Importance and interpretation of fast-response airway hygrometry during ventilation of anesthetized patients

Measurement of oxygen uptake (Vo2) should help detect non-steady state critical events and metabolic derangement during anesthesia. Vo2 requires measurement of respiratory relative humidity (RH) and temperature (T). We have developed a fast response T and humidity sensor (HS), which uses tiny wet and dry thermometers to determine RH by psychrometry, where low RH causes evaporation to decrease wet T below dry T. In laboratory bench studies, we determined that >/=5 l/min gas flow through the HS is required for valid psychrometry function. This study demonstrates that monitoring of flow through the HS enhances the accuracy of RH measurement and interpretation.

Phase One: Laboratory bench validation; We designed a special bench setup for the validation of metabolic gas exchange compared to precise ethanol combustion. Phase 2: Clinical study; During mechanical ventilation of 6 anesthetized surgical patients, airway flow was used to successfully select valid wet T and dry T during inspiration and expiration, from which respective RH's were calculated using principles of psychrometry.

The average (+/-SD) percent error for airway Vco2 (compared to the stoichiometric value) was -1.84 +/- 2.69% (Table 2). The average (+/-SD) percent error for airway Vo2 was 0.91 +/- 3.10%. Average RQ was 0.649 +/- 0.017. For all patients, average inspired RH was 36.1 +/- 11.8% (range of 17-52%), which differed significantly from expiration (103 +/- 9%). Among the 6-8 consecutive breaths for each patient, average standard deviations of expired RH were only 0.6%.

We conclude that airway flow monitoring enhances the interpretation and accuracy of the fast-response HS measurements during inspiration and expiration, allowing for the determination of Vo2 in patients during anesthesia.