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 (PgapCO2), gastric intramucosal pH (pHi) and arterial pH-pHi difference (pHgap). 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 (VT) from 7 to 10 ml/kg followed by an abrupt return to baseline VT level. Measurements and main results: Tonometer saline PtCO2 and hemodynamic data were collected hourly at various VT levels: H0 and H0' (baseline VT = 7 ml/kg), H1 (VT = 8 ml/kg), H2 (VT = 9 ml/kg), H3 (VT = 10 ml/kg), H4 (baseline VT). 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 mmHg to 13 ± 4 mmHg (p < 0.05). VT 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.
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