title: Failure of the straight-line DCS boundary when extrapolated to the hypobaric realm.
abstract: The lowest pressure (P2) to which a diver can ascend without developing decompression sickness (DCS) after becoming equilibrated at some higher pressure (P1) is described by a straight line with a negative y-intercept. We tested whether extrapolation of such a line also predicts safe decompression to altitude. We substituted tissue nitrogen pressure (P1N2) calculated for a compartment with a 360-min half-time for P1 values; this allows data from hypobaric exposures to be plotted on a P2 vs. P1N2 graph, even if the subject breathes oxygen before ascent. In literature sources, we found 40 reports of human exposures in hypobaric chambers that fell in the region of a P2 vs. P1N2 plot where the extrapolation from hyperbaric data predicted that the decompression should be free of DCS. Of 4,576 exposures, 785 persons suffered decompression sickness (17%), indicating that extrapolation of the diver line to altitude is not valid. Over the pressure range spanned by human hypobaric exposures and hyperbaric air exposures, the best separation between no DCS and DCS on a P2 vs. P1N2 plot seems to be a curve which approximates a straight line in the hyperbaric region but bends toward the origin in the hypobaric region.
authors.affiliation: Department of Physiology, State University of New York, Buffalo 14214.
authors.forename: J
authors.initials: J
authors.lastname: Conkin
authors.forename: H D
authors.initials: HD
authors.lastname: Van Liew
mesh_terms: D000531:Altitude; D003665:Decompression Sickness / Q000453:epidemiology*; D004242:Diving / Q000502:physiology* / Q000706:statistics & numerical data; D006801:Humans; D015994:Incidence; D008433:Mathematics; D008954:Models, Biological; D011312:Pressure