Factors Affecting Performance of Air Tonometry Using the TONOCAP®

Abstract

Saline tonometry has been replaced by automated air tonometry (TONOCAP ® ). As with saline tonometry there are some pitfalls to consider. We investigated the influence of different filling handicaps to the tonometry catheter sampling balloon on measurement of regional PCO 2 (PrCO 2 ). In an in vitro set-up, PrCO 2 was measured using the TONOCAP ® at intervals of 10 minutes from a 8F tonometry catheter sampling balloon placed in a container with constant PCO 2 (PcCO 2 ). Catheter alarms displayed by the TONOCAP® device were noted. The following experiments were performed: A) control measurement without modifications; B) adding deadspace of 7 ml into the sampling line; C) placing the sampling balloon in a 3 ml syringe; D) cross-clamping of the sampling balloon. Each experiment was performed four times using two catheters on two TONOCAP ® devices. Differences between PcCO 2 and PrCO 2 were calculated as Pc-rCO 2. Results are presented as mean±SD and were compared using ANOVA with Scheffe's correction. Pc-rCO 2 in the control set-up (A) amounted to 0.16±0.22%. Increasing catheter deadspace (B) resulted in a higher Pc-rCO 2 of 3.2±0.57% (P<0.0001). Restriction of balloon expansion (C) caused an Pc-rCO 2 gap of 1.17±0.36% (P<0.0001). With filling restriction (D) the Pc-rCO 2 gap increased to 3.87±0.29% (P<0.0001). The catheter alarms provided were not able to indicate all catheter problems and initial alarms disappeared although the problems continued. We conclude that impaired catheter deadspace to balloon volume relation can negatively influence accuracy of PrCO 2 measurement by the TONOCAP ®.