Pre-acclimatisation has also been suggested as a
preventative measure against altitude sickness and
involves spending a period of time at moderate
altitude prior to ascending to the final altitude
however, studies have shown conflicting results.
Whilst some studies demonstrate benefits associated
with pre-exposure to hypobaric hypoxia, others
demonstrate no significant effect (Forster, 1985;
Küpper and Schöffl, 2010; Guo et al., 2016; Luks et
al., 2017). One study (Forster, 1985) demonstrated
decreased incidence of altitude sickness in sea level
residents undertaking frequent and rapid ascents to
elevations of 4,200m when a “shift-worker” pattern
of 5-days spent at the designated height, followed
by the same period spent at sea-level was followed.
Another study (Guo et al., 2016) demonstrated a
significant increase in performance and reduction of
AMS incidence at 4400m after a 4-day short term
pre-exposure at 3700m. Furthermore, the use of
hypoxic chambers at sea-level for pre-acclimatisation
effects has been shown to reduce the incidence of
AMS with recommendations suggesting that the
pre-acclimatisation process is performed sleeping
at simulated altitudes following an ascent profile
that replicates the “gold-standard” for typical
acclimatisation (Küpper and Schöffl, 2010; Fulco et
al., 2013; Luks et al., 2017).
Oxygen Enrichment
As previously mentioned, reduction of PO2 to below
60mmHg – such as at high altitude – results in rapid
decline of SO2 leading to compromised oxygen
delivery to the tissues therefore supplementary
oxygen has long been considered preventative
against AMS. West (2004) compared the effects of
ventilatory acclimatisation and oxygen enrichment
on the improvement of oxygenation. The study
measured alveolar PO2 and reduction in equivalent
altitude as a measure of oxygenation and showed
that full ventilatory acclimatisation at altitudes
of up to 3600m reduced the equivalent altitude
sufficiently for supplementary oxygen not to be
necessary. At altitudes between 3600-4200m,
full acclimatisation reduces equivalent altitude to
approximately 3400m. Dependent on the pattern
of ascent, oxygen enrichment may or may not be
necessary for ascend to these altitudes. For ascent to
altitudes of greater than 5050m, oxygen enrichment
is suggested in order to optimise mental and physical
performance, where an oxygen supplementation
of 27% at 5000m is capable of reducing equivalent
altitude to around 3200m – generally well tolerated
(West, 2004). However, there is conflicting evidence
in terms of the use of placebo oxygen. Some studies
exploring the effects of placebo in the place of real
oxygen supplementation have shown that a placebo
is capable of mimicking the effects of oxygen at
altitudes as high as 4500-5500m, whilst other
studies have shown that placebo oxygen had no
effect on reducing the incidence of altitude sickness,
other than potentially decreasing fatigue (Fabrizio
et al., 2015; Benedetti et al., 2018; Moraga et al.,
2018).
Chemoprophylaxis
Acetazolamide is a carbonic anhydrase (CA) inhibitor.
Carbonic anhydrases are enzymes that catalyse the
conversion reaction between carbon dioxide + water
and carbonic acid (bicarbonate ions + hydrogen
ions). The active site of most carbonic anhydrase
enzymes contains a zinc ion and it is to this zinc
ion that acetazolmide binds to in order to inhibit
the enzymes action. Inhibition of renal carbonic
anhydrase by acetazolamide results in removal
of bicarbonate by urination as well as metabolic
acidosis (increased hydrogen ions in the blood
causing acidic blood) which increases the ventilatory
response and arterial PO2. Typical recommendations
for acetazolamide as a prophylactic state that
use should start a minimum of one day prior to
ascent, and a maximum of five (Wright et al., 1983).
Dexamethasone is a glucocorticoid (a type of
steroid hormone); its effects most likely mediated
through changes to capillary permeability and
cytokine release. Due to its significant side effects,
dexamethasone is not typically recommended for
prophylaxis unless acetazolamide is contraindicated.
In one double-blind, randomised trial, 47 climbers
were given either 250mg acetazolamide, 4mg
dexamethasone, or a placebo every 8 hours as a
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