title: Physiological effects of rapid reduction in carbon dioxide partial pressure in submarine tower escape.
abstract: BACKGROUND
  The objective of this study was to determine whether adverse effects from a rapid drop in inspired carbon dioxide partial pressure (PiCO₂) in the breathing gas could hinder or prevent submarine tower escape.
  
  
  METHODS
  A total of 34 male volunteers, mean (SD) age 33.8 (7.5) years, completed the trial. They breathed air for five minutes then 5% CO₂/16% O₂, 79% N₂ (5CO₂/16O₂) for 60 minutes before switching to breathing 100% O₂ for 15 minutes and then returned to air breathing. Breathing gases were supplied from cylinders via scuba regulators and mouthpieces. Blood pressure, cerebral blood flow velocity, electrocardiogram and end-tidal CO₂ and end-tidal O₂ were monitored throughout. Subjects were asked at intervals to indicate symptom type and severity.
  
  
  RESULTS
  Symptoms whilst breathing 5CO₂/16O₂ included breathlessness and headache. Following the switch to 100% O₂ seven subjects reported mild to moderate faintness, which was associated with a significant drop in cerebral blood flow compared to those who did not feel faint (P < 0.02). No subject vomited or fainted following this breathing-gas switch.
  
  
  CONCLUSIONS
  This study shows that the risk of fainting, sudden collapse or vomiting on switching to 100% O₂ following acute exposures to hypercapnia at a PiCO₂ of up to 5.0 kPa is less than 8%.
authors.affiliation: Principal Scientist, QinetiQ, Maritime Life Support, Haslar Marine Technology Park, Haslar Road, Gosport, Hampshire UK. PO12 2AG, Phone: +44-(0)2392-335151, Fax: +44-(0)2392-335197, E-mail: galoveman@qinetiq.com.
authors.forename: Geoffrey A M
authors.initials: GA
authors.lastname: Loveman
authors.affiliation: QinetiQ, Maritime Life Support, Gosport, UK.
authors.forename: Fiona M
authors.initials: FM
authors.lastname: Seddon
authors.affiliation: QinetiQ, Maritime Life Support, Gosport, UK.
authors.forename: Julian C
authors.initials: JC
authors.lastname: Thacker
authors.affiliation: QinetiQ, Maritime Life Support, Gosport, UK.
authors.forename: M Graham
authors.initials: MG
authors.lastname: White
authors.affiliation: QinetiQ, Maritime Life Support, Gosport, UK.
authors.forename: Karen M
authors.initials: KM
authors.lastname: Jurd
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D002245:Carbon Dioxide / Q000008:administration & dosage*; D002560:Cerebrovascular Circulation / Q000502:physiology; D004417:Dyspnea / Q000209:etiology; D006261:Headache / Q000209:etiology; D006801:Humans; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D009325:Nausea / Q000209:etiology; D010100:Oxygen / Q000008:administration & dosage*; D010313:Partial Pressure; D013366:Submarine Medicine; D011795:Surveys and Questionnaires; D013575:Syncope / Q000209:etiology; D013997:Time Factors; D055986:Vital Signs / Q000502:physiology; D014839:Vomiting / Q000209:etiology
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen
keywords: Doppler; Hypercapnia; cerebral blood flow; oxygen; physiology; submarine