Effect of lingual gauze swab placement and light exclusion on pulse oximeter agreement with arterial haemoglobin-oxygen saturation derived from blood-gas analysis in healthy anaesthetised horses.

OBJECTIVE: To compare peripheral arterial haemoglobin-oxygen saturation values obtained using pulse oximetry (SpO2) with and without a lingual gauze swab under ambient light and light-exclusion conditions, with arterial haemoglobin-oxygen saturation derived from arterial blood-gas analysis (SaO2). STUDY DESIGN: Prospective method-comparison study. ANIMALS: A group of 60 healthy adult horses undergoing elective general anaesthesia. METHODS: In American Society of Anesthesiologists (ASA) physical status classification I-II horses, SpO2 readings were obtained under six conditions during anaesthesia: no swab, no swab with light exclusion, dry swab, dry swab with light exclusion, wet swab and wet swab with light exclusion. Agreement with SaO2 values was assessed using the Bland-Altman method for repeated measurements. Bias (defined as SpO2 minus SaO2) and limits of agreement (LoA) were estimated from mixed-effects models with anaesthetic episode as the primary clustering unit. Confidence intervals (95% CI) were obtained using cluster bootstrapping. RESULTS: A total of 154 paired SpO2-SaO2 observations from 62 anaesthetic episodes with repeated measurements were analysed per experimental condition. SaO2 ranged from 94.4% to 100%, and PaO2 from 62.6 to 595.8 mmHg (8.3 to 79.4 kPa). SpO2 underestimated SaO2 across all conditions. Biases ranged from -1.6 to -2.7%, with the least bias observed using a dry swab under ambient light (-1.6 %, 95% CI -2.1 to -1.1). LoA width ranged from 10.3 to 13.3 % across configurations. Light exclusion produced more negative bias and wider LoA width than the ambient light configurations; however, these differences were small relative to the overall LoA width. CONCLUSIONS: In healthy adult horses undergoing elective anaesthesia, lingual gauze swab placement was not detrimental but did not meaningfully improve agreement between SpO2 and SaO2. CLINICAL RELEVANCE: SpO2 readings were imprecise across all configurations. Arterial blood gas analysis remains necessary when accurate assessment of arterial oxygenation is required.