7 - Vent/Perfusion relationships; Hypoxemia
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- Give the values of the PO2 gradient between the atmosphere and the tissues.
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Atmosphere = 150
Nasal cavity = 140
Alveoli = 100
Pulmonary capillary = 98
artery = 96
tissues = 20 - What is the alveolar gas equation (in its simplified form)?
- PAO2 = PiO2 - (PACO2/0.8)
- What is a normal value for the A-a difference? List three causes for inflation of this value.
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*2-12mmHg
1.diffusion barrier
2.Anatomical shunt
3.Perfusion-ventilation inequality - What is an anatomical shunt?
- Any circulation that bypasses the lungs and does not contribute to the oxygenation of blood leaving the left heart.
- What are two non-pathological examples of an anatomical shunt?
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*the thebesian veins, which drain into the left ventricle
*the bronchial veins, which drain into the pulmonary veins - What percentage of cardiac output is normally shunted around the lung? What is the percentage in cases of atrial septal defect?
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*1-2%
*50-60% - What is a pathological shunt? Does supplemental oxygen help patients with pathological shunts?
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*An airway is blocked, thus blood perfusing that area of the lung is not oxygenated
*no, the airways are still blocked and thus the blood does not come into contact with oxygen - How is PaCO2 affected in cases of pathological shunt?
- PaCO2 is usually normal, as increased PCO2 triggers hyperventilation thus allowing the lungs to breath off excess CO2.
- What two factors determine the PO2 of blood exiting a pulmonary capillary bed?
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*rate of perfusion (Q)
*rate of alveolar ventilation (V) - What is a normal value for the ratio of ventilation to perfusion?
- 0.8-1, indicating that ventilation and perfusion are nearly equal.
- How is an alveolar obstruction reflected in the ventilation-perfusion ratio? Track the PO2 of the blood as it enters and leaves the capillary perfusing this alveolus.
- In this case, V/Q will be less than 1 as alveolar ventilation is reduced while perfusion remains constant. The blood passing through remains at the same PO2. This is equivalent to a pathological shunt.
- How is capillary blockage reflected in V/Q? What happens to the PAO2 of the alveoli ventilating such a capillary?
- In this case, V/Q is greater than 1 because Q decreases while V remains constant. PAO2 rises to 150mmHg because no oxygen is being drawn out by blood.
- Describe the vertical gradient of V/Q through the lung. What value do the majority of alveoli operate at?
- It is highest in the upper regions (4) and decreases through the middle (1) and lower (0.7-0.8) regions. The majority of alveoli operate at V/Q = 1.
- What happens to the value of V/Q in patients with emphysema? What causes this effect?
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*V/Q becomes greater than 1
*alveoli coelesce such that some capillaries continue to serve unventilated areas and this reduces the effective perfusion
*similar to physiological dead space
*ventilation remains ~constant - What happens to the value of V/Q in patients with bronchitis? What causes this effect?
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*V/Q is less than 1
*airways are blocked and ventilation decreases
*perfusion remains constant
*this can be thought of as a pathological shunt - What happens to the vertical gradient of V/Q during exercise?
- The gradient disappears as the entire lung is used to provide oxygen to the body.
- List four pulmonary causes for pathophysiological hypoxemia.
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1.V/Q mismatch
2.Physiological shunt
3.Diffusion impairment
4.Hypoventilation - Name three clinical conditions that result in a V/Q mismatch. What happens to the A-a difference in these cases?
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*Asthma, bronchitis, cystic fibrosis
*A-a difference increases - Name two clinical conditions that cause physiological shunts. What happens to the A-a difference in these cases? Will supplemental oxygen help these patients?
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*ARDS, COPD, arterial-venous fistulas
*A-a difference increases
*supplemental oxygen will not help (except for a minimal increase in dissolved gas) - Where is the pathology in cases of hypoventilation? What happens to the A-a difference in these cases? Will supplemental oxygen help these patients?
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*Typically in the regulation of respiration, rather than in the lung itself
*no change in A-a
*supplemental O2 will help - Name two clinical conditions that cause diffusion impairment. What happens to the A-a difference in these cases? Does supplemental O2 help?
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*fibrosis, pulmonary edema
*A-a increases
*patients will respond to supplemental O2 - Name 3 non-pulmonary causes of hypoxemia.
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1.Low atmospheric PO2 (altitude)
2.Intercardiac right-to-left shunt
3.Low O2 carrying capacity of blood (anemia, CO poisoning)