Pheochromocytoma is a catecholaminesecreting tumor arising from the adrenal medulla or extra-adrenal paraganglia [1]. Surgical excision of the tumor is the only curative therapy. However, surgery carries a high risk of inducing a massive release of catecholamines into the circulation, leading to hypertensive crisis, cardiac arrhythmia, pulmonary edema, and cardiac ischemia. Prolonged peripheral vasoconstriction secondary to high catecholamine levels results in volume depletion [2], and acute reduction in catePerioperative sequential monitoring of hemodynamic parameters in patients with pheochromocytoma using the Non-Invasive Cardiac System (NICaS) abstract. Surgical treatment of pheochromocytoma is associated with a high risk of hemodynamic instability. To reduce the risk of perioperative complications, adequate medical treatment to normalize blood pressure and restore blood volume is required. Accurate evaluation of the circulating blood volume (CBV) in perioperative patients with pheochromocytoma is clinically important. In the present study, we adopted whole-body bioimpedance monitoring technique using the NonInvasive Cardiac System (NICaS), which can non-invasively measure cardiac output (CO) values. NICaS-derived CO values were evaluated in eight preoperative patients with pheochromocytoma and were compared with simultaneous CBV values measured by a conventional indicator dilution method using 131 I-labeled human serum albumin. In these patients with pheochromocytoma, the NICaS-derived CO values were significantly correlated with the CBV values measured by 131 I-labeled human serum albumin (4.86 ± 1.05 L/min vs 4.79 ± 1.02 L; r = 0.906; P = 0.002). Sequential NICaS-derived CO values confirmed that CBV increased after preoperative treatment with an α-blocker, with or without volume loading. The results of this study indicate that NICaS can be used to accurately and non-invasively evaluate the hemodynamic status. By sequential monitoring of NICaS-derived CO values, we are able to confirm whether adequate CBV in a patient with pheochromocytoma is obtained by preoperative medical treatment with α-blockers or volume loading, to avoid perioperative complications.Key words: Pheochromocytoma, Cardiac output (CO), NICaS, Circulating blood volume, α-blocker cholamine levels after pheochromocytoma surgery can cause hypotension and shock [3]. To minimize the perioperative complications, patients are treated with α-blockers and intravenous fluids to restore the circulating blood volume (CBV) before surgery [2][3][4].The indicator dilution technique using radioactive tracers has been used in nuclear medicine departments for over 50 years to monitor CBV in patients with pheochromocytoma [5]. A semi-automated blood volume analyzer, the BVA-100, also uses indicator dilution principles to measure plasma volume using radioactive iodinated human serum albumin. Although the BVA-100 system improves accuracy and eliminates the many time-consuming and difficult steps required for on-site standard testing, it is expensive, and is n...