HIGH ALTITUDE PHYSIOLOGY

CATEGORISATION FOR CATEGORISATION FOR DESCRIPTIVE CONVENIENCE: DESCRIPTIVE CONVENIENCE:

ALTITUDE TYPE FROM SEA-LEVEL (In feet)

HIGH 8,000 – 12,000

VERY HIGH 12,000 – 18,000

EXTREMELY HIGH

Above 18,000

SIGNIFICANT ATMOSPHERIC PRESSURE VARIATION WITH ALTITUDE:

ALTITUDE PRESSURE

(FEET) ( mm of Hg) (ATMOSPHERIC UNIT)

0 760 1

18,000 380 1/2

34,000 190 1/4

48,000 95 1/8

63,000 47 1/16

BASIC CONCEPT:BASIC CONCEPT:

Human body is specifically designed in such a Human body is specifically designed in such a way that it delivers adequate O2 to the way that it delivers adequate O2 to the tissues only when oxygen is supplied at a tissues only when oxygen is supplied at a pressure close to the sea-level (P = 760 mm pressure close to the sea-level (P = 760 mm Hg Hg PO2 =159 mm Hg) PO2 =159 mm Hg)

So, at high altitude there is hypoxic hypoxia So, at high altitude there is hypoxic hypoxia tissue oxygenation suffers tissue oxygenation suffers physiological physiological derangements.derangements.

““connecting a 24 volt motor to a 6 volt connecting a 24 volt motor to a 6 volt battery”—perfect comparison by J.S.Milledge.battery”—perfect comparison by J.S.Milledge.

COMMON HYPOXIC EFFECTS COMMON HYPOXIC EFFECTS WITH DIFFERENT ALTITUDES:WITH DIFFERENT ALTITUDES:

ALTITUDE LEVEL

INSPIRED AIR PO2

Hb-SATURATION

EFFECTS

In feet (metre)

In mm of Hg in % Stages (if any)

0 (i.e.sea-level)

160 ~ 97 % NIL

Upto 10,000(3,000)

110 ~ 90 % Usually none, +/- some nocturnal visual reduction ( of indifference)

10,000 – 15,000(3,000 – 4,500)

98 ~ 80 % Mod. Hypoxic symptoms cardiorespiratory manifestaions & early CNS involvements ( of reaction)

15,000 – 20,000(4,500 – 6,000)

70 < 70 % Severe hypoxic symp aggravated CNS involvement (of disturbance)

Above 20,000 & onwards

Further falls

below 60 %

Unconsciousness & alarming deterioration survival impossible without supplemental O2 (critical survival altitude)

ALTITUDES & HEIGHTSALTITUDES & HEIGHTS Normally most people on earth stay at Normally most people on earth stay at

around the around the Mean Sea LevelMean Sea Level (MSL) (MSL) However, groups of people stay at higher However, groups of people stay at higher

than normal attitudes.than normal attitudes. The The SherpasSherpas, a mountain tribe of , a mountain tribe of

Himalayas live at around 5500 Meters Himalayas live at around 5500 Meters above MSL, normally.above MSL, normally.

When people living around MSL go up to When people living around MSL go up to higher altitudes, changes occur in their higher altitudes, changes occur in their physiology, especially in Respiration.physiology, especially in Respiration.

BAROMETRIC PRESSURE CHANGES BAROMETRIC PRESSURE CHANGES IN HIGH ALTITUDEIN HIGH ALTITUDE

As the altitude increases above the As the altitude increases above the sea level, the corresponding sea level, the corresponding atmospheric pressure atmospheric pressure decreasesdecreases..

The partial pressure of Oxygen also The partial pressure of Oxygen also decreasesdecreases..

The arterial Oxygen saturation levels The arterial Oxygen saturation levels also also decreasedecrease with increase in the with increase in the altitude.altitude.

PARTIAL PRESSURES OF OXYGEN PARTIAL PRESSURES OF OXYGEN

ACUTE EFFECTS OF HYPOXIAACUTE EFFECTS OF HYPOXIA

As the altitude increases, the As the altitude increases, the barometric pressure decreases.barometric pressure decreases.

This causes a handicap which may This causes a handicap which may be:be:• AppreciableAppreciable• ConsiderableConsiderable• SeriousSerious• Causing Imminent CollapseCausing Imminent Collapse

ACCLIMATIZATIONACCLIMATIZATION

““Getting used to…”Getting used to…” People remaining at high altitudes for People remaining at high altitudes for

days, weeks or years become more days, weeks or years become more and more acclimatized to low POand more acclimatized to low PO22..

This causes the hypoxia to cause This causes the hypoxia to cause fewer deleterious effects on their fewer deleterious effects on their bodies.bodies.

They can thus work harder at higher They can thus work harder at higher altitudes without hypoxic effects.altitudes without hypoxic effects.

How does acclimatization occur?How does acclimatization occur?

Increased :Increased :• Pulmonary ventilation.Pulmonary ventilation.• Number of RBCs ( Hypoxia causes Number of RBCs ( Hypoxia causes

excess erythropoiesis).excess erythropoiesis).• Diffusing capacity of the lungs.Diffusing capacity of the lungs.• Vascularity of the peripheral tissuesVascularity of the peripheral tissues• Ability of the tissue cells to use Oxygen Ability of the tissue cells to use Oxygen

despite low POdespite low PO22. .

NATURAL ACCLIMATIZATIONNATURAL ACCLIMATIZATION

This occurs in people living from their This occurs in people living from their birth at high altitudes.birth at high altitudes.

Those living in the Andes & Those living in the Andes & Himalayan mountains, for instance.Himalayan mountains, for instance.

Acclimatization begins in them in Acclimatization begins in them in infancy.infancy.

The chest size is greatly increased.The chest size is greatly increased. Their hearts are considerably larger Their hearts are considerably larger

than those of lowlandersthan those of lowlanders

ACUTE MOUNTAIN SICKNESSACUTE MOUNTAIN SICKNESS

This is also called as “High Altitude This is also called as “High Altitude Pulmonary Edema.”Pulmonary Edema.”

This occurs in a small number of This occurs in a small number of lowlanders who ascend rapidly to high lowlanders who ascend rapidly to high altitudes.altitudes.

Begins from a few hours up to 2 days after Begins from a few hours up to 2 days after their ascent.their ascent.

Is serious and results in their death unless Is serious and results in their death unless they are given Oxygen or taken to a low they are given Oxygen or taken to a low altitude.altitude.

ACUTE MOUNTAIN SICKNESS : ACUTE MOUNTAIN SICKNESS : SYMPTOMS & SIGNSSYMPTOMS & SIGNS

Acute Cerebral Edema:Acute Cerebral Edema:• Hypoxia causes cerebral vasodilatationHypoxia causes cerebral vasodilatation• Increases capillary pressureIncreases capillary pressure• Causes fluid to leak out into the tissuesCauses fluid to leak out into the tissues• This leads to cerebral edema causing:This leads to cerebral edema causing:

Severe disorientationSevere disorientation Other cerebral dysfunctionsOther cerebral dysfunctions

Acute Pulmonary Edema:Acute Pulmonary Edema:• Severe hypoxia causes Pulmonary Severe hypoxia causes Pulmonary

arteriolar constriction.arteriolar constriction.• In some areas it is more and causes In some areas it is more and causes

edema.edema.• This can extend progressively to other This can extend progressively to other

areas of the lungs.areas of the lungs.• Can be reversed within hours on Oxygen Can be reversed within hours on Oxygen

therapytherapy

ACUTE MOUNTAIN SICKNESS : ACUTE MOUNTAIN SICKNESS : SYMPTOMS & SIGNSSYMPTOMS & SIGNS

CHRONIC MOUNTAIN SICKNESSCHRONIC MOUNTAIN SICKNESS

Seen in people who reside for long at high altitudes.

Red cell number and mass increases exceptionally.

Pulmonary arterial pressure becomes very high.

The heart becomes enlarged in the right side.

The peripheral arterial pressure begins to fall

Congestive Cardiac failure & death follows They need to be taken to low altitudes as

soon as possible. They recover in low altitudes within days or

weeks.

PHYSIOLOGICAL RESPONSES PHYSIOLOGICAL RESPONSES TO HIGH ALTITUDE HYPOXIA:TO HIGH ALTITUDE HYPOXIA:

• Arbitrarily Divided into following two---Arbitrarily Divided into following two---

I)I) Acute responses (aka accommodation)Acute responses (aka accommodation)

II)II) Long term responses ( aka acclimatization) Long term responses ( aka acclimatization) • ““Arbitrary” because ----Arbitrary” because ----

i)i) Acute are also beneficial for long-term coping Acute are also beneficial for long-term coping up.up.

ii) Acute are modified steadily & imperceptibly in ii) Acute are modified steadily & imperceptibly in such a way that after 2-3 days are considered such a way that after 2-3 days are considered as beginninng of acclimatization .as beginninng of acclimatization .

iii) Sharpness of division depends on rate of iii) Sharpness of division depends on rate of ascent . ascent .

IMP. CONCEPTS IN IMP. CONCEPTS IN ENVIRONMENTAL PHYSIOLOGY:ENVIRONMENTAL PHYSIOLOGY:

ACCOMMODATION AT HIGH ALTITUDE:ACCOMMODATION AT HIGH ALTITUDE: immediate reflex responses immediate reflex responses of the of the

body to acute hypoxic exposure.body to acute hypoxic exposure.

A)A) Hyperventilation:Hyperventilation: arterial PO2 arterial PO2 stimulation of peripheral stimulation of peripheral

chemoreceptors chemoreceptors increased rate & increased rate & depth of breathingdepth of breathing

B) Tachycardia:B) Tachycardia:Also d/t peripheral chemo. Response Also d/t peripheral chemo. Response

CO CO oxygen delivery to the tissues oxygen delivery to the tissues

Contd…..Contd…..C)C) Increased 2,3-DPG conc. in RBCIncreased 2,3-DPG conc. in RBC::within hours, ↑deoxy-Hb conc. within hours, ↑deoxy-Hb conc. locally ↑pH locally ↑pH

↑2,3-DPG ↑2,3-DPG ↓oxygen affinity of Hb ↓oxygen affinity of Hb tissue tissue O2 tension maintained at higher than normal O2 tension maintained at higher than normal levellevel

D) Neurological :D) Neurological :• Considered as “Considered as “warning signs”warning signs”• Depression of CNS Depression of CNS feels lazy, feels lazy,

sleepy ,headachesleepy ,headache• ‘‘Release Phenomena’Release Phenomena’ like effect of alcohol like effect of alcohol• At further heightAt further height cognitive impairment, cognitive impairment,

twitching, convulsion & finally unconsciousnesstwitching, convulsion & finally unconsciousness

ACCLIMATIZATION AT HIGH ALTITUDE:ACCLIMATIZATION AT HIGH ALTITUDE:

•Delivery of atmospheric O2 to the tissues normally involve 3 stages---with a drop in PO2 at each stage.•When the starting PO2 is lower than normal, body undergoes acclimatization so as to—(i)↓ pressure drop during transfer(ii)↑ oxygen carrying capacity of blood(iii) ↑ ability of tissues to utilize O2

•With longer stay at high altitude ,body is able to adjust by certain physiological adaptations..

A)Sustained Hyperventilation:A)Sustained Hyperventilation:• Prolonged hyperventilation Prolonged hyperventilation CO2 wash-out CO2 wash-out

respiratory alkalosis respiratory alkalosis renal compensationrenal compensation alkaline alkaline urineurine normalization of pH of blood & CSF normalization of pH of blood & CSF withdrawal of central chemo-mediated respiratory withdrawal of central chemo-mediated respiratory depression depression net result is ↑resting pulmonary net result is ↑resting pulmonary ventilation (by ~5 folds to 60L/min),primarily d/t ↑ in ventilation (by ~5 folds to 60L/min),primarily d/t ↑ in TV (upto 50% of VC)TV (upto 50% of VC)

• Such powerful ventilatory drive is Such powerful ventilatory drive is also possible as-also possible as-

(i)(i) ↑↑sensitivity of chemo- mechs to PO2 & PCO2sensitivity of chemo- mechs to PO2 & PCO2

(ii)(ii) Somewhat ↓ in work of breathing Somewhat ↓ in work of breathing make easy & make easy & less tiringless tiring

B) Other Respiratory B) Other Respiratory Changes:Changes:

↑ ↑ TLC :TLC : esp in high-landers(natives for esp in high-landers(natives for generations)generations) evidenced by relatively evidenced by relatively enlarged (barrel-shaped) chestenlarged (barrel-shaped) chest l/t l/t ↑ventilatory capacity in relation to body ↑ventilatory capacity in relation to body mass.mass.

↑ ↑ Diffusing capacity of lungsDiffusing capacity of lungs:: d/t d/t hypoxic pulmonary vasoconstriction hypoxic pulmonary vasoconstriction Pul. Hypertension Pul. Hypertension ↑ no. of ↑ no. of pulmonary capillariespulmonary capillaries

→→ existence of this effect is still existence of this effect is still debatable!!!debatable!!!

C)↑Vascularity of the C)↑Vascularity of the Tissues:Tissues:

More capillaries open up in tissues than at sea-level More capillaries open up in tissues than at sea-level ((normal ~25 % at restnormal ~25 % at rest—remaining as ‘reserve’).—remaining as ‘reserve’).

This combined This combined with systemic vasodilatationwith systemic vasodilatation(also a (also a hypoxic response)hypoxic response) more O2 delivery to tissues. more O2 delivery to tissues.

D) Cellular level changes:D) Cellular level changes: ↑ ↑ intracellular intracellular mitochondrialmitochondrial density density ↑ ↑ conc. of cellular conc. of cellular oxidative enzymesoxidative enzymes ↑ ↑ synthesis of synthesis of MbMb( O2-storing pigment)( O2-storing pigment)

→→ all aimed to improve O2 utilization. all aimed to improve O2 utilization.

E) Physiological E) Physiological Polycythemia:Polycythemia:

Cheyne-Stokes Cheyne-Stokes Respirations:Respirations:

• Above 10,000 ft (3,000 m) Above 10,000 ft (3,000 m) most people experience a most people experience a periodic breathing periodic breathing during sleepduring sleep. The pattern . The pattern begins begins with with a few shallow breathsa few shallow breaths increases to increases to deep sighing deep sighing respirations respirations falls off falls off rapidly. rapidly.

• Respirations Respirations may cease entirely may cease entirely for a few secs for a few secs & then & then shallow breaths begin again. shallow breaths begin again. During period of During period of breathing-arrestbreathing-arrest, person often becomes restless & may , person often becomes restless & may wake with a sudden feeling of suffocation. wake with a sudden feeling of suffocation.

• Can disturb Can disturb sleeping patternssleeping patterns exhausting the exhausting the climber. climber. AcetazolamideAcetazolamide is helpful in relieving this. is helpful in relieving this.

Not considered abnormal at high altitudes. But if Not considered abnormal at high altitudes. But if occurs first during an illness (other than Altitude occurs first during an illness (other than Altitude illnesses) or after an injury (particularly a head illnesses) or after an injury (particularly a head injury) injury) may be a sign of a serious disorder. may be a sign of a serious disorder.