Gas exchange Flashcards
Dalton’s law
The total pressure in a mixture of gases is the sum of presures exerted independently by each gas in the mixture
What is the pressure exerted by each gas?
Partial pressure
P
Partial pressure
PI
Pressure in inspired air
PA
Alveolar pressure
Pa
Arterial pressure
PB
Barometric pressure
Q or Q.
Volume of blood (perfusion) or
Volume of blood per unit time
Alveolar hypoventilation
A decrease in alveolar ventilation in relation to CO2 production
What are the effects of alveolar hypoventilation?
Elevates the pulmonary pressure of CO2
Decreases the pulmonary pressure of O2
What causes alveolar hypoventilation
Damage to CNS (drugs, trauma)
Peripheral nerve injury
Damage to pump (muscle paralysis, trauma to chest, bloated abdomen)
Lung resisting inflation (airway obstruction and decreased lung compliance
Alveolar hyperventilation
Decreases PACO2
Increased PAO2
Occurs in response to hypoxia, acidosis or hyperthermia
Ficks law (V. Gas)
V.gas = [A x D (P1-P2)] /T
Ficks law variables
V.gas- rate of diffusion
A- surface area
D= diffusion coefficient
T= thickness of tissue
How does thickness of tissue affect the rate of gas diffusion ?
Increased thickness will slow diffusion
How increases the rate of gas diffusion?
Increased surface area
Exercise-associated Hypoxemia
In strenuous exercise PVO2<40
Cardiac output is higher, blood flow is faster, less time available for equilibrium
What is the direction if diffusion?
Venous blood to alveoli
Is O2 or CO2 a better indicator of lung disease?
O2 because lung diseases have a minimal effect on CO2 diffusion
Perfusion
Blood flow in pulmonary capillaries
What happens with reduced alveolar ventilation?
Decreased PO2 and increased PCO2 in alveoli
Perfusion will reduce to match ventilation
Vasoconstriction
What happens with enhanced alveolar ventilation?
Increased PO2 and decreased CO2 in alveoli
Enhanced perfusion
Dilatuon
What does V./Q. Ratio determine?
Adequacy of pulmonary exchange
ideal: 0.8
Low V./Q. Ratio
Overperfused and under ventilated
Blood leaving is low in O2 and high in CO2
Ex: airway obstruction and right to left shunt
High V./Q. ratio
High ventilation
Blood after obstruction is higher in O2 and lower in CO2
Ex: vascular obstruction, blood doesn’t flow to thwart (dead space)
Shunt
Perfusion without ventilation
Wasted perfusion
Refractory to oxygen therapy
Hypoxemia without hyoercarbia
Type 1 respiratory failure
Dead space
Ventilation without perfusion
Wasted ventilation
Hypoxemia with hypercarbia
Type 2 respiratory failure
Pneumonia
Large portions of blood flows to units with low V./Q.
Blood passing though right to left shunts is increased
Isoshunt diagram
Plots the relationship between FiO2 and PAO2 against a set of virtual shunt lines
Used to estimate shunt % by O2 therapy
What causes right- to - left vascular shunt to allows blood to bypass the lung?
Atelectasis
Acute obstruction
Acute pneumonia
What affects PaO2 and PaCO2?
Inspired air
Alveolar ventilation
Alveolocapillary diffusion
Ventilation/ perfusion mismatch
