Negative pressure breathing enhances nitrogen elimination

The rate of nitrogen elimination during decompression is primarily dependent on tissue perfusion and, therefore, cardiac output (CO). Negative pressure breathing (NPB) is thought to increase CO by enhancing venous return and, consequently, the rate of nitrogen elimination. This notion was investigated in the present study. Nitrogen elimination was measured in five subjects lying supine while breathing a nitrogen free gas (79% Ar, 20% O2) supplied at -15 cm H2O (-1.5 kPa) or at atmospheric pressure. Over a 2-h washout period, NPB increased nitrogen elimination by 39.2 +/- 21.7% (mean +/- SD), increased calf blood flow by 34.1 +/- 34.6%, and elevated BP slightly during the last half of the experiment. Negative pressure breathing did not significantly change CO; the increase in nitrogen elimination may have been due to redistribution of blood flow. Negative pressure breathing appears to be a useful means of increasing nitrogen elimination and should be considered in situations such as decompression or treatment of decompression sickness where this effect may be beneficial.