Working lung and respiratory failure Flashcards
V/Q ratio
V= ventilation, Q= perfusion. Respiratory is the most efficient when V/Q ratio is 1.
Below 1= Underventilated.
Above 1= Underperfused.
Too low= hypoxaemia
Too high= wasted ventilation, dead space.
5 respiratory causes of hypoxaemia
- Mismatch- V/Q ratio is too low.
- Intrapulmonary shunt- poorly ventilated alveolus.
- Hypoventilation
- Reduced inspired partial pressure of O2.
- Diffusion limitation- diffusion distance too large.
V/Q mismatch
Hypoxaemia that arise due to V/Q ratio being too low, like 0.001.
This means that despite great perfusion to the alveoli, ventilation is poor. There is not enough air for gaseous exchange in the alveoli.
Intrapulmonary shunt
Air supply to alveoli is shunted. Can be due to:
Arteriovenous malformations
Pneumonia
Lung consolidation
V/Q ratio= 0
There is no air in the alveoli, therefore no ventilation can occur.
This decreases oxygenation of blood, causing hypoxaemia.
Venous admixture
Amount of poorly oxygenated blood in the veins needed to drop O2 levels from alveoli to arteries.
Why is there a drop in O2 moving from the alveoli to the arteries?
Anatomical shunt-
Bronchial veins drains into the pulmonary vein, without being oxygenated at the alveoli.
The thebesian veins drain into the the left ventricle without being oxygenated
Intracardiac shunt
Blood is able to move from the right to the left side of the heart due to defective passage.
Such as a congenital defect where the foramen ovale doesn’t close.
This reduces oxygen content in arteries.
Zone 1 of the lung
The apex of the lung- hardly any perfusion.
PA> Pa> Pv
Therefore capillaries are compressed and hardly any perfusion occurs.
Highest intrapleural pressure
Highest transpulmonary pressure
Contains the most distended alveoli- less air flow in during inspiration.
V/Q= 3.3 due to very low perfusion, CO2 cannot leave. Very high O2 and low CO2.
Zone 2 of the lung
The middle of the lung- interrupted perfusion.
Pa> PA> Pv
Blood is able to reach the alveoli but the alveolar pressure is higher than the venous pressure.
This causes an increase in arterial pressure until the venous pressure exceeds the alveoli pressure to allow perfusion.
This is lost when gaseous exchange happens so repeats.
V/Q= 1, the amount of ventilation that occurs is sufficient enough for good perfusion of blood,
Zone 3 of the lung
The base of the lung- constant perfusion.
Contains the smallest alveoli, so ventilation is in excess, compared to perfusion.
Pa>Pv>PA.
V/Q= 0.63
Contains the lowest intrapulmonary and intrapleural pressure,
Intrapleural pressure
Pressure in the pleural cavity.
This is negative due to the pressure being lower than the atmospheric pressure- this increases during inspiration.
Transpulmonary pressure
The difference in pressure between the alveolar pressure and the intrapleural pressure in the pleural cavity.
Intrapleural pressure is always large and negative. Alveolar pressure shifts between positive and negative when we breathe.
Hypoxic pulmonary vasoconstriction
When there is low pressure of oxygen in an alveolus/ area of the lung, it is poorly ventilated.
This causes the blood vessels to constrict and direct blood flow to somewhere that has
Shunt equation
The ratio of oxygen in the blood, Qs, to the cardiac output of the heart, Qt.
Qs= difference between oxygen content in capillaries and arteries. Qt= Difference between of oxygen content between capillaries and veins.
Hypoventilatory hypoxaemia
Low O2 in the blood due to impaired function of the respiratory control centre.
The will have a low V/Q value.
Causes:
Narcotic/ Opiate overdose.
Depressants, like alcohol.
CNS diseases that impair respiratory control
