BPK 306 Lecture Notes - Alveolar Pressure, Blood Gas Tension, Hypercapnia

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Respiratory Section #7
A. Physiological Dead Space: Bohr Eq’n
a. All the space within the respiratory system that is not involved in gas
b. Value can be larger than anatomical dead space b/c it includes lung areas
where ventilation is wasted (non anatomical dead space area that receives
poor BF)
c. Thus, Bohr eq’n estimates volume of lung that does not eliminate CO2
d. Physiological dead space always greater than anatomical dead space
e. Bohr Eq’n:
ii. VD/VT = (PACO2 – PECO2)/PACO2
iii. If expired PCO2 is low due to anatomical dead space, and
physiological dead space, then VD/VT ratio increases
iv. VD/VT: 0.25 to 0.35 in young adults
1. increases w/ age n abnormalities
2. Higher ratio, greater ventilatory work for given O2 uptake
3. Physiological dead space is major reason for CO2 retention
v. High VD can cause relative alveolar hypoventilation and
hypoemia. Best to treat with augmented ventilation than incr FIO2
(need to avoid incr in PaCO2 and respiratory acidosis)
B. Alveolar Ventilation Equation:
a. VArate = VErate – VDrate
b. VArate = (VCO2rate/PaCO2)*K
i. Now we can find VCO2rate and PaCO2 to find VArate
ii. PaCO2 is the most important measurement to evaluate pulmonary
problems. This value tells us if VArate is adequate for CO2
production rate
C. Alveolar Gas Equation
a. Analogous to alveolar ventilation equation with PCO2 is alveolar gas eq’n
with PO2
b. (VIrate*FIO2) – (VErate*FEO2) = VO2rate
c. PAO2 = PIO2 – (PACO2/R) + F
i. If we know PACO2 we can find PAO2
ii. PACO2 is estimated from blood gas measurement of PaCO2
iii. With concurrent blood gas measurement of PaO2, alveolar blood
PO2 diff can be found
iv. This will tell us if there’s any significant ventilation:perfusion
inequalities. Won’t tell us if it’s due to high or low VArate/Qrate
v. Bohr eq’n tells us if VArate/Qrate is high
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