Mathieu Raillard

Objectives
To record the terms, definitions, and abbreviations used in the literature, investigate the rationale for employing dynamic compliance (Cdyn) in studies of anaesthetised dogs with mechanically ventilated lungs, and identify the methods used to calculate Cdyn.

Databases used
A comprehensive search across Medline, PubMed, Scopus, and CAB Abstracts databases identified studies using keywords related to canine species, anaesthesia, Cdyn, and the respiratory system. Reference lists from recent publications (2010–2024) focusing on respiratory mechanics in dogs were also reviewed. Following duplicate removal, a two-step screening process was employed. This involved reviewing titles and abstracts, followed by full-text retrieval based on predefined eligibility criteria, concentrating on studies involving anaesthetised dogs with closed chests where Cdyn was measured. Data extraction included terms, definitions, measurement equipment, and study applications.

Conclusions
Of 362 initial documents, 186 duplicates were removed, leaving 176 for abstract screening. Of these, 122 full texts were retrieved, with 54 meeting inclusion criteria. Most studies were published between 1970 and 2002, with only five published after 2010. In 49/54 studies, dogs were used as animal models for translational research. Whole-body plethysmographs and pneumotachographs were commonly used to evaluate tidal volume for the calculation of Cdyn; the sampling site of airway pressure varied. In 43/54 papers, oesophageal or pleural pressure was measured to determine transpulmonary pressure, suggesting that Cdyn of the lung was monitored, although this was not always explicitly stated. The three most recent studies involved clinical patients, using Cdyn of the respiratory system displayed by respiratory modules integrated into ventilators or multiparametric physiologic monitors. Future research should establish clear protocols for measuring Cdyn to enhance understanding and characterisation for both research and clinical purposes.

Introduction: 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.

Methods: 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 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.

Results: 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.

Conclusion and clinical relevance: 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.

Introduction: Spirometry devices, which are components of many anaesthesia machines, are commonly used to assess lung mechanics during anaesthesia. Spirometry calibration usually adheres to manufacturer recommendations without established guidelines. Although more accurate and less variable than inbuilt spirometry in certain General Electric anaesthesia ventilators, near-patient spirometry lacks adequate evaluation.

Methods: We assessed near-patient spirometers’ performance using Pedi-lite and D-lite flow sensors. Certified 1 L calibration syringes were used on 67 monitors located in 14 veterinary hospitals. Three consecutive inspired and expired volume values displayed by the monitors for each volume of the calibration syringe were recorded. Volumes studied were 50, 100, 150, 250, 300 mL for Pedi-lite and 150, 300, 450, 500, 750 mL for D-lite. Measured and targeted volumes were averaged, agreement error calculated. Accuracy was assessed plotting agreement errors against calibration volumes. A linear mixed-effects model was used to obtain linear regression between the error and the calibration volume. Mean, differential and proportional bias, limits of agreement, claimed accuracy and 10% clinical tolerance were calculated and displayed. Differences among monitors were evaluated using the Friedman rank sum test, differences between inspired and expired volumes using the Wilcoxon signed-rank.

Results: Inter-monitor variability for inspired and expired volume readings using both sensors was high; intra-monitor variability was low. The error magnitude was independent of volumes evaluated. Using Pedi-lite, only a minority of measurements met manufacturer’s specification or a 10% clinical tolerance; both inspired and expired volumes were significantly underestimated. Using D-lite, superior performance was demonstrated for volumes between 300 and 750 mL (mean biases close to zero and the majority of measurements meeting manufacturer’s specifications and clinical tolerance). The difference between measured inspired and expired volumes with both sensors was significant.

Discussion: These results support caution when interpreting clinical measurements of lung volumes and mechanics in anaesthetised patients when using these sensors. This is particularly important in smaller patients where lung volumes are below 300 mL. Trends should be reliable.