Trachte: Ventilation Perfusion Relationships Flashcards
What are the causes of hypoxemia?
- hypoventilation
- Diffusion limitation
- shunt (left-right)
- Ventilation-perfusion mismatch
What are O2 levels in humidified air?
150 mmHg
What are O2 levels in the alveoli?
100 mmHg
What happens to O2 in the alveoli if it is being perfused normally? What replenishes oxygen?
It’s constantly being REMOVED by blood
Inhalation of fresh air
What is the oxygen level in tissues?
1-100 mmHg
What is hypoventilation?
If O2 is not replenished fast enough in the alveoli–>
alveolar PO2 DECLINES
What causes hypoventilation?
Drugs (opiates)
chest wall damage
paralysis
What does hypoventilation do to PCO2?
INCREASES PCO2
How do you calculate PCO2?
PCO2 = (VCO2/VA) x K
How do you calculate PAO2?
PAO2 = PIO2 – [PACO2/R]
PAO2 is alveolar PO2
PIO2 is inspired air PO2
PACO2 is alveolar PCO2
R is the respiratory quotient (produced CO2/consumed O2)
What is R and what is it dependent on?
THe respiratory quotient (produced CO2/consumed O2)
Dependent on SUBSTRATE
What is R w/ carbohydrate as substrate? w/ FA as substrate?
R = 1 with carbohydrate as substrate
R = 0.7 with fatty acid as substrate
How can impaired diffusion result in hypoxia?
- Reducing the area available for gas exchange
2. Increasing the diffusion distance
Where does a natural shunt occur?
Naturally occurs with mixing of O2 depleted blood from the bronchial circulation and a small amount of blood from the thesbian veins of the heart
What else can cause shunts?
- congenital cardiac abnormalities
2. PDA
How do you calculate shunt flow?
Qs/Qt = (CcO2 – CaO2)/CcO2 – CvO2)
How do you calculate capillary O2?
From PAO2
Can 100% O2 correct the hypoxemia resulting from a shunt?
NO
What are two of the influences on Alveolar PO2?
Ventilation and perfusion
What is the major cause of hypoxia in lung diseases?
Mismatch of ventilation and perfusion
**also affects alveolar PCO2
What happens to PO2 and PCO2 if there is no perfusion of a unit but the alveoli are still ventilated?
PO2 and PCO2 approach that of inspired gas (150 and 0 mmHg)
What happens to PO2 and PCO2 if there IS perfusion but NO ventilation?
PO2 and PCO2 approach that of venous blood
40 and 46 mmHg
Why do gradations of ventilation-perfusion exist?
Because the lung is perfused and ventilated differently in the APEX and BASE
What is the ventilation-perfusion ratio in the APEX? What are PO2 and PCO2?
HIGH
PO2= 132 mmHg
PCO2= 28 mmHg
What is the ventilation perfusion ratio in the base of the lung?
LOW
PO2= 89 mmHg PCO2= 42 mmHg
Does pH change from apex to base of the lung? Why?
YES
pH is influenced by CO2
What affect do ventilation-perfusion mismatches have on PO2 and PCO2?
How do lung diseases affect mismatches?
Slight lowering of arterial PO2 <4 mmHg
Slight elevation of PCO2
MUCH MORE DRAMATIC EFFECTS
Can hypoxia or hypercapnia be rectified by more rapid breathing?
Hypercapnia
What is a function of the rate of breathing?
CO2 elimination
What determines the differences in O2 and CO2 responses to more rapid breathing?
Different shapes of their dissociation curves
What is the dissociation curve like for CO2?
Linear in the physiological range
What is the dissociation curve like for O2?
sigmoidal and almost flat at the top of the range
Why is PCO2 close to normal even in lung disease?
B/c alterations in arterial PCO2 will result in more rapid breathing–> this increases the work of breathing
How do you assess ventilation perfusion abnormalities?
Use the alveolar gas equation to calculate what arterial PO2 should be and compare it w/ what is observed
