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.

Respiratory dysfunction is a cause of perioperative morbidity and mortality in anaesthesia in dogs. Appropriate interventions such as establishing an artificial airway, increasing the inspired fraction of oxygen, and using mechanical ventilation can help minimise risks. The effectiveness of ventilation is closely tied to the mechanics of breathing, and careful monitoring is key in optimising ventilator settings. Spirometry is commonly available in veterinary anaesthesia and can facilitate a more precise assessment of respiratory mechanics. It can provide valuable information on the Pressure-Volume (PV) relationship, which is crucial for understanding the mechanics of lung inflation and deflation. Compliance is a key aspect of the P-V relationship that reflects the elastic properties of the respiratory system. Compliance can be measured for the lungs, thoracic cage, or both (“compliance of the respiratory system”). It is defined as the change in lung volume relative to the corresponding change in pressure: alveolar to intrathoracic pressure for lung compliance, intrapleural to ambient pressure with relaxed respiratory muscles for thoracic cage compliance, and alveolar to ambient pressure for total compliance. Compliance can be measured in different ways (i.e., static, quasi-static, and dynamic), all offering unique perspectives on respiratory mechanics. Among these, dynamic compliance of the respiratory system (Cdyn) is a particularly attractive variable for its potential to offer continuous, real-time, non-invasive measurements. Despite a reported equation based on body mass for Cdyn, there is no comprehensive reference interval established in dogs. In addition, many previously undocumented factors are likely to have a marked influence on Cdyn. This thesis explores clinically relevant aspects of Cdyn in canine anaesthesia.

The first study was a survey of 128 veterinary anaesthesia and critical care professionals. The purpose of the survey was to provide insights into the use of spirometry in small animal anaesthesia. The results demonstrated that spirometry was more frequently used during mechanical ventilation than spontaneous breathing. Over 75% of respondents considered spirometry essential in “selected” or “most” cases. An interesting finding was that the shape of dynamic P-V loops was the most commonly evaluated feature. Additionally, many respondents reported using compliance in clinical practice. However, the survey results indicated a need for better calibration of the monitors and a clearer understanding of compliance values and the factors that influence them.

As a result, the second study was performed. This was a scoping review that evaluated 54 studies reporting Cdyn in anaesthetised dogs. The review revealed major variability in the definitions and abbreviations of Cdyn, as well as in the methods used for its measurement. Most studies published between 1970 and 2002 recruited dogs used as translational experimental models, employing relatively simple monitoring techniques and intrathoracic pressure measurement methods, therefore focusing specifically on dynamic compliance of the lung. However, this was not necessarily stated clearly. In contrast, the more recent studies shifted their focus to total compliance of the respiratory system displayed by modern monitors. In this scoping review, inconsistencies in the literature were identified, highlighting that better understanding and characterisation of Cdyn were necessary to improve its application in veterinary anaesthesia and clarify which variables should be reported in manuscripts in which Cdyn is an outcome variable. The survey and the review highlighted that a variety of spirometry technologies were potentially available in veterinary practice. Datex Ohmeda / GE Healthcare monitors, originally developed for human patients, were among the most commonly available technologies in veterinary anaesthesia at the time of the survey and thus were the subject of the third study.

The third study involved a performance assessment of 67 spirometry monitors, equipped with Pedi-lite and D-lite flow sensors across 14 veterinary institutions. The aim of the study was to assess the accuracy of the monitors using certified 1 Litre calibration syringes. The results indicated substantial variability between monitors, with most systems underestimating volumes. This variability was more pronounced when using the Pedi-lite sensor for volumes below 300 mL, raising concerns about the reliability of Cdyn measurements in smaller dogs. The findings suggested that close attention should be paid to the type of spirometry equipment used and its calibration, particularly in smaller animals.

To address the need for reference intervals in clinical practice, the final study was conducted to establish Cdyn values in a population of 515 client-owned dogs. The study involved 11 veterinary centres across six countries. The analysis revealed that Cdyn was influenced by several factors including the body condition score, the internal diameter of the endotracheal tube, the duration of the inspiration, the type of sensor used for the measurement, and the administration of an inspired fraction of oxygen >80% for more than 10 minutes before the measurement. Establishing a single reference interval applicable to all dogs proved challenging and would have been potentially misleading. Nevertheless, the study provided valuable insights into the factors that must be considered when evaluating Cdyn.

In conclusion, the survey and scoping review emphasised variability in current practices and the need for standardisation. The performance assessment study identified challenges associated with tidal volume measurement and Cdyn calculation in clinical practice. The multicentre study reinforced the necessity for individualised monitoring of respiratory function during anaesthesia, considering subject and equipment-specific variables. This thesis advocates for the standardisation of terminology related to the compliance of the respiratory system in canine anaesthesia. It provides a comprehensive list of essential information to report when using Cdyn in the future and offers valuable clinical insights into the complex aspects of Cdyn in anaesthetised and mechanically ventilated dogs.

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.