This prospective study was designed to evaluate the relationship and correlations between tidalvolume measurements by RUP using spirometry with a pitot-tube system as reference method. Five horses with a mean age of 13 ± 6 years and a mean bodyweight of 531 ± 77 kg were anaesthetised twice within two weeks. One time the animals were placed on the table in dorsal and the other time in left lateral recumbence. Each time the horses were artificially ventilated using 9 different ventilation modes. Those modes differed in tidalvolume (8, 10 and 12ml/kg) and PEEP (0, 10, 20 cmH2O). The sequence of the modes was randomised for each horse and anaesthesia. Before and between each of the 9 ventilation modes the animals were artificially ventilated with a baseline ventilation mode (15 ml/kg and PEEP 0 cmH2O). Data were collected electronically, synchronised between the measurement-systems and statistically evaluated (regression analysis, ANOVA and Bland-Altman).
Correlation of all data measured with RUP was poor to tidalvolume measured with spirometry (R2 =0.52). Reasons could be found in variability between the different ventilation modes. Beside individual differences in shape of thorax and abdomen between the horses, recumbence and PEEP showed significant influence on correlation. In left lateral recumbence and PEEP 0 cmH2O the thoracic RUP showed a higher correlation than in dorsal recumbence (R2 = 0.81 vs. R2 = 0.57); there was only a small difference in the abdominal RUP. Increase in PEEP decreased the correlation of the thoracic RUP significantly, but again had less influence on the abdominal RUP.
Other reasons for the variability between the ventilation modes can be found in potential displacement of the RUP tubes over time, changes in gas- and fluid filling of the abdominal organs and spontaneous respiration. A significant change of compliance of thorax and abdomen could be observed before and after a spontaneous breath occurred. The off-line calibration of all data after the measurements and the evaluation with the Bland-Altman analysis showed a bias of up to 2.2 litres for each measurement taken with RUP.
An increase in PEEP with the same tidalvolume resulted in a significant increase of the thoracic and abdominal RUP, and therefore also in the endexpiratory lung volume (EELV) compared to baseline ventilation. The thoracic RUP showed an increase in EELV after PEEP of 10 and 20 cmH2O even after return to baseline ventilation, as described in recruitment manoeuvres. In this study design and with the described calibration technique RUP was not useful in measuring tidalvolume. The influence of PEEP on the EELV was described accurately by RUP and therefore RUP may be used in monitoring and potentially measuring changes of EELV in anaesthetised, artificially ventilated horses in the future.