Gas Exchange: Hyper and Hypobaric Pressures

Question 1

What is the alveolar gas equation?

Answer 1

PAO2 = PiO2 x PaCO2/RQ

The respiratory quotient is often quoted as 0.8

Question 2

What is the significance of hypobaric pressures?

Answer 2

Hypobaric pressures = Pressures below normal atmospheric pressure.

Increasing altitude causes hypobaric conditions. PiO2 will be reduced despite normal FiO2 resulting in progressive alveolar hypoxia & hyperventilation.

Question 3

How can you calculate PiO2?

Answer 3

Inspired PO2 = FiO2 x (Barometric pressure – Saturated vapour pressure of water @ 37 degrees Celsius)

Question 4

What is the PiO2 at sea level?

Answer 4

Sea level: barometric pressure is 101kPa and the SVP of water is 6.3 kPa at 37 degrees.

Therefore:

PiO2 = 0.21 x (101-6.3) => 19.89

Question 5

What is the barometric pressure at 5000m and at Everest ~9000m?

Answer 5

5000m ~ 50Kpa

Everest ~ 9000m ~30 Kpa

Question 6

Describe how the body acclimitises to altitude?

Answer 6

Respiratory
Rapid ascent to a summit will result in an initial increased hypoxic drive of respiration but this is short lived and returns to normal within hours.

Long term restoration of PAO2 is through hyperventilation and hypocapnia.

Hypocapnia produces a respiratory alkalosis this results in renal bicarbonate excretion and a metabolic acidosis which increases the respiratory drive.

Both the peripheral and central chemoreceptors are important in the process.

Haemoglobin
Increased concentration to preserve oxygen content and carrying capabilities.

Also, an increased production of 2,3 DPG but the shift is counteracted by respiratory alkalosis.

Question 7

What adaptations do people that live at high altitude have?

Answer 7

Polycythaemia

Reduced ventilatory response to hypoxia

Question 8

3 categories

What are the different categories of mountain sickness and at what altitude do they occur?

Answer 8

Usually mountain sickness occurs above 6000ft ~1800m

Acute mountain sickness
* Mild: dyspnoea, headache, nausea, fatigue and sleep disturbance associated with Cheyne- Stokes respiration.
* Severe: High altitude pulmonary oedma (HAPO), High altitude cerebral oedma (HACO)

Chronic mountain sickness

Question 9

What are the symptoms of severe AMS and the treatment?

Answer 9

**High altitude pulmonary oedma **(HAPO): associated with exercise and caused by excessive pulmonary vasoconstriction.

Symptoms of SOB and a persistent cough of frothy white sputum.

High altitude cerebral oedma (HACO): Hallucinations and reduced GCS.

Treatment
* Immediate descent
* Oxygen as needed
* Nifedipine (acts as a pulmonary vasodilator) and acetazolamide (carbonic anhydrase inhibitor, therefore induces a metabolic acidosis causing a compensatory rise in respiratory rate)

Question 10

5

What are the signs and symptoms of chronic mountain sickness?

Answer 10

• Poor hypoxic response to ventilation
• CO2 retention
• Polycythaemia
• Cyanosis
• Clubbing

Question 11

What is the significance of hyperbaric pressures?

Answer 11

Hyperbaric pressures = Pressures above normal atmospheric pressure.

**Gases become dense **and nitrogen develops narcotic properties at 4atm.*

On decompression, gases in closed spaces expand and bubbles may form in blood or tissues causing decompression sickness, ‘the bends’.

*This is why below 30m divers use heliox (as well as its reduced density)

Question 12

How many meters below the sea do you need to descend for pressure to increase by 1 atmosphere?

Answer 12

Pressure in water increases by one atmosphere per 10 m of descent below the surface.

Question 13

What are the risks of rapid decompression (rapid ascent when diving)?

Answer 13

• Barotrauma in any air-filled space, such as the lungs or ear
• Arterial air embolus
• Potentially permanent neurological damage
• Bubbles forming in vessel poor tissues (cartilage) with avascular necrosis

Question 14

How is decompression sickness managed?

Answer 14

Slow decompression through slow ascent helps prevent decompression sickness.

Decompression sickness is treated by emergency recompression in a hyperbaric chamber and the administration of oxygen.

Question 15

Describe how hyperbaric oxygen therapy is used and the rationale behind it?

Answer 15

Describes the administration of 100 % oxygen at a pressure of 2-3 atm given for 1-2h daily through a tight-fitting mask for up to 30 days.

Rationale:
* Arterial oxygen only increases marginally (19 to 25 ml/dl)
* Venous oxygen increases dramatically at 3atm vs 1atm. As tissue oxygen approximates with venous oxygen, it aids with tissue hypoxia.

Indications include: carbon monoxide poisonining and anaerobic infections*

The use in infections is controversial and has fallen out of favour

Question 16

What is the effect on increased pressure on gases and how does this effect work of breathing?

Answer 16

As pressures increase gases will become more dense and therefore work of breathing will be increased.

Question 17

When scuba diving what happens to your FiO2 requirement?

Answer 17

The deeper you are the greater the atmospheric pressure therefore your FiO2 requirement goes down.

PAO2= PiO2 x CO2/RQ

PiO2 = FiO2 x (atmospheric pressure - water vapour pressure) therefore at atmospheric pressure decreases you need a lower FiO2 to maintain your PiO2

Question 18

What are the issues with breathing 100% O2?

Answer 18

Can produce oxygen toxicityresulting in:
* convulsions
* chest pain
* irreversible acute lung injury

Question 19

What are the anaesthetic implications of anaesthesia in hypobaric conditions?

Answer 19

Hypobaric Pressures:
* Pressurization and depressurization in aeroplanes occurs suddenly during take-off and landing. This may cause sudden expansion of air filled spaces within a patient, such as bullae and pneumothorax.
* Valves in breathing systems may stick
* Inappropriate light anaesthesia because of inadequate vaporization of volatiles due to low temperature, which accompanies the altitude
* Theoretically, boiling point of volatiles will fall but they have still been used safely in altitude.