Monitoring of tidal ventilation by electrical impedance tomography in anaesthetised horses.

Abstract

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 &#xb1; s.d.: age 11.5 &#xb1; 7.5 years and body weight 491 &#xb1; 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) of 11-16 mL/kg (baseline volume), EIT data and VT measured by conventional spirometry were collected over 1 min. Thereafter, VTwas changed in 1 L steps until reaching 10 L. After, VTwas 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) and the whole image (EIT) were calculated. Linear regression analysis was used to assess the relationship between spirometry data and EITand EITfor individual horses and pooled data. RESULTS: Both EITand EITsignificantly predicted spirometry data for individual horses with Rranging 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(R= 0.799; P<0.001) and EIT(R= 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.