The risk of anaesthetic and sedation-related death and 95% confidence intervals (95% CI) were calculated adjusting for clustering at the clinic level.
In the case–control study, cases were compared with anaesthetics or sedations recorded in the cohort that did not die within 48 h of the procedure (controls) (Appendix A).
In cases that died before the procedure was performed, intended duration was recorded as the mean duration for controls of the same procedure category.
The unmatched controls were randomly and prospectively selected at a 1:4 case:control ratio from the cohort of cats anaesthetized and sedated during the study.
Details of the patient, procedure, anaesthetic and sedative management, and personnel involved were recorded for cases and controls on self-administered questionnaires (Table 1).
Intended procedure type and duration were recorded, in addition to actual procedure and duration.
A nested case–control study was undertaken in 117 UK veterinary centres.
Sample size calculations indicated that approximately 150–170 cases would be required to detect risk factors with a prevalence of 5% in the controls and an odds ratio of ≥2.5 or a prevalence of 10% in the controls and an odds ratio of ≥2.0 (80% power, 5% level of statistical significance, case:control ratio of 1:4).The cohort study data were returned to the investigators on a monthly basis and were entered into a relational database (Access, Microsoft), exported to a spreadsheet (Excel, Microsoft) and cumulative frequencies of monthly anaesthetic and sedation events were calculated.This allowed the identification of individual cat anaesthetic and sedation events by veterinary centre and within that centre by animal number of the month (e.g.During the study, 79 178 anaesthetic and sedation procedures were recorded and the overall risk of anaesthetic and sedation-related death was 0.24% (95% CI 0.20–0.27).Factors associated with increased odds of anaesthetic-related death were poor health status (ASA physical status classification), increasing age, extremes of weight, increasing procedural urgency and complexity, endotracheal intubation, and fluid therapy.