Grissom CK, McAlpine JC, Harmston CH, Radwin MI, Giesbrecht GG, Scholand MB, Morgan JS. Hypercapnia effect on core cooling and shivering threshold during snow burial. Aviat Space Environ Med 2008; 79:735–42. Introduction: Hypercapnia during avalanche burial may increase core temperature cooling rate by decreasing the temperature threshold for shivering or by increasing respiratory heat loss. Methods: We studied the effect of hypercapnia on rectal core temperature (T_re) cooling rate, respiratory heat loss, heat production, and the T_re shivering threshold during snow burial (mean snow temperature -3.2 ± 2.7°C) in 11 subjects. In a 60-min hypercapnic burial subjects breathed a 5% carbon dioxide and 21% oxygen inhaled gas mixture and in a separate 60-min normocapnic burial subjects breathed ambient air. After extrication from snow burial subjects were passively rewarmed in a 15°C shelter and T_re afterdrop was measured. Results: The T_re over 1 h of burial in the hypercapnic study was 1.28 ± 0.4°C and in the normocapnic study was 0.97 ± 0.4°C (P 0.045). Minute ventilation, respiratory heat loss, total metabolic rate, and metabolic rate of the respiratory muscles were greater during the hypercapnic burial. There was no difference in shivering threshold between the hypercapnic and normocapnic conditions. Afterdrop in the hypercapnic study (0.69 ± 0.4°C at 21 ± 8.1 min after extrication) was not different than in the normocapnic study (0.86 ± 0.3°C at 23.1 ± 5.3 min after extrication). In both the hypercapnic and normocapnic studies afterdrop cooling rate was significantly greater during extrication than during snow burial. Discussion: Hypercapnia significantly increased T_re cooling rate by increasing respiratory heat loss but did not suppress shivering. Afterdrop may significantly contribute to hypothermia during rescue of avalanche burial victims.

Voelckel WG, Yannopoulos D, Zielinski T, McKnite S, Lurie KG. Inspiratory impedance threshold device effects on hypotension in heat-stroked swine. Aviat Space Environ Med 2008; 79:743–8. Introduction: Heat stroke is still an epidemiologically relevant disease with overall mortality rates as high as 15–33%. In heat stroke, hypotension is caused by relative central hypovolemia. We performed a feasibility study to determine if an inspiratory impedance threshold valve (ITD™) set to open at −12 cm H₂O would reverse systemic hypotension by enhancing venous return in hyperthermic, hypotensive, but spontaneously breathing animals. Methods: Seven anesthetized pigs weighing 30 ± 2 kg were warmed with a heating device until a mean rectal temperature of 44°C was reached, and mean arterial blood pressure (MAP) was ≤ 60 mmHg. The animals were then treated with the ITD. An intravenous bolus of 200 cc of 4°C normal saline was delivered 20 min after the ITD was placed, and external cooling was started. Results: Heat stroke criteria were achieved within 105 ± 15 min. MAP had decreased from 105 ± 5 to 57 ± 5 mmHg, and respiratory rates had increased from 33 ± 2 to 101 ± 13 breaths/min. Addition of the ITD significantly improved MAP to 85 ± 4 mmHg, and reduced respiratory rate to 54 ± 6 breaths/min within 2 min. The effect was sustained until fluid replacement and external cooling were delivered 20 min later. At that point, MAP returned to baseline within 30 min, and 6/7 animals survived for an additional 30 min. Conclusions: Use of an inspiratory impedance threshold device resulted in an immediate rise in blood pressure in animals in heat stroke and preserved blood pressure for at least 20 min prior to cooling and fluid replacement.

Macias BR, Minocha R, Cutuk AA, Hill J, Shiau J, Hargens AR. Intramuscular pressures in antigravity muscles using gravity-independent, pneumatic hardware. Aviat Space Environ Med 2008; 79:749–53. Introduction: Resistive exercise helps prevent muscle atrophy in microgravity, but better exercise equipment is needed. Therefore, the purpose of this study was to determine if a pneumatic, gravity-independent leg-press device (LPD) provides sufficient force to leg musculature. We hypothesized that intramuscular pressure (IMP), a quantitative index of muscle force, is greater in the antigravity superficial posterior and deep posterior compartments than in the non-antigravity anterior compartment during bilateral leg-press exercise. Methods: Millar® pressure transducers were inserted into the anterior, lateral, superficial posterior, and deep posterior muscle compartments of the left leg of eight healthy subjects (three women, five men). Subjects were supine on the Keiser SX-1, a pneumatic LPD. Then maximal voluntary contraction (MVC) was determined; each subject performed three consecutive voluntary contractions at ∼18%, 50%, and 100% MVC while continuously measuring IMP. Repeated measures ANOVA were used to determine differences of IMPs between compartments and loads. Results: The magnitudes of IMP (mean ± SEM) at 18 ± 3% (abbreviated ∼18%), 50%, and 100% MVC in the superficial and deep posterior compartments were significantly greater than that in the anterior compartment during exercise (P < 0.05). Additionally, IMPs in all four compartments significantly rose as resistance increased at ∼18%, 50%, and 100% MVC (P < 0.05). Conclusion: The LPD provides significantly increased resistance to all four compartments, but with greater loading of the antigravity compartments as compared to the non-antigravity compartment. Since antigravity muscles of the leg are contained primarily in the superficial and deep posterior compartments, the LPD may help prevent muscle atrophy associated with microgravity.

Vacchiano C, Moore J, Rice GM, Crawley G. Fexofenadine effects on cognitive performance in aviators at ground level and simulated altitude. Aviat Space Environ Med 2008; 79:754–60. Introduction: Antihistamines are used for the treatment of allergic rhinitis (AR) symptoms. However, the cognitive effects of some antihistamines can dramatically impair individuals in occupations that require sustained vigilance. Methods: The cognitive effects of fexofenadine were compared to a placebo (passive control) and cetirizine (active control) in healthy naval flight personnel. All subjects received one dose of each treatment in one of six possible sequences with two washout periods in between, and were assessed for aviation-related cognitive skills using the Aeromedical Vigilance Test (AVT) at both ambient atmospheric conditions and normobaric hypoxic conditions. Drowsiness was self-assessed by participants using a visual analog scale (VAS). Results: There was no significant difference between fexofenadine and placebo over the entire 60-min test period, under ambient atmospheric conditions, or under either hypoxic condition. Compared with placebo, cetirizine significantly increased AVT errors over the entire 60-min test period, at 10,000 ft, and at 15,000 ft. No statistical difference was found between treatments under ambient atmospheric conditions, although cetirizine treatment resulted in a greater change from baseline in adjusted average number of AVT errors (0.2124 ± 0.06) than fexofenadine treatment (0.1989 ± 0.07) and placebo (0.0745 ± 0.07). Furthermore, at 10,000 ft there were significantly more AVT errors with cetirizine than with fexofenadine. There were no significant increases in self-reported drowsiness (VAS) for both cetirizine and fexofenadine compared with placebo. Conclusion: Fexofenadine is comparable to placebo in its effect on the cognitive skills important for piloting an aircraft, while cetirizine impairs cognition and may affect piloting ability.

Pontier J-M, Blatteau J-E, Vallée N. Blood platelet count and severity of decompression sickness in rats after a provocative dive. Aviat Space Environ Med 2008; 79:761–4. Introduction: Previous animal studies reported that platelet count (PC) is decreased following decompression. Adherence and aggregation of platelets to the bubble surface has been demonstrated in severe decompression sickness (DCS). The present study was designed to clarify the relationship between post-dive platelet levels and the severity of DCS in a rat model. Methods: A total of 57 male Sprague-Dawley rats were assigned to either one experimental group with a hyperbaric exposure (N = 22) or one control group (N = 27). Rats were compressed to 1000 kPa (90 msw) for 45 min while breathing air and decompressed to surface in 38 min with stops at 200, 160, and 130 kPa. Onset of neurological DCS and death time were recorded during a 120-min observation period after surfacing. In the control group, rats were maintained at atmospheric pressure in the same chamber for an equivalent period of time. Blood samples for PC were taken 30 min before and immediately after exposure in two groups. Results: Blood PC after hyperbaric exposure had significantly decreased, whereas PC had increased in the control group. We found a correlation between % fall in PC and latency to death time. The platelet loss tended to decrease when fatal DCS was delayed. Rats suffering from severe DCS with a short latency to death presented a pronounced decline in platelets. Discussion: The present study highlighted a relationship between the post-dive decrease in PC and DCS severity in rats. Platelet consumption could offer a new index for evaluating decompression stress.

Mantoni T, Rasmussen JH, Belhage B, Pott FC. Voluntary respiratory control and cerebral blood flow velocity upon ice-water immersion. Aviat Space Environ Med 2008; 79:765–8. Introduction: In non-habituated subjects, cold-shock response to cold-water immersion causes rapid reduction in cerebral blood flow velocity (∼50%) due to hyperventilation, increasing risk of syncope, aspiration, and drowning. Adaptation to the response is possible, but requires several cold immersions. This study examines whether thorough instruction enables non-habituated persons to attenuate the ventilatory component of cold-shock response. Methods: There were nine volunteers (four women) who were lowered into a 0°C immersion tank for 60 s. Middle cerebral artery mean velocity (CBFV) was measured together with ventilatory parameters and heart rate before, during, and after immersion. Results: Within seconds after immersion in ice-water, heart rate increased significantly from 95 ± 8 to 126 ± 7 bpm (mean ± SEM). Immersion was associated with an elevation in respiratory rate (from 12 ± 3 to 21 ± 5 breaths · min⁻¹) and tidal volume (1022 ± 142 to 1992 ± 253 ml). Though end-tidal carbon dioxide tension decreased from 4.9 ± 0.13 to 3.9 ± 0.21 kPa, CBFV was insignificantly reduced by 7 ± 4% during immersion with a brief nadir of 21 ± 4%. Discussion: Even without prior cold-water experience, subjects were able to suppress reflex hyperventilation following ice-water immersion, maintaining the cerebral blood flow velocity at a level not associated with impaired consciousness. This study implies that those susceptible to accidental cold-water immersion could benefit from education in cold-shock response and the possibility of reducing the ventilatory response voluntarily.

Ramsey CS, Werchan PM, Isdahl WM, Fischer J, Gibbons JA. Acceleration tolerance at night with acute fatigue and stimulants. Aviat Space Environ Med 2008; 79:769–73. Introduction: The impact of pharmacological agents on aviators concerns all flight surgeons. This study tested the related hypotheses that acute fatigue reduces +G_z tolerance and endurance, and that stimulants can partially reverse this impact. Additionally, the researchers attempted to develop a test battery sensitive enough to detect subtle differences in aviator cognition and performance among conditions. Methods: To determine the effect of fatigue on +G_z tolerance and the impact of stimulant use, 10 male centrifuge subjects, mean age 32, from Brooks City-Base, TX, were tested in a repeated measures study under five nighttime conditions following an average of 22 h of sustained wakefulness during their circadian nadir. Using a within-subject design, subjects received placebo, dextroamphetamine 10 mg, modafinil 200 mg, methylphenidate 10 mg, and pemoline 37.5 mg at night, and were tested during a daytime control session. Cognitive/performance tests were administered before each centrifuge run. Results: No difference in +G_z tolerance or endurance was detected among conditions. The cognitive/performance tests also did not detect any differences. Subject perception that anti-G straining maneuver (AGSM) difficulty was greater during the night placebo condition than during the daytime control, methylphenidate and modafinil night conditions reached statistical significance (P = 0.005, 0.012, 0.022, respectively). Discussion: Physiological changes during the circadian nadir following acute sleep deprivation do not appear to negatively impact +G_z tolerance. A standardized protocol sufficiently sensitive to detect subtle behavioral and performance effects would be useful to test and compare the effect of other pharmacological agents on aviators.

Newman DG, Callister R. Cardiovascular training effects in fighter pilots induced by occupational high G exposure. Aviat Space Environ Med 2008; 79:774–8. Introduction: A fundamental difference in the cardiovascular response to acceleration between a group of fighter pilots (FP) and a group of non-pilots (NP) has been demonstrated previously. This study investigated the longitudinal effects of repetitive occupational +G_z exposure on the cardiovascular response to acceleration. Methods: There were 6 FP and 6 NP subjects who underwent rapid +75° head-up tilt (HUT) on two separate occasions. The FP group were tested after a non-flying period of 5 wk (Test 1), and tested again after a period of repetitive exposure to high +G_z missions (Test 2). The NP group did not fly at all between Test 1 and Test 2. Mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), and total peripheral resistance (TPR) were all determined non-invasively. SV was determined using impedance cardiography and calculated via the Kubicek equation. For each variable, resting values and the response to tilt for both HUT tests within and between each group were compared. Results: In the FP group, resting MAP was higher (86 mmHg) in Test 2 compared with Test 1 (78 mmHg). Between groups, FP resting MAP was only different from the NP resting MAP in Test 2. The FP HR response to HUT increased significantly between the two tests. Conclusions: These findings suggest a +G_z-induced cardiovascular training effect in the FP group. Repetitive exposure to +G_z results in an increased resting MAP and an elevated HR response to tilt, which may provide benefits to operational fighter pilots.

De Loose V, Van den Oord M, Burnotte F, Van Tiggelen D, Stevens V, Cagnie B, Witvrouw E, Danneels L. Individual, work-, and flight-related issues in F-16 pilots reporting neck pain. Aviat Space Environ Med 2008; 79:779–83. Introduction: Neck pain is a common problem in F-16 pilots. A cross-sectional survey was used to determine the self-reported 1-yr prevalence of neck pain and to compare individual, work-related, and flight-related characteristics in F-16 pilots with and without neck pain. Method: There were 90 male F-16 pilots of the Belgian Air Force and The Royal Netherlands Air Force who voluntarily completed an anonymous survey. Results : The 1-yr prevalence of neck pain was 18.9%. Pilots were divided into two groups: healthy (HG) and neck pain group (NPG). This study could not identify individual or specific flight-related differences between these two groups. High force demands, often sitting for a long time, frequently holding the neck in a forward bent posture, and being physically tired were all physical work-related factors that were reported significantly more often in the NPG. The NPG also reported significantly more psychosocial factors, such as being mentally tired at the end of the day and being annoyed by others at the workplace. Discussion: Since the specific flight-related factors were not significantly different between the HG and the NPG, physical and psychosocial factors could have been important factors in the development or maintenance of neck pain in F-16 pilots. Conclusion: The results of this study highlight for the first time that, in addition to flight-related issues, other aspects must be considered in analyzing neck pain. These other aspects stress the importance of a broader approach when considering neck pain, even in this population that is exposed to very high loads during flight.

Leon GR, Koscheyev VS, Stone EA. Visual analog scales for assessment of thermal perception in different environments. Aviat Space Environ Med 2008; 79:784–6. Introduction: Subjective thermal perception in stable and dynamic environments has been measured primarily by categorical rating scales. At present, there is an increasing use of visual analog scales (VAS) to assess whole body and body region thermal perceptions in laboratory and other thermal environments. Discussion: The rationale behind the use of VAS is that individuals are more accurate in conveying their subjective experience if they are not forced to make ratings according to numbers or categories. Unipolar scales have the advantage of focusing on a single subjective perception, such as the amount of cold the subject experiences. With bipolar scales (e.g., cold to hot), the sensitivity in measuring the extent of a specific thermal experience may be diminished because the subject needs to make a discontinuous change from one thermal perception to another. Conclusions: The robust psychometric properties of VAS indicate their usefulness in measuring thermal experience. Unipolar VAS with a large equal interval scoring range will enable the investigator to measure small gradients of change in perceptual experience. However, the subjective perception of thermal status is not highly accurate at thermal extremes, and caution is needed in sole reliance on subjective report in these conditions.

Layne CS, Forth KE. Plantar stimulation as a possible countermeasure to microgravity-induced neuromotor degradation. Aviat Space Environ Med 2008; 79:787–94.Since the early days of human spaceflight it has been reported that extended exposure to gravitational unloading results in a myriad of neuromotor adaptations that, while appropriate for microgravity, are maladaptive upon return to Earth. If not countered, these adverse effects of microgravity can result in negative health consequences and place crewmembers at risk for injury. The most commonly used countermeasures in today's space programs are those requiring active participation in prescribed exercise regimes that are time intensive, not completely effective, and have led to relatively low compliance. In this paper we review evidence that suggests a “passive” countermeasure in the form of dynamic foot stimulation (DFS) to the plantar surfaces of the feet may be a useful supplement to more traditional exercise countermeasures. This includes reports from both Russian and American investigators using both human and animal models indicating the overall effectiveness of DFS, the specific stimulation parameters involved, and a physiological explanation for the outcomes associated with the exposure to microgravity. Additionally, the use of DFS has the potential to benefit those at risk for muscle atrophy, including those experiencing extended bedrest, the elderly, and those with spinal cord injuries.

Henderson DJ, Ridgway DM, Kamath S, Harries R, Samy AK. Acute lower limb ischemia associated with use of an immersion suit. Aviat Space Environ Med 2008; 79:795–6.External compression is a rare cause of acute lower limb ischemia. Workers required to wear immersion suits during helicopter simulation training are exposed to external compressive forces which can alter the hemodynamics in arterial bypass conduits. Herein a case of arterial thromboembolization to the lower limb following the wearing of an immersion suit, in a patient who had undergone arterial bypass surgery 13 yr previously is presented. The potential for this episode of acute leg ischemia being a direct result of the compressive forces exerted by the immersion suit and the possible implications for wearers of immersion suits following arterial graft surgery is discussed.

Vann RD, Moon RE, Freiberger JJ, Denoble PJ, Dear GL, Stolp BW, Massey EW. Decompression illness diagnosis and decompression study design. Aviat Space Environ Med 2008; 79:797–8.

Windsor JS, McMorrow RC, Rodway GW. Oxygen on Everest: the development of modern open-circuit systems for mountaineers. Aviat Space Environ Med 2008; 79:799–804.The use of supplemental oxygen on Mt. Everest is now commonplace. From 1990 to 2006, more than 95% of those summiting the mountain did so using supplemental oxygen at some point during their ascent. The open circuit systems currently in use can be traced back to the device first used by George Finch on Mt. Everest in 1922. Wearing equipment weighing 33 lb (15 kg), Finch and his colleague Geoffrey Bruce set a world altitude record by reaching a height of 27,250 ft (8175 m). However, it would be with a lighter system weighing just 22 lb (10 kg) that Sir Edmund Hillary and Tenzing Norgay made the first ascent of the mountain in 1953. In the years since then considerable improvements in weight, comfort, and efficiency have been made; however, the original “open” principles first used by Finch almost a century ago still remain steadfastly in place.