PECO2 = Partial Pressure of Expired CO2

Roxanne Rapan

Helpful URLs:
	http://infonet.welch.jhu.edu/~omie/res_phys/index.html
	http://www.mtsinai.org/pulmonary/books/physiology

Definition:
	The partial pressure of carbon dioxide gas in expired air.

Relevance:
	Used to calculate VD/VT in the Bohr dead space equation, and
normal alveolar ventilation. Since anatomic dead space does not exchange
gas, all CO2 in expired gas comes from alveolar gas exchange. CO2 in
inspired air is negligible.
			VD/VT=(PACO2-PECO2)/PACO2

Normal ratios are 0.3 ± 0.03 mmHg. A elevated ration indicates either a
depressed tidal volume (as in shallow breathing) or increased dead space
volume (as in a diseased lung) both of which lead to decreased alveolar
ventilation.

Sample Problem:
	Calculate the dead space/tidal volume ratio when PACO2 is measured
to be 46 mmHg and PECO2 is 27 mmHg.
(0.41)
	What can be said about this patient?
(Elevated ratio, indicating low tidal volume or diseased lung. Shows
decreased alveolar ventilation.)

Sample Problems:

Answers:

For more information on this topic, please refer to West , page .

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