8- Ventilation and Perfusion balance Flashcards
what are 2 main causes of inefficiencies of the respiratory system?
- respiratory dead spaces
2. venous shunting
What is a respiratory dead space?
any part of the respiratory system that doesn’t actively take place in gas exchange.
What are respiratory dead spaces useful for?
- delivering air to alveoli
- making sure we don’t have big changes in co2 when breathing in and out (helps to even out changes)
What is tidal volume?
volume of air going in and out in a single breath
what is the average tidal volume of adults?
between 500 and 750ml
How much tidal volume remains in the anatomical dead space?
25%
What is an anatomical dead space?
the volume of inspired breath which has not mixed with the gas in the alveoli (respiratory system excluding the alveoli)
what is the alveolar dead space?
alveoli that does not take place in gas exchange (usually found in pts with lung disease)
what is physiological dead space?
anatomical dead space + alveolar dead space = physiological dead space
in healthy people what is the physiological dead space?
equal to anatomical (i.e. no alveolar dead space)
in lung disease what is the physiological dead space?
greater than the anatomical, i.e a lot of alveolar dead space
What 2 reasons may the alveoli fail to exchange gas\
- no blood perfusion
- no ventilation
What is venous shunting?
Blood missing out from being oxygenated
where does venous shunting occur?
- bronchial circulation
- cardiac circulation
What is bronchial circulation?
supplies blood to the airways such as trachea, bronchi, smaller airways and lung paranchyma
what is cardiac circulation?
blood draining from the myocardium into the cardiac veins and then cardiac sinus
how much blood is lost in the thesbian veins?
1%
How much blood is lost in the bronchial circulation?
1%
in total how much blood is lost through venous shunting?
2%
What is the ventilation/perfusion balance?
ensuring you have the perfect match in blood flow from the capilaries surrounding the alveoli to ventilation
What is Va/Q?
Va= alveolar ventilation
Q= blood flow
Va/Q is the ventilation perfusion balance
How does gravity/posture effect the ventilation perfusion balance?
affects the intraplural pressure, i.e when you stand up the variation in pressure cause the airways to sag they rest more on the diaphragm, greater difference in visceral and parietal pressure
what is the state of the apex when you are stood up?
- alveoli are distended (overinflated)
- capillaries more empty (less blood flow)
What is the state of the base when you are stood up?
- alveoli less distended so better ventilation
- blood flow greater so better perfusion
Where is blood flow and ventilation more efficient when stood up?
at the base of the lungs
when can Va/Q mismatch be exacerbated?
in people with respiratory disease
What happens in patients with vasoconstriction?
No perfusion (blood flow), contributes to total physiological dead space
what happens in patients with broncho-constriction?
No ventilation (gas exchange)
what is hypoxia?
when a region of the body is deprived of an oxygen supply
What is the effect of hypoxia on alveolar blood perfusion?
Potassium channels detect low o2, constrict vessels and redirect blood.
What can happen if there is too much constriction?
can lead to an increase in arterial pressure, right side of the heart has to pump harder to beat the pressure, can lead to right sided heart failure (cor pulmonale)
how does overventilation effect Pa02 and PaCO2 in blood?
- little increase in oxygen ventillation
- significantly reduces CO2 concentration in blood leaving the alveoli
How does underventillation effect PaO2 and PaCO2 in blood?
- slightly reduces O2
- PaCO2 will increase
How does the body respond to bronchoconstriction?
blood will become diluted from ventilated alveoli, PO2 will drop, PCO2 will rise,ph falls, respiratory acidosis, detected casues increase in rate and depth of breathing
What centres detects a drop in PaO2
automatic control centre in medulla and pons, chemoreceptors in carotid and aortic bodies
Effect of high PaCO2 and low PaO2 on breathing?
increased rate and depth of breath
How would spirometry recordings present in a patient with asphyxia?
less O2
more CO2
(frequency increases)
How would spirometry recordings present in a patient with hypercarbia?
no changes in O2
more CO2
(respiratory frequency will increase)
How would spirometry recordings present in a patient with hypoxia?
less O2
no changes in CO2
(No change in frequency)
What happens to blood gases in type 1 respiratory failure?
Low Pa02
Low PaCO2
What is type 1 respiratory failure?
blocking perfusion to the lungs (asthma, fibrosis, embolism)
What happens to blood gases in type 2 respiratory failure?
Low PaO2
High PaCO2
What is normal PaO2?
90-100mmHg
What is normal PaCO2?
36-46mmHg
What is type 2 respiratory disease?
- reduced ventilatory drive
- reduced neuromuscular power
- resetting the chemoreceptors
