title: Dependence of the gradient between arterial and end-tidal P(CO(2)) on the fraction of inspired oxygen.
abstract: BACKGROUND
  End-tidal P(CO(2)) (Pe'(CO(2))) is routinely used in the clinical assessment of the adequacy of ventilation because it provides an estimate of Pa(CO(2)). How well Pe'(CO(2)) reflects Pa(CO(2)) depends on the gradient between them, expressed as ΔPa-e'(CO(2)). The major determinant of ΔPa-e'(CO(2)) is alveolar dead space (Vd(alv)). The fraction of inspired O(2) (Fi(O(2))) is not thought to substantially affect ΔPa-e'(CO(2)) in anaesthetized patients. We hypothesized that a high Fi(O(2)) may indeed increase ΔPa-e'(CO(2)) by preferentially vasodilating well-perfused alveoli, resulting in the redistribution of blood flow to these alveoli from poorly perfused alveoli and an increase in Vd(alv). We therefore investigated the effects of changes in Fi(O(2)) on ΔPa-e'(CO(2)) and Vd(alv).
  
  
  METHODS
  With Institutional Review Board approval and informed consent, we studied 20 ASA I-II supine patients undergoing elective lower abdominal surgery under combined general and epidural anaesthesia. At constant levels of ventilation, Fi(O(2)) levels of 0.21, 0.33, 0.5, 0.75, and 0.97 were applied in a random order and ΔPa-e'(CO(2)) and Vd(alv) were calculated.
  
  
  RESULTS
  The ΔPa-e'(CO(2)) values were, in order of ascending Fi(O(2)), {mean [standard error of the mean (SEM)]} 0.13 (0.04), 0.28 (0.08), 0.29 (0.09), 0.44 (0.11), and 0.53 (0.09) kPa. The corresponding values of Vd(alv) were 25.5, 33.8, 35.8, 48.9, and 47.4 ml. Each successive hyperoxic level showed a significant increase in ΔPa-e'(CO(2)) except between the 0.33-0.5 and 0.75-0.97 Fi(O(2)) levels.
  
  
  CONCLUSIONS
  These data demonstrate that ΔPa-e'(CO(2)), in anaesthetized patients depends on Fi(O(2)).
authors.affiliation: TOYOTA-KAI Medical Corporation KARIYA TOYOTA General Hospital, Kariya, Aichi, Japan. hh-yamauchi@na.commufa.jp
authors.forename: H
authors.initials: H
authors.lastname: Yamauchi
authors.forename: S
authors.initials: S
authors.lastname: Ito
authors.forename: H
authors.initials: H
authors.lastname: Sasano
authors.forename: T
authors.initials: T
authors.lastname: Azami
authors.forename: J
authors.initials: J
authors.lastname: Fisher
authors.forename: K
authors.initials: K
authors.lastname: Sobue
mesh_terms: D000005:Abdomen / Q000601:surgery; D000328:Adult; D000368:Aged; D000767:Anesthesia, Epidural; D000768:Anesthesia, General; D002245:Carbon Dioxide / Q000097:blood*; D005260:Female; D006801:Humans; D018496:Hyperoxia / Q000097:blood; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative; D010100:Oxygen / Q000097:blood; D010102:Oxygen Inhalation Therapy; D011650:Pulmonary Alveoli / Q000098:blood supply; D011652:Pulmonary Circulation; D012121:Respiration, Artificial; D016683:Supine Position / Q000502:physiology; D013990:Tidal Volume / Q000502:physiology
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen