Page 247 - Industrial Ventilation Design Guidebook

208 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL. CONSIDERATIONS

TABLE 5.6 Effect of Dead Space Volume, Tidal Volume, and
Breathing Frequency on Alveolar Ventilation at a Fixed Minute
Ventilation (W = 58.0 L/min). Modified from Cherniack. 26

V A (L/min) V D (m/L) V T(mL) f(min~' )

3.2 150 250 32
4.0 250 500 16
4.8 200 500 16
5.6 150 500 16
6.8 100 1000 8
7 A, alveolar gas volume; V D, dead space volume; V r, tidal volume; f, breathing
frequency

Air entering alveolar spaces but not partaking in gas exchange due to poor
perfusion of individual alveoli is not part of V A but adds to the total dead
space volume (V D). This additional dead space leads to the concept of a
physiological dead space that includes not only an anatomical component
(conducting airways) but also a functional component (poorly perfused or
nonperfused alveoli). Diseases affecting either conducting airway geometry
or pulmonary perfusion can thus alter V D. The total volume of air ( « 500
mL) inspired (or expired) during each breath is known as the tidal volume,
V T and can be described by

1
where f = breathing frequency (breaths min" ) so that

Expired minute ventilation, V E, defines the gas volume inspired or ex-
pired in 1 minute and is given by

Typical V E for normal quiet breathing is approximately 6-8 L/min. In ex-
treme circumstances, individuals can live for brief periods with minute ventila-
tion rates as low as 1-2 L/min or as high as 300 L/min. Table 5.6 shows the
dependence of V^ on V D, Vj-, and f for a given V E.
Alveolar ventilation supplies O 2 to the bloodstream while alveolar capil-
lary perfusion provides alveolar gas with CO 2. Resting individuals consume
approximately 250 mL O 2/min and produce approximately 200 mL CO 2/min
because, stoichiometrically, metabolic processes require a greater supply of O 2
than the quantity of CO 2 produced. Defining the respiratory exchange ratio,
R, as

then R = 0.8 during normal resting breathing andV A = 4 L/min is required to
lower the arterial CO 2 partial pressure to 40 torr and raise arterial O 2 partial