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.