Monitoring of tidal ventilation by electrical impedance tomography in anaesthetised horses

Summary Background Electrical impedance tomography ( EIT ) is a method to measure regional impedance changes within the thorax. The total tidal impedance variation has been used to measure changes in tidal volumes in pigs, dogs and men. Objectives To assess the ability of EIT to quantify changes in tidal volume in anaesthetised mechanically ventilated horses. Study design In vivo experimental study. Methods Six horses (mean ± s.d.: age 11.5 ± 7.5 years and body weight 491 ± 40 kg) were anaesthetised using isoflurane in oxygen. The lungs were mechanically ventilated using a volume‐controlled mode. With an end‐tidal carbon dioxide tension in the physiological range, and a set tidal volume ( VT vent ) of 11–16 mL/kg (baseline volume), EIT data and VT measured by conventional spirometry were collected over 1 min. Thereafter, VT vent was changed in 1 L steps until reaching 10 L. After, VT vent was reduced to 1 L below the baseline volume and then further reduced in 1 L steps until 4 L. On each VT step data were recorded for 1 min after allowing 1 min of stabilisation. Impedance changes within the predefined two lung regions of interest ( EIT ROI ) and the whole image ( EIT thorax ) were calculated. Linear regression analysis was used to assess the relationship between spirometry data and EIT ROI and EIT thorax for individual horses and pooled data. Results Both EIT ROI and EIT thorax significantly predicted spirometry data for individual horses with R 2 ranging from 0.937 to 0.999 and from 0.954 to 0.997 respectively. This was similar for pooled data from all six horses with EIT ROI (R 2 = 0.799; P<0.001) and EIT thorax (R 2 = 0.841; P<0.001). Main limitations The method was only tested in healthy mechanically ventilated horses. Conclusions The EIT can be used to quantify changes in tidal volume.