Gastric intramucosal PCO₂ and pH variability in ventilated critically ill patients

Objective: Gastric intramucosal PCO₂ (PiCO₂) and pH (pHi) are currently used as indices of the adequacy of splanchnic perfusion and as end points to guide therapeutic intervention. However, little is known about their spontaneous variability over time. The present study was designed to define the magnitude of spontaneous variability of PiCO₂ and pHi in sedated medical intensive care unit (ICU) patients using an automated recirculating air tonometer and to test whether high-level positive end-expiratory pressure (PEEP) or inverse inspiratory/expiratory (I:E) ratio ventilation resulted in a greater variability than low PEEP with conventional I:E ratio ventilation. Design: Prospective study. Setting: Medical ICU in a tertiary medical center. Patients: Twenty-three acute respiratory failure patients. Interventions: After being sedated, patients were randomized to undergo pressure control ventilation at the following three settings: A, high PEEP (15 cm H₂O) with conventional I:E ratio (1:2), and B, low PEEP (5 cm H₂O) with inverse I:E ratio (2;1) alternately, and then C, low PEEP (5 cm H₂O) with conventional I:E ratio (1:2). Each ventilation setting period lasted 1 hr. Measurements and Main Results: The PiCO₂ and pHi were measured at baseline (time 0), and at 15, 30, 45, and 60 mins thereafter. The corresponding coefficients of variation (CVs) of PiCO₂ for overall pooled group and settings A, B, and C were 4.0%, 4.4%, 3.4%, and 4.2%, respectively. The corresponding CVs of pHi for overall pooled group and settings A, B, and C were 0.36%, 0.37%, 0.33%, and 0.4%, respectively. Analysis of variance showed no significant difference in the CVs of PiCO₂ or pHi between the three settings. The 95% confidence interval is approximately ±8% variability for PiCO₂ and ±0.7% variability for pHi. Conclusions: In critically ill medical ICU patients with stable hemodynamics, the spontaneous variability of PiCO₂ or pHi are not substantial. High PEEP (15 cm H₂O) and inverse ratio ventilation (2:1), which does not change the cardiac output or hemodynamics, does not contribute to increased spontaneous variability in PiCO₂ or pHi.