Chapter 2: Ventilation Flashcards
How can you measure the static volumes of the lung? What volume of the lung can it not measure?
- water spirometer
- electronic devices nowadays available
- cannot measure the functional residual capacity or the residual volume
How can you measure the functional residual capacity and residual volume?
- Gas dilution technique
- Body plethysmograph
How do you calculate the functional residual capacity using the gas dilution technique - explain how it works and give the formula
subject exhales and then breathes in gas from the spirometer with a known volume and concentration of helium (C1xV1) - Helium is virtually insoluble in blood (i.e., no helium is lost) - after breathing the cc of helium equilibrizes between subject and spirometer (C2 x (V1+V2)), where V2 is the functional residual capacity
no helium lost so C1 x V1 = C2 x (V1+V2)
rearrange euqation to V2 = V1 x (C1-C2 / C2)
What is the principle of Boyle’s law?
pressure x volume is constant at constant temperature
How do you measure FRC with body plethysmography
subject is in an eclosed box and inhales with a blocked mouth piece - subject will expand lungs and increase lung volume but decrease pressure - since box is enclosed the volume in the box drops and the pressure increases (boyle’s law)
P1V1 = P2 (V1 - change in V) - box
change in volume same in chest and pressure gradient measured, so:
P3V2 = P4 (V2 + change in V) - V2 is FRC
How do the FRC measurements from gas dilution and plethysmography differ and in what situation could this matter?
gas dilution technique only measures communicating gas
plethysmography measures the total volume
- if the is lung disease leading to airway obstruction and air trapping, gas dilution may not measure this volume
Fill in the gaps
How does the inspired and expired volume differ?
inspired volume is slightly higher, because more O2 is absorbed than CO3 released
What is the alveolar ventilation?
volume of FRESH gas entering the respiratory zone of the lungs each minute
What is the dead space ventilation?
dead space volume x respiratory frequency
Why is increasing the tidal volume more effective at increasing the alveolar ventilation?
Because increasing the tidal volume decreases the fraction of VD of each breath (dead space fraction)
Describe the alveolar ventilation equation
- anatomic dead space does not produce/increase CO2 in expired air - must all come from alveolar ventilation
- CO2 output/production = Alveolar ventilation x FCO2 (fractional cc of CO2)
- so: alveolar ventilation = CO2 output/FCO2
where FCO2 = PCO2/K - so Alveolar ventilation = CO2 output/PCO2 x K
can use PaCO2 for PACO2 in healthy individuals
What is the name of the anatomic dead space measurement and how does it work?
Fowler’s method
subject takes one breath at 100% O2 - on exhalation the N2 cc is measured and plotted agaisnt time
the anatomic dead space is the volume up to where the N2 equalizes (dashed line in picture)
What is the name of the method to measure physiologic dead space? Explain how it works
Bohr’s method
principle: all expire CO2 comes from the alveolar gas, not from dead space
- following boyle’s law –> VT x FECO2 = VA x FACO2 - i.e., tidal volume times expired CO cc equals alveolar volume times alveolar CO2
- VT = VA + VD –> VA = VT - VD –> can be pluged into first euqation
- VT x FECO2 = (VT - VD) x FACO2
- so, VD/VT = (PACO2 - PECO2) / PACO2
- A is alveolar, E is mixed expired
- in health individuals can substitue PaCO2 for PACO2
- can substitute ETCO2 for PECO2
What is the definition of physiologic dead space?
The volume of gas not eliminating CO2
