Mathieu Raillard ^1* Martina Mosing ² Anthea Louise Raisis ¹ Adam Auckburally ³ Georgina Beaumont ⁴ Frances Downing ⁵ Charlotte Heselton ⁴ Paul David MacFarlane ⁶ Karine Portier ^7,8 Josephine Robertson ⁹ Joao Henrique Neves Soares ¹⁰ Barbara Steblaj ¹¹ Elliot Wringe ⁵ Olivier Levionnois ^12*

• ¹ Murdoch University, Perth, Australia
• ² Clinical Unit of Anaesthesiology and Perioperative Intensive-Care Medicine, Department for Small Animals and Horses, University of Veterinary Medicine Vienna, Vienna, Vienna, Austria
• ³ Southern Counties Veterinary Specialists, Ringwood, United Kingdom
• ⁴ Manchester Veterinary Specialists, Worsley, Manchester, United Kingdom
• ⁵ Davies Veterinary Specialists, Herts, United Kingdom
• ⁶ Langford vets, University of Bristol, Langford, United Kingdom
• ⁷ Centre de Recherche et de Formation en Algologie Comparée (CREFAC), Campus vétérinaire de Lyon, VetAgro Sup, Marcy l’Etoile, Auvergne-Rhone-Alpes, France
• ⁸ Centre de Recherche en Neurosciences de Lyon, INSERM, CRNL U1028 UMR5292, Université Claude Bernard Lyon 1, Lyon, Rhône-Alpes, France
• ⁹ Small Animal Hospital, University of Glasgow, Glasgow, Scotland, United Kingdom
• ¹⁰ Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, Davis, California, United States
• ¹¹ Section Anaesthesiology, Department of Diagnostics and Clinical Sciences, Vetsuisse Faculty, University of Zurich, Zurich, Zurich, Switzerland
• ¹² Division of Anaesthesiology and Pain therapy, Vetsuisse Faculty, University of Bern, Bern, Bern, Switzerland

In clinical practice, evaluating dynamic compliance of the respiratory system (Cdyn) could provide valuable insights into respiratory mechanics. Reference values of Cdyn based on body weight have been reported, but various factors may affect them and the evidence is scanty. This study aimed to establish a reference interval for Cdyn and identify associated variables. Data were collected from 515 clientowned dogs requiring anaesthesia, excluding those with lower airway disease. The dogs were anesthetized, the tracheas intubated, and lungs ventilated at clinicians' discretion across 11 centres in six countries, with no restrictions on anaesthesia protocols or ventilation settings, except avoiding inspiratory pauses. Three Cdyn measurements from three consecutive breaths per dog were recorded using a standardized form, which also documented factors affecting Cdyn identified through literature and an online survey. Various spirometry technologies were used. The substantial variance in Cdyn measurements led to a comprehensive analysis using a multiple linear regression model. Multicollinearity (variables highly correlated with each other) was addressed by investigating, transforming, or excluding factors. Initial simple linear regression assessed each variable's individual effect on Cdyn, followed by a multiple linear regression model constructed via stepwise forward selection and backward elimination. The best-fitting model identified a linear relationship between Cdyn and body mass when the following conditions were met: high BCS (Body Condition Score), orotracheal tubes <7% smaller than predicted, the use of a D-lite flow sensor, and the absence of a high FIO2 (>80%) exposure for more than 10 minutes before Cdyn measurement. In cases where these conditions were not met, additional factors needed to be incorporated into the model. Low (1/9, 2/9, 3/9) and medium (4/9, 5/9) BCS, an orotracheal tube of the predicted size or larger and longer inspiratory times were associated with increased Cdyn. The use of alternative spirometry sensors, including Ped-lite, or prolonged exposure to high FIO2 levels resulted in decreased Cdyn. Establishing a reference interval for Cdyn proved challenging. A single reference interval may be misleading or unhelpful in clinical practice. Nevertheless, this study offers valuable insights into the factors affecting Cdyn in healthy anesthetized dogs, which should be considered in clinical assessments.