Performance of automated air tonometry under hypothermia

Abstract

SummaryIntestinal tonometry is used during hypothermic cardio-pulmonary bypass surgery to assess splanchnic perfusion. In an in vitro set-up the performance of automated air tonometry (TONOCAP Ò ) was tested for normo-and hypothermia. A 14-FG tonometry catheter was built into a testing chamber (100 cm 3 ) perfused with blood from a cardio-pulmonary bypass circuit with P a CO 2 held at 5.6-5.8 kPa (alphastat). P r CO 2 from the balloon of the tonometry catheter was measured at intervals of 10 min at 37°C and at 25°C by the TONOCAP Ò . Bias (precision) of P r CO 2 -P a CO 2 alpha-stat and P r CO 2 ) P a CO 2 pH-stat at 37°C blood temperature were low at 0.23 kPa (0.21) each. Tonometrically measured P r CO 2 at 25°C significantly differed from P a CO 2 alpha-stat bias (precision) of 2.00 kPa (0.11) but was similar to P a CO 2 pH-stat (0.30 kPa (0.11)). P r CO 2 values as measured by the TONOCAP Ò represent pH-stat approach. Identical blood gas management (pH-or alpha-stat) should be used for calculation of mucosal -arterial CO 2 difference (P r-a CO 2 gap) or calculation of intramucosal pH. Partial pressure of carbon dioxide in the gastric lumen (regional PCO 2 = P r CO 2 ) can be used as a non-invasive method for assessing adequacy of gut mucosal perfusion [1]. The P r CO 2 value represents the balance between CO 2 production and its removal by gastrointestinal mucosal perfusion and alveolar ventilation [2].The introduction of automated air tonometry has significantly facilitated the measurement of gastric mucosal P r CO 2 in the clinical setting [3,4]. The automated air tonometer (TONOCAP Ò , Datex-Engstrom, Tonometrics Division, Helsinki, Finland) allows semi-continuous periodic assessment of gastric or sigmoid P r CO 2 with compatible tonometry catheters. The automated air tonometer fills the balloon at the distal end of the tonometry catheter with air. Following an equilibration period of 10 min, the air is automatically sampled and analysed for CO 2 by means of an infrared sensor. In order to avoid depletion of luminal CO 2 and to reduce equilibration times, the aspirated gas mixture is re-filled into the sampling balloon [5].There is an increasing interest in assessment of gastric luminal P r CO 2 during cardiac surgery with hypothermic cardio-pulmonary bypass [6][7][8]. Thus the aim of the present study was to evaluate performance of automated air tonometric assessment of P r CO 2 during normo-and hypothermia using the TONOCAP Ò in an in vitro set-up. Materials and methods

Assessment of a new prototype hydrogel CO 2 sensor; comparison with air tonometry

“…Temperature correction will always be needed, however. This is a disadvantage compared to air tonometry which is not sensitive to temperature [14,15].…”