Resp 14 - Hypoxia

Question 1

What parameter defines hypoxaemia?

Answer 1

PO2 < 8 kPa

Question 2

Describe how the partial pressure of oxygen changes from inspired air to the tissues.

Answer 2

21.3 —> 20 (conducting airways) —> 13.5 (alveoli as the air is humidified and mixes with the air in airways) —> 13.5 (PaO2 immediately past exchange surface) —> 13.3 (diluted by return of bronchial circulation, 99% saturation) —> 5.3 (mixed venous blood, 75% saturation)

Question 3

How does PAO2 and PaO2 change with age?

Answer 3

Decreases

Question 4

How can the partial pressure of oxygen in the alveoli (after mixing) be changed?

Answer 4

It’ll continue to move down its concentration into the blood until it reaches equilibrium.

Question 5

How is the ODC shifted to the right?

Answer 5

In environments that are associated with increased metabolism (e.g. exercise) - this is associated with:
o Increase in acidity
o Hypercapnia
o Increased 2,3-DPG concentration

Shifted to the left is the opposite

Question 6

How is the ODC squished?

Answer 6

Anaemia

Polycythemia causes it to get shifted up

Question 7

What percentage of cardiac output goes to perfuse the bronchial tree?

Answer 7

1%

Question 8

State the normal arterial and venous PO2.

Answer 8

Arterial = 13.3
Venous = 5.3
NOTE: only 25% of haemoglobin desaturates when going from arteries to veins

Question 9

How does dissolved oxygen affect the oxygen that is bound to haemoglobin?

Answer 9

The PO2 (dissolved oxygen) only counts for a small part of oxygen transport in the blood (the rest is bound to haemoglobin) but it is like the conductor of an orchestra. Uptake of dissolved oxygen into tissues is NOT keeping you alive directly, but it is accompanied by a LARGE unloading of oxygen from haemoglobin, which provides a sufficient supply of oxygen to the tissues.

Question 10

The amount of gas that’ll diffuse across a membrane is proportional to:

Answer 10

o Surface area for gas exchange
o Diffusion constant (CO2 diffuses faster than O2)
o Diffusion gradient - hypoxic air reduces this gradient

–> Fick’s law

Question 11

What is the oxygen cascade? What does it look like when you are breathing hypoxic air?

Answer 11

It describes the decreasing tension of the air from inspired to respiring cells.

When breathing hypoxic air, inspired PO2 is much lower so each step has a lower PO2. Exercise capacity is greatly reduced

Question 12

State four factors affecting the oxygen cascade.

Answer 12

Ventilation/Perfusion mismatch
Alveolar Ventilation
Diffusion Capacity
Cardiac Output

Question 13

State the different energy sources for different durations of exercise.

Answer 13

(in order of increasing duration of exercise - longest at the bottom)
Intramuscular ATP
Phosphocreatine
Lactic Acid 
Aerobic

Question 14

State the five challenges of altitude.

Answer 14

Hypoxia
Solar Radiation
Thermal
Hydration
Dangerous

Question 15

What’s the difference between accommodation and acclimatisation.

Answer 15

Accommodation = ACUTE response to this kind of change - rapid physiological change in response to a change in oxygen in the oxygen environment 
Acclimatisation = physiology becomes more efficient so that you can get more out of the environment

Question 16

What have the benefits of increased ventilation and PaO2 been attributed to?

Answer 16

Renal compensation for the respiratory alkalaemia

Increased sensitivity to hypoxia

Question 17

Where is erythropoietin produced?

Answer 17

Renal cortex

Question 18

What does low PaO2 stimulate and what is the name given to this response?

Answer 18

Hypobaric Hyperventilation

Decreased PaO2 –> Increase Ventilation –> Increase PAO2 –> Increase PaO2

Question 19

What is acetazolamide?

Answer 19

Carbonic anhydrase inhibitor. This will create excess bicarbonate which will take up H+ from blood and so less H+, less acid so pH rises.
This is used to accelerate the slow renal compensation to hypoxia-induced hyperventilation (increased secretion of HCO3-)

Question 20

What is acclimation?

Answer 20

Like acclimatisation but in an artificial environment like breathing hypoxic gases or hypobaric chamber

Question 21

State four innate adaptations of native highlanders.

Answer 21

Barrel Chest - more O2 into body
Increased Haematocrit - more O2 carried in blood
Large Heart - greater pulmonary perfusion
Increased Mitochondrial Density - More O2 utilised

Question 22

What causes chronic mountain sickness and what are the consequences of CMS?

Answer 22

CMS is thought to be due to secondary polycythaemia in response to hypoxia - RBCs are overproduced to the point where they are no longer beneficial - thicken the blood. This spontaneously happens in acclimatised individuals. Symptoms - cyanosis, fatigue.

Consequences - heart failure, eventual death

Question 23

What causes acute mountain sickness and what are the consequences?

Answer 23

Maladaptation to the high altitude environment usually right after ascent. Symptoms are pretty vague, ‘hangover’ - dizziness, vomiting, irritability, insomnia, fatigue. Probably associated with mild cerebral oedema.

Consequences: HACE or HAPE

Question 24

What causes High Altitude Cerebral Oedema (HACE)?

Answer 24

Cause: rapid ascent or inability to aclimatise;
Vasodilation of vessels in response to hypoxaemia (to increase blood flow).
More blood going to the capillary beds and so more leakage —> cerebral oedema —> rise in ICP —> herniation

Symptoms: confusion, ataxia, behavioural change
Consequences: irreversible brain damage, coma, death

Question 25

What causes High Altitude Pulmonary Oedema (HAPE)?

Answer 25

Pulmonary arteries VASOCONSTRICT in response to hypoxaemia - leading to pulmonary hypertension.
This means that hydrostatic pressure in the capillaries is increased so more fluid moves out of the capillaries and into the tissue.

Symptoms: dyspnea, dry cough, bloody sputum

Question 26

What are the two main types of respiratory failure? State the parameters that define them.

Answer 26

Type 1 = Hypoxaemic (PO2 < 8 kPa)
Type 2 = Hypercapnic (PCO2 > 6.7 kPa)
There is also a third type: Mixed = PO2 < 8kPa AND PCO2 > 6.7 kPa

Question 27

What causes type 1 and type 2 respiratory failure?

Answer 27

Type 1 = V/Q mismatch - hypoventilated alveoli or hypoperfused
Type 2 = Increased CO2 production and decreased elimination - obesity, decreased CNS drive, increased work of breathing - basically air stagnates because poor ventilation, poor conc. gradient

Question 28

How do the kidneys control blood pH?

Answer 28

By secretion and retention of weak acids

They can eliminate or replenish H+ by altering the plasma bicarbonate concentration (HCO3-)