12. Altitude and Diving Flashcards
Altitude
Highest habitation
How high does atmospheric pressure half
How does this affect oxygen
What would happen if i was taken to the top of everest
The highest permanent habitation
in the world is found in the Andes
mountain range at 4877 m (16 000 ft)
above sea level.
In the northern Andes, the majority of inhabitants live above 2743 m (9000 ft).
The capital cities of Bolivia (La Paz), Ecuador (Quito) and Colombia (Bogotá) are all high-altitude cities.
La Paz is the highest
capital city in the world at 3630 m (11 910 ft).
> Atmospheric pressure
halves every 5500 m (18 000 ft).
> The percentage of oxygen in the
atmosphere at sea level is about 21%
and the barometric pressure is around 101 kPa.
As altitude increases,
the percentage remains the same
but the number of oxygen molecules
per breath is reduced.
At 3600 m (12 000 ft) the barometric pressure
is only about 64 kPa (480 mmHg),
so there are roughly 40% fewer oxygen
molecules per breath
thus the body must adjust to
having less oxygen.
> A t 19 200 m (63 000 ft) barometric pressure
is 6.25 kPa, meaning inspired PiO2 is zero
(as the partial pressure of water is 6.3 kPa and PiO2 = FiO2 × (Patm − PH2O)).
If a human being who resides
at sea level were to be
suddenly taken to the top of Mount Everest
(8848 m/29 028 ft) he or she
would succumb to hypoxia and lose consciousness.
The body requires a period of acclimatisation during which physiological adaptation occurs in response to the relative lack of oxygen.
Describe the acute and chronic physiological responses to high altitude
6 responses
The alveolar gas equation is
key to understanding the fundamental
physiological response to high altitude:
PACO2 PAO2 =PiO2 – \_\_\_\_\_\_
R
Where:
PAO2 Alveolar partial pressure of oxygen
PiO2 Inspired pressure of oxygen = FiO2 ⋅ (PATM − PH2O)
PACO2 Alveolar partial pressure of carbon dioxide (approximates with PaCO2)
R Respiratory quotient = CO2 production / O2 consumption (N = 0.8)
1 Hyperventilation
2
> Oxyhaemoglobin dissociation curve:
3
> Polycythaemia:
4
> Cardiovascular responses:
5
> Hypoxic pulmonary vasoconstriction:
6
> Angiogenesis and enzyme changes:
> Hyperventilation:
On ascent to altitude
there is an increase in minute ventilation
as a result of hypoxic stimulation
of the peripheral chemoreceptors
located in the aortic and carotid bodies.
The hyperventilation results in a
lowered arterial PaCO2,
which increases alveolar pressure of oxygen,
as can be seen from the alveolar gas
equation.
The hypocarbia secondary to
hyperventilation results in CSF
alkalosis.
However, this is transient as
bicarbonate is excreted from the
CSF over 24 – 48 hours and renally excreted.
> Oxyhaemoglobin dissociation curve:
At moderate altitudes there is a right shift
in the oxyhaemoglobin dissociation curve
caused by increased levels of 2,3-DPG,
thereby favouring oxygen unloading.
At high altitudes there is an overall
left shift in the oxyhaemoglobin
dissociation curve favouring
oxygen uptake in the pulmonary capillaries.
> Polycythaemia:
Increased erythropoietin secretion
results in a slow increase in red cell count
in order to increase oxygen-carrying capacity.
However, this also results in a raised haematocrit, which can lead to thrombosis.
> Cardiovascular responses:
Increase in heart rate and stroke volume
from sympathetic stimulation
from the effects of hypoxia
in an attempt to maintain oxygen delivery to the tissues.
Overall rise in myocardial work.
> Hypoxic pulmonary vasoconstriction:
Results in an increase in
pulmonary vascular resistance,
which can lead to right heart failure.
6
> Angiogenesis and enzyme changes:
Increase in capillary density with time,
thereby reducing oxygen diffusion distance.
This is associated with a change in intracellular
oxidative enzymes favouring cellular
respiration under hypoxic conditions.
How does high altitude affect volatile anaesthesia?
See Chapter 77, ‘Vaporisers’,
for an in-depth explanation on this topic.
> Gas and vapour analysers measure
partial pressure and assume sea
level atmospheric pressure (101 kPa).
E .g. an oxygen analyser measuring 21 kPa
will assume atmospheric pressure to be 101 kPa
and provide a percentage of oxygen on the
display of 21% or 0.21;
however, if the analyser is used at an altitude
where atmospheric pressure is only 70 kPa,
the analyser will under-read,
displaying 21% when it should be 33%.
> TEC vaporisers function normally at altitude.
The output of these vaporisers is a
constant partial pressure of volatile
agent not a constant volume percentage.
E .g. Vaporiser dialled to deliver 1% isoflurane:
• Gas from the vaporising chamber is
fully saturated with volatile agent
(i.e. it has achieved its
saturated vapour pressure (SVP)
at that ambient temperature).
• SVP is not affected by
ambient pressure
(i.e. does not change with altitude).
• 1% isoflurane at sea level will
have a partial pressure of 1%.
• 1% isoflurane at altitude – the volatile agent from the vaporising chamber will be diluted into a less dense gas stream and therefore the concentration of isoflurane will be
higher but the partial pressure remains
the same as it would be at sea level.
Clinical effect is dependent on the
partial pressure and therefore
remains the same.
What is acute mountain sickness (AMS)?
How common
Depends on
Symptoms
AMS is very common at high altitude.
At over 3000 m (10 000 ft), 75% of people
will have mild symptoms.
The occurrence of AMS is
dependent upon the elevation,
the rate of ascent and
individual susceptibility.
Many people will experience
mild AMS during the
acclimatisation process.
The symptoms usually start 12–24 hours
after arrival at altitude and
begin to decrease in severity
around the third day.
The symptoms of mild AMS include:
> Headache
> Nausea and dizziness
> Loss of appetite
> Fatigue
> Shortness of breath
> Disturbed sleep
> General feeling of malaise
What is high-altitude pulmonary oedema (HAPO)?
HAPO results from increased
pulmonary extravascular lung water,
which prevents effective oxygen exchange.
As the condition progresses,
severe hypoxaemia develops,
which leads to cyanosis,
impaired cerebral function
and death.
Symptoms of HAPO include:
> Shortness of breath at rest
> Tightness in the chest and
a persistent cough bringing up white, watery
or frothy fluid
> Marked fatigue and weakness
> A feeling of impending suffocation at night
> Confusion and irrational behaviour
What is high-altitude cerebral oedema (HACO)?
HACO is a potentially
life-threatening complication
of high altitude resulting from swelling of
brain tissue secondary to fluid leakage.
Symptoms of HACO include:
> Headache
> Weakness
> Disorientation
> Loss of coordination
> Decreasing levels of consciousness
> Loss of memory
> Hallucinations and psychotic behaviour
> Coma
Describe the treatment of AMS.
The only cure for mountain sickness is either acclimatisation or descent.
Symptoms of mild AMS can be treated with
analgesics for headache
(e.g. ibuprofen),
acetazolamide and
dexamethasone.
> Acetazolamide: carbonic anhydrase inhibitor, which reduces bicarbonate formation and increases hydrogen ion concentration in the body,
leading to development of a metabolic acidosis, which causes a respiratory compensation response resulting in an increase in minute ventilation and thus further lowering of PaCO2.
> Dexamethasone:
corticosteroid with
predominantly glucocorticoid
actions.
It has anti-inflammatory properties
and is useful in reducing cerebral oedema.
Many pilgrims at the annual festival at Gosainkunda
Lake in Nepal suffer from HACO following a rapid rate of ascent, and respond remarkably well to dexamethasone.
Other treatments for altitude
sickness include the following
> Nifedipine:
calcium channel blocker,
most commonly used as an antihypertensive.
It also has the effect of rapidly reducing pulmonary artery pressure by
inhibiting hypoxic pulmonary vasoconstriction,
thereby improving oxygen transfer.
It can therefore be used to treat HAPO,
though unfortunately its effectiveness
is not anywhere as good as that of
dexamethasone in HACO.
> Furosemide: loop diuretic,
may be used to treat pulmonary oedema
acutely.
However, furosemide may also lead to
collapse from low-volume
shock if the victim is already dehydrated.
> 100% oxygen also reduces the effects of altitude sickness.
Diving
How much does atm pressure increase
what are the problems seen
During diving the opposite problems to
altitude are seen.
Barometric pressure increases by 1 atm for
every 10 m descent
(e.g. at a depth of 30 m barometric pressure will be 4 atm).
The following specific issues relate to respiratory physiology during diving:
> Effects of compression and decompression
> Inert gas narcosis
> Decompression sickness
> Oxygen toxicity
> High-pressure nervous syndrome
