Pulmonary 6: Ventilation-Perfusion Relationships Flashcards
Define ventilation. (Work out the equation for adults/children.
V= f x Vt
(15min x 500mL=7.5L/min)
children = 3-5mL/kg
What are typical (volumes) gas:blood ratio? (flows) gas: blood?
Diagram a lung showing typical volumes and flows. (Although there is variation around these values).
Tidal volume, total ventilation, anatomic dead space, frequency, alveolar gas volume, alveolar ventilation, pulmonary capillary blood, pulmonary blood flow
Slide 5
Volumes
gas: blood
43: 1
Flows
Gas: blood
almost 1:1
What is the functional unit of the lung?
What would happen if all inspired air was diverted to L lung and all the right CO diverted to R lung, what would occur?
The functional unit of the lung is the alveolus including its capillary blood flow.
If all the inspired air was diverted to the L lung and all the right CO was diverted to the R lung, no gas exchange could take place.
What do V/Q ratios dictate? Provide units.
What is the result of collective alveolar units perfusing blood? How do PO2 and PCO2 change?
What is an ideal lung?
Ventilation/perfusion (V/Q) ratios dictate the amount of oxygen added to and CO2 removed from pulmonary capillary blood.
V=mL air/min =mL O2/min
Q=mL blood/min
V/Q= mL O2/mL blood
The collective effort of all alveolar units operating on perfusing blood elevates the PO2 from 40-100mmHg and decrease the PCO2 from 46 to 40mmHg.
Ideal lung (V/Q=1)
What are the PO2 and PCO2 values in:
dry inspired air humidified tracheal air alveolar air mixed venous blood systemic arterial blood
dry inspired air
PO2: 160
PCO2: 0
humidified tracheal air
PO2:150
PCO2: 0
alveolar air
PAO2: 100
PACO2: 40
mixed venous blood
PvO2: 40
PvCO2: 46
systemic arterial blood
PAO2: 100
PACO2: 40
How do you calculate the PO2 of inspired air?
PO2 falls as the gas moves from the atmosphere
in which we live to the mitochondria where it is utilized. The PO2 of air is 20.93% of the total dry gas pressure (that is, excluding water vapor). At sea
level, the barometric pressure is 760 mm Hg, and at the body temperature of 37°C, the water vapor pressure of moist inspired gas (which is fully saturated with water vapor) is 47 mm Hg. Thus, the PO2 of inspired air is (20.93/100) × (760 − 47), or 149 mm Hg (say 150)
Draw a scheme of O2 partial pressures from air to tissues (atmosphere to mitochondria). Draw ideal/hypoventilation lines. How does PO2 change in air, lung blood, and tissues?
What effect does hypoventilation have on PO2 in alveolar gas?
Why does PO2 fall to about 100mmHg by time reaches alveoli? (What determines PO2 of alveolar gas?)
Slide 6
Scheme of the O2 partial pressures from air to tissues. The solid line shows a hypothetical perfect situation, and the broken line depicts hypoventilation. Hypoventilation depresses the PO2 in the alveolar gas and, therefore, in the tissues.
shows by time O2 has reached alveoli, PO2 has fallen to about 100mmHg (about one-third). This is because the PO2 of alveolar gas is determined by a balance between the removal of O2 by pulmonary capillary blood and its continual replenishment by alveolar ventilation on the other.
When systemic arterial blood reaches tissue capillaries, O2 diffuses to mitochondria where PO2 is much lower.
Describe the V/Q ratio at the level at a single alveolus and at the level of the whole lung.
What is a normal V/Q ratio and what does that mean?
single alvelolus-
alveolar ventilation: capillary flow
whole lung-
total alveolar ventilation: cardiac output
normal is 0.8 (V/Q ratio less than one means perfusion exceeds ventilation)
Draw a graph with PO2 on the horizontal axis and PCO2 on the vertical axis.
How does V/Q ratio change. In what situations does it increase/decrease from normal? What are the values with normal/increased/decreased ratio?
Explain.
Slide 8 and 9
shunt alveoli are perfused but not ventilated, V/Q ratio =0; alveolar equals mixed venous blood PCO2 and PO2; no gas exchange occurs
as ventilation increases toward a V/Q ratio of 1, normal alveolar and blood gases are achieved.
When perfusion decreases while ventilation is maintained (V/Q ratio increased > 1) alveolar PO2 and PCO2 approach the level of inspired gas.
Alveoli that are ventilated but not perfused (infinite V/Q ratio) contribute to physiological dead space- no gas exchange
What area of the lung receives the largest percentage of ventilation and perfusion?
With distance up the vertical lung will there be a greater drop in blood flow or in ventilation?
The base of the lung receives the largest percentage of ventilation and perfusion. Conversely, the apex receives the lowest percentage of ventilation and perfusion.
With distance up the vertical lung, the drop in blood flow is greater than the drop in ventilation, again due to the fact that gravity exerts a greater effect on the high density blood than on low density air.
From apex to base how does ventilation and perfusion change and which changes faster?
Will apex/base V/Q ratios be greater or less than 1? So how does the ventilation perfusion ratio change as you go up the lung?
Show on a graph rib number on horizontal axis, I/min % of lung volume on L vertical axis and ventilation perfusion ratio on R vertical axis. Draw lines for blood flow, ventilation and V(A)/Q ratio
From apex to base ventilation increases more slowly than perfusion
apex V/Q >1
base V/Q
Analyze the graph on slide 11.
Where are ventilation and blood flow ideally matched? What does this mean in regards to blood flow vs ventilation at the apex/base?
Only in the middle region of the vertical lung are ventilation and blood flow ideally matched.
Base/bottom of lung there is much more blood flow than ventilation, apex/top of lung there is much more ventilation than blood flow.
Ratio is very low at base, ideal in middle, and very high at apex
What is the V/Q ratio like at the apex of the lung? What does this mean in regards to turnover of alveolar gas?
How is PAO2 affected? PACO2?
At the apex of the lung where V/Q is high, the turnover of the alveolar gas with fresh air is more frequent than normal.
This causes alveolar PAO2 to be higher and alveolor PACO2 to be lower than in the middle regions of the lung.
What is the V/Q ratio like at the base of the lung? What does this mean in regards to turnover of alveolar gas?
How is PAO2 affected? PACO2?
At the base of the lung where V/Q is low, the turnover of the alveolar gas with fresh air is less frequent than normal. This causes the alveolar PAO2 to be lower and the alveolar PACO2 to be higher than found in the middle regions of the lung.
On a graph comparing PO2 on horizontal axis and PCO2 on vertical axis, where on the curve would you see low VA/Q or high VA/Q?
Slide 12
