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150 mL

350 mL

150 mL

350 mL

End exp.

End insp.

FIGURE 9 Tidal volume (Vt) includes a volume that reaches the alveoli and is effective at gas exchange (Va), and dead space volume that remains in the conducting airways and is not effective at gas exchange (Vd).

effective at bringing fresh gas to the alveoli because (1) the first gas inspired is gas left in conducting airways from the last expiration, which has already had O2 removed and CO2 added to it in the lungs; and (2) the last part of an inhalation does not get past the conducting airways and into the gas exchanging alveoli. The volume in the conducting airways that is not effective at gas exchange is called anatomic dead space (Vd). The portion of ventilation effective for gas exchange is that portion actually reaching the alveoli, or alveolar ventilation (Va). In a normal individual, anatomic Vd is about 150 mL (or about 1 mL/lb body mass) so Va is only about 4.2 L/min:

The effects of changing fR on Va are different from those of changing Vt, in contrast to the case for Ve. Doubling or halving fR will double or halve Va, respectively. However, doubling VT will more than double Va if Vd is constant. Similarly, decreasing Vt can decrease Va disproportionately. Differential effects of Vt and fR on Va have important implications for artificial ventilation. It also means that the optimal breathing pattern depends on gas exchange, in addition to respiratory mechanics.

Anatomic dead space can be measured with a single breath method as shown in Fig. 10. Nitrogen concentration is continuously measured at the mouth of a person who inspires a breath of pure O2, and then slowly exhales to RV. Expired volume is measured at the same time. The first gas expired from the dead space contains the pure O2 that was just inhaled. After the dead space

FIGURE 10 Fowler's method for measuring dead space as described in the text. After a single inspiration of 100% O2, expired N2 concentration is plotted against expired volume. A vertical line drawn so area A = area B, and this line intersects the volume axis at the anatomic deadspace volume (Vd). In real life, the alveolar plateau may slope and have inflection points from uneven ventilation between lung regions (see Chapter 21).

FIGURE 10 Fowler's method for measuring dead space as described in the text. After a single inspiration of 100% O2, expired N2 concentration is plotted against expired volume. A vertical line drawn so area A = area B, and this line intersects the volume axis at the anatomic deadspace volume (Vd). In real life, the alveolar plateau may slope and have inflection points from uneven ventilation between lung regions (see Chapter 21).

gas, alveolar gas that still contains nitrogen is expired. There is not a perfectly sharp transition between the anatomic dead space gas and alveolar gas because of diffusive mixing at the interface between conducting and respiratory airway zones. However, the average volume at this interface can be determined as shown in Fig. 10, and this equals the anatomic dead space volume.

In reality, anatomic Vd can increase with Vt as conducting airways lengthen and dilate during inspiration, and vice versa during expiration. Also, as discussed in Chapter 21, Va may be reduced even more by physiologic dead space, which can exceed anatomic dead space.

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