Hypoxaemia Flashcards
Shunt
Occurs when blood passes between venous and arterial circulation without contact with the alveoli.
Results in a systemic arterial O2 content which is in between the level in the pulmonary alveoli and systemic veins.
Increasing inspired O2 does not change the hypoxia as O2 never reaches the alveoli to be exchanged.
CO2 is not increased because chemoreceptors cause hyperventilation to compensate for hypercarbia
Causes of hypoxaemia
Hypoventilation
Shunts Impaired diffusion
V/Q mismatch
Low inspired oxygen levels e.g. high altitude
Why does hypoventilation cause hypoxaemia?
In hypoventilation there is a reduction in the flow of air to the lungs. The blood flow to the alveoli remains the same but less gas exchange occurs. The oxygen tension decreases and CO2 tension increases. Rise in CO2 levels cause arterial pH to drop.
Causes of shunting of blood
VSD/ASD
arterio-venous fistula
intrapulmonary shunt - caused when the airway is obstructed
How does high altitude affect ventilation
Hyperventilation is caused by hypoxic stimulation of peripheral chemoreceptors
Low pCO2 causes alkalosis which inhibits the increase in ventilation.
CSF alkalosis is compensated by the choroid plexus which export HCO3- returning the pH to normal. Breathing is controlled at a new lower pCO2.
Blood alkalosis is compensated within a few days by renal HCO3- secretion.
The sensitivity of the carotid bodies to hypoxia is increased during acclimatisation.
What long-term physiological changes occur in high altitudes?
Hypoxaemia causes renal release of EPO which stimulates RBCs. Polycythemia increases O2 capacity of the blood.
Increase in 2,3 BPG to lower Hb affinity for O2
Density of muscle capillaries increases. Increase in oxidative enzymes.
Pulmonary hypoxic vasoconstriciton
Effect of V/Q mismatch on blood gas levels
Gas levels in the alveoli are determined by V/Q ratio. Mismatch in V/Q ratio reduces efficeincy of gas exchange.
Increasing ventilation increases O2 and reduces CO2 in end capillary blood.
Increasing perfusion increases CO2 and reduces O2 in end capillary blood.
Reduction in ventilation (low V/Q ratio) occurs in consolidation. Blood flow through the area is less oxygenated. PE results in a higher V/Q ratio, perfusion to the lung is reduced.
Describe the changes in respiration that occur in exercise
Oxygen utilisation of CO2 production increases ventilation up to 10 times during exercise.
Increase on pCO2 causes an increase in ventilation (dynamic phase) which increases pO2, followed by a plateau phase where ventilation is matched to meet metabolic needs.
When available oxygen is exhaused e.g. heavy exercise, cells respire anaerobically, producing lactic acid, which further accelerates an increase in ventilation by causing metabolic acidosis.
The O2 dissociation curve moves to the right in tissues due to reduced pH and increased temperature. This reduces Hb affinity for O2 in the tissues so delivevry is increased.
What is a pulmonary arterio-venous malformation?
Abnormal direct connections between the pulmonary arteries and veins.
Acts as an arteriovenus shunt so blood passing through them does not come into contact with gas in the alveoli. Therefore causes hypoxaemia. Often multiple. Common in hereditary Haemorrhagic Telangiectasia.
Also allows passage of substances like bacteria (forms abscesses) and emboli (stroke)
Describe the adaptations that occur in gradual exposure to increasing altitude
As altitude increases, peripheral chemoreceptors detect hypoxia. Central chemoreceptors prevent excessive increases in ventilation which would produce hypocapnia.
Small increases in ventilation in response to high altitude cause CSF and blood to become alkaline. Cells in the choroid plexus excrete HCO3- to maintain the pH of the CSF. Kidneys excrete HCO3- to return blood pH to normal.
Breathing is controlled at a new, lower pCO2.
