Ventilation-Scharf Flashcards
What is the total ventilation per minute on average?
7500 mL/min
What is the average normal respiratory rate?
17.5 +/- 2.5
What is average tidal volume?
500mL/breath
What is the approximation of dead space/conducting airway volume?
1cc/lb so 150mL for 150lb healthy person, 1/3 of tidal volume.
What is the average alveolar ventilation/minute volume
(Tidal volume-dead space)*resp rate, about 5250mL/min
Volume subdivisions of lung volumes
IC=TLC-FRC
ERV=FRC-RV
IRV=TLC-IC+TV
VC=TLC-RV
Gas dilution spirometer calculation of FRC
C1xV1=C2x(V1+V2)
V2 is the volume of the lung, can measure anytime (start of FRC)
How does a plethysmograph work?
Relies on Boyle’s law. Patient is inside a closed container. Once breathed out, close mouthpiece and pant. Pressure changes in the box reflecting changes in alveoli. Measures the Thoracic Gas Volume (same as FRC, including dead space).
Fowler’s method of measuring FRC (or dead space)
Uses single breath N2 test by giving 100% O2.
Phase I: Inhaling O2, N2 decreases
Phase II: N2 at zero because no N2 in the dead space (no blood) before expiration during beginning of exhale.
Phase III: N2 increases because it comes from alveoli, followed by end of expiration.
Bohr’s method for measuring dead space fraction of total volume
PACO2 is assumed to be the same as blood.
Equation to calculate the partial pressure of CO2 relative to alveolar flow
PCO2 is equal to arterial PCO2, determined by ratio of alveolar ventilation (CO2 flow) to arterial production (alveolar flow)
To halve PCO2, double the ventilation rate.
Anatomic vs Physiologic dead space
Anatomic = volume of conducting airways
Physiologic = based on gas exchange
How do alveoli compare at the apex and the base?
More open at the apex but stiffer and harder to ventilate.
