A Method to Calibrate the Carbon Dioxide (Chemical) Stimuli of Pneumatic Esthesiometer Externally

To determine the feasibility of using a portable carbon dioxide (CO₂) sensor to calibrate a pneumatic esthesiometer and then to calibrate the chemical stimuli.

The chemical stimuli in ocular surface experiments are combinations of medical air and added CO₂ (%CO₂). These stimuli were calibrated using a portable CO₂ sensor (COZIR CM-0041) and data logger, delivered for 100 seconds by using the Waterloo Belmonte esthesiometer. The distances between the sensor and esthesiometer tip were 0 (to measure feasibility), 3, 5, and 10 mm. In experiment I, 100% CO₂ was tested using four different flow rates (50, 100, 150, and 200 mL/min) at three working distances. In experiment II, flow rates of 20 to 100 mL/min and concentrations of 20% to 100% CO₂ were tested in 20 steps at 3 working distances.

The CO₂ sensor correctly reported the esthesiometer extremes of 0% and 100% CO₂ when placed at the esthesiometer tip. There were progressive, systematic increases in concentrations reaching/reported by the sensor with increasing flow rates and nominal concentrations and progressive decreases in measurements with increases in working distance.

CO₂ concentrations in pneumatic esthesiometers can be calibrated and, as expected, vary with flow rate and distance, highlighting the importance of calibration and standardization of CO₂ stimuli in these instruments.

Calibrated CO₂, a chemical sensory stimulus in humans, may be used in testing the surface of the eye as well as other membranes within which the CO₂ can be dissolved (e.g., mucous) to produce an acidic stimulus.