Ability of the Analgesia Nociception Index Monitor to Distinguish Between Excessive Analgesia and Inadvertent Parasympathetic Nerve Stimulation During Surgery of Large Cerebellopontine Angle Tumours

Bradycardia and asystole are potential life threatning complications during surgery of large cerebellopontine angle (CPA) tumors (>2 x 2 cm diameter), with compression of the pons.The incidence of such complications are unknown. One of the plausible mechanisms is inadvertent parasympathetic nerve stimulation (IPNS) due to the proximity of this cranial nerve to the CPA tumor. Monitoring parasympathetic nerve activity may provide further insight to the implication of this cranial nerve in the cardiac complications observed during surgery of large CPA tumors. The analgesia nociception index (ANI) monitor assesses the balance between analgesia and nociception through the detection of parasympathetic tone. Despite the abundant clinical reports about this index, to the knowledge of the investigators, only a few studies have been published in the neurological setting. Moreover, there are no data reporting the parasympathetic profile (measured by the ANI monitor) in situations of IPNS and EA. Are these profiles the same or different? Such is the main question this study thrives to answer. Understanding the behavior of parasympathetic nerve activity in this context could help provide the appropriate management strategy.

In order to answer this question, participants undergoing elective large CPA tumor surgery were included in this prospective observational study. Standard cardiorespiratory monitoring including heart rate (HR) was done. Target-controlled anesthesia with Propofol and Remifentanil was guided by a bispectral index of 30-40 and an ANI of 50-70 respectively. Data was continuously recorded with event markers at the onset of bradycardia (HR < 45 bpm), asystole and the coincidence of ANI > 80 with Remifentanil site effect > 6 ng.ml-1 (defined as excessive analgesia).