British Journal of Anaesthesia, Vol 80, Issue 3 337-341, Copyright © 1998 by The Board of Management and Trustees of the British Journal of Anaesthesia
PJGM. Steverink, J. J. Kolkman, ABJ. Groeneveld and J. W. de Vries
Tonometry of PCO2 is a promising method for assessing the oxygen supply to
demand ratio of the gastrointestinal mucosa in critically ill patients. A
balloon-tipped tonometer is introduced into the stomach or sigmoid colon,
and saline is instilled into the balloon. After a time to allow partial
equilibration with intraluminal PCO2, saline is aspirated and PCO2 is
measured. Intermittent instillation and aspiration of saline 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. We evaluated the role of the
catheter deadspace as a source of error during PCO2 tonometry. The increase
in PCO2 in sigmoid-type tonometers with a normal length (normal tonometer
(NT)) and in those with a 50% increase in length and thus deadspace
(extended tonometer (ET)), in a saline bath at a PCO2 of 4.8 kPa was
assessed. Saline dwell times were 10, 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 following contamination of the catheter
deadspace after immersion in saline baths at PCO2 values of 4.8 and 9.6
kPa, respectively, before each measurement cycle. In another experiment,
the tonometer was rinsed between measurement cycles to remove deadspace
saline containing carbon dioxide and to obviate contamination of instilled
saline. PCO2 was measured in a blood-gas analyser, taking into account
measurement bias in saline. Failure to remove deadspace saline between
measurement cycles resulted in an overestimation of 10% and 6% for the NT
and 16% and 10% for the ET, at saline dwell times of 10 and 20 min,
respectively, at a deadspace PCO2 of approximately 4.0 kPa. At a deadspace
PCO2 of approximately 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
min, respectively. Rinsing the NT/ET resulted in accurate assessment of
PCO2 at all dwell times, but the dwell time-dependent increase in PCO2 was
slightly slower in the ET, particularly at 10 min, after a sink effect of
the increased deadspace. Hence, a previously unrecognized deadspace effect
caused error during PCO2 tonometry, particularly with short dwell times.
This potentially large error can be avoided by rinsing 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.
CLINICAL INVESTIGATIONS
Catheter deadspace: a source of error during tonometry
Department of General and Large Animal Surgery, Faculty of Veterinary Medicine, University of Utrecht, Utrecht, The Netherlands; Department of Gastroenterology, Free University Hospital Amsterdam, Amsterdam, The Netherlands; Medical Intensive Care Unit, Free University Hospital Amsterdam, Amsterdam, The Netherlands; Institute of Anaesthesiology, University Hospital Utrecht, The Netherlands
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