title: Intensive buffering can keep pH above 7.2 for over 4 h during apnea: an experimental porcine study.
abstract: BACKGROUND
  Ventilation with low tidal volumes reduces mortality in acute respiratory distress syndrome. A further reduction of tidal volumes might be beneficial, and it is known that apneic oxygenation (no tidal volumes) with arteriovenous CO(2)  removal can keep acid-base balance and oxygenation normal for at least 7 h in an acute lung injury model. We hypothesized that adequate buffering might be another approach and tested whether tris-hydroxymethyl aminomethane (THAM) alone could keep pH at a physiological level during apneic oxygenation for 4 h.
  
  
  METHODS
  Six pigs were anesthetized, muscle relaxed, and normoventilated. The lungs were recruited, and apneic oxygenation as well as administration of THAM, 20 mmol/kg/h, was initiated. The experiment ended after 270 min, except one that was studied for 6 h.
  
  
  RESULTS
  Two animals died before the end of the experiment. Arterial pH and arterial carbon dioxide tension (PaCO(2) ) changed from 7.5 (7.5, 7.5) to 7.3 (7.2, 7.3) kPa, P < 0.001 at 270 min, and from 4.5 (4.3, 4.7) to 25 (22, 28) kPa, P < 0.001, respectively. Base excess increased from 5 (3, 6) to 54 (51, 57) mM, P < 0.001. Cardiac output and arterial pressure were well maintained. The pig, which was studied for 6 h, had pH 7.27 and PaCO(2)  27 kPa at that time.
  
  
  CONCLUSIONS
  With intensive buffering using THAM, pH can be kept in a physiologically acceptable range for 4 h during apnea.
authors.affiliation: Hedenstierna laboratory, Anesthesia and Intensive Care, Department of Surgical Sciences, Uppsala University, Uppsala, Sweden. staffan.hostman@gmail.com
authors.forename: S
authors.initials: S
authors.lastname: Höstman
authors.forename: J
authors.initials: J
authors.lastname: Engström
authors.forename: G
authors.initials: G
authors.lastname: Hedenstierna
authors.forename: A
authors.initials: A
authors.lastname: Larsson
mesh_terms: D000136:Acid-Base Equilibrium / Q000502:physiology; D000818:Animals; D001049:Apnea / Q000097:blood* / Q000628:therapy; D062186:Arterial Pressure / Q000502:physiology; D001774:Blood Chemical Analysis; D002021:Buffers; D002245:Carbon Dioxide / Q000097:blood; D002302:Cardiac Output / Q000502:physiology; D004231:Diuresis; D004573:Electrolytes / Q000097:blood; D006454:Hemoglobins / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D012121:Respiration, Artificial; D012964:Sodium / Q000652:urine; D013552:Swine; D014325:Tromethamine / Q000627:therapeutic use*; D014882:Water-Electrolyte Balance / Q000502:physiology
chemical_list: D002021:Buffers; D004573:Electrolytes; D006454:Hemoglobins; D014325:Tromethamine; D002245:Carbon Dioxide; D012964:Sodium