Influence of alveolar ventilation changes on calculated gastric intramucosal pH and gastric-arterial PCO2 difference

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 (P_gapCO₂), gastric intramucosal pH (pHi) and arterial pH-pHi difference (pH_gap). Design: Clinical prospective study. Setting: A medical intensive care unit in a university hospital. Patients: Ten critically ill, mechanically ventilated patients requiring hemodynamic monitoring with pulmonary artery catheter. Interventions: 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. Measurements and main 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 pH_gap (p < 0.05) increased but P_gapCO₂ 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 P_gapCO₂ decreased from 16 ± 5 mmHg to 13 ± 4 mmHg (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 P_gapCO₂ is not affected by ventilation variations unless CO changes are associated. Received: 15 June 1998 Final revision received: 21 October 1998 Accepted: 16 November 1998