Analyser position for end-tidal carbon dioxide monitoring in a rebreather circuit

A diving rebreather currently nearing release incorporates an infra-red CO2 analyser at the end of the exhale hose and uses the expired gas CO2 measurement made at this position to detect hypercapnia. This configuration may allow exhaled anatomic and mouthpiece dead space gas to mix with alveolar gas in the exhale hose thus falsely lowering the CO2 measurement, especially at low tidal volumes.

A test circuit was constructed using a typical rebreather mouthpiece and exhale hose connected into an anaesthetic machine breathing loop. True end-tidal PCO2 was measured in gas sampled from the mouth and compared breath-by-breath to the PCO2 measured in gas sampled at the end of the exhale hose. Two subjects each completed 60 breaths at tidal volumes of 500, 750, 1000, 1500 and 2000 ml.

There was a small (≤ 0.21 kPa) mean difference between true end-tidal CO2 and end-of-hose CO2 at tidal volumes of 1000 ml or more. However, at lower tidal volumes, the mean difference increased and, at 500 ml, it was 1.04 kPa and 0.70 kPa in subjects 1 and 2 respectively.

Measurement of the peak exhaled PCO2 at the end of a rebreather exhale hose may provide a reasonable estimation of the true end-tidal CO2 at large tidal volumes, but may significantly underestimate the true end-tidal CO2 at low tidal volumes.