Humans At Altitude

Question 1

What are the common abbreviations?

Answer 1

P or p – partial pressure
A – alveolar 
a – arterial
v - venous
C – content
F – fraction
i – inspired
S - saturation

Question 2

What is Pb- Barometric pressure?

Answer 2

Downward force exerted by earths atmosphere at a given point.

Typical constituents of barometric
pressure:
– PB = *PN2 + PO2 + PCO2 + PH2O
– * All other gases

At sea level PB = 101 kPa

Question 3

How does altitude effect pressure?

Answer 3

Exponential decrease in pressure with altitude

Although FiO2 remains the same (~21%) the reduction in PB will result in the partial pressure of O2 declining with altitude

Question 4

Effects of Hypoxia?

Answer 4

Ordered by altitude:

Collapse

Lethargy, general weakness

Breathing difficulties, anorexia, GI problems

Insomnia, pulmonary stress, vomiting, nausea

Lightheaded, headache

Question 5

What 3 systems need adjusting in acclimatisation?

Answer 5

Cardiac

Respiratory

Haematological

Question 6

How to calculate systemic 02 delivery?

Answer 6

DO2: cardiac output multiplied by o2 content of arterial blood (Ca02).

Question 7

First line of defence upon high altitude due to decrease in inspired oxygen?

Answer 7

Increased cardiac output to balance decrease in reduced arterial oxygen content. Cardiac output is increased by increasing heart rate. Occurs due to increase in circulating catecholamines.

Question 8

Methods of restoring Ca02 from effects of hypoxia?

Answer 8

Ventilatory changes:

Detected in peripheral chemoreceptors in carotid and aortic bodies

Sends signals to brain stem respiratory nuclei

Involuntary increase in depth and rate of breathing: hyperventilation

Haematological changes:

Hb =total Hb(g) /total plasma volume(L)

• Reduce plasma volume
• Increase total haemoglobin

Question 9

Alveolar gas equation?

Answer 9

PAO2 = PiO2 – (PACO2/R)

R = respiratory exchange ratio= CO2 production/ O2 consumption

PACO2 and PAO2 are inversely related. So, during a fixed rate of CO2 production, a fall in PACO2 causes an increase in PAO2.

Question 10

How does hyperventilation work in relation to the alveolar gas equation?

Answer 10

Normal Ventilation:
Sea Level: PAO2 = 150 – (40/0.8) = 100 mmHg
Summit : PAO2 = 100 – (40/0.8) = 50 mmHg

(50% reduction)

Hyperventilation:

Blow off CO2 thereby causing an increase in PAO2

This causes alkalosis, detected by central chemoreceptors (medulla).

HCO3 lost from CSF returning CSF pH to normal. Removes inhibition so hyperventilation remains acutely elevated.

Compensatory renal response manifested after 2-3 days. This returns blood pH back to normal. Reduced reabsorption of HCO3 in the proximal tubule Subsequently it is flushed out in urine

Question 11

How is total haemoglobin increased?

Answer 11

Cells in kidney and liver detect reduced PaO2 via 02 sensing TF call Hypoxic Inducible Factor (HIF).

This stimulates Erythropoietin production.(EPO)

Stims bone marrow to produce more RBC.

Question 12

What reduced affinity for o2 to HB and shifts oxyhemoglobin curve to the right?

Answer 12

Decrease in pH (more H+)
Increase in co2
Increased temperature
Increased DPG

Bonus info:

At moderate altitude the curve is probably shifted right (lower affinity, easier offloading)

At extreme altitude curve is probably shifted left (high affinity, easier loading)