Factors affecting performance of air tonometry using the TONOCAP

Area of Science:

• Medical Devices
• Respiratory Physiology
• Clinical Engineering

Background:

• Automated air tonometry (TONOCAP) is increasingly used, replacing older saline tonometry methods.
• Potential pitfalls in TONOCAP usage require thorough investigation for reliable clinical application.
• Understanding factors affecting regional PCO2 (PrCO2) measurement is crucial for patient monitoring.

Purpose of the Study:

• To evaluate the impact of various catheter filling and volume restrictions on the accuracy of TONOCAP PrCO2 measurements.
• To assess the reliability of TONOCAP device alarms in detecting catheter-related measurement errors.

Main Methods:

• An in vitro setup measured PrCO2 using a TONOCAP device and an 8F catheter under controlled conditions.
• Experiments included adding deadspace, restricting balloon expansion with a syringe, and cross-clamping the balloon.
• Differences between known PCO2 (PcCO2) and measured PrCO2 were analyzed using ANOVA.

Main Results:

• Increased catheter deadspace led to a significant Pc-rCO2 gap of 3.2% (P < 0.0001).
• Restricting balloon expansion caused a Pc-rCO2 gap of 1.17% (P < 0.0001).
• Filling restrictions resulted in the largest Pc-rCO2 gap of 3.87% (P < 0.0001), with alarms failing to detect all issues.

Conclusions:

• The relationship between catheter deadspace and balloon volume critically affects TONOCAP's PrCO2 measurement accuracy.
• Clinical implementation of TONOCAP requires careful attention to catheter integrity to avoid erroneous readings.
• TONOCAP's built-in alarms may not reliably detect all catheter-related measurement interferences.