Indications

Gut mucosal hypoperfusion is an early consequence of hypovolaemia. Covert circulatory inadequacy due to hypovolaemia may be detected as gut mucosal acidosis and has been related to post-operative complications after major surgery. In critically ill patients there is some evidence that prevention of gut mucosal acidosis improves outcome. The sigmoid colon tonometer is useful to detect ischaemic colitis early (e.g. after abdominal vascular surgery).

Technique Saline tonometry

In the original technique the tonometer balloon was prepared by degassing and filling with 2.5ml 0.9% saline. The saline was withdrawn into a syringe connected to the sampling tube prior to insertion. After insertion the saline was passed back into the balloon. The PCO2 of the saline in the balloon equilibrated with the PCO2 of the gut lumen over a period of 30-90min. At steady state it was assumed that the PCO2 of the gut lumen and gut mucosa were in equilibrium. Time correction factors were derived for partial equilibration between the balloon saline and the gut lumen. The measurement was completed by sampling the saline from the balloon and an arterial blood sample for measurement of arterial [HCO3-].

Gas tonometry

Using air in the tonometry balloon allows more rapid equilibration between the tonometer and the luminal PCO2. A modified capnometer automatically fills the balloon with air and samples the PCO2 after 5-10min equilibration. Subsequent cycles of balloon filling do not use fresh air so CO2 equilibration is quicker. Tonometric PCO2 may be compared with end-tidal PCO2 (measured with the same capnometer) as an estimate of arterial PCO2. With a normal capnogram, a balloon PCO2 significantly higher than end-tidal PCO2 implies gut mucosal hypoperfusion.

pH versus regional PCO2

The pH of the gut mucosa (pHi) may be calculated using a modified Henderson-Hasselbach equation:

jiTeml [HCOj 1

where K is the time dependent equilibration constant. However, most of the variation in the measurement is due to variation in regional PCO2. Comparing regional PCO2 with PaCO2 gives as much information as making the calculation of pHi and overcomes the problematic assumption that arterial [HCO3-] is equivalent to mucosal capillary [HCO3-].

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