Chp. 2: Ventilation

Question 1

Basic lung volumes

Answer 1

VT: 500mL
Total ventilation: 7500mL/min
Anatomic dead space: 150mL
RR: 15/min
Alveolar ventilation: 5250mL/min
Alveolar gas: 3000mL
Pulmonary capillary blood: 70mL
Pulmonary blood flow: 5000mL/min

**VERY SMALL volume of capillary blood compared with that of alveolar gas

Question 2

FRC

Answer 2

Volume of gas in lung after normal (tidal) expiration

Question 3

Vital capacity

Answer 3

Maximal inspiration followed by maximal expiration. Exhaled volume is VC

Question 4

Residual volume

Answer 4

Gas remaining in lung after maximal expiration

Question 5

What volumes CANNOT be measured with a spirometer?

Answer 5

FRC, residual volume, total lung capacity

Question 6

How can FRC, residual volume, and total lung capacity be measured?

Answer 6

Gas dilution technique

Question 7

Gas dilution technique general principle

Answer 7

Helium is insoluble in blood. After some breaths, helium concentrations in spirometer and lung become the same. C1V1 = C2V2

Question 8

What volumes CAN be measured with a simple spirometer>

Answer 8

Vital capacity, tidal volume

Question 9

What is an alternate method for measurement of FRC?

Answer 9

Body plethysmography

Question 10

Boyle’s Law

Answer 10

Pressure x Volume is constant at constant temperature

P1V1 = P2V2

Question 11

Body plethysmography vs. helium dilution

Answer 11

BP: Depends on Boyle’s Law. Measures total volume of gas in lung, including any trapped behind closed airways

HD: Measures only communicating gas or ventilated lung volume

**VIRTUALLY THE SAME in healthy individuals, but with lung disease, ventilated volume may be considerably less than total volume because of gas trapped behind obstructed airways

Question 12

Why is volume of air entering lung slightly greater than volume exiting??

Answer 12

Because more O2 is taken in than CO2 given out

Question 13

Why does alveolar volume expand by the full size of the tidal volume, given that 150mL is anatomic dead space?

Answer 13

Because 150mL of gas left over in the anatomic dead space at the end of the previous exhalation is drawn into the alveoli with each breath before the fresh gas enters

Question 14

Valve box

Answer 14

Used to measure total (or minute) ventilation

Question 15

How can alveolar ventilation be increased?

Answer 15

By raising either VT or RR or both

Question 16

Is increasing RR or VT more effective at increasing alveolar ventilation?

Answer 16

VT because this reduces the fraction of each breath occupied by the anatomic dead space

Question 17

Alveolar ventilation equation

Answer 17

VA = (VCO2/PCO2) x K

Question 18

What is the relationship between alveolar ventilation and PCO2?

Answer 18

If alveolar ventilation is halved, PCO2 will double (assuming CO2 production is unchanged)

Question 19

What increases the anatomic dead space?

Answer 19

Large inspirations, because of traction or pull exerted on bronchi by surrounding lung parenchyma

Also depends on size and posture of subject

Question 20

How can anatomic dead space be measured?

Answer 20

Fowler’s method

Breathe through valve box with sampling tube with rapid nitrogen analyzer. Single inspiration of 100% O2, N2 concentration rises as dead space gas is increasingly washed out by alveolar. Then, alveolar plateau

Plot N2 against expired volume

**anatomic dead space measurement

Question 21

Bohr’s Method

Answer 21

Another way of measuring dead space

VD/VT = (PACO2 - PECO2) / PACO2

Normal ratio of dead space to VT is 0.2-0.35

**physiologic dead space measurement

Question 22

Why might the physiologic dead space be larger than anatomic in lung disease?

Answer 22

Inequality of blood flow and ventilation within lung

Question 23

Physiologic dead space

Answer 23

Volume of gas that does not eliminate CO2

Question 24

Does the upper or lower lung ventilate better?

Answer 24

Lower

Demonstrated with radioactive xenon gas

Question 25

KEY CONCEPTS of ventilation

Answer 25

1) Lung volumes that can't be measured with a simple spirometer include total lung capacity, FRC, and residual volume, These can be determined by helium dilution or body plethysmography. 2) Alveolar ventilation is volume of fresh (non-dead space) gas entering the respiratory zone per minute. Determined from alveolar ventilation equation, ie, the CO2 output divided by fractional concentration of CO2 in expired gas 3) Concentration of CO2 (and therefore its partial pressure) in alveolar gas and arterial blood is inversely related to alveolar ventilation 4) Anatomic dead space is the volume of conducting airways and can be measured from nitrogen concentration following a single inspiration of 100% O2 (Folwer's method) 5) Physiologic dead space is volume of lung that does not eliminate CO2. Measured by Bohr's method using arterial and expired CO2 6) Lower regions of lung are better ventilated than upper because of gravity