altitude medicine shawn dowling sept 2008 shawn dowling sept 2008
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ALTITUDE MEDICINEALTITUDE MEDICINE
Shawn DowlingShawn DowlingSept 2008Sept 2008
Shawn DowlingShawn DowlingSept 2008Sept 2008
ObjectivesObjectives
Discuss basic Discuss basic altitude physiologyaltitude physiology
How to recognize How to recognize altitude disordersaltitude disorders
Management Management PreventionPrevention
Discuss basic Discuss basic altitude physiologyaltitude physiology
How to recognize How to recognize altitude disordersaltitude disorders
Management Management PreventionPrevention
Will not discuss:Will not discuss: Subacute/Chronic Subacute/Chronic
Altitude IllnessesAltitude Illnesses Cold-related Cold-related
illnessesillnesses Altitude-exacerbated Altitude-exacerbated
illnessesillnesses
Will not discuss:Will not discuss: Subacute/Chronic Subacute/Chronic
Altitude IllnessesAltitude Illnesses Cold-related Cold-related
illnessesillnesses Altitude-exacerbated Altitude-exacerbated
illnessesillnesses
High Altitude IllnessesHigh Altitude Illnesses
DefinitionDefinition Cerebral (AMS Cerebral (AMS HACE spectrum) and HACE spectrum) and
pulmonary (HAPE) syndromes which pulmonary (HAPE) syndromes which develop in un- or underacclimatized develop in un- or underacclimatized persons after ascent to high altitudepersons after ascent to high altitude
Are often PREVENTABLE and can usually Are often PREVENTABLE and can usually be managed if S/Sx are recognized early be managed if S/Sx are recognized early and Tx are implemented in a timely fashionand Tx are implemented in a timely fashion
DefinitionDefinition Cerebral (AMS Cerebral (AMS HACE spectrum) and HACE spectrum) and
pulmonary (HAPE) syndromes which pulmonary (HAPE) syndromes which develop in un- or underacclimatized develop in un- or underacclimatized persons after ascent to high altitudepersons after ascent to high altitude
Are often PREVENTABLE and can usually Are often PREVENTABLE and can usually be managed if S/Sx are recognized early be managed if S/Sx are recognized early and Tx are implemented in a timely fashionand Tx are implemented in a timely fashion
Moderate Altitude Moderate Altitude
2400-3000m (8,000-10,000ft)- i.e. Lake 2400-3000m (8,000-10,000ft)- i.e. Lake Louise/Sunshine Valley Louise/Sunshine Valley
Minor impairment of arterial oxygen transport Minor impairment of arterial oxygen transport (S(SaaOO22>>90%), P90%), PAAOO22>60 torr>60 torr Usually not clinically important unless significant Usually not clinically important unless significant
underlying medical disorderunderlying medical disorder AMS common with rapid ascent above 2500 AMS common with rapid ascent above 2500
metersmeters
2400-3000m (8,000-10,000ft)- i.e. Lake 2400-3000m (8,000-10,000ft)- i.e. Lake Louise/Sunshine Valley Louise/Sunshine Valley
Minor impairment of arterial oxygen transport Minor impairment of arterial oxygen transport (S(SaaOO22>>90%), P90%), PAAOO22>60 torr>60 torr Usually not clinically important unless significant Usually not clinically important unless significant
underlying medical disorderunderlying medical disorder AMS common with rapid ascent above 2500 AMS common with rapid ascent above 2500
metersmeters
High AltitudeHigh Altitude
3000-5500m (10,000-18,000ft) 3000-5500m (10,000-18,000ft) i.e. Mount Columbia (12, 365ft – highest i.e. Mount Columbia (12, 365ft – highest
point in Alberta)point in Alberta)Maximum arterial saturation < 90%; Maximum arterial saturation < 90%;
PPAAOO22<60 torr<60 torrMost common range for serious altitude Most common range for serious altitude
illnessillness
3000-5500m (10,000-18,000ft) 3000-5500m (10,000-18,000ft) i.e. Mount Columbia (12, 365ft – highest i.e. Mount Columbia (12, 365ft – highest
point in Alberta)point in Alberta)Maximum arterial saturation < 90%; Maximum arterial saturation < 90%;
PPAAOO22<60 torr<60 torrMost common range for serious altitude Most common range for serious altitude
illnessillness
Extreme AltitudeExtreme Altitude
5500-8850m (18,000-29,035ft) from Mount 5500-8850m (18,000-29,035ft) from Mount Logan (19550ft) to Mount Everest (29035ft)Logan (19550ft) to Mount Everest (29035ft)
Marked hypoxemia and hypocapniaMarked hypoxemia and hypocapnia Maximum arterial saturation 50-75%; PMaximum arterial saturation 50-75%; PAAOO22
28-40 torr28-40 torr Deterioration eventually outstrips Deterioration eventually outstrips
acclimatization, complete acclimatization not acclimatization, complete acclimatization not possiblepossible
5500-8850m (18,000-29,035ft) from Mount 5500-8850m (18,000-29,035ft) from Mount Logan (19550ft) to Mount Everest (29035ft)Logan (19550ft) to Mount Everest (29035ft)
Marked hypoxemia and hypocapniaMarked hypoxemia and hypocapnia Maximum arterial saturation 50-75%; PMaximum arterial saturation 50-75%; PAAOO22
28-40 torr28-40 torr Deterioration eventually outstrips Deterioration eventually outstrips
acclimatization, complete acclimatization not acclimatization, complete acclimatization not possiblepossible
EpidemiologyEpidemiology
Study Group # at Risk per Year
Sleeping Altitude
% AMS(# affected)
% HAPEor HACE
Western USAVisitors
40 Million 2400-2800 meters
15 (6 million) .01(4000?)
Mt. EverestTrekkers
6,000 3000-5200 meters
35 (2100) 1.0 (60?)
Mt. McKinleyClimbers
1,200 3000-5300 meters
30 (300) 2-3 (25-35)
Mt. RainierClimbers
9,000 3000 meters
67 (6000) ?
Effects of Acute Exposure to Low Atmospheric Pressures on Alveolar Gas Concentrations and Arterial Oxygen Saturation *
Arterial ArterialBarometric Po2 in Pco2 in Po2 in Oxygen Pco2 in Po2 in Oxygen Pressure Air Air Alveoli Saturation Alveoli Alveoli Saturation
Altitude (ft) (mm Hg) (mm Hg) (mm Hg) (mm Hg) (%) (mm Hg) (mm Hg) (%)
Breathing Air Breathing Pure Oxygen
0
10,000
20,000
30,000
40,000
50,000
760
523
349
226
141
87
159
110
73
47
29
18
40 (40)
36 (23)
24 (10)
24 (7)
104 (104)
67 (77)
40 (53)
18 (30)
97 (97)
90 (92)
73 (85)
24 (38)
40
40
40
40
36
24
673
436
262
139
58
16
100
100
100
99
84
15
* Numbers in parentheses are acclimatized values.. Holy Crap!!
More About Gas!More About Gas!More About Gas!More About Gas!
At sea level we are At sea level we are underneath an ocean underneath an ocean of airof air
% O% O22 is always 21% is always 21% But as you climb But as you climb
higher, the Phigher, the Patmatm drops drops and thus your POand thus your PO22 dropsdrops
At sea level we are At sea level we are underneath an ocean underneath an ocean of airof air
% O% O22 is always 21% is always 21% But as you climb But as you climb
higher, the Phigher, the Patmatm drops drops and thus your POand thus your PO22 dropsdrops
PiPiO2O2 = 0.21(P = 0.21(PBB-47)-47) PPBB=barometric pressure=barometric pressure
At sea levelAt sea level PiPiO2O2= 160 mm Hg = 160 mm Hg
At 2500mAt 2500m PiPiO2O2 = 119 mm Hg = 119 mm Hg
Aconquilcha,Chile 5340mAconquilcha,Chile 5340m PiPiO2O2 = 82 mm Hg = 82 mm Hg
On top of Everest (8848m)On top of Everest (8848m) PiPiO2O2 = 43 mm Hg = 43 mm Hg
PiPiO2O2 = 0.21(P = 0.21(PBB-47)-47) PPBB=barometric pressure=barometric pressure
At sea levelAt sea level PiPiO2O2= 160 mm Hg = 160 mm Hg
At 2500mAt 2500m PiPiO2O2 = 119 mm Hg = 119 mm Hg
Aconquilcha,Chile 5340mAconquilcha,Chile 5340m PiPiO2O2 = 82 mm Hg = 82 mm Hg
On top of Everest (8848m)On top of Everest (8848m) PiPiO2O2 = 43 mm Hg = 43 mm Hg
Thanks Rigby
Breathing pure Oxygen
Breathing air
Altitude (thousands of feet)
Art
eria
l oxy
gen
satu
ratio
n (p
er c
ent)
Figure 43-1
Breathing pure Oxygen
Breathing air
Altitude (thousands of feet)
Art
eria
l oxy
gen
satu
ratio
n (p
er c
ent)
Figure 43-1
AcclimatizationAcclimatization
Complex adaptation by essentially Complex adaptation by essentially every system to minimize hypoxia and every system to minimize hypoxia and maintain cellular functions despite maintain cellular functions despite decreased PiO2decreased PiO2
Given sufficient time most people can Given sufficient time most people can acclimatize to 5500m/18150ft, beyond acclimatize to 5500m/18150ft, beyond that progressive deterioration occursthat progressive deterioration occurs
Complex adaptation by essentially Complex adaptation by essentially every system to minimize hypoxia and every system to minimize hypoxia and maintain cellular functions despite maintain cellular functions despite decreased PiO2decreased PiO2
Given sufficient time most people can Given sufficient time most people can acclimatize to 5500m/18150ft, beyond acclimatize to 5500m/18150ft, beyond that progressive deterioration occursthat progressive deterioration occurs
So WHAT IS the problem with Altitude?So WHAT IS the problem with Altitude?
HYPOXIAHYPOXIA
WHAT ARE THE PHYSIOLOGIC WHAT ARE THE PHYSIOLOGIC COMPENSATIONS?COMPENSATIONS? Essentially all the physiologic changes are Essentially all the physiologic changes are
attempting to improve arterial and therefore attempting to improve arterial and therefore cellular oxygenationcellular oxygenation
HYPOXIAHYPOXIA
WHAT ARE THE PHYSIOLOGIC WHAT ARE THE PHYSIOLOGIC COMPENSATIONS?COMPENSATIONS? Essentially all the physiologic changes are Essentially all the physiologic changes are
attempting to improve arterial and therefore attempting to improve arterial and therefore cellular oxygenationcellular oxygenation
Pulmonary Response to PiO2Pulmonary Response to PiO2
Hypoxemic Ventilatory ResponseHypoxemic Ventilatory Response Begins w/i minutes of ascent above 1500mBegins w/i minutes of ascent above 1500m
Hypoxia - pulmonary vasoconstrictorHypoxia - pulmonary vasoconstrictor Allows for selective shunting away from areas of Allows for selective shunting away from areas of
hypoxia w/preferential areas of hypoxia w/preferential areas of vasodilationvasodilation
pulmonary vascular resistancepulmonary vascular resistance pulmonary artery pressure (cold and exercise PAP pulmonary artery pressure (cold and exercise PAP
while descent, O2 and certain Rx PAP)while descent, O2 and certain Rx PAP)
diffusion capacity and lung volumediffusion capacity and lung volume
Hypoxemic Ventilatory ResponseHypoxemic Ventilatory Response Begins w/i minutes of ascent above 1500mBegins w/i minutes of ascent above 1500m
Hypoxia - pulmonary vasoconstrictorHypoxia - pulmonary vasoconstrictor Allows for selective shunting away from areas of Allows for selective shunting away from areas of
hypoxia w/preferential areas of hypoxia w/preferential areas of vasodilationvasodilation
pulmonary vascular resistancepulmonary vascular resistance pulmonary artery pressure (cold and exercise PAP pulmonary artery pressure (cold and exercise PAP
while descent, O2 and certain Rx PAP)while descent, O2 and certain Rx PAP)
diffusion capacity and lung volumediffusion capacity and lung volume
Hypoxemic Ventilatory ResponseHypoxemic Ventilatory Response
Ventilation increases proportionally to the Ventilation increases proportionally to the degree of hypoxia detected at the degree of hypoxia detected at the chemoreceptors chemoreceptors Lower PaCO2 = higher PLower PaCO2 = higher PAAO2O2 remember … Premember … PAAO2 = PiO2 - PaCO2/RO2 = PiO2 - PaCO2/R As PCO2 drops a respiratory alkalosis occurs (what As PCO2 drops a respiratory alkalosis occurs (what
does this do to the medulla?) does this do to the medulla?) kidneys compensate by excreting bicarb(over 6-8/7)kidneys compensate by excreting bicarb(over 6-8/7)
Vigourous HVR helps acclimatization Vigourous HVR helps acclimatization whereas a poor HVR may lead to altitude-whereas a poor HVR may lead to altitude-illnessillness
Ventilation increases proportionally to the Ventilation increases proportionally to the degree of hypoxia detected at the degree of hypoxia detected at the chemoreceptors chemoreceptors Lower PaCO2 = higher PLower PaCO2 = higher PAAO2O2 remember … Premember … PAAO2 = PiO2 - PaCO2/RO2 = PiO2 - PaCO2/R As PCO2 drops a respiratory alkalosis occurs (what As PCO2 drops a respiratory alkalosis occurs (what
does this do to the medulla?) does this do to the medulla?) kidneys compensate by excreting bicarb(over 6-8/7)kidneys compensate by excreting bicarb(over 6-8/7)
Vigourous HVR helps acclimatization Vigourous HVR helps acclimatization whereas a poor HVR may lead to altitude-whereas a poor HVR may lead to altitude-illnessillness
Cerebral CirculationCerebral Circulation
Hypoxemia = increases cerebral blood flowHypoxemia = increases cerebral blood flow Hypocapnea = decreases cerebral blood flowHypocapnea = decreases cerebral blood flow Net result is an increase (modest) in cerebral Net result is an increase (modest) in cerebral
blood flow at PaO2 <60 (altitude >12,500 blood flow at PaO2 <60 (altitude >12,500 feet)feet) Contributes to pathophysiology of AMS and HACEContributes to pathophysiology of AMS and HACE
Hypoxemia = increases cerebral blood flowHypoxemia = increases cerebral blood flow Hypocapnea = decreases cerebral blood flowHypocapnea = decreases cerebral blood flow Net result is an increase (modest) in cerebral Net result is an increase (modest) in cerebral
blood flow at PaO2 <60 (altitude >12,500 blood flow at PaO2 <60 (altitude >12,500 feet)feet) Contributes to pathophysiology of AMS and HACEContributes to pathophysiology of AMS and HACE
CVS ResponseCVS Response
Catecholamine release: Epi and NorepiCatecholamine release: Epi and Norepi Sympathetic Tone: Sympathetic Tone: CO= HR x SVCO= HR x SV BP= CO x PVRBP= CO x PVR
This responses allows for better tissue This responses allows for better tissue perfusion (but, also decreases exercise perfusion (but, also decreases exercise performance) – over time epi/norepi performance) – over time epi/norepi levels drop and CO/HR/PVR return to levels drop and CO/HR/PVR return to normalnormal
Catecholamine release: Epi and NorepiCatecholamine release: Epi and Norepi Sympathetic Tone: Sympathetic Tone: CO= HR x SVCO= HR x SV BP= CO x PVRBP= CO x PVR
This responses allows for better tissue This responses allows for better tissue perfusion (but, also decreases exercise perfusion (but, also decreases exercise performance) – over time epi/norepi performance) – over time epi/norepi levels drop and CO/HR/PVR return to levels drop and CO/HR/PVR return to normalnormal
HematologicHematologic
Blood volume increases immediately: Blood volume increases immediately: Primarily due to hemo[ ] from diuresisPrimarily due to hemo[ ] from diuresis
Hypoxia stimulates renal EPO productionHypoxia stimulates renal EPO production Takes weeks for it to effect Red Cell MassTakes weeks for it to effect Red Cell Mass
Oxygen Dissociation curve – combination of Oxygen Dissociation curve – combination of inc 2,3 DPG (rightward shift) and alkalosis inc 2,3 DPG (rightward shift) and alkalosis (leftward shift) leads to minimal net effect(leftward shift) leads to minimal net effect
Blood volume increases immediately: Blood volume increases immediately: Primarily due to hemo[ ] from diuresisPrimarily due to hemo[ ] from diuresis
Hypoxia stimulates renal EPO productionHypoxia stimulates renal EPO production Takes weeks for it to effect Red Cell MassTakes weeks for it to effect Red Cell Mass
Oxygen Dissociation curve – combination of Oxygen Dissociation curve – combination of inc 2,3 DPG (rightward shift) and alkalosis inc 2,3 DPG (rightward shift) and alkalosis (leftward shift) leads to minimal net effect(leftward shift) leads to minimal net effect
AcclimatizationAcclimatizationAcute Chronic
Ventilation, O2 Diffusion capacity
Ventilation
Heart Rate Red Cell Mass (from EPO release)
Cerebral Blood Flow Metabolism (tissue changes)
Diuresis, Hct (from volume contraction)
High Altitude IllnessHigh Altitude Illness
What are RF?What are RF?What are positive predictors?What are positive predictors?What worsens altitude illnesses?What worsens altitude illnesses?What does not effect altitude illnesses?What does not effect altitude illnesses?
What are RF?What are RF?What are positive predictors?What are positive predictors?What worsens altitude illnesses?What worsens altitude illnesses?What does not effect altitude illnesses?What does not effect altitude illnesses?
High Altitude IllnessHigh Altitude Illness What are Risk Factors?What are Risk Factors?
Rate of ascent/Starting altitudeRate of ascent/Starting altitude Altitude reachedAltitude reached Sleeping at altitudeSleeping at altitude Individual physiology/Genetics?Individual physiology/Genetics?
What are Positive Predictors?What are Positive Predictors? Hx of prior ascent w/o SxHx of prior ascent w/o Sx Elderly (less AMS/HACE), Women (less HAPE)Elderly (less AMS/HACE), Women (less HAPE)
What are Risk Factors?What are Risk Factors? Rate of ascent/Starting altitudeRate of ascent/Starting altitude Altitude reachedAltitude reached Sleeping at altitudeSleeping at altitude Individual physiology/Genetics?Individual physiology/Genetics?
What are Positive Predictors?What are Positive Predictors? Hx of prior ascent w/o SxHx of prior ascent w/o Sx Elderly (less AMS/HACE), Women (less HAPE)Elderly (less AMS/HACE), Women (less HAPE)
What worsens altitude-illnesses?What worsens altitude-illnesses? Respiratory depressants/AlcoholRespiratory depressants/Alcohol Pre-existing Medical Illnesses (very few): Pre-existing Medical Illnesses (very few):
i.e. Pulm HTN, certain CHD, i.e. Pulm HTN, certain CHD, ExertionExertion HypothermiaHypothermia
What does not affect risk?What does not affect risk? Physical fitnessPhysical fitness CAD/HTNCAD/HTN Mild COPDMild COPD PregnancyPregnancy DMDM
What worsens altitude-illnesses?What worsens altitude-illnesses? Respiratory depressants/AlcoholRespiratory depressants/Alcohol Pre-existing Medical Illnesses (very few): Pre-existing Medical Illnesses (very few):
i.e. Pulm HTN, certain CHD, i.e. Pulm HTN, certain CHD, ExertionExertion HypothermiaHypothermia
What does not affect risk?What does not affect risk? Physical fitnessPhysical fitness CAD/HTNCAD/HTN Mild COPDMild COPD PregnancyPregnancy DMDM
Case #1Case #1
You’ve decided to hike to Machu Picchu You’ve decided to hike to Machu Picchu and have flown directly from Lima (sea and have flown directly from Lima (sea level) into Cuzco (3515m/11,600ft). level) into Cuzco (3515m/11,600ft). That day you start the hike up and That day you start the hike up and notice you develop a nasty HA, and notice you develop a nasty HA, and nausea. nausea.
You’ve decided to hike to Machu Picchu You’ve decided to hike to Machu Picchu and have flown directly from Lima (sea and have flown directly from Lima (sea level) into Cuzco (3515m/11,600ft). level) into Cuzco (3515m/11,600ft). That day you start the hike up and That day you start the hike up and notice you develop a nasty HA, and notice you develop a nasty HA, and nausea. nausea.
What’s your Diagnosis?What’s your Diagnosis?
1.1. Acute Mountain SyndromeAcute Mountain Syndrome2.2. High Altitude Cerebral EdemaHigh Altitude Cerebral Edema3.3. High Altitude Pulmonary EdemaHigh Altitude Pulmonary Edema4.4. Other Medical IllnessOther Medical Illness
1.1. Acute Mountain SyndromeAcute Mountain Syndrome2.2. High Altitude Cerebral EdemaHigh Altitude Cerebral Edema3.3. High Altitude Pulmonary EdemaHigh Altitude Pulmonary Edema4.4. Other Medical IllnessOther Medical Illness
PearlPearl
Any illness at altitude is altitude Any illness at altitude is altitude illness until proven otherwise.illness until proven otherwise.
Any illness at altitude is altitude Any illness at altitude is altitude illness until proven otherwise.illness until proven otherwise.
Lake Louise Criteria for AMSLake Louise Criteria for AMS
Presence of a Presence of a HAHA in an un-acclimatized in an un-acclimatized person who has arrived at an altitude person who has arrived at an altitude >2000m (usu >2500m)>2000m (usu >2500m) + 1 of + 1 of GI (anorexia, N,V)GI (anorexia, N,V) FatigueFatigue InsomniaInsomnia Dizziness/lightheadedDizziness/lightheaded
Sx usually begin between 6-10 hrs but Sx usually begin between 6-10 hrs but may be as early as 1 hourmay be as early as 1 hour
Presence of a Presence of a HAHA in an un-acclimatized in an un-acclimatized person who has arrived at an altitude person who has arrived at an altitude >2000m (usu >2500m)>2000m (usu >2500m) + 1 of + 1 of GI (anorexia, N,V)GI (anorexia, N,V) FatigueFatigue InsomniaInsomnia Dizziness/lightheadedDizziness/lightheaded
Sx usually begin between 6-10 hrs but Sx usually begin between 6-10 hrs but may be as early as 1 hourmay be as early as 1 hour
PearlsPearls
Never ascend with symptoms Never ascend with symptoms of AMSof AMS
Never leave someone with AMS Never leave someone with AMS alonealone
-Letting someone with AMS hike alone is the equilavent -Letting someone with AMS hike alone is the equilavent of going for a hike in the Kananaski’s alone the of going for a hike in the Kananaski’s alone the
night of night of Party at the retreat.Party at the retreat.
Never ascend with symptoms Never ascend with symptoms of AMSof AMS
Never leave someone with AMS Never leave someone with AMS alonealone
-Letting someone with AMS hike alone is the equilavent -Letting someone with AMS hike alone is the equilavent of going for a hike in the Kananaski’s alone the of going for a hike in the Kananaski’s alone the
night of night of Party at the retreat.Party at the retreat.
AMS PathophysiologyAMS Pathophysiology
Altitude Hypoxia
CBF Blood-brain barrierpermeability Na+K+ATPase
ICFCBV Vasogenic edema
Mild brain swelling
CSF volumebuffering
Adequate Inadequate
AMSNo AMS
Treatment?Treatment?
Any or CombinationAny or Combination Stop, Rest and Continue once Sx resolveStop, Rest and Continue once Sx resolve Acetazolimide (alone or in combination)Acetazolimide (alone or in combination) DexamethasoneDexamethasone O2, immediate descentO2, immediate descent NifedipineNifedipine + Symptomatic Treatment+ Symptomatic Treatment
Any or CombinationAny or Combination Stop, Rest and Continue once Sx resolveStop, Rest and Continue once Sx resolve Acetazolimide (alone or in combination)Acetazolimide (alone or in combination) DexamethasoneDexamethasone O2, immediate descentO2, immediate descent NifedipineNifedipine + Symptomatic Treatment+ Symptomatic Treatment
Treatment?Treatment?
Any or CombinationAny or Combination1.1. Stop, Rest and Continue once Sx resolveStop, Rest and Continue once Sx resolve2.2. Acetazolamide (alone or in combination)Acetazolamide (alone or in combination)3.3. Dexamethasone (if severe)Dexamethasone (if severe)4.4. O2, rapid/immediate descentO2, rapid/immediate descent5.5. NifedipineNifedipine6.6. + Symptomatic Treatment+ Symptomatic Treatment
Any or CombinationAny or Combination1.1. Stop, Rest and Continue once Sx resolveStop, Rest and Continue once Sx resolve2.2. Acetazolamide (alone or in combination)Acetazolamide (alone or in combination)3.3. Dexamethasone (if severe)Dexamethasone (if severe)4.4. O2, rapid/immediate descentO2, rapid/immediate descent5.5. NifedipineNifedipine6.6. + Symptomatic Treatment+ Symptomatic Treatment
Treatment Options of AMSTreatment Options of AMS
Stop and rest at altitude that Sx 1Stop and rest at altitude that Sx 1stst develop +/- Acetazolamide -> May develop +/- Acetazolamide -> May proceed once Sx abateproceed once Sx abate
+ Tx Sx+ Tx Sx Nausea: Prochlorperazine 10mg po/im Nausea: Prochlorperazine 10mg po/im
(stimulates HVR)(stimulates HVR) HA: Tylenol/NSAID’sHA: Tylenol/NSAID’s Insomnia: acetazolamide 125mg PO QHSInsomnia: acetazolamide 125mg PO QHS
Stop and rest at altitude that Sx 1Stop and rest at altitude that Sx 1stst develop +/- Acetazolamide -> May develop +/- Acetazolamide -> May proceed once Sx abateproceed once Sx abate
+ Tx Sx+ Tx Sx Nausea: Prochlorperazine 10mg po/im Nausea: Prochlorperazine 10mg po/im
(stimulates HVR)(stimulates HVR) HA: Tylenol/NSAID’sHA: Tylenol/NSAID’s Insomnia: acetazolamide 125mg PO QHSInsomnia: acetazolamide 125mg PO QHS
Treatment of AMSTreatment of AMSTreatment of AMSTreatment of AMS Mild SymptomsMild Symptoms Does not need descent Does not need descent
if mild Sx and constant if mild Sx and constant supervisionsupervision
Stop ascent until betterStop ascent until better Acetazolamide (250 Acetazolamide (250
BID)BID) Tylenol/ASA for SxTylenol/ASA for Sx Anti-emetic PRNAnti-emetic PRN Consider OConsider O22 at 1-2 at 1-2
LPMLPM
Mild SymptomsMild Symptoms Does not need descent Does not need descent
if mild Sx and constant if mild Sx and constant supervisionsupervision
Stop ascent until betterStop ascent until better Acetazolamide (250 Acetazolamide (250
BID)BID) Tylenol/ASA for SxTylenol/ASA for Sx Anti-emetic PRNAnti-emetic PRN Consider OConsider O22 at 1-2 at 1-2
LPMLPM
Moderate or Moderate or Unresolving AMSUnresolving AMS
Descent 500 mDescent 500 m Consider:Consider:
OO22 at 1-2 LPM at 1-2 LPM Hyperbaric therapyHyperbaric therapy Dexamethasone Dexamethasone
4mg PO q6h until 4mg PO q6h until able to descendable to descend
May ascend after May ascend after symptoms resolvesymptoms resolve
Moderate or Moderate or Unresolving AMSUnresolving AMS
Descent 500 mDescent 500 m Consider:Consider:
OO22 at 1-2 LPM at 1-2 LPM Hyperbaric therapyHyperbaric therapy Dexamethasone Dexamethasone
4mg PO q6h until 4mg PO q6h until able to descendable to descend
May ascend after May ascend after symptoms resolvesymptoms resolve
If If someone is someone is getting worse, go down getting worse, go down at onceat once
If Sx of HACE are present DESCEND If Sx of HACE are present DESCEND IMMEDIATELYIMMEDIATELY
If If someone is someone is getting worse, go down getting worse, go down at onceat once
If Sx of HACE are present DESCEND If Sx of HACE are present DESCEND IMMEDIATELYIMMEDIATELY
Quick word about AcetazolamideQuick word about Acetazolamide
What’s the MOA?What’s the MOA?What’s the MOA?What’s the MOA?
Quick word about AcetazolamideQuick word about Acetazolamide
What’s the MOA?What’s the MOA? Causes a bicarb diuresis by inhibiting Causes a bicarb diuresis by inhibiting
Carbonic Anhydrase at the kidney Carbonic Anhydrase at the kidney This leads to a metabolic acidosis which This leads to a metabolic acidosis which
counteracts the alkalosis from counteracts the alkalosis from hyperventilationhyperventilation
Allowing an individual to have a better HVR Allowing an individual to have a better HVR and therefore acclimatize betterand therefore acclimatize better
Which allergy must you ask about?Which allergy must you ask about?
What’s the MOA?What’s the MOA? Causes a bicarb diuresis by inhibiting Causes a bicarb diuresis by inhibiting
Carbonic Anhydrase at the kidney Carbonic Anhydrase at the kidney This leads to a metabolic acidosis which This leads to a metabolic acidosis which
counteracts the alkalosis from counteracts the alkalosis from hyperventilationhyperventilation
Allowing an individual to have a better HVR Allowing an individual to have a better HVR and therefore acclimatize betterand therefore acclimatize better
Which allergy must you ask about?Which allergy must you ask about?
Pt adamant that the HA is from Pt adamant that the HA is from dehydrationdehydration Can give Dx/Tx of 1L of fluid + advil or Can give Dx/Tx of 1L of fluid + advil or
tylenol – if this completely resolves HA, not tylenol – if this completely resolves HA, not likely to be AMSlikely to be AMS
Pt adamant that the HA is from Pt adamant that the HA is from dehydrationdehydration Can give Dx/Tx of 1L of fluid + advil or Can give Dx/Tx of 1L of fluid + advil or
tylenol – if this completely resolves HA, not tylenol – if this completely resolves HA, not likely to be AMSlikely to be AMS
Case #2Case #2
Despite the advice to refrain from Despite the advice to refrain from continuing to ascend, you continue up continuing to ascend, you continue up and begin to feel disoriented and and begin to feel disoriented and noticed you seem to be walking with a noticed you seem to be walking with a drunken’ sort of gaitdrunken’ sort of gait
Dx?Dx?
Despite the advice to refrain from Despite the advice to refrain from continuing to ascend, you continue up continuing to ascend, you continue up and begin to feel disoriented and and begin to feel disoriented and noticed you seem to be walking with a noticed you seem to be walking with a drunken’ sort of gaitdrunken’ sort of gait
Dx?Dx?
High Altitude Cerebral Edema (HACE)High Altitude Cerebral Edema (HACE)
Least common but most lethal altitude illnessLeast common but most lethal altitude illness Usually occurs above 3600m(12,000 feet)Usually occurs above 3600m(12,000 feet) Symptoms usually develop over 1-3 daysSymptoms usually develop over 1-3 days
reported range 12 hours to 9 daysreported range 12 hours to 9 days Usually also have symptoms of AMS / HAPEUsually also have symptoms of AMS / HAPE
Least common but most lethal altitude illnessLeast common but most lethal altitude illness Usually occurs above 3600m(12,000 feet)Usually occurs above 3600m(12,000 feet) Symptoms usually develop over 1-3 daysSymptoms usually develop over 1-3 days
reported range 12 hours to 9 daysreported range 12 hours to 9 days Usually also have symptoms of AMS / HAPEUsually also have symptoms of AMS / HAPE
HACE DxHACE Dx
• Presence of a change in mental status and/or Presence of a change in mental status and/or ataxia in a person with AMSataxia in a person with AMS
OrOr
• Presence of both mental status changes and Presence of both mental status changes and ataxia in a person without AMSataxia in a person without AMS
• Presence of a change in mental status and/or Presence of a change in mental status and/or ataxia in a person with AMSataxia in a person with AMS
OrOr
• Presence of both mental status changes and Presence of both mental status changes and ataxia in a person without AMSataxia in a person without AMS
HACE S & SxHACE S & Sx
Think of the effect of hypoxia and ICPThink of the effect of hypoxia and ICP Global encephalopathyGlobal encephalopathy AtaxiaAtaxia Altered mentationAltered mentation SeizuresSeizures Occasional CN palsies (secondary to increased Occasional CN palsies (secondary to increased
ICP)ICP) PapilledemaPapilledema Retinal hemorrhageRetinal hemorrhage ComaComa Death is due to brain herniationDeath is due to brain herniation
Think of the effect of hypoxia and ICPThink of the effect of hypoxia and ICP Global encephalopathyGlobal encephalopathy AtaxiaAtaxia Altered mentationAltered mentation SeizuresSeizures Occasional CN palsies (secondary to increased Occasional CN palsies (secondary to increased
ICP)ICP) PapilledemaPapilledema Retinal hemorrhageRetinal hemorrhage ComaComa Death is due to brain herniationDeath is due to brain herniation
HACE PathophysHACE Pathophys
Altitude Hypoxia
CBF Blood-brain barrierpermeabilityNa+K+ATPase
ICFCBV Vasogenic edema
Mild brain swelling
CSF volumebuffering
Adequate Inadequate
AMSNo AMS
Severe hypoxemia
Fluid retention
HACE
Tx of HACETx of HACE
DESCENT (until pt improves)DESCENT (until pt improves)O2O2DXM 8mg PO/IM/IV initially then 4mg DXM 8mg PO/IM/IV initially then 4mg
Q6HQ6H+/- acetazolamide 250mg PO BID +/- acetazolamide 250mg PO BID What if the weather does not allow for a What if the weather does not allow for a
safe descent and you happen to be part safe descent and you happen to be part of a well equipped expedition?of a well equipped expedition?
DESCENT (until pt improves)DESCENT (until pt improves)O2O2DXM 8mg PO/IM/IV initially then 4mg DXM 8mg PO/IM/IV initially then 4mg
Q6HQ6H+/- acetazolamide 250mg PO BID +/- acetazolamide 250mg PO BID What if the weather does not allow for a What if the weather does not allow for a
safe descent and you happen to be part safe descent and you happen to be part of a well equipped expedition?of a well equipped expedition?
Gamow BagGamow Bag
Hyperbaric ChambersHyperbaric Chambers Lightweight (14.9 lb)Lightweight (14.9 lb) Manually pressurizedManually pressurized Generate 103mm Hg (2 psi) above ambient Generate 103mm Hg (2 psi) above ambient
pressurepressure Simulates descent of 4000-5000 feet at moderate Simulates descent of 4000-5000 feet at moderate
altitudesaltitudes Simulates descent of 9000 feet at top of Mt. EverestSimulates descent of 9000 feet at top of Mt. Everest
After short course of treatment patient often able After short course of treatment patient often able to descend on their own to descend on their own
This is primarily a temporizing measure - This is primarily a temporizing measure - Not an Not an alternate to descendingalternate to descending
Hyperbaric ChambersHyperbaric Chambers Lightweight (14.9 lb)Lightweight (14.9 lb) Manually pressurizedManually pressurized Generate 103mm Hg (2 psi) above ambient Generate 103mm Hg (2 psi) above ambient
pressurepressure Simulates descent of 4000-5000 feet at moderate Simulates descent of 4000-5000 feet at moderate
altitudesaltitudes Simulates descent of 9000 feet at top of Mt. EverestSimulates descent of 9000 feet at top of Mt. Everest
After short course of treatment patient often able After short course of treatment patient often able to descend on their own to descend on their own
This is primarily a temporizing measure - This is primarily a temporizing measure - Not an Not an alternate to descendingalternate to descending
Case #3Case #3
During a visit to Nepal, Mr. Hackalung, During a visit to Nepal, Mr. Hackalung, decides to organize a hike to Everest decides to organize a hike to Everest Base camp (5430m/18000ft).Base camp (5430m/18000ft).
On day 3 you notice you notice he On day 3 you notice you notice he develops a dry cough and during the develops a dry cough and during the lunch break he’s quite SOBlunch break he’s quite SOB
You’re the group docYou’re the group doc What’s the most likely Dx?What’s the most likely Dx?
During a visit to Nepal, Mr. Hackalung, During a visit to Nepal, Mr. Hackalung, decides to organize a hike to Everest decides to organize a hike to Everest Base camp (5430m/18000ft).Base camp (5430m/18000ft).
On day 3 you notice you notice he On day 3 you notice you notice he develops a dry cough and during the develops a dry cough and during the lunch break he’s quite SOBlunch break he’s quite SOB
You’re the group docYou’re the group doc What’s the most likely Dx?What’s the most likely Dx?
Case #3Case #3
Fortunately for you, Fortunately for you, you realize that you you realize that you develop X-ray vision develop X-ray vision at altitude – at altitude – convenient skill – convenient skill – except that you’re except that you’re on an all guy on an all guy expedition but…- expedition but…- here is what you here is what you see.see.
Fortunately for you, Fortunately for you, you realize that you you realize that you develop X-ray vision develop X-ray vision at altitude – at altitude – convenient skill – convenient skill – except that you’re except that you’re on an all guy on an all guy expedition but…- expedition but…- here is what you here is what you see.see.
Responsible for most high altitude deathsResponsible for most high altitude deathsRelatively Abrupt OnsetRelatively Abrupt OnsetUsually occurs on 2Usually occurs on 2ndnd night at altitude, Rare night at altitude, Rare
after 4 daysafter 4 daysWomen less susceptible than menWomen less susceptible than menPresence of HAPE increases likelihood of Presence of HAPE increases likelihood of
cerebral illnesses – by worsening hypoxemia: cerebral illnesses – by worsening hypoxemia: 50% have AMS & 14% have HACE50% have AMS & 14% have HACE
Responsible for most high altitude deathsResponsible for most high altitude deathsRelatively Abrupt OnsetRelatively Abrupt OnsetUsually occurs on 2Usually occurs on 2ndnd night at altitude, Rare night at altitude, Rare
after 4 daysafter 4 daysWomen less susceptible than menWomen less susceptible than menPresence of HAPE increases likelihood of Presence of HAPE increases likelihood of
cerebral illnesses – by worsening hypoxemia: cerebral illnesses – by worsening hypoxemia: 50% have AMS & 14% have HACE50% have AMS & 14% have HACE
High Altitude Pulmonary Edema (HAPE)
HAPE DiagnosisHAPE Diagnosis
At least two of the At least two of the following Sx:following Sx: Dyspnea at restDyspnea at rest CoughCough Weakness or Weakness or
decreased exercise decreased exercise performanceperformance
Chest tightness or Chest tightness or congestioncongestion
At least two of the At least two of the following Sx:following Sx: Dyspnea at restDyspnea at rest CoughCough Weakness or Weakness or
decreased exercise decreased exercise performanceperformance
Chest tightness or Chest tightness or congestioncongestion
At least two of the At least two of the following signs:following signs: Central cyanosisCentral cyanosis Audible crackles or Audible crackles or
wheezing in at least wheezing in at least one lung fieldone lung field
TachypneaTachypnea tachycardiatachycardia
At least two of the At least two of the following signs:following signs: Central cyanosisCentral cyanosis Audible crackles or Audible crackles or
wheezing in at least wheezing in at least one lung fieldone lung field
TachypneaTachypnea tachycardiatachycardia
HAPE PathophysHAPE PathophysPAO2
Platelet and FibrinMicroemboli
SympatheticDischarge
Hypoxic PulmonaryVasoconstriction
Central NervousSystem
VascularObstruction
Pulmonary Blood Volume
PulmonaryHypertension
Pulmonary VenousConstriction?
Local Overperfusion
Failure of Capillary Membrane
Permeability Leak
HAPE
Increase CWP & PAP
Doc, how do you want to treat him?Doc, how do you want to treat him? You have access to most drugs, but no You have access to most drugs, but no
monitoring equipment.monitoring equipment.
Doc, how do you want to treat him?Doc, how do you want to treat him? You have access to most drugs, but no You have access to most drugs, but no
monitoring equipment.monitoring equipment.
HAPE Tx – Increase PiO2HAPE Tx – Increase PiO2
Definitive TxDefinitive Tx Descent or Simulated Descent or Simulated
descent (if unable to descent (if unable to Descend)Descend)
Supplemental O2 Supplemental O2 (SaO2>90%)(SaO2>90%)
May be a role for observant May be a role for observant management (bed rest/O2) if mild management (bed rest/O2) if mild case and experienced and well-case and experienced and well-equipped physician presentequipped physician present
Definitive TxDefinitive Tx Descent or Simulated Descent or Simulated
descent (if unable to descent (if unable to Descend)Descend)
Supplemental O2 Supplemental O2 (SaO2>90%)(SaO2>90%)
May be a role for observant May be a role for observant management (bed rest/O2) if mild management (bed rest/O2) if mild case and experienced and well-case and experienced and well-equipped physician presentequipped physician present
Temporizing Temporizing MeasuresMeasures
Nifedipine 10mg PONifedipine 10mg PO Ventolin Ventolin DiamoxDiamox PEEPPEEP Viagra – currently Viagra – currently
being studiedbeing studied DXM – no benefitDXM – no benefit
Temporizing Temporizing MeasuresMeasures
Nifedipine 10mg PONifedipine 10mg PO Ventolin Ventolin DiamoxDiamox PEEPPEEP Viagra – currently Viagra – currently
being studiedbeing studied DXM – no benefitDXM – no benefit
PreventionPrevention
Rational Ascent (MOST IMPORTANT)Rational Ascent (MOST IMPORTANT) First night not higher than 2400m (8000 feet)First night not higher than 2400m (8000 feet) CLIMB HIGH, SLEEP LOW. CLIMB HIGH, SLEEP LOW. At altitudes above At altitudes above
3000 meters (10,000 feet), your sleeping elevation 3000 meters (10,000 feet), your sleeping elevation should not increase more than 300 meters (1000 should not increase more than 300 meters (1000 feet) per night,feet) per night, daytime increase of 600m. daytime increase of 600m.
EEvery 1000 meters (3very 1000 meters (33300 feet)00 feet) of sleeping of sleeping elevation gainelevation gain you should spend a second night. you should spend a second night.
Ginko bilobaGinko biloba Limited studies have been performed, but the Limited studies have been performed, but the
results look very promising for prophylaxis of AMS. results look very promising for prophylaxis of AMS. 120 mg po BID starting 5 days before ascent, and 120 mg po BID starting 5 days before ascent, and continuing at altitude.continuing at altitude.
Rational Ascent (MOST IMPORTANT)Rational Ascent (MOST IMPORTANT) First night not higher than 2400m (8000 feet)First night not higher than 2400m (8000 feet) CLIMB HIGH, SLEEP LOW. CLIMB HIGH, SLEEP LOW. At altitudes above At altitudes above
3000 meters (10,000 feet), your sleeping elevation 3000 meters (10,000 feet), your sleeping elevation should not increase more than 300 meters (1000 should not increase more than 300 meters (1000 feet) per night,feet) per night, daytime increase of 600m. daytime increase of 600m.
EEvery 1000 meters (3very 1000 meters (33300 feet)00 feet) of sleeping of sleeping elevation gainelevation gain you should spend a second night. you should spend a second night.
Ginko bilobaGinko biloba Limited studies have been performed, but the Limited studies have been performed, but the
results look very promising for prophylaxis of AMS. results look very promising for prophylaxis of AMS. 120 mg po BID starting 5 days before ascent, and 120 mg po BID starting 5 days before ascent, and continuing at altitude.continuing at altitude.
AcetazolamideAcetazolamide Not routinely indicated but use consider Not routinely indicated but use consider
if; if; Forced rapid ascent (1 day) to altitudes Forced rapid ascent (1 day) to altitudes
over 3000mover 3000m.. A rapid gain in sleeping elevation - for A rapid gain in sleeping elevation - for
example gaining 1000 m in one day. example gaining 1000 m in one day. A history of recurrent AMS.A history of recurrent AMS.
Dose: 125 mg BID, 24 hrs before ascent Dose: 125 mg BID, 24 hrs before ascent + 2-3/7 after ascent+ 2-3/7 after ascent
AcetazolamideAcetazolamide Not routinely indicated but use consider Not routinely indicated but use consider
if; if; Forced rapid ascent (1 day) to altitudes Forced rapid ascent (1 day) to altitudes
over 3000mover 3000m.. A rapid gain in sleeping elevation - for A rapid gain in sleeping elevation - for
example gaining 1000 m in one day. example gaining 1000 m in one day. A history of recurrent AMS.A history of recurrent AMS.
Dose: 125 mg BID, 24 hrs before ascent Dose: 125 mg BID, 24 hrs before ascent + 2-3/7 after ascent+ 2-3/7 after ascent
DXMDXM Indication – Hx of AMS, but diamox Indication – Hx of AMS, but diamox
probably preferredprobably preferred Dose: 2mg PO Q6H, start day before Dose: 2mg PO Q6H, start day before
ascent and continue until at max altitude x ascent and continue until at max altitude x 48 hrs48 hrs
Nifedipine ERNifedipine ER Indication – Hx of HAPEIndication – Hx of HAPE 20-30mg PO BID, start day before ascent 20-30mg PO BID, start day before ascent
and continue until at max altitude x 48 hrsand continue until at max altitude x 48 hrs
DXMDXM Indication – Hx of AMS, but diamox Indication – Hx of AMS, but diamox
probably preferredprobably preferred Dose: 2mg PO Q6H, start day before Dose: 2mg PO Q6H, start day before
ascent and continue until at max altitude x ascent and continue until at max altitude x 48 hrs48 hrs
Nifedipine ERNifedipine ER Indication – Hx of HAPEIndication – Hx of HAPE 20-30mg PO BID, start day before ascent 20-30mg PO BID, start day before ascent
and continue until at max altitude x 48 hrsand continue until at max altitude x 48 hrs
AvoidAvoid AlcoholAlcohol Sleeping Pills (diamox is the Rx of choice)Sleeping Pills (diamox is the Rx of choice) Narcotics (other than at moderate doses)Narcotics (other than at moderate doses)
Avoid overexertionAvoid overexertionStay warmStay warm
AvoidAvoid AlcoholAlcohol Sleeping Pills (diamox is the Rx of choice)Sleeping Pills (diamox is the Rx of choice) Narcotics (other than at moderate doses)Narcotics (other than at moderate doses)
Avoid overexertionAvoid overexertionStay warmStay warm
PearlsPearls
SUMMARY: DESCEND IF UNSURE SUMMARY: DESCEND IF UNSURE All other Tx are only temporizing for life-All other Tx are only temporizing for life-
threatening illnessesthreatening illnesses
SUMMARY: DESCEND IF UNSURE SUMMARY: DESCEND IF UNSURE All other Tx are only temporizing for life-All other Tx are only temporizing for life-
threatening illnessesthreatening illnesses
What Should make you think of Other diagnoses?What Should make you think of Other diagnoses?
Onset of Sx >3/7 after ascent to a given Onset of Sx >3/7 after ascent to a given altitudealtitude
Lack of a HALack of a HARapid response to rest/stopping ascentRapid response to rest/stopping ascentLack of response to descent, O2, or Lack of response to descent, O2, or
dexamethasone(unless HAPE)dexamethasone(unless HAPE)
Onset of Sx >3/7 after ascent to a given Onset of Sx >3/7 after ascent to a given altitudealtitude
Lack of a HALack of a HARapid response to rest/stopping ascentRapid response to rest/stopping ascentLack of response to descent, O2, or Lack of response to descent, O2, or
dexamethasone(unless HAPE)dexamethasone(unless HAPE)
SummarySummary
Early recognition is importantEarly recognition is importantAMSAMS
stop ascent, rest, +/- diamox, Tx Sxstop ascent, rest, +/- diamox, Tx SxHACEHACE
DESCENT +/- DXM, diamoxDESCENT +/- DXM, diamoxHAPEHAPE
DESCENT +/- nifedipine, ventolinDESCENT +/- nifedipine, ventolin
Early recognition is importantEarly recognition is importantAMSAMS
stop ascent, rest, +/- diamox, Tx Sxstop ascent, rest, +/- diamox, Tx SxHACEHACE
DESCENT +/- DXM, diamoxDESCENT +/- DXM, diamoxHAPEHAPE
DESCENT +/- nifedipine, ventolinDESCENT +/- nifedipine, ventolin
ReferencesReferences
High-Altitude Illness. Emerg M Clinics of N High-Altitude Illness. Emerg M Clinics of N America, 2004.America, 2004.
High-Altitude Illness. NEJM, July 2001.High-Altitude Illness. NEJM, July 2001. Rosen’s.Rosen’s. Guyton. Textbook of Medical PhysiologyGuyton. Textbook of Medical Physiology Aric’s Rounds 2003Aric’s Rounds 2003 Rigby’s NRC lectureRigby’s NRC lecture
High-Altitude Illness. Emerg M Clinics of N High-Altitude Illness. Emerg M Clinics of N America, 2004.America, 2004.
High-Altitude Illness. NEJM, July 2001.High-Altitude Illness. NEJM, July 2001. Rosen’s.Rosen’s. Guyton. Textbook of Medical PhysiologyGuyton. Textbook of Medical Physiology Aric’s Rounds 2003Aric’s Rounds 2003 Rigby’s NRC lectureRigby’s NRC lecture
So what’s UP with Altitude?So what’s UP with Altitude?
Barometric pressure decreases as Barometric pressure decreases as altitude rises (hypobaria) and with altitude rises (hypobaria) and with higher latitudeshigher latitudes
Hypobaria decreases the partial Hypobaria decreases the partial pressure of O2: 0.021 x Atm Pressure pressure of O2: 0.021 x Atm Pressure = PiO2, note- [02] remains at 21%= PiO2, note- [02] remains at 21%
Changes in weather (eg: low pressure Changes in weather (eg: low pressure front) can change relative altitude (up to front) can change relative altitude (up to 500-2500 feet)500-2500 feet)
Barometric pressure decreases as Barometric pressure decreases as altitude rises (hypobaria) and with altitude rises (hypobaria) and with higher latitudeshigher latitudes
Hypobaria decreases the partial Hypobaria decreases the partial pressure of O2: 0.021 x Atm Pressure pressure of O2: 0.021 x Atm Pressure = PiO2, note- [02] remains at 21%= PiO2, note- [02] remains at 21%
Changes in weather (eg: low pressure Changes in weather (eg: low pressure front) can change relative altitude (up to front) can change relative altitude (up to 500-2500 feet)500-2500 feet)