Performance of the Parasympathetic Tone activity (PTA) index to predict changes in mean arterial pressure in anaesthetized horses with different health conditions

Background: The parasympathetic tone activity (PTA) index is based on heart rate variability recently and has been developed in animals to assess their relative parasympathetic tone and their analgesia nociception balance. The present study aimed to evaluate the variation of PTA in anaesthetized horses according to haemodynamic status and health conditions and to determine the performance of dynamic variations of PTA (∆PTA) to predict mean arterial pressure (MAP) variations.Results: Thirty-nine client-horses admitted to the Veterinary Campus of Lyon were anaesthetized for elective or emergency surgery and divided into “Colic” and “Elective” groups. During anaesthesia, dobutamine was administered as treatment of hypotension (MAP < 60 mmHg). No significant variation of PTA and MAP were detected immediately before and after the time of cutaneous incision. PTA increased 5 min before each hypotension (+15% in Elective and +11.4% in Colic group), conversely, PTA decreased 1 min after the administration of dobutamine (-12.7% in Elective and -9% in Colic group). Horses of the Colic group had lower PTA values than those in Elective group, whereas MAP didn’t differ between groups. Globally, to predict a 10% increase in MAP, ΔPTA performance was associated with: AUC ROC [95% CI] = 0.77 [0.70 to 0.83] (p<0.0001), with a sensitivity of 88.2% and a specificity of 57.7% for a threshold value of −1%. Besides, to predict a 10% decrease in MAP, ΔPTA performance was associated with: AUC ROC [95% CI] =0.80 [0.73 to 0.85] (p<0.0001), with a sensitivity of 62.5% and a specificity of 94.6% for a threshold value of 25%.Conclusions: The PTA index in anaesthetized horses appears to be influenced by the haemodynamic status and the health condition. The shift toward lower PTA values in colic horses may potentially reflects a sympathetic tone predominance. Of clinical significance, a PTA increase of >25% in 1 min showed an acceptable performance to predict a MAP decrease of >10% within 5 min. Even though these results require further evaluation, this index may thus help to predict potential autonomic dysfunctions in sick animals.