Respiratory Pathophysiology (Sources: Revision Notes) Flashcards
What is hypoxaemia?
Low arterial oxygen tension, resulting from a pathology in the transfer of oxygen from the atmosphere to the left side of the heart
What is hypoxia?
Results from inadequate arterial oxygen tension or inadequate delivery or oxygen
Can relate to any tissue
What is the oxygen cascade?
The sequential reduction in oxygen tension that occurs with each step of the transfer of oxygen from the environment to arterial blood, under normal physiological conditions
What are the mechanisms of hypoxemia?
- Low inspired oxygen
- Alveolar hypoventilation
- Diffusion impairment
- Ventilation/perfusion mismatch
What are the clinically relevant mechanisms of low inspired O2 in the ICU?
If working at altitude - e.g. aeromedical work
Hypoxic gas mixtures - e.g. oxygen supply failure
Describe the hypoxia associated with alveolar hypoventilation
Reduction in global ventilation leads to decrease in ventilation/perfusion (V/Q) and consequent hypoxia
Vharacterised by a normal A-a gradient and is corrected by delivery of high FiO2
What pathophysiology may result in diffusion impairments?
Increase thickness of the alveolar membrane
Decrease capillary transit time and therefore insufficient opportunity for oxygen diffusion and uptake e.g. hyper dynamic states
Reduction in pulmonary capillary blood volume e.g. hypovolaemia
Describe how V/Q mismatch can result in hypoxia?
In health the V/Q varies from 0.6 at the bases to 3 at the apices. Overall it’s about 1 - meaning that almost all blood is returning to the left heart oxygenated
Reduction in ventilation relative to lung perfusion in a given lung unit results in reduction of V/Q. Physiological hypoxic pulmonary vasoconstriction will reduce flow to poorly ventilated units, however some flow persists. Blood passing through low V/Q units bypasses (or shunts) gas exchange and is returned to the left heart poorly oxygenated.
At low shunt fractions, increase in FiO2 may compensate for the reduced ventilation and provide adequate arterial oxygenation, at > 30% shunt fraction, however, increasing fio2 will not help.
What is a true shunt?
Occurs if blood passes from right to left of the heart via a route with no contact with gas exchange
This may be intra-pulmonary, in lung units with zero ventilation or intra-cardiac
How do you calculate the PO2 of inspired gas?
FiO2 x Patm
Describe the Po2 of gas entering the trachea?
Becomes humidified. The Po2 therefore needs to take into account the saturated vapour pressure of water at 37 degrees = 6.3kPa
PO2 - FiO2 = (Patm - P H20)
What is the alveolar gas equation?
PAO2 = (FiO2 x (Patm-Ph2o)) - PACO2/resp Quotient
What is Fick’s law?
Relates to diffusion
Q= A/T x D (P1-P2)
Therefore diffusion is dependent on the concentration gradient across the membrane, the surface area, the thickness and the diffusion co-efficient
How is oxygen carried in the blood?
Bound to Hb
Low solubility in plasma
Each RBC contains 2-3 million haemoglobin molecules, each capable of binding four oxygen molecules
What factors lead to a left shift of the OHDC?
Factors that result in a left shift result in an increased affinity of Hb for O2
- decrease in temp
- Decreased PaCo2
- Decreased 2,3, diphosphoglyceric acid
- increase in pH
