Physiology Gas Exchange Flashcards
What is the formula indicating a gas exchange problem is present?
A-a gradient
Should be less than 10mmHg
If increased, a gas exchange problem is present - don’t get appropriate loading of oxygen into arterial blood
What are possible causes for a gas exchange problem?
Impaired diffusion
Shunt
V/Q mismatch
What are the causes of impaired diffusion?
Thickening of the diffusion barrier
Decrease in the surface area
Exercise
Breathing low oxygen mix
Describe why significant impairment of the diffusion of oxygen is needed before there is a reduction of PaO2 at rest.
Each erythrocyte spends 0.74s in the pulmonary capillaries.
Equilibration between alveolar gas and capillary blood occurs within 0.25s.
Equilibration occurs even when transit time is reduced.
How do we know if there is a problem with oxygen diffusion?
Problems with exercise.
Describe the equilibrium between alveolar and capillary PaCO2.
Takes 0.25s
Carbon dioxide diffusion is rarely impaired.
More diffusible than oxygen
Which diffusion would first have issues?
Diffisuion of oxygen
Describe the issue surrounding shunting.
Some blood goes not get oxygenated - lowers the PaO2.
What is a shunt?
Blood that re-enters the arterial system without going through the ventilated areas of the lung
What can be causes of a shunt?
Anatomical - e.g. cardiac defect, bronchial artery blood, myocardial blood via Thebesian veins
Pathological - e.g. pneumonia - alveoli full of exudate (pus) so not ventilated. Blood still perfuses these alveoli, but not oxygenated.
State the important factors to note about a shunt causing hypoxaemia.
Shunted blood is essentially venous blood which dilutes the oxygenated arterial blood and reduces the PaO2.
A-a gradient is increased by the shunt.
Shunt responds poorly to additional oxygen.
Shunt size can be calculated from the shunt equation.
Does impaired diffusion respond well to increased oxygen?
Does a shunt respond well to increased oxygen? Y
Yes - an oxygen diffusion problem does.
No - a shunt does not.
What determines the PO2 of the gas in each alveolus?
Ratio of ventilation and perfusion to that alveolus.
Anytime ventilation and perfusion aren’t matched, gas exchange will be altered leading to an abnormal A-a gradient.
= Wasted ventilation/perfusion
When might non-uniform distribution of alveolar ventilation and perfusion occur?
ALVEOLAR VENTILATION ISSUES
Uneven resistance to airflow
Non-uniform compliance in different parts of the lung
What are the causes of uneven resistance to airflow?
Airway collapse e.g. emphysema, asthma, bronchitis, compression by tumours or oedemas
What are the causes of uneven compliance?
Fibrosis, regional variation in surfactant production, pulmonary congestion or oedema, emphysema, atelectasis (collapse of lung tissue), pneumothorax or compression by tumours or cysts
Why might non-uniform distribution of alveoli ventilation and perfusion occur?
PERFUSION
Embolization or thrombosis Compression of the pulmonary vessels by high alveolar pressures Tumours Exudates Pneumothorax Destruction/occlusion of pulmonary vessels by various diseases Pulmonary vascular hypotension Collapse or overexpansion of alveoli
Explain why V/Q mismatch occurs in normal lungs.
Both ventilation and perfusion increase from the lung apex to the base.
Change in perfusion is greater than the change in ventilation.
A distribution of V/Q ratios exist even in a normal lung
Describe the perfusion status of alveoli at different parts of the lung.
Alveoli at apex are underperfused/overventilated
Alveoli at base are underventilated/overperfused.
What causes V/Q mismatch.
Any disease that alters ventilation or perfusion (arterial blood not fully oxygenated)
Is a V/Q mismatch responsive to supplementary oxygen?
Yes
Enriching air in low volumes
Alveoli can oxygenate blood - do not get wasted perfusion
List the factors that cause arterial hypoxaemia.
Reduced PB or FIO2 Hypoventilation Impaired diffusion Shunt V/Q mismatch
