(2) ALVEOLAR VENTILATION notes Flashcards
Typical lung volumes and flows (Fig. 1)
Conducting airways
has no role in what
Constitute
no role in gas exchange
has no role in no alveoli
*Anatomic dead space*
At end of normal expiration, alveolar gas volume =
= Functional Residual Capacity (FRC, 3L)
Quiet breathing
Tidal Volume (VT , 500 mL)
Pulmonary blood flow is
cardiac output (5L/min at rest)
Alveolar ventilation also 5 L/min at rest so that VA/Q = 1 example
ventilation and perfusion are matched.
Spirometry cannot be used to measure FRC or VR; need to use methods such as
Gas dilution techniques (use helium; equation in West text yields TLC not FRC)
Body plethysmography
Factors that affect vital capacity, residual volume and total lung capacity (Fig. 3).
Loss of elasticity of lung
Stiffness of lung
Stiffening of rib cage
Loss of alveoli
At any given instance pulmonary capillaries contain __________of blood, spread over lung surface area of
______
70ml.
2 50-100m .
Dead space ventilation (VD)
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.
Alveolar Ventilation (VA)
Can be estimated from VE &VD; because VA = VE - VD
However, VD is not easily measurable
- 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
Besides measuring VD and calculating VA, VA can also be measured from the CO2 concentration in expired gas by
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
Ventilation
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.
Ventilation. How measured?
Since PCO2 = (FCO2) x (PB) VA = (VCO2/PCO2)(PB)
Besides measuring VD and calculating VA, VA can also be measured from the CO2 concentration in expired gas by
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
Measuring VD - anatomic dead space - Fowler method (Fig. 4) Oxygen washout. Mid-point = VD
Measuring physiological dead space - Bohr method (Fig. 5)
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
=
- 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
F. Regional differences in alveolar ventilation (VA; Fig. 5)
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.
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.
