Characterisation of dynamic compliance of the respiratory system in anaesthetised dogs

Mathieu C Raillard

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.

Characterisation of dynamic compliance of the respiratory system in anaesthetised dogs

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