Limitations of high-frequency variability index in pain monitoring during pheochromocytoma resection

Dear Editor, The high-frequency variability index (HFVi) is an index of heart rate variability using the same algorithm as the analgesia nociception index and indicates sympathetic and parasympathetic tone. It is also used as an indicator of analgesia, with numbers close to 100 indicating parasympathetic dominance and analgesia and numbers close to 0 indicating inadequate analgesia.[1] However, HFVi has been reported to be affected by factors other than pain that can alter heart rate, such as bleeding and noradrenaline or adrenaline administration.[2,3] Herein, we report a case in which difficulty was experienced in assessing analgesia by HFVi during pheochromocytoma resection. A 56-year-old male, 170 cm in height and 67 kg in weight, was diagnosed as having secondary hypertension due to left-secreting pheochromocytoma based on elevated urinary metanephrines as well as metaiodobenzylguanidine scintigraphy findings. The patient had been taking the α1-blocker doxazosin 16 mg/day orally. Laparoscopic adrenalectomy was planned under general anaesthesia with a regional block. An HFVi sensor (High-Frequency HR Variability Index Monitor; Mdoloris Medical Systems, Loos, France) was attached to the chest before the induction of anaesthesia. Anaesthesia was induced with the target-controlled intravenous infusion of propofol with an effect site concentration of 3 µg/ml, fentanyl 100 µg and rocuronium 50 mg, followed by bilateral rectus sheath block and left transverse abdominis plane block using a total of 60 ml of 0.25% ropivacaine. Anaesthesia was maintained with intravenous propofol and remifentanil infusion at 0.2 µg/kg/min, with the patient state index value between 25 and 55. Until the periadrenal manipulation, there were no significant blood pressure or heart rate fluctuations, and the HFVi values remained between 40 and 70. During the ligation of a small artery and clamping of an adrenal artery, large increases in blood pressure and heart rate were observed, followed by a decrease in HFVi and a temporary increase when intravenous phentolamine was administered [Figure 1]. After tumour removal, the patient’s blood pressure and heart rate decreased, and noradrenaline administration was initiated, with the HFVi remaining stable around 90. Before the end of surgery, fentanyl intravenous infusion was initiated, and 1 g acetaminophen and 50 mg flurbiprofen were administered intravenously. The patient was then transferred to the intensive care unit, but his numeric rating scale score for pain was 4, and he required additional analgesics postoperatively.Figure 1: Trend of change of HR, BP, PSI and HFVi values. BP = blood pressure, HFVi = high-frequency variability index, HR = heart rate, PSI = patient state indexIn the present case, HFVi appeared to be affected by the secretion of endogenous catecholamines and noradrenaline initiated after resection of pheochromocytoma, suggesting that HFVi may not be a reliable indicator of analgesic effect in patients with pheochromocytoma. After removing a pheochromocytoma, a patient’s blood pressure and heart rate are typically low, often necessitating noradrenaline. It was previously reported that the use of noradrenaline increased HFVi values.[2] Similarly, in the current report, noradrenaline affected the HFVi values and particularly increased them. However, it is reported that placing adrenaline-soaked gauze decreased HFVi values.[3] Other factors that may affect the parasympathetic nervous system include medications and the presence of autonomic neuropathy. However, it is not known how these factors affect HFVi values, and some reports indicate that HFVi values were not affected by abdominal insufflation.[4] Explaining the variations in HFVi with changes in catecholamine levels and administration of vasoactive drugs remains a challenge in cases of pheochromocytoma. In addition, HFVi can be a misleading indicator of analgesic therapy in these cases despite the elevation in values. Understanding this is crucial to avoid inadequate analgesia in such cases. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient consented to his images and other clinical information to be reported in the journal. The patient understands that his name and initials will not be published and due efforts will be made to conceal his identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.