In vitro and in vivo evaluation of a new large animal spirometry device using mainstream CO2 flow sensors

Effects of PEEP on the relationship between tidal volume and total impedance change measured via electrical impedance tomography (EIT)

Electrical impedance tomography (EIT) is used in lung physiology monitoring. There is evidence that EIT is linearly associated with global tidal volume (VT) in clinically healthy patients where no positive end-expiratory pressure (PEEP) is applied. This linearity has not been challenged by altering lung conditions. The aim of this study was to determine the effect of PEEP on VT estimation, using EIT technology and spirometry, and observe the stability of the relationship under changing lung conditions. Twelve male castrated cattle (Steer), mean age 7.8 months (SD ± 1.7) were premedicated with xylazine followed by anaesthesia induction with ketamine and maintenance with halothane in oxygen via an endotracheal tube. An EIT belt was applied around the thorax at the level of the fifth intercostal space. Volume controlled ventilation was used. PEEP was increased in a stepwise manner from 0 to 5, 10 and 15 cmH₂O. At each PEEP, the VT was increased stepwise from 5 to 10 and 15 mL kg^-1. After a minute of stabilisation, total impedance change (VT_EIT), using EIT and VT measured by a spirometer connected to a flow-partitioning device (VT_Spiro) was recorded for the following minute before changing ventilator settings. Data was analysed using linear regression and multi variable analysis. There was a linear relationship between VT_EIT and VT_Spiro at all levels of PEEP with an R² of 0.71, 0.68, 0.63 and 0.63 at 0, 5, 10 and 15 cmH₂O, respectively. The variance in VT_EIT was best described by peak inspiratory pressure (PIP) and PEEP (adjusted R² 0.82) while variance in VT_Spiro was best described by PIP and airway deadspace (adjusted R² 0.76). The relationship between VT_EIT and VT_Spiro remains linear with changes in tidal volume, and stable across altered lung conditions. This may have application for monitoring and assessment in vivo.

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

Simple Summary

The aim of this study was to explore the usefulness of electrical impedance tomography (EIT), a novel monitoring tool measuring impedance change, to estimate tidal volume (volume of gas in litres moved in and out the airways and lungs with each breath) in anaesthetised horses. The results of this study, performed in clinical cases, demonstrated that there was a positive linear relationship between tidal volume measurements obtained with spirometry and impedance changes measured by EIT within each subject and this individual relationship could be used to estimate tidal volume that showed acceptable agreement with a measured tidal volume in each horse. Thus, EIT can be used to observe changes in tidal volume by the means of impedance changes. However, absolute measurement of tidal volume is only possible after establishment of the individual relationship.

Abstract

This study explores the application of electric impedance tomography (EIT) to estimate tidal volume (VT) by measuring impedance change per breath (∆Z_breath). Seventeen healthy horses were anaesthetised and mechanically ventilated for elective procedures requiring dorsal recumbency. Spirometric VT (VT_SPIRO) and ∆Z_breath 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 (VT_EIT) in litres from ∆Z_breath 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 VT_SPIRO and ∆Z_breath (part 1). Estimated VT_EIT was assessed for agreement with measured VT_SPIRO using Bland Altman analysis (part 2). Marked variability in slope and intercepts was observed across horses. Strong positive correlation between ∆Z_breath and VT_SPIRO was found in each horse (R² 0.9–0.99). The agreement between VT_EIT and VT_SPIRO 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.

Monitoring of tidal ventilation by electrical impedance tomography in anaesthetised horses

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 R² 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 (R² = 0.799; P<0.001) and EIT_thorax (R² = 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.