Lung volumes and how to measure them

Question 1

How are lung volumes measured?

Answer 1

Spirograph

Vitalograph

Question 2

What is a spirograph?

Answer 2

• Measures breathing volumes at rest

- Not portable ; used to make physiological measurements

Question 3

What is a vitalograph?

Answer 3

• Measures peak gas flow on maximal exhalation

- Portable;used for clinical monitoring

Question 4

What are the volumes and capacities measured by spirometry?

Answer 4

• Resting tidal volume
• Residual volume
• Expiratory reserve volume
• Inspiratory reserve volume
• Total lung capacity
• Inspiratory capacity
• Vital capacity
• Functional residual capacity

Question 5

Which volumes cannot be measured by spirometry?

Answer 5

Functional residual capacity (FRC)

Respiratory Dead Space

Question 6

What is respiratory dead space?

Answer 6

Parts of the lung that do not participate in gas exchange; Nose, mouth, trachea, conducting airways and alveolar areas affected by disease. Becomes greater when patients are intubated in hospital on ventilator support

Question 7

How do you measure functional residual capacity?

Answer 7

Helium inhalation method- helium is virtually insoluble in blood so little is lost

1st; Amount of helium= C1 x V1
Then; Amount of helium= C2 x (V1 + V2)

Since C1 x V1 = C2 x (V1 + V2)
V2 = V1(C1-C2)/C2

Question 8

How is dead space varied ?

Answer 8

Disease, Mechanical ventilation, Alveolar collapse, Species

Question 9

Why is dead space important?

Answer 9

• It affects respiratory acid base balance by influencing CO2 clearance
• It affects the amount of work associated with breathing
• It determines how efficiently the alveoli are ventilated

Question 10

Dead space can be measured by oxygen inhalation method, how is this done?

Answer 10

1) single inspiration of pure oxygen
2) O2 will mix with gas in lung that we know will be composed of 75% N2
3) O2 inhalation flushes N2 from anatomical dead space areas
4) As subject exhales, breath is initially O2 rich (anatomical dead space) but eventually equilibrates to the mixed N2/O2 concentration in the lung

Question 11

Dead space can be measured by carbon dioxide dilution method (bohr method). How is this done?

Answer 11

Dead space dilutes carbon dioxide entering the alveolus from the blood so;

Dead space/tidal volume=arterial PCO2 - exhaled PCO2/arterial PCO2

Question 12

How is vitalograph measured?

Answer 12

Maximum inhalation then exhale as hard and as fast as possible

FVC- forced vital capacity
FEV1- forced exp vol in 1s
FEV1- normally 80% of FVC