This study includes the in vitro and in vivo validation of a new spirometer to monitor ventilation in anaesthetised horses. The principle of the revolver is based on the distribution of flow into four equal flows through a flow-divider. Four NICO2® sensors, which are used in human spirometry, are attached into four connection tubes which connect the flow divider with the identical flow collector. In order to measure flow or tidalvolume one NICO2® sensor is connected with the dedicated NICO2® monitor.
In the in vitro part of the study the revolver/NICO2® system showed a good agreement between measured tidal volumes and reference volumes. The measurement of tidal volumes (1-7L) was not significantly influenced by the use of different gas mixtures (21% O2, 50 % O2, 100 % O2), as long as the composition of the used gas was entered into the software of the NICO2® monitor.
The relative volume error was 1,18 ± 0,77 % (21% O2, 3 L). The calculated conversion factor was close to 4 (3.91 to 4.1), which was also approved by the similar pressure differences and resistance of all four NICO2® sensors.
In the in vivo part of the study five horses were anaesthestised twice within two weeks: one time in lateral recumbency, one time in dorsal recumbency. All horses were artificially ventilated with a baseline modus (15mL/kg, 0 PEEP) and 9 different ventilation modes (8, 10, 15 ml/kg and 0, 10, 20 PEEP). Tidalvolumes measured with the revolver/NICO2® system were compaired to tidalvolumes measured with the reference method (H-lite/S/5TM) using the Bland-Altman-analysis. The range of measurement included tidal volumes from 2565 up to 8722 ml. The revolver/NICO2® system underestimates the volume measurement of the H-lite / S/5TM (bias ± precision -327.339 ± 291 mL). Neither the recumbency of the horses nor the used ventilation modes affected the accuracy of the revolver/NICO2® system. Condensed water accumulating inside of the connection tubes during general anesthesia did not lead to errors in the measurement.
The results of the in vitro validation and also the in vivo validation show that the principle of flow distribution was successfully implemented and the conversion factor to be used is four. The revolver can be used in an anaesthetic system for large animals due to its low resistance.