Monitoring gastrointestinal intraluminal PCO2: problems with airflow methods

Gastrointestinal intraluminal PCO2 (PiCO2) information is used to assess the adequacy of trauma patient resuscitation and to assist in choosing resuscitative interventions. Therefore, determining the limitations and potential caveats of different PiCO2 monitoring systems is clinically important. This study compared two PCO2 monitoring systems. The airflow device adds and then removes air samples to quantitate PCO2, whereas the fiber-optic device does not. METHODS: Airflow (TRIP Tonometer/Tonocap) and fiber-optic (Neotrend) systems were used. In vitro they were compared with each other and to two end-tidal CO2 monitors measuring the PCO2 of humidified air containing 5% and then 10% CO2. In vivo the two systems' catheters were surgically juxtaposed in 15 dogs' stomachs; paired PiCO2 readings were taken throughout hemorrhage and resuscitation. RESULTS: In vitro, paired PCO2 values from the airflow and fiber-optic devices correlated with each other (r = 0.99) and with end-tidal values (r = 0.99 with airflow, r = 0.95 with fiber-optic). In vivo, paired values differed significantly (P < 0.0001), correlating poorly for two devices simultaneously measuring the same variable (r = 0.61). Fiber-optic PiCO2 values were higher than airflow values (mmHg +/- SEM): 69.3 +/- 4.8 vs. 61.3 +/- 5.6 at the start of hemorrhage, 141.3 +/- 12.9 vs. 87.7 +/- 7.9 by end of hemorrhage, and 104.3 +/- 9.6 vs. 82.8 +/- 7.0 by end of resuscitation for fiber-optic and airflow, respectively. CONCLUSIONS: Despite agreement in vitro, airflow methods can influence PiCO2 values obtained in vivo. Passive sensing methods used to monitor PiCO2, such as fiber-optic methods, are preferable because they neither deliver O2 to, nor remove CO2 from the local microenvironment.