Use of Electrical Impedance Tomography (EIT) to Estimate Tidal Volume in Anaesthetized Horses Undergoing Elective Surgery

Abstract

This study explores the application of electric impedance tomography (EIT) to estimate tidal volume (VT) by measuring impedance change per breath (∆Zbreath). Seventeen healthy horses were anaesthetised and mechanically ventilated for elective procedures requiring dorsal recumbency. Spirometric VT (VTSPIRO) and ∆Zbreath were recorded periodically; up to six times throughout anaesthesia. Part 1 assessed these variables at incremental delivered VT of 10, 12 and 15 mL/kg. Part 2 estimated VT (VTEIT) in litres from ∆Zbreath at three additional measurement points using a line of best fit obtained from Part 1. During part 2, VT was adjusted to maintain end-tidal carbon dioxide between 45–55 mmHg. Linear regression determined the correlation between VTSPIRO and ∆Zbreath (part 1). Estimated VTEIT was assessed for agreement with measured VTSPIRO using Bland Altman analysis (part 2). Marked variability in slope and intercepts was observed across horses. Strong positive correlation between ∆Zbreath and VTSPIRO was found in each horse (R2 0.9–0.99). The agreement between VTEIT and VTSPIRO was good with bias (LOA) of 0.26 (−0.36–0.88) L. These results suggest that, in anaesthetised horses, EIT can be used to monitor and estimate VT after establishing the individual relationship between these variables.

Comparison of Effects of an Endotracheal Tube or Facemask on Breathing Pattern and Distribution of Ventilation in Anesthetized Horses

“…Tidal impedance variation (TIV; arbitrary units). TIV represents the total ΔZ over one breath and can be used as the EIT surrogate of tidal volume in large animals ( 17 , 18 ).…”

Thoracic Electrical Impedance Tomography—The 2022 Veterinary Consensus Statement

“…Impedance change has been shown to have a direct relationship with tidal volume (VT) measured by spirometry in cattle, pigs, dogs and horses (39,43,(49)(50)(51). However, there are inconsistencies in the literature in the terms used to describe this variable.…”

“…It is recommended that the term tidal variation (TIV) is used when the change in impedance is presented in AU, while the term tidal volume (VT EIT ) is used when the change in impedance is converted to units of volume (e.g., L or mL), either using a predefined linear regression line or after calibration using spirometry (49). It should be specified in materials and methods if tidal variation (TIV in AU or tidal volume (VT EIT ) in L or mL estimated from measured Z) is used to estimate gas volume per breath.…”

“…Silent Spaces have been used to assess ventilatory changes in horses with heart disease (28), horses undergoing surgery (49) or anaesthetised horses undergoing mechanical ventilation with or without ARM under anaesthesia (38,57), dogs undergoing PEEP challenges (32,40), anaesthetised rhinoceroses (21) and an anaesthetised orangutan (25).…”

“…Examples in the veterinary literature describing the use of CoV include its use in evaluating distribution of ventilation in standing sedated horses (22), horses with cardiac disease (28), horses undergoing experimental bronchoconstriction (46), horses with asthma before and after exercise (53), anaesthetised horses with mechanical ventilation with or without various ARM (22,38,57), horses undergoing various surgical procedures (49), anaesthetised dogs in various recumbencies (42), dogs undergoing ARM or increasing PEEP (32,40) and anaesthetised rhinoceroses (21).…”