Enteral feed delays response times of a tissue PCO₂ sensor

Introduction

Real time assessment of gut luminal PCO₂ is possible with rapidly responsive tissue CO₂ sensors [1]. The impact of the presence of feeds in the gut on the rapidity of response of the sensor to a change in mucosal CO₂ tension has not been evaluated.

Methods

The speed of onset of response and the 90% response time of a commonly used tissue gas sensor the Paratrend 7 (Diametrics Medical, UK) to a change in ambient CO₂ tension were compared in normal saline (control) and an enteral feed solution (Nutrison, Nutricia, Zoetermeer, Holland). Probe onset and 90% response times were determined for a step up and step down change in CO₂ tensions in saline and feed solutions by bubbling the following three pairs of gases A) 2% CO₂ and 10% CO₂ B) 10% CO₂ and 5% CO₂ and C) 5% CO₂ and 2% CO₂ through these solutions maintained at 37°C in a bubble tonometer. After calibration, the sensor was equilibrated in saline bubbled with the first gas of each pair. After equilibration the second gas of each pair was bubbled through the solution. This was repeated for a total of six equilibrations between each pair of gases. The experiment was then repeated with the feed solution.

Results

See Table

Conclusion

The presence of enteric feed significantly slows down the onset time and response time of the sensor to a change in ambient CO₂ tension. Altered viscosity and CO₂ binding by the feed are possible mechanisms for the altered response of the sensor. The reduction in response time may impact on the ability of tissue CO₂ sensors to provide accurate real time data in clinical practice.

Table The presence of enteric feed