Lecture 27/28: Ventilation-perfusion Flashcards
Describe the different zones and how pulmonary perfusion changes
Zone 1: usually not seen physiologically, seen with hemorrhage or positive pressure venilation. PA>Pa>Pv. leads to capillary compression.
Zone 2: apex of the lung. Pa>PA>Pv. some compression of the capillary (closer to the veins). At the bottom of zone 2, there is recruitment due to increased hydrostatic pressure.
Zone 3: middle of the lung. Pa>Pv>PA. At this point, there is recruitment of capillaries since at all points through the blood vessel, the pressure is higher than in the alveoli. At the bottom of Zone 3, there is some distension. Resistance falls, so perfusion increases.
Zone 4: extra-alveolar capillaries. Pa>Pv>PA. at the bottom of the lung, intrapleural pressure is more positive due to the weight of the lungs, so the capillaries aren’t pulled open as much, so there is less perfusion.
Alveolar vessels have continually increasing flow from apex to base, extra-alveolar vessels have continually decreasing flow
How does pulmonary vascular resistance change with changes in arterial pressure? How do changes in alveolar vessel and extra-alveolar resistance get affected by lung volume?
Pulmonary vascular resistance decreases with increased arterial pressure because of distension and recruitment. Alveolar vessel resistance increases as lung volume increases since the alveoli compress some of the capillaries. Extra-alveolar vessel resistance decreases as lung volume increases due to the more negative intrapleural pressure.
What are the local effects on perfusion of hypoxia, hypercapnia, the ANS, and metabolic factors?
Basically the opposite of what you would expect in the systemic circulation. Hypoxia will cause vasoconstriction through mitochondrial O2 sensor, inhibits K+ channel causing depolarization and contraction. hypercapnia will cause vasodilation. ANS will have mild dilation due to parasympathetic innervation, sympathetic innervation will decrease compliance (NOT increase resistance)
metabolic factors have relatively low response
What is the alveolar ventilation equation?
VA = k*VCO2/PACO2, where k is usually 0.863
What do you have to account for when calculating the partial pressure of inspired oxygen?
Calculate the water pressure (ex. moist air in the trachea) so you subtract that from the barometric pressure
Describe what a graph of alveolar PCO2 vs alveolar ventilation looks like, and how it changes with exercise
As ventilation increases, PCO2 will decrease (exponential function). Exercise will cause a right shift. So for a given ventilation, there is higher PCO2.
Describe the alveolar gas equation
PAO2 = PiO2- PACO2/R where R is the respiratory quotient (exchange ratio between CO2 and O2, usually less than 1)
This describes how much oxygen is exchanged for carbon dioxide at the end of inspiration
How can you calculate breath by breath changes in alveolar PO2? (Assuming FRC is about 3000, and PAO2 after expiration is 97mmHg)
Have PAO2 after expiration (97mmHg), so to calculate PAO2 after inspiration, multiply the 97FRC, and add on the new amount of inspired oxygen (PiO2(TV-Vd)) (tidal volume - dead space)
How does ventilation change as you move from the apex to the base of the lungs?
Ventilation increases as you move from the apex to the base of the lungs. We see a volume vs intrapleural pressure graph, and it gives us an left hook exponential function due to gravity and posture. since alveoli are more collapsed in the base, the change in interpleural pressure will have a bigger effect.
How does reduced compliance, increased resistance, and mass in the lung affect the ventilation?
Reduced compliance will decrease the volume change, since there is lower FRC and lower TV.
Increased resistance will cause a smaller slope of volume change, but will catch up over time. Mass in the lung will cause a lower FRC, but will have the same proportional TV.
How does the ventilation-perfusion ratio (VA/Q) change with height in the lung? How does it affect gas composition?
Both drop off as you go towards the apex, perfusion (Q) drops off faster, so you end up with a left uppercut (dabbing)
Inspired air has a VA/Q of infinity since there is no perfusion, similarly, venous blood has a VA of zero since there is no ventilation
Describe the V/Q mismatch and compensation for when there is no perfusion, and when there is no ventilation
When there is no perfusion on one side, V/Q approaches infinity. Therefore, the other lung must decrease V/Q to compensate, and the lung without perfusion must compensate to decrease V/Q. The other lung decreases V/Q by increasing perfusion. The blocked lung must decrease ventilation, so it senses the drop in CO2, this causes bronchiolarconstriction, so there is less ventilation. There is also less perfusion of type 2 alveolar pneumocytes, so there is less surfactant release, which causes collapse and thus less ventilation.
when there is no ventilation on one side, V/Q approaches zero. Blocked side compensates by trying to decrease perfusion as well, other side compensates by trying to increase ventilation. Blocked side drops perfusion since there is no O2 sensed, so there is arterial vasoconstriction. Air follows the path of least resistance and increase ventilation to the other side
