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A Breath of Fresh Air—Ventilation 

A Breath of Fresh Air—Ventilation
Chapter:
A Breath of Fresh Air—Ventilation
Author(s):

James R. Munis

DOI:
10.1093/med/9780199797790.003.0013
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date: 09 May 2021

The sine qua non of ventilation is arterial carbon dioxide. If you want to know about ventilation, just check the PaCO2. If it is low or normal, ventilation is fine, regardless of any other parameter, including respiratory rate, tidal volume, or dead space ratio. However, if PaCO2 is high, then alveolar ventilation (VA) is impaired (relative to the carbon dioxide load being presented to the lungs). In a conventional breathing circuit, dead space ends at the Y-shaped junction of the inspiratory and expiratory arms of the circuit and the endotracheal tube. On the machine side of that junction, the inspiratory and expiratory limbs see only fresh inspired or expired gas, respectively, but not both. You should know 2 other things about ventilation. One is the Bohr equation, which estimates the ratio of dead space to tidal volume. The anatomic dead space is estimated as the expired volume that coincides with half maximal nitrogen content. The second thing is the effect of gravity on the distribution of ventilation within the lung.

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