Postoperative fluid management in elderly, high-risk patients with multiple comorbidities requires careful assessment to prevent fluid overload. We report on two octogenarian male patients who developed hemodynamic instability in the intensive care unit following lengthy major surgeries. In these cases, conventional dynamic preload indices like Stroke Volume Variation were rendered unreliable by spontaneous breathing and atrial fibrillation, making fluid therapy decisions challenging. To overcome this, fluid responsiveness was directly evaluated using a mini fluid challenge （100 mL crystalloid） with a HemoSphere™ monitor. The test demonstrated low fluid responsiveness in both patients, indicating that a significant fluid bolus was unnecessary. This objective assessment allowed clinicians to avoid potentially harmful fluid overload, and both patients achieved hemodynamic stability with conservative management. The mini fluid challenge is a valuable tool for guiding safe fluid therapy in complex high-risk patients, especially when traditional indicators are compromised.

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　A male patient in his 60s with large squamous cell carcinoma of the floor of the mouth was scheduled for tumor resection and free-flap reconstruction. Preoperative chemotherapy （cetuximab） was discontinued because of an infusion reaction, and surgery was performed. During tumor manipulation, electrocardiography showed sudden tachycardia and T-wave elevation, with potassium levels rapidly increasing to 6.0 mmol/ L. Considering possible potassium release from tumor cells into the bloodstream, diuretics and glucose–insulin therapy were promptly initiated. Potassium levels were controlled, allowing the continuation of surgery and successful tumor excision. Post-excision hyperkalemia recurrence was not observed, and the procedure was completed. The interior of large tumors is prone to necrosis because of reduced blood flow. During manipulation, resecting a large tumor without achieving preoperative chemotherapy-induced tumor shrinkage can release tumor cell-derived potassium and metabolic products into the bloodstream, similar to that in tumor lysis syndrome. Therefore, the surgical team must ensure the safe completion of the procedure.

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