(2) ALVEOLAR VENTILATION notes

Question 1

Typical lung volumes and flows (Fig. 1)

Answer 1

Question 2

Conducting airways

has no role in what

Constitute

Answer 2

no role in gas exchange

has no role in no alveoli

*Anatomic dead space*

Question 3

At end of normal expiration, alveolar gas volume =

Answer 3

= Functional Residual Capacity (FRC, 3L)

Question 4

Quiet breathing

Answer 4

Tidal Volume (VT , 500 mL)

Question 5

Pulmonary blood flow is

Answer 5

cardiac output (5L/min at rest)

Question 6

Alveolar ventilation also 5 L/min at rest so that VA/Q = 1 example

Answer 6

ventilation and perfusion are matched.

Question 7

Spirometry cannot be used to measure FRC or VR; need to use methods such as

Answer 7

Gas dilution techniques (use helium; equation in West text yields TLC not FRC)

Body plethysmography

Question 8

Factors that affect vital capacity, residual volume and total lung capacity (Fig. 3).

Answer 8

Loss of elasticity of lung

Stiffness of lung

Stiffening of rib cage

Loss of alveoli

Question 9

At any given instance pulmonary capillaries contain __________of blood, spread over lung surface area of

______

Answer 9

70ml.

2 50-100m .

Question 10

Dead space ventilation (VD)

Answer 10

Need to measure dead space volume (or estimate it)

Can be estimated from dead space volume & breathing rate; VD = VD . f

Example

VD = 150 mL, f = 15 breaths/min, therefore

VD = 2.25 L/min.

Question 11

Alveolar Ventilation (VA)

Answer 11

Can be estimated from VE &VD; because VA = VE - VD

However, VD is not easily measurable

3. Example

VT =500mL,VD =150mL,f=15/min, VA = 7.5 L/min - 2.25 L/min.

= 5.25 L/min. or VA =(VT -VD).f

Question 12

Besides measuring VD and calculating VA, VA can also be measured from the CO2 concentration in expired gas by

Answer 12

VCO2 = (VA ) x (%CO2/100)
where VCO2 = volume of CO2 exhaled per unit time. Thus VA = (VCO2 ) x (100%/CO2)

But remember %CO2 /100 = fractional content of gas that is CO2, FCO2

Question 13

Ventilation

Answer 13

How measured.

Can be calculated from tidal volume (VT) & breathing rate; VE = VT . f

Example
VT = 500 mL, f = 15 breaths/min, therefore VE = 7.5 L/min.

However, this does not tell us what volume of air is ventilating the alveoli, alveolar ventilation rate.

Question 14

Ventilation. How measured?

Answer 14

Since PCO2 = (FCO2) x (PB) VA = (VCO2/PCO2)(PB)

Question 15

Besides measuring VD and calculating VA, VA can also be measured from the CO2 concentration in expired gas by

Answer 15

VCO2 = (VA ) x (%CO2/100)
where VCO2 = volume of CO2 exhaled per unit time. Thus VA = (VCO2 ) x (100%/CO2)

But remember %CO2 /100 = fractional content of gas that is CO2, FCO2

Question 16

Measuring VD - anatomic dead space - Fowler method (Fig. 4) Oxygen washout. Mid-point = VD

Answer 16

Question 17

Measuring physiological dead space - Bohr method (Fig. 5)

Answer 17

Assumption - All CO2 comes from alveolar gas and none from dead space.

Equation for determining this is

V (PACO − PECO ) D22

=

But since in normal subjects PACO2 = PaCO2 (partial pressure of CO2 in arterial blood), the equation becomes Bohr equation

V (PaCO − PECO ) D22

=

5. NOTE: Two different things measured; Fowler - anatomic dead space and Bohr - physiological dead space. In normal individuals these volumes are nearly the same but in persons with lung disease particularly involving unperfused alveoli, physiological dead space may be much larger

Question 18

F. Regional differences in alveolar ventilation (VA; Fig. 5)

Answer 18

Question 19

In chronic obstructive lung disease (COPD, emphysema or chronic bronchitis), now the fourth leading cause of death in the United States (after heart disease, cancer, and accidents), maldistribution of ventilation may become so severe that the patient requires supplemental oxygen at rest.

Answer 19

Some lung units may fill or empty very slowly. When chest physicians speak of fast or slow alveoli, they are referring to the time constants of various lung ventilation units.