Demonstration of increasing standard pH value of lacrimal fluid with increase of flow rate

Abstract

Purpose: To test the hypothesis that pH value of human tears is affected by flow rate. Methods: After stimulation with a drop of 70% alcohol into the ipsilateral nasal cavity tears from the left eyes of 30 healthy volunteers were collected with glass capillaries while flow rate of tear secretion was registered. The pCO2, actual and standard pH were determined in the collected samples with the Astrup method. Standard pH is defined as the pH of the sample that has been equilibrated at 37°C with a gas mixture having pCO2=40 mmHg. Results: The flow rate correlated positively and significantly with both the standard pH value (r=0.503, p<0.01) and the pCO2 (r=0.517, p<0.01) while there was no correlation between the actual pH and the flow rate. Conclusion: Loss of CO2 from the sample is a cause of serious uncertainty in tear pH determination. A positive flow rate-pH relationship was found for human tears when the uncertainty was removed by determination of the standard pH.