Chapter 5: Ventilation-Perfusion Relationships Flashcards

1
Q

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

A

FiO2 x (barometric pressure - water vapor pressure)

at sea level: barometric 760, water vapor 47

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2
Q

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

A

100 mm Hg

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

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3
Q

Name the alveolar ventilation equation

A

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

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4
Q

How do you calculate the alveolar PO2?

A

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

R respiratory quotient = 0.8

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5
Q

What is the definition of a shunt?

A

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

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6
Q

What contributes to shunt in the normal lung?

A
  • 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
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6
Q

h

Name the shunt equation

A

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

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7
Q

What is the normal shunt fraction in a healthy individual?

A

about 5%

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8
Q

Explain why a shunt is only minimally responsive to oxygen

A
  • 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

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9
Q

How is the CO2 affected in patients with pulmonary shunt?

A
  • the shunted blood has increased CO2 - sensed by chemoreceptors - increased minute ventilation - back to normal CO2
  • hypoxemia may increase the respiratory drive - low CO2
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10
Q

Explain the O2 - CO2 diagram

A
  • 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
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11
Q

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

A
  • both increase from top to bottom
  • however, blood flow increases more rapidly than ventilation I.e v/a highest at apex
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12
Q

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

A

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

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13
Q

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

A
  • 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
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14
Q

What is the normal V/Q?

A

1.0

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15
Q

How do high V/Q regions affect PCO2?

A
  • 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

16
Q

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

A

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

17
Q

What is the normal alveolar-arterial PO2 difference?

A

10-15 mm Hg