Hypoxia Flashcards
Normoxia
Adequate supply of oxygen
Hypoxia
Insufficient oxygen to main normal tissue function
Hypoxemia
Low levels of oxygen in the arterial blood
Anoxia
Absence of oxygen in the blood or tissue
Hypobaric hypoxia
Barometric pressure decreases as altitude increases
Normobaric hypoxia
Experimentally manipulate fraction of inspired O2 without manipulating barometric pressure
Hypoxic hypoxia
Reduction in arterial PO2
Examples of hypoxia hypoxia
High altitude
Hyperventilation
Anemic hypoxia
Reduction in O2 carrying capacity of the blood
Anemic hypoxia examples
Decreased RBC levels (hemorrhage)
Reduced hemoglobin concentration (anemia)
Reduced O2 binding capability of hemoglobin (carbon monoxide inhalation)
Circulatory hypoxia
Decreased blood flow to tissues
Circulatory hypoxia examples
Heart failure
Local vasoconstriction (cold)
Histotoxic hypoxia
Inability of tissues to use distribution of electron transport chain in mitochondira
Histotoxic hypoxia examples
Cyanide poisoning
PIO2 equation
(PB- 47mmHg) x 0.2093
Alveolar gas equation
(PAO2 - (PACO2/R) + F
Alveolar ventilation equation
VCO2/ PACO2 x 0.863
Flow equation
Pressure x conductance
Cardiac output equation
MAP x Vascular conductance
MAP/ vascular resistance
MAP equation
Cardiac output x vascular resistance
What causes vasodilation?
Hypoxia
What causes vasoconstriction?
Increased sympathetic nerve activity
What decreases vascular resistance?
Hypoxia-mediated vasodilation stimulated by arterial O2
What increases vascular resistance?
Sympathetically mediated vasoconstriction stimulated by arterial PO2
Pulmonary Vascular Physiological Responses
Hypoxic pulmonary vasoconstriction
Increase pulmonary artery pressure
High-altitude Pulmonary Edema
Pulmonary vasoconstriction
Pressure induced leak
Pulmonary edema
Clinical features of HAPE
What counteracts reduced O2 in hypoxia?
Greater ease of O2 offloading from Hb (smaller pressure gradient required to release O2)
Neurological responses to hypoxia
Hyperventilation
Brain blood flow is altered
Reduced tissue PO2 and decreased cerebral oxygenation (accelerates supraspinal fatigue)
Hyperventilation and hypocapnia relationship
Hyperventilation causes hypocapnia and decreases PACO2
Hypercapnia causes
Cerebrovascular vasodilation
Hypocapnia causes
Cerebrovascular vasoconstriction
Determinants of physiological responses to hypoxia
Degree of hypoxia
Individual variability
Physical activity
Temperature
Illness/disease
Drugs
Describe the first demonstration of the dangers of rapid high-altitude ascent
Zenith balloon ascent
Reached an altitude of 28,000 feet (8600 meters). Two never regained consciousness and died during, one survived to write about it but became deaf.
Why study hypoxia?
Aviation physiology
Military (airforce, navy)
Tourism
Railway and highway mountain passes
Rescue at high altitude
Shift work (mining, observations/telescopes)
How many people live at high altitude?
> 1500m - greater than 500 million
2500m - greater than 83 million
3500m - greater than 14 million
Where has most high altitude research been done?
Climbing
Aviation
Male researchers testing each other
Women deemed “distractions” and high-altitude residents as “unintelligent”
Acclimatization
Physiological adjustments derived from exposure to a natural environment
Decrease in PAO2 causes?
Carotid body - increase ventilation - decrease PACO2 - back to beginning
What is essential for normal respiratory acclimatization?
Peripheral chemoreceptors (Carotid bodies)
Animals who have had their carotid bodies denervated fail to acclimatize properly (similar to humans with unilateral and/or bilateral resection of their carotid bodies)
Ventilatory response to exercise
Larger ventilatory response causes increase in climbing performance (however, there is a trade off at extreme altitude)
The most successful climbers had what response to hypoxia?
Smaller responses - because of lowered HVR, their ventilatory efficiency was higher.
Describe the trade off of high HVR
Useful at moderate altitudes, but at extreme altitudes the metabolic cost of excessive breathing is counterproductive.
Describe the ventilation process
Ventilation - pulmonary O2 diffusion - circulatory O2 delivery - muscle O2 diffusion - muscle O2 utilization - muscle ATP turnover
Alveolar to arterial PO2 causes
Thickening of the diffusion barrier, decrease in the surface area, breathing low O2 mix
Circulation changes with altitude
Cerebral circulation increases CBF on initial exposure
Hypoxic pulmonary vasoconstriction
Increased sympathetic outflow
Increased heart rate
Reduced stroke volume (due to reduced blood volume)
How many people does acute mountain sickness affect?
Up to 25% travellers to 2500m and 75% with rapid ascent to 5000m
When does AMS occur?
Within 6-48 hours of ascent
Symptoms of AMS
Headache and one of:
Malaise
Nausea
Vomitting
Anorexia
Light headedness/dizziness
What causes AMS?
Increase in intracranial pressure
How to minimize risk of AMS?
Slow, gradual ascent (let yourself acclimatize); no more than 300m higher/night above 3000m
Acetazolamide (diamox)
Acetazolamide effects
Produces metabolic acidosis and stimulates ventilation
What causes high-altitude cerebral edema?
Increased intracranial pressure
When does high-atltitude cerebral edema occur?
1-2 days after onset of AMS symptoms
Symptoms of high-altitude cerebral edema
Impaired level of consciousness
Hallucinations
Swelling of optic disk
Treatment for high-altitude cerebral edema
Medications (dexamethasone)
O2
Descent
When does high-altitude pulmonary edema occur?
24-72 hours after rapid ascent
Symptoms of high-altitude pulmonary edema
Breathlessness
Cough
Lung crackles
Treatment of high-altitude pulmonary edema
O2
Medications
Descent
High-altitude pulmonary edema impairs what?
Diffusion from alveoli to capillary
Function of the gamow bag
Generates internall pressure using 10-15 pumps/minute (2-3 with CO2 bladder)
How long must gamow bag be used?
1-2 hours for minimal treatment for AMS
4 hours for HAPE
6 hours for HACE
Aids to acclimatization
Climb high, sleep low
High carbohydrate diet
Guides pace - avoid overexertion
Avoid alcohol and sleeping medications
Pre-acclimatization strategies
The highest city in the world
La Rinconada
Arterial O2 content equation
Increased [Hb] x O2 saturation + dissolved O2
How many people does chronic mountain sickness effect?
~5-20% of high-altitude residents (Andean)
Chronic mountain sickness causes
Excessive erythrocytosis (Hb > 21)
Increased cardio and cerebral vascular risk
Outcomes from excessive erythrocytosis and chronic mountain sickness
Pulmonary hypertension
Right heart failure
Neurological problems
Increased cardiovascular risk
Mortality
Treatment for chronic mountain sickness
Descent (not always possible)
Bloodletting
Acetazolamide
Drugs that lower blood viscosity
Why is bloodletting not preffered?
RBC production can bounce back even worse than before
Describe benefits of the tibetan advantage compared to lowlanders
More effective breathing pattern
Increased lung diffusion capacity
Some cardiac and brain adaptations
Higher capillary density
Greater mitochondria
Describe “unairness” of olympic events
Western countries objected to “Training advantages” of Kenyans and Ethiopians (were competing and winning in international distance running)
Most people couldn’t spend >4-6 weeks at altitude
Why altitude train?
Erogenic benefit of acclimatization
Hypoxic exercise > normoxic exercise
Variety (added stressor to boost ceiling in elite athletes)
Correlation and causation relationship
Correlation DOES NOT equal causation
VO2 max exercise at altitude
VO2 max remains reduced at high altitude, as well as reduced max cardiac output (reduced max heart rate and stroke volume)
Live high-train high
Acclimatization and hypoxic exercise
Live high - train low
Acclimatization
