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

Abstract

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 (VTEIT), using EIT and VT measured by a spirometer connected to a flow-partitioning device (VTSpiro) 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 VTEIT and VTSpiro 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 VTEIT was best described by peak inspiratory pressure (PIP) and PEEP (adjusted R2 0.82) while variance in VTSpiro was best described by PIP and airway deadspace (adjusted R2 0.76). The relationship between VTEIT and VTSpiro remains linear with changes in tidal volume, and stable across altered lung conditions. This may have application for monitoring and assessment in vivo.

Keywords: Anaesthesia; Cattle; Dead space; Pulmonary monitoring; Volumetric capnography.

Conflict of interest statement

The authors declare that they have no conflict of interest.

© 2021. The Author(s), under exclusive licence to Springer Nature B.V. part of Springer Nature.

Figures

Fig. 1

Schematic showing the positioning of the ventilator, flow partitioning device, spirometer and EIT belt when the animal is positioned in dorsal recumbency. Airway deadspace is illustrated highlighting the difference in point of measurement

Fig. 2

Electrical impedance tomography (EIT) graphs recorded in one of twelve steers during the study: a Impedance changes visible on the EIT image were evaluated with no predefined region of interest representing the lung area (VTThorax). b Measurement of the global impedance change VTEIT (AU = arbitrary unit) using EIT recorded over time in seconds (s). The baseline tidal volume in this animal was 5 L. Tidal volume increased from 5 to 10 and 15 ml kg−1 in a stepwise manner at each level of PEEP. PEEP was increased from 0, 5, 10, and 15 in a stepwise manner

Fig. 3

Flow diagram shwing the number of steers included in this study and drop out at each measurement point. VTset, tidal volume set by ventilator in ml kg−1, PEEP, positive end expiratory pressure

Fig. 4

Relationship and confidence interval between VTEIT and VTSpiro measured by electrical impedance tomography (EIT) and spirometry measurements in twelve anaesthetised mechanically ventilated steers at PEEP 0, PEEP 5, PEEP 10 and PEEP 15. Spirometry was measured and as a volume (mL) and EIT as impedance change ΔZ (AU = arbitrary unit) represented by full circles