Monitoring changes in distribution of pulmonary ventilation by functional electrical impedance tomography in anaesthetized ponies.

OBJECTIVE: To assess changes in the distribution in pulmonary ventilation in anaesthetized ponies using electrical impedance tomography (EIT).

STUDY DESIGN: Prospective experimental study.

ANIMALS: A group of eight adult pony geldings.

METHODS: General anaesthesia was induced and maintained using isoflurane in oxygen with volume-controlled ventilation [tidal volume (VT ) (10 mL kg-1 ), respiratory rate (8-12 breaths minute-1 )] to maintain end-tidal carbon dioxide between 35 and 40 mmHg (4.66-5.32 kPa). Regional distribution of ventilation was assessed with EIT (plane of fifth to sixth intercostal space) at 30, 60 and 90 minutes after intubation. The resulting functional images were divided into four regions of interest (ROI, A-D) to determine: 1) the ratio of VT distribution between dependent to nondependent lung (D/ND) and 2) changes in distribution within the lungs in the latero-lateral direction. The centre of ventilation (COV) was calculated. The dimension of the chest in the latero-lateral (W) and dorso-ventral (H) direction was measured at the height of the withers and the W/H ratio was calculated. The influence of time on the D/ND ratio, COV and ROI A-D were analysed with anova followed by post hoc Bonferroni tests. Pearson correlation coefficient and the coefficient of determination (r2 ) were calculated to evaluate the relationship between the W/H ratio and D/ND ratio, and COV. Statistical significance was set at p < 0.05.

RESULTS: The median D/ND ratio (T30-T90) was 0.67 (0.40-1.01) and for COV 31.15% (11.20-45.20), confirming the expected uneven distribution of ventilation. The D/ND ratio had a moderate negative correlation with the W/H ratio (r = -0.68, r2  = 0.46, p < 0.001), while the COV did not correlate with the W/H ratio (r = -0.04).

CONCLUSIONS AND CLINICAL RELEVANCE: Uneven ventilation in mechanically ventilated right laterally recumbent anaesthetized ponies occurs within 30 minutes and changed little over the following 60 minutes.