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VentPerf relations

Ventilation-Perfusion relationships- CJ- 1/15/2013

QuestionAnswer
Processes that determine arterial PO2 Atmospheric Po2, Ventilation, Diffusion, Shunt, V/Q inequality
Arterial hypoxemia Low blood Po2
4 causes of hypoxemia Hypoventilation, V/Q inequality, Diffusion impairment, Shunt
Alveolar PO2 calculation Inspired Po2- (PaCO2/R) R= VCO2/VO2
A-a difference calculation PAO2-PaO2
Normal PIO2 149 mmHg
Normal PaCO2 40 mmHg
Normal R .8
Normal PAO2 100 mmHg
Hypoventilation does what to PAO2 Decreases
Hyperventilation does what to PAO2 Increases
Hypoventilation does what to PACO2 Increases
Hyperventilation does what to PACO2 Decreases
Impaired diffusion does not normally cause hypoxemia alone because Spare space in the capillary equilibrates and there is usually another reason for the impaired diffusion that lends more to hypoxemia (ex. Fibrosis)
Define shunt Perfusion is normal but there is no Ventilation
Define dead space Ventilation is normal but there is no perfusion
Sources of shunt in a normal patient Bronchial arteries and drainage of coronary circulation
Hypoxemia that is not corrected with 100% O2 is due to Shunt
Explain why pure oxygen does not treat shunt The area of the lung that is ventilating normally is already on the "plateau" of the O2 dissociation curve so pure oxygen does no add more oxygen content and the shunted area is not exposed to increased O2 that is being introduced
Blood flow in the lung increases from _to_ Apex to base
Ventilation in the lung increases from _to_ Apex to base
Which is more dramatic, the change in blood flow or the change in ventilation in the lung from apex to base? Blood flow (remember he said it can approach zero blood flow because the alveoli are stretched open because of gravity which constricts capillaries)
The V/Q ratio decreases from _to_ Apex to base
The alveolar composition at the apex of the lung is similar to Inspired air, High PO2 and Low PCO2
The alveolar composition at the base of the lung is similar to Pulmonary arterial blood, Low PO2 and high PCO2
In a shunted lung, alveolar composition would be O2=40 and CO2=45, Same as arterial blood
In dead space, alveolar composition would be O2=150 and CO2=0, Same as inspired air
V/Q inequality depresses arterial Po2 because Most blood goes through areas where PAO2 is low(base) and because of the shape of the O2 dissociation curve (plateau for medium and high V/Q and "steep" for low V/Q)
Why is arterial PCO2 often normal with V/Q inequality? V/Q inequality interferes with CO2 transfer as well as O2 transfer so the respiratory system responds to increased CO2 by increasing ventilation. This decreases PCO2 (shape of the curve, linear and steep!) but does not effect the O2 (already plateaued)
If PaCO2 is high without an A-a difference, hypoxemia is due to Hypoventilation only
If PaCO2 is normal and there is no A-a difference, hypoxemia is due to Decreased inspired oxygen
If PaCO2 is normal, there is an A-a difference and the PaO2 is correctable with pure O2, hypoxia is due to Increased V/Q inequality
If PaCO2 is normal, there is an A-a difference and the PaO2 is NOT correctable with pure O2, hypoxia is due to Shunt
If PaCO2 is high and there is an increased A-a difference, hypoxia is due to Hypoventilation plus shunt or V/Q inequality
Created by: mcasto