Page 92 - JSOM Spring 2023

known in some communities as “tactical dehydration,” which maintain body temperature in the cool water, thermal stress
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is the deliberate practice of reducing fluid intake in the hours may result in additional fluid loss through sweat. This wa-
prior to flight to avoid having to urinate during flight. As some ter must be replenished. Importantly, several studies have also
flight profiles can extend into multiple hours, mild to moder- indicated that pre-dive oral hydration may reduce the risk of
ate dehydration can lead to decreases in attention, memory, decompression sickness, perhaps by minimizing dehydration
and psychomotor skills. 6,13 Further, dehydration can predis- and preventing diving-induced hypovolemia. 22,23 Since drink-
pose aircrew and Operators to hypoxia and, potentially, de- ing while diving is problematic, pre-dive and between-dive hy-
compression sickness. 14 dration are the best courses of action.

Factors including high altitude, environmental heat stress, dy- Urinary Relief Systems
namic flight profile, and increases in insensible fluid loss when
breathing on aircraft or closed oxygen systems (low humidity Hydration has long been known to be important to optimizing
breathing gas) exacerbate fluid loss leading to acute dehydra- human performance. However, for some specialized commu-
tion and ultimately metabolic acidosis. Symptoms of meta- nities, such as aircrew and divers, urinary relief systems have
bolic acidosis include confusion, fatigue, and muscular fatigue. been a second thought. Operation Enduring Freedom pushed
Depending on the severity, small changes in the amounts of the limits of Navy and Air Force aircrew’s physiological limits
acid or base can drastically change blood pH within a mat- as flight operations pushed jet aircrew into missions reaching
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ter of seconds to minutes. The human body compensates with nine hours. Whereas many aircraft have relief tubes, which
multiple, complex buffer systems that can take hours to days. are easy for men to use, choices are limited for women, which
Therefore, the situation may not be correctable during the makes the thought of a long mission a painful reminder of
mission/flight. the lack of relief systems. One system currently available for
women is the Female Urinary Diversion Device (FUDD), which
Dynamic flight operations have the propensity to generate has been tested in women who were deployed to austere loca-
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high accelerative forces and loads, known as G-forces. There tions in support of Operation Enduring Freedom. The results
are three axes of acceleration, Gx, Gy, and Gz, and, for the were positive with women reporting it was easy to use, store,
purposes of this article, we are concerned with the Gz axis as and carry. Whether it could be adapted for female pilots and
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it applies to the acceleration acting in the head to feet axis. aircrew as a relief system has not been studied. Because dehy-
Positive Gz (+Gz) forces are from the head to the feet (the dration has become a high-profile concern, interest in develop-
most common direction of acceleration in aviation), negative ing adequate urinary relief systems for both men and women
Gz (-Gz) forces are from the feet to the head. Dehydration has increased in the United States Air Force and Navy.
is known to reduce G-tolerance in individuals, making them
more susceptible to G-related disturbances, such as visual dis- Currently, several systems are authorized by the Navy and
ruptions (gray-out and blackout), near loss of consciousness, the Air Force including urine collection bags, funnel sys-
and G-induced loss of consciousness (G-LOC). G-LOC can tems, diapers, and a collection device that can pump urine
occur in aircrew during sustained, high-Gz turns in which ex- into a 1.7-quart collection bag as soon as it is activated by
ternal forces of gravity counteract and/or reduce the flow of the presence of urine. Although developed for aircrew, they
blood to the brain, creating cerebral hypoxia and, ultimately, may be suitable for other communities, check with the appro-
unconsciousness. The reduction in G-tolerance is likely related priate program offices for more specifics and for acquisition
to the reduction in circulating plasma volume and increases in information.
cardiovascular strain. 15
Summary
Although more frequently associated with tactical jet aircraft,
G-related symptoms have been experienced in military helicop- Water is essential for physiological function. Optimal fluid
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ters. In one study, sweat loss was measured in various mili- balance is of great importance to Operators as clothing, ac-
tary aircraft under similar conditions, and although tactical jet tivity, and environmental conditions place severe demands
aircraft had the highest rate of fluid loss per hour, helicopter on the body to maintain fluid balance. Substantial water loss
pilots had higher amounts of fluid loss per flight due to longer can occur under any number of conditions, directly impacting
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duration (time) flights. Additionally, G-induced disturbances physical and cognitive performance and ultimately the ability
could potentially occur in parachutists under canopy (espe- to accomplish the mission. Operators should be conscious of
cially if dehydrated) when using spiral turns to descend rapidly their water loss and develop personalized strategies to ensure
(a long, hard pull on a steering toggle can cause a sustained, adequate hydration before, during, and after physical activity.
tight turn greater than 360°) when using high-performance
parachutes. This type of turn can create a large bank angle and Disclosures
swing the parachutist’s feet out of the turn, which can poten- The authors have no financial relationships or conflicts to
tially reduce blood flow to the brain. disclose.

Divers Disclaimer
Divers are exposed to various conditions that may contribute The opinions and assertions expressed herein are those of the
to dehydration. Water immersion alone results in diuresis, authors and do not reflect the official policy or position of the
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decreased plasma volume, and increased urinary excretion Uniformed Services University or the Department of Defense.
of sodium, potassium, calcium, and magnesium, particularly This work was prepared by military and civilian employees
when the water is cold. Likewise, hyperbaric conditions in of the US Government as part of the individual’s official du-
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the absence of immersion can cause a hyperbaric diuresis. ties and therefore is in the public domain and does not pos-
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Moreover, when saturation divers wear hot water suits to sess copyright protection (public domain information may be
90 | JSOM Volume 23, Edition 1 / Spring 2023