CATHETER DEADSPACE - A SOURCE OF ERROR DURING TONOMETRY

Tonometry of PCO2 is a promising method for assessing the oxygen suppl y to demand ratio of the gastrointestinal mucosa in critically ill pat ients. A balloon-tipped tonometer is introduced into the stomach or si gmoid colon, and saline is instilled into the balloon. After a time to allow partial equilibration with intraluminal PCO2, saline is aspirat ed and PCO2 is measured. Intermittent instillation and aspiration of s aline allows serial PCO2 measurements, provided correction factors are used to calculate the PCO2 value expected at full equilibration from the PCO2 values measured after short dwell times. The technique is not yet widely applied, partly because of methodological controversies. W e evaluated the role of the catheter deadspace as a source of error du ring PCO2 tonometry. The increase in PCO2 in sigmoid-type tonometers w ith a normal length (normal tonometer (NT)) and in those with a 50% in crease in length and thus deadspace (extended tonometer (ET)), in a sa line bath at a PCO2 of 4.8 kPa was assessed. Saline dwell times were 1 0, 20, 30, 45, 60 and 90 min and the time-dependent PCO2 increase was determined at deadspace PCO2 values of approximately 4.0 and 8.0 kPa f ollowing contamination of the catheter deadspace after immersion in sa line baths at PCO2 values of 4.8 and 9.6 kPa, respectively, before eac h measurement cycle. In another experiment, the tonometer was rinsed b etween measurement cycles to remove deadspace saline containing carbon dioxide and to obviate contamination of instilled saline. PCO2 was me asured in a blood-gas analyser, taking into account measurement bias i n saline. Failure to remove deadspace saline between measurement cycle s resulted in an overestimation of 10% and 6% for the NT and 16% and 1 0% for the ET, at saline dwell times of 10 and 20 min, respectively, a t a deadspace PCO2 of approximately 4.0 kPa. At a deadspace PCO2 of ap proximately 8.0 kPa, PCO2 was overestimated by 17%, 11% and 5% for the NT and 31%, 20% and 11% for the ET, at dwell times of 10, 20 and 30 m in, respectively. Rinsing the NT/ET resulted in accurate assessment of PCO2 at all dwell times, but the dwell time-dependent increase in PCO 2 was slightly slower in the ET, particularly at 10 min, after a sink effect of the increased deadspace. Hence, a previously unrecognized de adspace effect caused error during PCO2 tonometry, particularly with s hort dwell times. This potentially large error can be avoided by rinsi ng the tonometer before each measurement cycle, allowing accurate PCO2 tonometry even at 10-min saline dwell times, provided that correction factors are used that are specific for catheter size. These findings may help to widen the clinical applicability of tonometry.