Evaluation of histamine-provoked changes in airflow using electrical impedance tomography in horses

Abstract

Background: Electrical impedance tomography (EIT) generates thoracic impedance images of the lungs and has been used to assess ventilation in horses. This technique may have application in the detection of changes in airflow associated with equine asthma.

Objectives: The objective was to determine if histamine-induced airflow changes observed with flowmetric plethysmography (Δflow) could also be explained using global and regional respiratory gas flow signals calculated from EIT signals.

Study design: Experimental in vivo study.

Methods: Six horses, sedated using detomidine were fitted with a thoracic EIT belt and flowmetric plethysmography hardware. Saline (baseline = BL) and increasing concentrations of histamine (C1-4) were nebulised into the face mask until a change in breathing pattern was clinically confirmed and Δflow increased greater or equal to 50%. After nebulisation Δflow and EIT images were recorded over 3 minutes and peak global inspiratory (InF_global ) and expiratory (ExF_global ) flow as well as peak regional expiratory and inspiratory flow for the dorsal and the ventral area of the right and left lungs were evaluated. Delta flow, InF_global and ExF_global at subsequent concentrations were indexed to baseline (y_i = C_i /BL-1). Indexed and nonindexed variables were evaluated for a difference from baseline at sequential histamine doses (time). Multiple linear regression assessment of variance in delta flow was also investigated.

Results: Consistent with histamine-provoked increases in Δflow, the global flow indices increased significantly. A significant increase in regional inspiratory flow was seen in the right and left ventral lung and dorsal right lung. Multiple regression revealed that the variance in ExF_global , and right and left ventral expiratory flow best explained the variance in Δflow (r² = .82).

Main limitations: Low number of horses and horses were healthy.

Conclusions: Standardised changes in airflow during histamine challenge could be detected using EIT gas flow variables.

Keywords: EIT; bronchoconstriction; bronchoprovocation; horse; regional gas flow; spirometry.