Ventilation-Perfusion Relationships Flashcards
What is the shorthand notation for alveolar ventilation?
Va
What is the shorthand notation for perfusion?
Q
How do you work out Va?
Minute ventilation must first be calculated:
= tidal volume (500) X RR (15)
= 7500
Dead space ventilation:
= dead space (150) X RR (15)
=2250
Therefore alveolar ventilation=
= 7500 - 2250 = 5250ml/min
How would you work out alveolar perfusion (pulmonary blood flow)?
Q = stroke volume (70) X HR (70) = 4900ml/min
If Va = 5250 and Q = 4900, what can be said about the V/Q relationship?
They roughly = 1 therefore they are matched.
What is a normal V/Q value?
0.8, as normal Va = 4L/min and Q = 5L/min
What would the V/Q situation be if one lung is blocked and the blood supply to that lung is also blocked, but the Va and Q to the other is normal?
The V/Q would still be 0.8 and there would still be potentially normal gas exchange.
What would the V/Q situation be if the Va to one lung is blocked but it’s Q is normal…
…and the Va to the other lung is normal but it’s Q is blocked?
What does this tell us about the ventilation and perfusion in order for the V/Q value not to be misleading?
There would still be a normal V/Q value of 0.8 but there will be a mismatch.
Therefore V and Q must be matched at the alveolar-capillary level!
What is dead space in terms of V and Q?
What happens to the equilibration of gasses across the capillary wall?
What does this mean in terms of PAO2 and PACO2?
It is a ventilated alveolus (Va) with a blocked blood supply (Q).
This means gases can not equilibrate across the capillary wall.
PAO2 and PACO2 will therefore remain as pre-diffused values close to that of tracheal pp values.
What is a shunt in terms of V and Q?
What does this mean for PAO2 and PACO2?
A shunt is when V is blocked but Q is not.
PAO2 and PACO2 will therefore equilibrate with venous PO2 and venous PCO2 values.
What maximal value can V/Q take in a dead space scenario?
Infinity
What maximal value can V/Q take in a shunt scenario?
Zero
What happens in a normal right to left shunt, give an example?
98% of venous blood passes through the lungs to be oxygenated.
2% joins the left side of the circulation without undergoing gas exchange.
An example of this is the bronchial veins and also the veins draining the left ventricle (the thebesian veins).
What condition could provide a pathophysiological shunt?
Pneumonia.
How would you calculate a 20% shunt?
20% shunt means 80% of the blood experiences gas exchange, 20% passes unchanged.
You would calculate 80% of the normal arterial O2 and CO2 CONTENT, and 20% of venous (unchanged) O2 and CO2 content.
You could then calculate PaO2 and PaCO2 From a graph given the content values.
