title: Oxygen-conserving effect of the diving response in the immersed human.
abstract: BACKGROUND
  The human cardiovascular diving response has been shown to have an oxygen-conserving effect during simulated breath-hold diving by apnoea with face immersion. However, it is not known if facial immersion enhances the response to the same extent as that in the diver with the body immersed and if this leads to oxygen conservation.
  
  
  METHODS
  Seventeen subjects each completed a total of 12 apnoeas of fixed, near-maximal duration. Four series of three apnoeas were conducted: dry body with apnoea (DA), dry body with face-immersion apnoea (DFIA), immersed body with apnoea (IA), and immersed body with face-immersion apnoea (IFIA). Air and water temperatures were 23 degrees Celcius. Heart rate, skin blood flow, arterial blood pressure, arterial haemoglobin saturation, lung volume and end-tidal fractions of carbon dioxide and oxygen were recorded non-invasively.
  
  
  RESULTS
  Face immersion led to a greater reduction in heart rate during apnoea, regardless of body immersion (DA-DFIA 9.3%, 95% confidence interval (CI) 3.54, 0.1; IF-IFIA 7.9%, 95% CI 4.8, 0.2). Both DFIA and DA resulted in skin vasoconstriction, which was more pronounced during DFIA (16%, 95% CI 8.4, 0.3). During body immersion, skin vasoconstriction was reduced considerably, and neither IA nor IFIA reduced blood flow further. Mean arterial pressure increased more in the immersed condition than on dry land. Arterial saturation remained higher after DFIA (0.4%, 95% CI 0.2, 0.01) and IFIA (0.4%, 95% CI 0.4, 0.01) series, suggesting an oxygen-conserving effect of the more powerful diving response associated with face immersion.
  
  
  CONCLUSIONS
  We conclude that the oxygen-conserving effect of the diving response in the immersed diver is the same as that observed in the dry, horizontal, simulated diving model.
authors: Postgraduate student at the time of the study, and is currently undertaking doctoral research in stress physiology at Tufts University, Boston, USA.
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