Pleno scenario Bbloc 7

dr. SwannyPhysiology Dept.

Respiration in High Altitude

Case of Ir. Cek nang, 56 years, who went to resort of 3200

meters above sea level.

Overview of Gas Exchange in the Lungs

Adapted from: Costanzo, LS. Physiology, 1st ed. 1998.

Mathematical Description of Gas Exchange

PaCO2 – Partial pressure of CO2 in the arterial blood.

PACO2 – Partial pressure of CO2 in the alveolar gas.

PaO2 – Partial pressure of O2 in the arterial blood.

PAO2 – Partial pressure of O2 in the alveolar gas.

Mathematical Description of Gas Exchange (cont)

• Alveolar Ventilation Equation – Primarily describes the relationship between PaCO2 and alveolar ventilation.

• Alveolar Gas Equation – Primarily describes the relationship between PAO2, FIO2, and PaCO2.

Oxygen Transport

Oxygen is carried in the blood in 2 forms:

• Dissolved O2 (~2% of total O2 content)

• O2 bound to hemoglobin (~98% of total O2 content)

Oxygen Transport (cont)

Total O2 content of blood (CaO2) =

+

Dissolved O2

O2 bound to hemoglobin

Oxygen-HemoglobinDissociation Curve at Rest

Hypoxia (↓ PaO2)

Respiratory failure manifesting as hypoxia is known as oxygentation failure.

Signs of oxygenation failure:• Tachypnea• Alkalemia (acidemia if concurrent ventilatory

failure)• Increased pulsus paradoxus• Cyanosis• Agitation Somnolence

Shifting the Curve

ALTITUDE MEDICINE

How high is high altitude?• High altitude: 1500-3000m above sea level• Very high altitude: 3000-5000m• Extreme altitude: above 5000m• “Death zone”: above 8000m

Why does high altitude affect us?

•All the medical effects of high altitude are caused by oxygen deprivation.

•The reduction in oxygen is best described by the fall in inspired partial pressure of oxygen

•These altitudes can have serious physical effects on humans.

• The three major areas that are affected are:• mental performance• physical performance • quality of sleep

High Altitude Diseases

• Altitude Sickness- Hypoxia– Low oxygen pressure– Ambient pressure and partial pressure of oxygen decrease

with altitude.

• AMS- Acute Mountain Sickness– Monge’s Disease– Rarely occurs below 2500 m– Headache, drowsiness, general fatigue, difficulty breathing

during physical exertion, nausea, loss of appetite.

Altitude Stressors

• Hypoxia • Over 2000m • PO2 at 5000m half that of sea level

• Cold • Drop 6.5° per 1000m

• Aridity • Radiation

Hypoxic responses

• Respiratory – ↑rate

• Cardiovascular– ↑rate and stroke volume– Pulmonary vasoconstriction and cerebral vasodilatation

(increases PA pressure)• Hematology

– ↑red blood cell mass and plasma viscosity – O2 Hb dissociation curve shift to left to increase affinity for

O2 (so less delivered to tissues)

Acclimatization

• Acute hypoxic stress is poorly tolerated but given time to adapt the body can handle certain degrees of hypoxia very well

• How? Changes in ventilation, blood, fluid balance, and cardiovascular parameters.

• Process by which people gradually adjust to high altitude

• Determines survival and performance at high altitude

• Series of physiological changes

O2 delivery

hypoxic tolerance +++

• Acclimatization depends on

• severity of the high-altitude hypoxic stress

• rate of onset of the hypoxia

• individual’s physiological response to hypoxia

Acclimatisation

• Hypoxic ventilatory response = VE

• Starts within the 1st few hours of exposure 1500m

• Mechanism

Ventilatory acclimatisation

Ascent to altitude

Hypoxia

Carotid body stimulation

Respiratory centres stimulation

Increased ventilation

Improved hypoxia

Decreased PCO2

CO2 + H2O H2CO3 HCO3- + H+

•↑alkaline bicarbonate excretion in the urine

but slow process !

• Progressive increase in the sensitivity of the carotid bodies

• After several hr to days at altitude (interval of ventilatory acclimatization): cerebrospinal fluid pH adjustment to the respiratory alkalosis

new steady state

Adjustment of respiratory alkalosis

Lung diffusion• Definition

Process by which O2 moves from the alveolar gas into the pulmonary capillary blood, and CO2 moves in the reverse direction

• High altitude O2 diffusion, because– a lower driving pressure for O2 from the air to the

blood– a lower affinity of Hb for O2 on the steep portion of the

O2/Hb curve– and inadequate time for equilibration

• At high altitude– interstitial oedema heterogeneity +++

V/Q heterogeneity

O2

• At rest

- Inhaled air is not evenly distributed to alveoli- Composition of gases is not uniform throughout lungs- Different areas of the lungs have different perfusion- Differences are less in recumbent position

Blood Acclimatization

• Increase in EPO within 2 hours - benefit in exercise tolerance only with long term stays. Not important for alt. Sickness

• Increase 2,3-DPG (shifts oxyhemoglobin diss curve R). However resp alkalosis shifts curve L so likely no net effect

• If at altitude for a while, red cells (some advise aspirin while at altitude)

Fluid Balance Acclimatisation

Peripheral venous constriction → increased central volume → decreased ADH and aldosterone → diuresis → decreased plasma volume and hyperosmolality.

• Antidiuresis is sign of acute mountain sickness

Cardiovascular Acclimatisation

• Increased HR and sympathetic tone compensates for lower stroke volume

• hypoxia → pulmonary circulation constriction• helpful for pneumonia, effusions etc but not

for global hypoxia• leads to pulmonary hypertension

Changes during sleep

• Some people have trouble sleeping above 2500m

• Many have problems above 4500m• Periodic breathing may occur• Avoid sedatives and alcohol – reduce

breathing further at night

When acclimatisation fails

• Altitude syndromes– Acute mountain sickness (AMS): the least-threatening

and most common– High altitude pulmonary oedema – High altitude cerebral oedema

• All these syndromes have – several features in common – respond to descent or oxygen

potentially lethal form of altitude illness

Acute Mountain Sickness

• Constellation of symptoms in context of recent gain in altitude– Similar to a hangover

• 50% at 3500m• Most at 5000m

Altitude PathophysiologyHyperventilation to

compensate for decreased FiO2

Alkalosis

Suppression of breathing during sleep

Episodic respiratory arrests during sleep

Altitude Pathophysiology

Hypoxia

Vasodilation

Increased cerebral blood

flow

Cerebral edema

Increased Capillary

Permeability

Pulmonary edema

Peripheral edema Increased

cold injury risk

Altitude Sickness Symptoms

• Mild Acute Mountain Sickness– Headache– Malaise– Anorexia– Nausea, vomiting

– Dizziness– Dyspnea on exertion– Dry cough– Decreased urine output

Any AMS symptoms are due to altitude until proven otherwise

Precautions and Prevention

•Acclimatization- adapting to the oxygen deprivation of high altitude to some extent

Although acclimatization ameliorates the hypoxia of high altitude to some extent, professionals still recommend supplementary oxygen.

Precautions and Prevention•Plenty of rest

•Avoid alcohol

•Avoid tobacco

•Avoid marijuana

•Stay hydrated

•Avoid heavy meals

Hypoxia – Treatment

• Treat the underlying process• Increase FIO2 with supplemental oxygen• Reposition the body • Secretion management• Bronchodilation• Diuresis• Increase mean lung volume• Reduce O2 requirement• Consider transfusion if [Hb] < 7.0 g/dL

AMS - Treatment

• Mild– Rest and stop

ascent– Descend if not

improved after 24 hours

– Drink fluids– Simple analgesics

• Moderate/Severe– Descend ≥ 100m– Acetazolamide – Dexamethasone– Hyperbaric O2

(Gamow bag)

Thank you