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 (P(gap)CO2), 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: HO and HO' (baseline V(T) = 7 ml/kg), H1 (V(T) = a 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(gap)- CO2 remained unchanged. Cardiac output (CO) was not affected by ventilatory change. During the 'hypoventilation phase' (H3-H4), pHi fell from 7.27 ± O.11 to 7.23 ± 0.09 (p < 0.01) and P(gap)CO2 decreased from 16 ± 5 mmHg to 13 ± 4 mmHg (p < 0.05). V(T) reduction was associated with a significant cardiac out put elevation (p < 0.05). Conclusions: PaCO2 and PtCO2 are similarly influenced by the changes in alveolar ventilation. Unlike pHi, the P(gap)CO2 is not affected by ventilation variations unless CO changes are associated.