Fluctuating high-frequency variability index in a patient implanted with pacemaker: A case report

High-frequency variability index, which is calculated based on heart rate variability derived from the R-wave peak to R-wave peak interval in the electrocardiogram, is used in clinical settings to adjust intraoperative opioid administration. Theoretically, patients implanted pacemaker with fixed rate have no fluctuation in high-frequency variability index due to constant R-wave peak to R-wave peak interval. We experienced a case with fluctuating high-frequency variability index in a patient implanted with pacemaker. A 75-year-old female implanted with a cardiac pacemaker owing to a complete atrioventricular block, underwent total knee arthroplasty. In the operating room, the pacemaker rate was set to 70 bpm in VOO mode and her surgery was started. Unexpectedly, the high-frequency variability index fluctuated between 35 and 80 during her surgery. While the exact reasons for these fluctuations are unclear, electrical impulses generated by the pacemaker and transmitted via electrodes to the myocardium may induce myocardial contractions, contributing to the observed variations in high-frequency variability index. Thus, it is crucial to note that a fluctuating high-frequency variability index in patients with implanted cardiac pacemakers does not reliably reflect their autonomic tone.