Transcript Hypoxia and Hyperventilation

Hypoxia and Hyperventilation
Iraj yasaei MD
Flight physician
physiology of hypoxia, is at the basis of
high-altitude medicine, plays an important
role in aviation field environment.
Evangelista Torricelli (1608 –
1647) was the first person to
realize that the atmosphere
above us creates a pressure
that can, for example, support a
column of mercury.
Pascal(newton Pa=N/㎡
per square
meter
kilopascal
kPa
bar
b
Millimeter of
mercury(Torr)
mmHg(Torr)
Atmosphere
(standard)
atm
Pound per
square inch
psi (lbf/in2)
Relationship of Altitude to Barometric Pressure
PRESSURE
ALTITUDE
FEET
mm/HG
ATMOSPHERES
0
760
1
18,000
380
1/2
34,000
190
1/4
48,000
95
1/8
63,000
47
1/16
Composition of the Air
• 78 Percent Nitrogen N2
• 21 Percent Oxygen
• 1 Percent Other
– .03 percent CO2 ≈ 0
PERCENT COMPOSITION OF
THE ATMOSPHERE REMAINS
CONSTANT
BUT PRESSURE
DECREASES
WITH ALTITUDE
The effect of altitude on O2 tension
(breathing air)
100
80
60
40
CO2
20
Alveolar oxygen tension
120
O2
0
0
5000
10000
Altitude
15000 20000 25000
• The most important feature of compensatory
mechanism of body for reduced alveolar oxygen
pressure (Hypoxia) :
• In acute exposure…is……hyperventilation
• In chronic exposure…is…. Polycythemia
Alveolar PO2 and PCO2 of acclimatized
humans at high altitude.
•Sea level is at the top right
of the graph, and the
summit of Mount Everest is
at the bottom left.
•Note that after a certain
altitude has been
exceeded, alveolar PO2
does not decrease further,
It is defended at a level of
about 35 mm Hg by the
process of extreme
hyperventilation, which
reduces the PCO2 to less
than 10 mm Hg.
HYPOXIA
State of oxygen [O2] deficiency
in the blood cells and tissues
sufficient to cause
impairment of function
Stages of Hypoxia
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•
•
•
Indifferent Stage
Compensatory Stage
Disturbance Stage
Critical Stage
Indifferent Stage
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Altitudes: up to 10,000 ft
Oxygen saturation 90 – 98 percent
No awareness of symptoms
Symptoms:
No noticeable impairment
decrease in night vision , acuity and color
perception at 4000 ft reported
Compensatory Stage
• Altitudes: 10,000-15,000 ft
• Oxygen saturation: 80 – 90 percent
• Symptoms:
• Nausea, dizziness, lethargy, headache, fatigue
• Decreased efficiency, increased irritability, poor
judgment and impaired coordination
• An increase in RR, HR, and BP compensate for
lack of O2
Disturbance Stage
• Altitudes: 15,000-20,000 ft
• Oxygen saturation: 70 – 80 per cent
• Compensatory mechanism no longer effective
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•
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Symptoms:
short term memory, reaction time, speech and handwriting impaired
Slow mental function (calculation impaired)
Behavior: aggressive, euphoric, over confident
Impaired muscular coordination, fine movement impossible
Muscular spasm and tetany
Drowsiness, decreased level of consciousness
Hyperventilation, cyanosis
Critical Stage
• Altitudes: above 20,000 ft
• Oxygen saturation 60 – 70 percent
• All features of previous stage + loss of consciousness,
convulsions and death
Time of Oxygen
1 Minute
2 Minutes
3 Minutes
4 Minutes
5 Minutes
6 Minutes
Put Back on Oxygen
Types of Hypoxia
•
•
•
•
Hypemic (anemic)
Stagnant (circulatory)
Histotoxic (cellular)
Hypoxic (hypobaric)
Hypobaric hypoxia
•A deficiency in
alveolar-capilary
membrane
•V/Q mismatch
•Sever copd
Reduced
pO2
in the lungs
(high
altitude)
Red
blood cells
Body tissue
Ultrastructural changes in the wall of a
pulmonary capillary
•The arrows at the
top show a
disruption in the
alveolar epithelial
layer;
•the arrows at the
bottom show a break
in the capillary
endothelial layer,
with a platelet
apparently adhering
to the exposed
basement
membrane.
These changes are
caused by the high
mechanical stress in
the capillary
wall.
CAUTION!!!!
Failure to recognize your
signs and symptoms may
result in an aircraft mishap
Hypemic Hypoxia
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+
+
An oxygen
deficiency due to
reduction in the
oxygen carrying
capacity of the
blood
•Anaemia
•Heavy smoking
•Hypovolomia
•Blood donation
Adequate
oxygen
•Cardiogenic shock
•Venous pooling
•Arterial spasm
•ischemia
Stagnant
Hypoxia
Reduced
blood
flow
Blood moves
slowly and
Red blood cells
not reaching
tissue needs
fast enough
Histotoxic Hypoxia
Adequate
oxygen
Inability of the
cell to accept
or use oxygen
Red blood cells
retain oxygen
cyanide poisoning
Hyperventilation
significance
• Incapacitation of healthy crew member or
passenger
• Confusion with hypoxia
Remember treat it as hypoxia except prove
otherwise
Factors modifying hypoxia
symptoms
• Pressure altitude
• Physical activity
• Rate of ascent
• Individual factors
• Time at altitude
• Physical fitness
• Temperature
Self-imposed stresses
DEATH
Drugs
Exhaustion
Alcohol
Tobacco
Hypoglycemia
keep self imposed stresses out of the aircraft
Alveolar Gas Equation
• PAO2 = FiO2 (PB – PH2O) – PACO2 [FiO2 + (1-FiO2)/R]
The below version of equation is useful for clinical purposes; the R
value is 0.8, arterial and alveolar PCO2 are same and Water vapor
pressure is 47 mm Hg.
PAO2 = FIO2(PB-47) - 1.2(PaCO2)
PB
FIO2
PIO2
PaCO2
PAO2
R
= barometric pressure
= fraction of inspired oxygen
= pressure of inspired oxygen in the trachea
= arterial PCO2, equal to = alveolar PACO2
= alveolar PO2, PaO2 = arterial PO2,
= respiratory quotient (CO2 excretion over O2 uptake in the
lungs)
The effect of altitude on O2 tension
(breathing 100% O2)
120
80
60
40
20
Alveolar oxygen tension
100
O2
CO2
0
30000
46000
34000
38000
Altitude
42000
• The partial pressure of oxygen in dry air is the fraction of inspired
oxygen (FIO2) times the barometric pressure; at sea level this is
0.21x(760) = 160 mm Hg. With increasing altitude, the barometric
pressure falls and FIO2 remains constant.
• In the upper airways (nose, larynx, trachea), water vapor is added to
the inspired air. Water vapor pressure is 47 mm Hg at normal body
temperature; this pressure affects all dry (nonvapor) gas pressures
(oxygen, nitrogen, carbon dioxide).
• Thus tracheal (inspired O2) PiO2 = 0.21(76047) = 150 mm Hg.
• As air travels toward the alveoli, carbon dioxide increases; PCO2 at
the alveolar level = arterial PCO2 = 40 mm Hg (normal alveolar
ventilation).
• Since the lungs are an open system in continuous
contact with the atmosphere, total alveolar gas pressure
(sum of partial pressure of gases in alveoli) must be
equal to barometric pressure. But since inspired PCO2 is
zero and alveolar PCO2 is always equal to 40 mm Hg,
the partial pressure of some other gas must fall. Water
vapor does not change since it is a function of body
temperature. So PiO2=PAO2+PACO2
• This expression is special case of alveolar air equation
which reflects only three gas tensions.
• What about nitrogen? When nitrogen is present it is
necessary to introduce a correction factor into the
equation. the magnitude of this correction factor varies
with the respiratory exchange ratio.
•
Under normal conditions, approximately 250 ml of
oxygen are added to the pulmonary circulation per
minute (the VO2), while 200 ml of carbon dioxide are
removed (the VCO2). The ratio of VCO2/VO2 is the
respiratory quotient (R or RQ), so the normal R is
approximately 0.8. Thus, as air moves from the trachea
to the alveoli, PiO2 will fall 1.2 mm Hg for every I mm Hg
increase in PaCO2. If tracheal PiO2 is 150 mm Hg and if
PACO2 is 40 mm Hg, alveolar partial pressure of oxygen
(PaO2) is 102 mm Hg.
• PAO2 = PiO2 – PACO2 (FiO2 + (1 – FiO2)/R)
• PAO2 = FIO2(PB-47) - 1.2(PaCO2)