Fluid
and Electrolyte Balance and Endurance Exercise:
What
can we learn from recent research?
Ian
R. Rogers, MD, FACEM
In the
not too distant past, the conventional (42km) marathon was
considered to be the limit of human endurance. In the last
two decades, we have seen this "limit" challenged.
It is now commonplace to see non-elite athletes competing
in ultra-marathons, long-distance cycle events, and multidisciplinary
"multi-sports" or "adventure sports"
races. These new sporting disciplines have brought with
them a whole new raft of previously unreported medical illnesses.
The most important of these is exercise-associated hyponatremia.
Much of the research on this subject has been from work
done in the setting of the Ironman triathlon (3.8km swim,
180km cycle, 42km run). It is, however, applicable to a
wide range of endurance exercises and provides important
information about appropriate fluid and electrolyte balance
for any form of wilderness endurance activity.
The
first reports of exercise-associated hyponatremia appeared
in the early and mid 1980s, during Ironman triathlons and
ultra-marathon running races. More recently it has been
reported in a broad range of endurance and wilderness activities
including cycling, mountaineering, canyon walking, and all-night
dance parties. When first reported, it was thought that
exercise- associated hyponatremia was due to excessive losses
of sodium in the sweat that could not be adequately replaced
during exertion. Repeated studies have now shown this not
to be the case. The common feature of all studies has been
excessive fluid intake. This fluid is retained in the extracellular
space and in particular the intravascular space. Effectively,
it dilutes the sodium, which is largely an extracellular
cation, and leads to the observed clinical effects of the
syndrome. So exercise-associated hyponatremia is due to
dilution from fluid overload.
The
symptoms of hyponatremia are largely caused by the fluid
accumulation and edema formation, especially in the brain.
This means that the early symptoms can be relatively non-
specific and include nausea and vomiting, lethargy, malaise,
headache, and fatigue. These symptoms typically occur at
serum sodium levels of 125-134mmol/l (the normal is 135-145mmol/l).
It can be difficult to differentiate these symptoms in the
field from a whole range of other medical problems such
as dehydration, heat illness, hypothermia, and hypoglycemia.
As the serum sodium becomes lower still (less than 125mmol/l)
more specific symptoms develop. Prominent among these is
marked confusion followed by seizures and coma. It is clear
that women are substantially more at risk of the syndrome
than men, though just why is uncertain. Exercise-associated
hyponatremia should be suspected following prolonged exercise
(more than 4 hours), when fluid intake has been high (and
greater than expected losses), and when typical symptoms
develop.
It is
not usual to be able to measure serum sodium in a field
setting, so treatment needs to be commenced on the basis
of clinical suspicion. Milder cases should self-correct
with rest and limitation of access to further fluid. The
subject will then usually excrete the excess fluid in the
urine. Severe hyponatremia (serum sodium less than 125mmol/l)
is an emergency warranting medical evacuation. It is critical
to withhold IV fluids as this will just worsen the dilution
and hyponatremia. Patients who are having seizures or are
comatose require intravenous hypertonic saline which is
usually only available in a hospital.
Prevention
is obviously preferable to treatment and is of particular
relevance to a wilderness setting. It has now been shown
that hyponatremia can be prevented in the Ironman triathlon
by limiting access to education, fluid, and increasing salt
intake. There is no reason to believe that a similar strategy
is not also applicable to wilderness endurance activity.
This
begs the question of what is the appropriate amount and
type of fluid intake for endurance activity. It is important
to realize that longstanding advice about appropriate fluid
intake for exercise was formulated on research done on much
shorter events when the "limit of human endurance"
was much less. The applicability of this to longer events
is questionable. The American College of Sports Medicine
in its position statement, currently recommends a fluid
intake during exercise of 600-1200 mls/hr. The fluid intake
of most of the reported cases of exercise associated hyponatremia
has been at the middle or upper end of this range challenging
this as an appropriate fluid intake. A more realistic intake
is likely to be 500-750mls/hr. Whether this fluid should
contain salt as well remains unproven. Intuitively, it seems
prudent to use a proprietary sports drink containing 20-30mmol/l
of sodium if this is available, rather than just water.
While
the old mantra, "If you donít drink you die" is
not yet dead, it has certainly been challenged. We can no
longer assume that excess fluid taken during prolonged exercise
will just be passed out in the urine. Like most things in
life, balance is the key and the balance is likely to be
at a fluid intake not much above 500 mls per hour in most
situations, unless predicted losses are very substantial.
REFERENCES
Backer
HD, Shopes E, Collins SL, Barkan H. Exertional heat illness
and hyponatremia in hikers. Am J Emerg Med 1999;
17-532-539.
Hiller
WDB, OíToole ML, Massimino F, Hiller RE, Laird RH. Plasma
electrolyte and glucose changes during the Hawaiian Ironman
Triathlon. Med Sci Sport Exerc 1985; 17:S219.
Murray
B. Fluid Replacement: The American College of Sports Medicine
position stand. Med Sci Sports Exerc 1996; 9:4.
Speedy
DB, Noakes TD, Rogers IR, et al. Hyponatremia in ultradistance
triathletes. Med Sci Sport Exerc 1999; 31:809-815.
Speedy
DB, Rogers IR, Noakes TD, et al. Diagnosis and prevention
of hyponatremia at an ultradistance triathlon. Clin J
Sport Med 2000; 10:52-58.
Speedy
DB, Noakes TD, Schneider C. Exercise associated hyponatremia:
a review. Emerg Med 2001; 13:17-27
Ian
is an Emergency Medicine physician in Verdun, Nedlands,
Western Australian. He is a frequent contributor to the
Journal and popular speaker at WMS conferences.
Wilderness
Medicine Letter,
Volume 18, Number 3, Spring 2001
