Abstract
The aim was to examine the effects of recumbency and anaesthesia on distribution of ventilation in beagle dogs using Electrical Impedance Tomography (EIT). Nine healthy beagle dogs, aging 3.7±1.7 (mean±SD) years and weighing 16.3±1.6 kg, received a series of treatments in a fixed order on a single occasion. Conscious dogs were positioned in right lateral recumbency (RLR) and equipped with 32 EIT electrodes around the thorax. Following five minutes of equilibration, two minutes of EIT recordings were made in each recumbency in the following order: RLR, dorsal (DR), left (LLR) and sternal (SR). The dogs were then positioned in RLR, premedicated (medetomidine 0.01, midazolam 0.1, butorphanol 0.1 mg kg-1 iv) and pre-oxygenated. Fifteen minutes later anaesthesia was induced with 1 mg kg-1 propofol iv and maintained with propofol infusion (0.1–0.2 mg kg-1 minute-1 iv). After induction, the animals were intubated and allowed to breathe spontaneously (FIO2 = 1). Recordings of EIT were performed again in four recumbencies similarly to conscious state. Centre of ventilation (COV) and global inhomogeneity (GI) index were calculated from the functional EIT images. Repeated-measures ANOVA and Bonferroni tests were used for statistical analysis (p < 0.05). None of the variables changed in the conscious state. During anaesthesia left-to-right COV increased from 46.8±2.8% in DR to 49.8±2.9% in SR indicating a right shift, and ventral-to-dorsal COV increased from 49.8±1.7% in DR to 51.8±1.1% in LLR indicating a dorsal shift in distribution of ventilation. Recumbency affected distribution of ventilation in anaesthetized but not in conscious dogs. This can be related to loss of respiratory muscle tone (e.g. diaphragm) and changes in thoracic shape. Changing position of thoraco-abdominal organs under the EIT belt should be considered as alternative explanation of these findings.
Highlights
The distribution of ventilation is a main determinant of ventilation and perfusion matching and the oxygenation of arterial blood [1]
During anaesthesia left-toright Centre of Ventilation (COV) increased from 46.8±2.8% in DR to 49.8±2.9% in SR indicating a right shift, and ventral-to-dorsal COV increased from 49.8±1.7% in DR to 51.8±1.1% in left lateral (LLR) indicating a dorsal shift in distribution of ventilation
Ventral-to-dorsal COV increased (p = 0.014) from DR to LLR (49.8 ± 1.7% and 51.8 ± 1.1%, respectively) indicating a dorsal shift in ventilation distribution
Summary
The distribution of ventilation is a main determinant of ventilation and perfusion matching and the oxygenation of arterial blood [1]. A disadvantage of EIT over the other methods is that it cannot measure total lung volume, ventilation per unit of lung volume cannot be obtained by EIT alone. This method measures total changes in regional air content that is consistent with regional distribution of tidal volume (hereinafter “distribution of ventilation”). The fact that this information is obtained non-invasively and continuously suggests that EIT may have a huge potential as a monitoring tool for ventilated patients in the ICU [5] or during operations. In spite of the fact, that canine patients are being ventilated in ICU [6] with a variety of lung conditions and they would potentially benefit from lung monitoring with EIT, it is not yet a common technique in veterinary medicine
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