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

In 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.

Data 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.

The 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 FIO₂ levels resulted in decreased C_dyn.

Establishing 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.