chapter 2 ventilation

Question 1

how is RV/FRC/TLC measured?

Answer 1

1. Helium dilution method
   V2 = V1 x (C1-C2)/C2

2. Body plethysmograph (Boyle's Law) 
box:
P1V1 = P2 (V1-dV) 
person:
P3V2 = P4 (V2-dV)
d = delta
P3 is mouth pressure before inspiratory effort 
P4 is mouth pressure after inspiratory effort 

Body plethysmograph measures total volume of gas in lung (including air trapped behind closed airways)
Helium dilution method measures only communicating gas

Helium is used bc of its very low solubility in blood

Question 2

What is alveolar ventilation

Answer 2

amount of fresh inspired air available for gas exchange

Question 3

How do you measure alveolar ventilation

Answer 3

1. Calculate the dead space ventilation and subtract from total ventilation
   . . .
   Va = VE - VD
2. Measure from CO2 in expired gas
   alveolar ventilation equation
   . .
   VA = VCO2/ PCO2 x K

Question 4

What is normal anatomic dead space?

What increases it?

Answer 4

150 mL
Volume of the conducting airways

Increases with large inspirations bc of traction exerted on bronchi by surrounding lung parenchyma

Question 5

How do you estimate physiologic dead space?

Answer 5

Bohr’s method
VD/VT = (PaCO2-PECO2)/PaCO2

(note: PACO2 = PaCO2 bc there is no A-a gradient for CO2)
normal ratio is 0.2 to 0.35 in resting breathing

This is the volume of gas that doesn’t eliminate CO2

Question 6

How do you estimate anatomic dead space?

Answer 6

Fowler’s method

inspire 100% O2 (washout N2) - uniform gas concentration is seen (represents pure alveolar gas)
Plot N2 concentration against volume - draw vertical line where area before and after are equal
Dead space is the area up to the vertical line

Question 7

What is the normal difference between physiologic and anatomic dead space measurements?

Answer 7

should be the same in normal subjects

Question 8

What lung volumes cant be measured with a simple spirometer?

Answer 8

TLC, FRC, RV

Question 9

which part of lung is best ventilated?

Answer 9

Bases are best ventilated per unit lung

lower regions of the lung are better ventilated than the upper regions because of the effects of gravity on the lung