Hypoxia Flashcards
Define these common terms:
A a v P or p C F
Alveolar Arterial Venous Partial pressures Content Fraction
What is barometric pressure (PB)?
The total pressure of all gases in the air (including inert)
Define partial pressure of a gas in a mixture
The pressure a gas would exert if it occupied that volume alone (Dalton’s Law)
OR the total pressure of a gas mix is the sum of each
gases partial pressure (P)
How do gases move in large airways (conducting zone)?
Gases move by convention
How do gases move in smaller airways?
By diffusion - from a high partial pressure to a low partial pressure
E.g. diffusion of o2 from alveoli to pulmonary capillaries is partly dependent on the partial pressure gradient
What is the uptake of o2 in pulmonary capillaries (and extraction of o2 at tissues) influenced by?
1) partial pressure gradient
2) transit time of blood through capillary e.g. too quick, equilibrium may fail
3) (and also the relationship between PO2 and Hb saturation)
What is the systemic o2 delivery equation?
DO2 = CO x CaO2
What is the aim of acclimatisation?
To maintain an adequate o2 delivery to the tissues, in the face of low inspired PO2
What kind of a response is acclimatisation?
An integrated and slowly developing response (hours-days-weeks), requiring adjustments to the following systems:
- cardiac
- respiratory
- haematological
What are the three adjustments in response to hypoxia?
- respiratory (hyperventilation and acid/base balance adjustments)
- cardiovascular (HR)
- haematological (erythropoiesis)
What is the equation for PB?
• PB = *PN2 + PO2 + PCO2 + PH20
*Includes inert gases
What are the pressure units?
Pressure units
1kPa = 7.5mmHg
What is dry PO2 in sea level?
PO2 = fractional concentration (FO2) x PB
- FO2 = 0.209 (approx. 21%) in dry air
- PB at sea level = 101kPa (760mmHg)
• So dry PO2 at sea level = 0.209 X 101 = 21kPa (159mmHg)
What happens when dry ambient air passes through airways?
Becomes quickly saturated with H20 vapour as it passes through airways
- H20 exerts it’s own PP which needs to be taken into account
What does PH20 change with and what is its value at body temp?
Changes with body temp.
At body temp = 6.25kPa (47 mmHg)
What is the process of o2 uptake into the alv-cap membrane?
O2 into solution —> diffusion through membrane —> combination with Hb on RBC
What is hypoxia?
Inadequate supply of o2 to the tissues
= cellular function may become compromised
Why does PO2 decrease from air to alveoli?
- addition of water vapour
- has exchange with co2
Why is oxygen needed?
Acts as the final e- acceptor in the ETC
So, helps in the creation of the H+ gradient either side of the inner membrane, which drives oxidative phosphorylation (OXPHOS) and the production of ATP
What decreases with increasing altitude?
PB decreases = PIO2 decreases too
This is because:
PIO2 = FO2 x (PB x H20)
= HALVES PAO2
Why does FO2 stay the same with increasing altitude?
Proportion of gases stays the same, just fewer molecules overall
Give an example of PB decreasing with altitude
(London) Sea level = 760 mmHg
Everest base camp = 403.5 mmHg
From participants breathing ambient air at various altitudes.
Grocott et al (2009) NEJM
What is the first line of defence to hypoxia exposure?
Increasing CO - to maintain DO2 in the face of lower PIO2
1) HR increases immediately
- 4500m: HR 10-15% higher than sea level
- 7600m: HR ~100% higher than sea level
2) SV remains constant or decreases
What is the second response to hypoxic exposure?
Restoration of CaO2.
Via 2 mechanisms:
1) ventilatory adjustments
2) haematological adjustments
Main ventilatory compensation mechanism = the hypoxic ventilatory response
What is hyperventilation?
Where CO2 is eliminated in expired air at a faster rate than is produced (i.e. blow off CO2) - decrease in PaCO2
Why does hyperventilation occur?
Due to hypoxic stimulation of arterial chemoreceptors
Most important feature of acclimatisation to high altitudes
Why is hyperventilation beneficial?
As a person climbs to higher altitudes, the hypoxic ventilatory drive increases, and PCO2 decreases
- above a certain altitude, the HV response aims to defend PO2 above a certain threshold (eg 35 mmHg) by extreme HV
- this can reduce PCO2 to <10 mmHg!
- keep going higher, eventually won’t be able to compensate
- won’t be able to HV anymore + eventually become very hypoxic + pass out
What is the equation for PAO2?
PAO2 = FIO2 x (PB x PH20) - (PaCO2/R)
What is the equation for PIO2?
PIO2 = FIO2 x (PB - PH2O)
