title: Influence of alveolar ventilation changes on calculated gastric intramucosal pH and gastric-arterial PCO2 difference.
abstract: OBJECTIVE
  To evaluate the influence of changes in alveolar ventilation on the following tonometry-derived variables: gastric intramucosal CO2 tension (PtCO2), gastric arterial CO2 tension difference (PgapCO2), gastric intramucosal pH (pHi) and arterial pH-pHi difference (pHgap).
  
  
  METHODS
  Clinical prospective study.
  
  
  METHODS
  A medical intensive care unit in a university hospital.
  
  
  METHODS
  Ten critically ill, mechanically ventilated patients requiring hemodynamic monitoring with pulmonary artery catheter.
  
  
  METHODS
  Gastric tonometer placement. A progressive increase in tidal volume (V(T)) from 7 to 10 ml/ kg followed by an abrupt return to baseline V(T) level.
  
  
  RESULTS
  Tonometer saline PtCO2 and hemodynamic data were collected hourly at various V(T) levels: H0 and H0' (baseline V(T) = 7 ml/kg), H1 (V(T) = 8 ml/kg), H2 (V(T) = 9 ml/kg), H3 (V(T) = 10 ml/kg), H4 (baseline V(T)). During the "hyperventilation phase" (H0-H3), pHi (p<0.01) and pHgap (p<0.05) increased but PgapCO2 remained unchanged. Cardiac output (CO) was not affected by ventilatory change. During the "hypoventilation phase" (H3-H4), pHi fell from 7.27+/-0.11 to 7.23+/-0.09 (p<0.01) and PgapCO2 decreased from 16+/-5 mm Hg to 13+/-4 mm Hg (p<0.05). V(T) reduction was associated with a significant cardiac output elevation (p<0.05).
  
  
  CONCLUSIONS
  PaCO2 and PtCO2 are similarly influenced by the changes in alveolar ventilation. Unlike pHi, the PgapCO2 is not affected by ventilation variations unless CO changes are associated.
authors.affiliation: Service de Réanimation Médicale, Hôpital de l'Archet, Nice, France. gbernard@unice.fr
authors.forename: G
authors.initials: G
authors.lastname: Bernardin
authors.forename: P
authors.initials: P
authors.lastname: Lucas
authors.forename: H
authors.initials: H
authors.lastname: Hyvernat
authors.forename: P
authors.initials: P
authors.lastname: Deloffre
authors.forename: M
authors.initials: M
authors.lastname: Mattéi
mesh_terms: D001784:Blood Gas Analysis / Q000379:methods; D002245:Carbon Dioxide / Q000378:metabolism*; D016638:Critical Illness; D005260:Female; D005753:Gastric Mucosa / Q000098:blood supply* / Q000378:metabolism*; D006439:Hemodynamics; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D008365:Manometry / Q000379:methods; D008875:Middle Aged; D011446:Prospective Studies; D012121:Respiration, Artificial; D012720:Severity of Illness Index
chemical_list: D002245:Carbon Dioxide