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Lecture

Respiratory System Part 7

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Department
Biomedical Physio & Kines
Course
BPK 306
Professor
Mike Walsh
Semester
Fall

Description
Respiratory Section #7 VII. PULMONARY GAS EQUATIONS: A. Physiological Dead Space: Bohr Eq’n a. All the space within the respiratory system that is not involved in gas exchange 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 volume e. Bohr Eq’n: i. VT*FECO2 = VA*FACO2 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 incI FO2 (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 vi. 5 levels to the eq’n: 1. The eq’n 2. Decrease PACO2  increase PAO2 3. Understanding pressure in alveoli (PT = PN2 +PO2 + PCO2). Remember that at end of expiration and inspiration, alveolar pressure is atmospheric 4. Understand that VArate is rate of entry for O2 and rate of
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