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March 8, 1995

Permissive Hypercapnia: Some Equations-Reply

Author Affiliations

University of Texas Medical Branch Galveston

JAMA. 1995;273(10):775. doi:10.1001/jama.1995.03520340029025

In Reply.  —We appreciate Dr Helmholz's comments regarding our article on permissive hypercapnia. Regarding his critique of equation 1, we concur that the alveolar air equation as delineated in his letter is the more accurate form.1 However, for purposes of clarity and simplicity, we elected to use the more familiar clinical form of the alveolar gas equation as a reasonable approximation.Regarding equation 2, we disagree with Helmholz's assertion that PACO2 is independent of barometric pressure. The relationship between volume and pressure is adjusted by a constant, K, whose value is valid only under a set level of temperature and pressure. In his example, K=0.863=(TB/TS) (PB)/(1000 mL/L), where TB is body temperature (310°K), Ts is standard temperature (273°K), and PB is standard barometric pressure (760 mm Hg).2 Therefore, changes in these conditions alter the value of K and affect partial pressure as well as gas volumes. The

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