# Alveolar and Airway Components of the Tidal Volume in Mechanically Ventilated Dogs: An Exploratory Cross-Sectional Study

## TL;DR

This study explores how tidal volume in ventilated dogs varies, finding that alveolar volume is the main factor, with breed differences in airway anatomy.

## Contribution

The study identifies alveolar tidal volume as the primary source of variability in tidal volume among anesthetized dogs.

## Key findings

- Alveolar tidal volume (VTalv) contributes more to tidal volume variability than airway dead space (VDaw).
- Brachycephalic breeds have smaller VDaw but similar VTalv compared to non-brachycephalic breeds.
- Ideal body weight correlates with higher VDaw and lower VTalv.

## Abstract

Mechanical ventilation is widely used during anesthesia in dogs, but the tidal volume (VT) required can vary substantially between individuals. Tidal volume is composed of two main components: airway dead space (VDaw), which fills the conducting airways, and alveolar tidal volume (VTalv), which reaches the alveoli where gas exchange occurs. In this study, these components were measured in anesthetized dogs undergoing routine mechanical ventilation. The primary aim was to determine whether VDaw or VTalv is the main contributor to interindividual variability in VT, and how these components relate to body weight and breed. The results showed that VTalv accounted for most of the variability in VT, whereas VDaw contributed less. Brachycephalic breeds exhibited smaller VDaw but similar VTalv compared with non-brachycephalic breeds. These findings highlight the importance of considering alveolar volume and breed-specific airway anatomy when setting VT, potentially improving individualized ventilation strategies in dogs.

Background: Tidal volume (VT) during mechanical ventilation in dogs varies considerably between individuals, but the contributions of airway dead space (VDaw) and alveolar tidal volume (VTalv) to this variability remain unclear. Methods: In this prospective, observational, exploratory study, 95 anesthetized dogs receiving volume-controlled ventilation with VT initially set at 15 mL kg−1 of predicted ideal body weight (IBW) were analyzed. When necessary, TV was titrated within a predetermined quasi-static driving pressure range (i.e., >6 and <10 cmH2O) to maintain normocapnia. Volumetric capnography was used to measure expired tidal volume (VTe), VDaw, and VTalv. The relative contributions of VDaw and VTalv to VTe variability were assessed across body sizes and breeds, including brachycephalic dogs. In addition, the effects of ideal body weight (IBW) on the variability of VTe, VDaw, and VTalv were evaluated. Results: VTalv accounted for a greater proportion of the observed VTe variability than VDaw, indicating that interindividual differences in VTalv were the primary determinant of VT variability. Brachycephalic dogs exhibited smaller VDaw but similar VTalv compared with non-brachycephalic dogs. Ideal body weight was correlated with proportionally higher VDaw and lower VTalv. Recumbency was not correlated with the observed variability in VT components. Conclusions: VTalv was the main determinant of VT variability among anesthetized dogs, and the brachycephalic breeds studied exhibited smaller VDaw but similar VTalv compared with non-brachycephalic breeds. These findings highlight the importance of considering VTalv and breed-specific airway anatomy when individualizing ventilatory settings.