the patient was a 70-year - old man with a history of coronary artery bypass grafting and mitral annuloplasty due to ischemic heart disease , functional mitral regurgitation , type 2 diabetes mellitus , and chronic kidney disease ( ckd ) . , gem dr , dual chamber pulse generator , vt zone : 150188  bpm , vf zone : > 188  bpm ) for the primary prevention of sudden cardiac death and was admitted to our center for the incidence of icd shocks . his admission surface 12-lead electrocardiogram ( ecg ) revealed an increase of the t - wave amplitude compared to his previously recorded surface 12-lead ecg ( fig . his serum potassium and creatinine levels were 7.4  meq / l and 1.9  mg / dl , respectively . interrogation of the icd revealed 8 icd shock therapies for 4 ventricular fibrillation ( vf ) episodes that occurred during sinus rhythm ( sr ) on the visit day . ventricular intracardiac electrograms showed that the large amplitude of the t - waves during sr resulted in oversensing of the t - waves as r - waves ( fig . the patient was treated with calcium gluconate , and spironolactone was discontinued immediately after admission . the patient s serum potassium level normalized to 4.4  meq / l in 5 days . the increased t - wave amplitude recorded by both the surface 12-lead ecg and icd electrograms resolved simultaneously along with the normalization of the plasma potassium level ( figs . the patient was discharged with prescriptions of furosemide , temocapril , and calcium resonium , and no further inappropriate icd therapies were observed over a period of 66 months . t - wave oversensing is caused by alternations or variations in intracardiac electrograms for reasons such as hyperglycemia , drugs , sympathetic tones , ventricular pacing , and hyperkalemia . several similar cases of hyperkalemia - induced t - wave oversensing with inappropriate icd therapies have been reported , . however , the relationship between 12-lead surface electrograms and internal icd electrograms during hyperkalemia was not fully elucidated . a previous study reported that t - wave oversensing is more commonly observed after paced beats than spontaneous beats and this was attributed to the polarization voltage . in the present case , t - wave oversensing originated from normal atrio - ventricular conduction , resulting in an inappropriate icd shock . the amplitudes of the intracardiac r - waves and t - waves were measured as approximately 78  mv and 3  mv , respectively . thus , the oversensing of t - waves with large amplitudes could not be prevented by a simple algorithm in this model . possible approaches include prolongation of the number of intervals for tachycardia detection , adjustment of the vt and vf zone ranges , elevation of the sensing threshold , change of sensing vector , and use of all - time v pacing to mask t - waves in the blanking period . however , these strategies run the risk of ignoring true ventricular tachyarrhythmias and exhausting the battery . if these strategies are ineffective , ventricular lead repositioning / replacement may also be considered in order to obtain a greater r - wave amplitude , making oversensing less likely . in recently published reports , patients with icds including the latest t - wave oversensing discriminator program have experienced a dramatically lower rate of inappropriate icd shocks compared to previous studies . furthermore , the incidence of t - wave oversensing causing inappropriate icd shocks in this study was much smaller than previously reported . therefore , device replacement should be considered if needed . interestingly , the t - wave amplitude on icd electrograms corresponded with that on 12-lead surface electrograms , and the amplitude on icd electrograms was dramatically changed compared to that on 12-lead surface electrograms . differences between the action potential characteristics of ventricular endocardial and epicardial tissues were reported previously , . of note , the ik1 channel is distributed in the endocardial tissue , and is strongly associated with the repolarization process and extracellular potassium concentration . furthermore , the differential characteristics of ik1 may contribute to the differences in action potential configuration between endocardial and epicardial myocytes , . thus , we speculated that the change in the repolarization process during hyperkalemia was evident on the icd electrogram as compared to the surface 12-lead ecg . hyperkalemia should be carefully monitored in icd recipients , especially those with ckd , to prevent inappropriate icd shocks due to t - wave oversensing .
there have been reports of hyperkalemia - induced t - wave oversensing in patients with implantable cardioverter defibrillators ( icds ) . 
 however , a comparison of t - wave amplitudes and morphologies between the surface 12-lead electrocardiogram ( ecg ) and icd electrogram has not been reported . 
 we present the case of a 70-year - old man who received inappropriate icd shocks due to hyperkalemia - induced t - wave oversensing . 
 the t - wave amplitudes on both the icd electrogram and 12-lead ecg corresponded and normalized after normalization of the potassium level .