Ventilation and Perfusion Flashcards
What is the effect of low PaO2 on pulmonary circulation and why?
Vasoconstriction; shunts blood away from poorly oxygenated areas of lung to well-ventilated regions of lung.
What does it mean for gas to be perfusion limited?
Gas equilibrates early along the length of the capillary to alveolar air; diffusion can only be Increased if blood flow increases.
What gases are exchanged in a “perfusion limited” manner?
O2 (normally), CO2, and N20.
What does it mean for gas to be diffusion limited?
Gas does not equilibrate by the time blood reaches the end of the capillary.
In what conditions is gas exchange diffusion limited?
fibrosis, emphysema; CO.
What is the formula for diffusion?
Vgas = Area/Thickness * D(P1-P2)
*D(P1-P2) = difference in partial pressures.
What variable of the diffusion formula changes in fibrosis?
T increases (thicker wall)
What variable of the diffusion formula changes in emphysema?
Area for diffusion decreases.
What can cause oxygen to become diffusion limited?
Fibrosis (increase thickness) High altitude (changes the P1-P2 gradient) Exercise - more Hb needing to be saturated for a given Area of exchange.
What is the Ohms law equivalent for vascular resistance?
Change in pressure = Flow * Resistance,
or R = change in pressure/flow
What is the formula for pulmonary vascular resistance in relation to pressure and flow?
(P pulmonary artery - P atrium)/cardiac output
How does pulmonary vascular resistance relate to viscosity, vessel length, and vessel radius?
R = 8viscositylength/r^4
- resistance increases with viscosity and with length
- resistance decreases to the 4th power with increase in radius
What determines alveolar gas?
PAO2 = PIO2 - (PaCO2/R)
PIO2 = oxygen in inspired air PaO2 = arterial PaCO2 R = resp. quotient; CO2 produced/O2 consumed
What are normal values for PAO2 formula?
= 150mmg - PaCO2/0.8
What is the A-a gradient? What is it useful for?
PAlveolarO2 - ParterialO2 = 10-15mmg.
The A-a gradient tells you the difference between how much oxygen is in the alveolus, and how much is in the blood. An abnormally increased A-a gradient can identify a source of hypoxemia
Under what hypoxemic conditions is A-a gradient increased?
Right to left shunting (causes abnormally poorly oxygenated blood)
V/Q mismatch (abnormally matched perfusion and ventilation)
Fibrosis (diffusion limitation)
What are causes of hypoxemia with a normal A-a gradient?
High altitude, hypoventilation (eg opiod use)
What is the definition of hypoxemia?
Decreased PaO2 (pressure of oxygen in arteries)
What is the definition of hypoxia?
Decreased oxygen delivery to tissues
What are three causes of hypoxia other than hypoxemia?
Decrease in cardiac output, anemia, Co poisoning
What is V/Q and what is the ideal V/Q?
V = ventilation, Q = perfusion, ideally V/Q = 1
Where is there wasted ventilation in the lung?
Apex of lung, where V/Q = 3. (Zone 1)
Where is there wasted perfusion of the lung?
Bases of lungs, where V/Q = 0.6 (Zone 3)
How does ventilation and perfusion change from top to bottom of the lung?
Both ventilation and perfusion are greater at the base of the lung. However, there is a greater drop in perfusion at the apex of the lung than there is in ventilation; hence there is wasted ventilation at the apex.
How does exercise change ventilation/perfusion relationships in the lung?
Increases cardiac output, and vasodilation of apical capillaries; greater perfusion of the apices, V/Q ratio approaches 1.
What is going on when V/Q = 0
V has dropped to “0”, ie 0 ventilation. This represents an airway obstruction. Blood flowing past it would not be picking up any oxygen, similar to as if it were skipping the lungs altogether, like in R-L heart shunt. This is called SHUNT.
Does shunt correct with oxygen?
No.
What is going on when V/Q = infinity?
Q has dropped to 0, ie, perfusion block. Assuming < 100% dead space, 100% O2 improves PaO2.
