Chapter 5: Ventilation-Perfusion Relationships Flashcards
What is the PO2 of inspired room air and how do you calculate it?
FiO2 x (barometric pressure - water vapor pressure)
at sea level: barometric 760, water vapor 47
To what does the PO2 drop to, when it reaches the alveoli?
100 mm Hg
because it it’s made up of leftover air and replenished air from breath
Name the alveolar ventilation equation
VCO2 - CO2 production, VA - alveolar ventilation, K - constant
How do you calculate the alveolar PO2?
PAO2 = [FiO2 x (barometric pressure - water vapor pressure)] - PCO2/R
R respiratory quotient = 0.8
What is the definition of a shunt?
Blood reaching the arterial system without going through the venitlated areas of the lungs
What contributes to shunt in the normal lung?
- 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
h
Name the shunt equation
Qs - shunt
Qt - total blood flow
CcO2 - pulmonary end-capillary O2 content
CaO2 - arterial O2 content
CVO2 - mixed venous O2 content
What is the normal shunt fraction in a healthy individual?
about 5%
Explain why a shunt is only minimally responsive to oxygen
- 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
How is the CO2 affected in patients with pulmonary shunt?
- the shunted blood has increased CO2 - sensed by chemoreceptors - increased minute ventilation - back to normal CO2
- hypoxemia may increase the respiratory drive - low CO2
Explain the O2 - CO2 diagram
- 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
How do ventilation and perfusion change from top to bottom of the lungs?
- both increase from top to bottom
- however, blood flow increases more rapidly than ventilation I.e v/a highest at apex
How does the V/Q ratio change from the top to the bottom of the lungs?
V/Q highest at the top/apex - ventilation but almost no perfusion
V/Q increases towards the bottom as perfusion increases more rapidly
Why do low V/Q regions affect the lungs more than high V/Q
- 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
What is the normal V/Q?
1.0
How do high V/Q regions affect PCO2?
- 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
Explain why increasing ventilation with ventilation/perfusion inequality can effectively normalice CO2 but not O2
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
What is the normal alveolar-arterial PO2 difference?
10-15 mm Hg
