Negative pressure oxygen breathing and head-down tilt increase nitrogen elimination

Negative pressure breathing (NPB) increases the rate of nitrogen elimination, which is thought to be due to an increase in cardiac output due to augmented venous return to the heart. Hyperoxia, however, decreases the rate of nitrogen elimination. The effect of hyperoxia on the increase in nitrogen elimination during NPB is not known. We hypothesized that NPB as and head down tilt (HDT), which is also thought to increase cardiac output, would counteract the detrimental effects of hyperoxia on nitrogen elimination. Nitrogen elimination was measured in 12 subjects while they lay supine breathing 100% O ₂ supplied at atmospheric pressure (control), -10 cm H ₂O (NPOB _-10), and -15 cm H ₂O (NPOB _-15). Nitrogen elimination was also measured in the subjects while they breathed 100% O ₂ supplied at atmospheric pressure in the supine position with a 6° HDT. Over a two- hour washout period, NPOB significantly increased nitrogen elimination by more than 14%, although there was no significant difference between NPOB _-10 and NPOB _-15. HDT also significantly increased nitrogen elimination by almost 8%. Neither NPOB nor HDT significantly affected cardiac output but calf blood flow was significantly lower during NOPB _-15. Combining NPB or HDT with 100% oxygen breathing appear to be useful means of increasing nitrogen elimination and should be considered in situations where this effect may be beneficial, such as with oxygen prebreathing prior to decompression.