Mathieu Raillard ¹ Martina Mosing ² Anthea Louise Raisis ¹ Adam Auckburally ³ Karla Borland ⁴ Susana Canfrán ⁵ Frances Downing ⁶ Alejandra García De Carellán Mateo ⁷ Paul David MacFarlane ⁸ William McFadzean ⁹ Tristan Merlin ¹⁰ Karine Portier ^11,12 Josephine Robertson ¹³ Joao Henrique Neves Soares ¹⁴ Barbara Steblaj ¹⁵ Aurora Zoff ¹⁶ Olivier Levionnois ^17*

• ¹ School of Veterinary and Life Sciences, Murdoch University, Perth, Western Australia, Australia
• ² Small Animal Clinic, Department for Small Animals and Horses, University of Veterinary Medicine Vienna, Vienna, Vienna, Austria
• ³ Southern Counties Veterinary Specialists, Ringwood, United Kingdom
• ⁴ Anderson Moores, Hursley, United Kingdom
• ⁵ Hospital Clínico Veterinario Complutense, Universidad Complutense de Madrid, Madrid, Spain
• ⁶ Davies Veterinary Specialists, Herts, United Kingdom
• ⁷ Hospital Veterinario de la Universidad Católica de Valencia “San Vicente Mártir”, Valencia, Spain
• ⁸ Langford vets, University of Bristol, Langford, United Kingdom
• ⁹ Cave Veterinary Specialists, West Buckland, England, United Kingdom
• ¹⁰ Eastcott Veterinary Clinic & Hospital, Swindon, United Kingdom
• ¹¹ VetAgro Sup (Campus Vétérinaire), Centre de Recherche et de Formation en Algologie Comparée (CREFAC), University of Lyon, Marcy l'Etoile, France
• ¹² Université Claude Bernard Lyon, Centre de Recherche en Neurosciences de Lyon, INSERM, CRNL U1028 UMR5292, Trajectoire, Lyon, Bron, France
• ¹³ Small Animal Hospital, University of Glasgow, Glasgow, United Kingdom
• ¹⁴ Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, Davis, California, United States
• ¹⁵ Division of Anaesthesiology, Department for Clinical Diagnostics and Services, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland
• ¹⁶ North Downs Specialist Referrals, Bletchingley, United Kingdom
• ¹⁷ Division of Clinical Anaesthesiology, Department of Clinical Veterinary Medicine, Vetsuisse Faculty, University of Bern, Bern, Bern, Switzerland

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. 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 signedrank. 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. 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.