Characterization of dynamic compliance of the respiratory system in healthy anesthetized dogs

IntroductionIn clinical practice, evaluating dynamic compliance of the respiratory system (C$_{dyn}$) could provide valuable insights into respiratory mechanics. Reference values of C$_{dyn}$ 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 C$_{dyn}$ and identify associated variables.MethodsData were collected from 515 client-owned dogs requiring anesthesia, excluding those with lower airway disease. The dogs were anesthetized, the tracheas intubated, and lungs ventilated at clinicians' discretion across 11 centers in six countries, with no restrictions on anesthesia protocols or ventilation settings, except avoiding inspiratory pauses. Three C$_{dyn}$ measurements from three consecutive breaths per dog were recorded using a standardized form, which also documented factors affecting C$_{dyn}$ identified through literature and an online survey. Various spirometry technologies were used. The substantial variance in C$_{dyn}$ 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 C$_{dyn}$, followed by a multiple linear regression model constructed via stepwise forward selection and backward elimination.ResultsThe best-fitting model identified a linear relationship between C$_{dyn}$ 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 C$_{dyn}$ 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 C$_{dyn}$. The use of alternative spirometry sensors, including Ped-lite, or prolonged exposure to high FIO$_{2}$ levels resulted in decreased C$_{dyn}$.Conclusion and clinical relevanceEstablishing a reference interval for C$_{dyn}$ proved challenging. A single reference interval may be misleading or unhelpful in clinical practice. Nevertheless, this study offers valuable insights into the factors affecting C$_{dyn}$ in healthy anesthetized dogs, which should be considered in clinical assessments.