, at the end of which rescue was presumed to have occurred. Physiological data was collected from the submersible occupants in order to assess the degree of thermal stress experienced in this exercise. The experiment was terminated after a duration of approximately 25 h at 1 atm internal pressure due to exhaustion of two of the three on-board power supplies, causing the CO2 scrubbers to be inoperative and the CO2 content in the breathing gas to increase to toxic levels. Only two of the three submersible occupants experienced cold stress, one in the forward sphere and one in the aft sphere. At the end of 24 h, the core temperatures of both individuals had decreased by 0.5 degrees C and, during this time, skin temperatures, particularly of the extremities, had steadily and slowly decreased. Neither individual was hypothermic, but it was considered likely that after a 3-d exposure, at least two of the crew members would have had core temperatures of 35 degrees C or lower, assuming that CO2 poisoning had not occurred earlier.

Does the evoked response measure inert gas narcosis?

The purpose of this review is to examine the validity of change in the cortical evoked response as a measure of inert gas narcosis in humans. Three criteria are defined which must all be met if a nonbehavioral measure is to be accepted as an indicator of narcosis. The evoked response is assessed in terms of these criteria. Two classes of experiments which have used the evoked response in hyperbaric ocnditions are identified. The first class allows the evoked response to be assessed against more than one of these criteria. The outcome of every experiment in this class supports the view that the evoked response is not a valid measure of narcosis. The second class of experiment assumed that the evoked response is a measure of narcosis and were not designed to assess validity appropriately. Arguments by Kinney and associates in support of the assumption of validity are shown to be unsound. Possible explanations for inability to demonstrate validity are discussed and it is suggested that factors other than narcotic potency of the breathing gas mixture determine or at least play a major role in determining amplitude of the evoked response.

Effect of hyperbaric air on long-term memory organization and recall.

Three experiments are reported which investigated the effects of hyperbaric air on long-term memory. In the first, word lists were learned at 1 and 8.6 ATA using a variable input-free recall paradigm. It was found that learning was affected but not clustered memory organization, and it was concluded that disorganization of memory is not a factor contributing to the learning deficit found with hyperbaric air. In the second and third experiments it was found that the recall of words, which had been learned when non-narcotic, was disrupted at 10 ATA by hyperbaric air and that this disruption was not overcome by providing memory-cues at the time of recall. Two possible explanations for these results are discussed. A hypothesis is put forward to reconcile the results of various hyperbaric memory experiments by pointing out that a relationship between stress and learning found with nitrous oxide could be applicable to these studies also.