Chapter 5: Ventilation-Perfusion Relationships

Question 1

What is the PO2 of inspired room air and how do you calculate it?

Answer 1

FiO2 x (barometric pressure - water vapor pressure)

at sea level: barometric 760, water vapor 47

Question 2

To what does the PO2 drop to, when it reaches the alveoli?

Answer 2

100 mm Hg

because it it’s made up of leftover air and replenished air from breath

Question 3

Name the alveolar ventilation equation

Answer 3

VCO2 - CO2 production, VA - alveolar ventilation, K - constant

Question 4

How do you calculate the alveolar PO2?

Answer 4

PAO2 = [FiO2 x (barometric pressure - water vapor pressure)] - PCO2/R

R respiratory quotient = 0.8

Question 5

What is the definition of a shunt?

Answer 5

Blood reaching the arterial system without going through the venitlated areas of the lungs

Question 6

What contributes to shunt in the normal lung?

Answer 6

• some of the bronchial artery blood is collected by the pulmonary veins after supplying the bronchi
• small amount coronary venous blood drains directly into the LV

Question 6

h

Name the shunt equation

Answer 6

Qs - shunt
Qt - total blood flow
CcO2 - pulmonary end-capillary O2 content
CaO2 - arterial O2 content
CVO2 - mixed venous O2 content

Question 7

What is the normal shunt fraction in a healthy individual?

Answer 7

about 5%

Question 8

Explain why a shunt is only minimally responsive to oxygen

Answer 8

• the shunted blood does not reach the oxygen even if more supplemented
• however, the blood reaching ventilated alveoli are now maximally oxygenated contributing to a slightly higher PaO2

the added O2 is mostly dissolved as the blood perfusing ventilated alveoli is nearly fully saturated

Question 9

How is the CO2 affected in patients with pulmonary shunt?

Answer 9

• the shunted blood has increased CO2 - sensed by chemoreceptors - increased minute ventilation - back to normal CO2
• hypoxemia may increase the respiratory drive - low CO2

Question 10

Explain the O2 - CO2 diagram

Answer 10

• describes the CO2 and O2 changes in the alveoli (this end-capillary) depending on the V/Q
• PO2 plotted on the x axis and PCO2 on the y axis
• if there is high V/Q - ventilation is high but alveoli not perfused - the PO2 will be high PCO2 low
• if there is low V/Q - perfusion is high but low ventilation - PCO2 will be high and PO2 low - if no ventilation at all –> these will be the same as mixed venous blood

Question 11

How do ventilation and perfusion change from top to bottom of the lungs?

Answer 11

• both increase from top to bottom
• however, blood flow increases more rapidly than ventilation

Question 12

How does the V/Q ratio change from the top to the bottom of the lungs?

Answer 12

V/Q highest at the top/apex - ventilation but almost no perfusion
V/Q increases towards the bottom as perfusion increases more rapidly

Question 13

Why do low V/Q regions affect the lungs more than high V/Q

Answer 13

• low V/Q (i.e., poor ventilation normal perfusion) leads to admixture of low PO2 blood - lowering overall PO2 in arterial circulation
• high V/Q (i.e., normal ventilation but poor perfusion) do not achieve much higher O2 concentration - even though high PO2 - due to the flattening of the Oxygen dissociation curve - contribute only little improvements to the overall arterial PO2

Question 14

What is the normal V/Q?

Answer 14

1.0

Question 15

How do high V/Q regions affect PCO2?

Answer 15

• high V/Q regions are inefficient at elimitating CO2 - because not enough perfusion to bring the CO2 to the alveolus
• creates alveolar dead space
• would increase PCO2 - however chemoreceptors sense increase PCO2 and will cause a compensatory increase in minute ventilation –> PCO2 typically remains unchanged

alveolar + anatomic dead space = physiologic dead space

Question 16

Explain why increasing ventilation with ventilation/perfusion inequality can effectively normalice CO2 but not O2

Answer 16

PCO2 decreases significantly while PO2 increases only marginally
* explained by the different shapes of the oxygen and CO2 dissociation curves
* CO2 dissociation curve is almost straight linear - i.e., increase in ventilation will increase the CO2 output
* O2 dissociation curve flattens towards the top –> high V/Q regions will not benefit much (i.e., “too saturated already”) but only areas with low V/Q benefit appeciably from more ventilation

high V/Q - CO2 responsive to more ventilation - O2 is not

Question 17

What is the normal alveolar-arterial PO2 difference?

Answer 17

10-15 mm Hg