Appraisal of the 'penumbra effect' using lingual pulse oximetry in anaesthetized dogs and cats

Objective: Factors described as contributors to the 'penumbra effect' in relation to pulse oximetry include optical shunting, circulatory anastomoses and probe parallelity. This study aimed to clarify the main underlying mechanism involved.

Study design: Prospective clinical trial.

Animals: A total of 30 dogs and 15 cats (client-owned).

Methods: In anaesthetized dogs and cats, a pulse oximeter probe was placed on the tongue to measure haemoglobin oxygen saturation (SpO₂) and perfusion index. In 15 dogs, the probe was positioned at the root (baseline) of the tongue, then at 0.5 and 1 cm rostral to it, to investigate the effect of circulatory anastomoses on SpO₂ values. In cats (which do not have lingual arteriovenous anastomoses), the probe was positioned at the root and apex of the tongue. To assess the effect of probe parallelity on SpO₂ values in dogs, two lines were drawn parallel to the planes of the light-emitting diode and the detector surfaces and the intersection angle calculated using ImageMeter Pro, Google Play. In a further 15 dogs, the probe was placed at the tongue edge (0% optical shunt), with 50% optical shunt, then with the 50% optical shunt shielded. Data were analysed using Friedman's test, Student t test and Pearson's correlation coefficient (p < 0.05).

Results: In dogs, SpO₂ values were significantly higher at 1.0 cm than at baseline (p < 0.0001). In cats, there were no significant differences in SpO₂ values at each location. There was no significant difference in SpO₂ between 0% and 50% optical shunt in dogs. SpO₂ had a moderate negative correlation with tongue thickness and negligible correlation with intersection angle.

Conclusions and clinical relevance: Circulatory anastomoses are probably responsible for observed changes in SpO₂ as the probe is placed towards an extremity, rather than optical shunting or probe parallelity.