Heart failure is a progressive disorder that affects 6-10% of the population over the age of 65.¹ Biventricular pacemakers are approved for use in patients with advanced heart failure that is refractory to medical therapy when cardiac contraction is dyssynchronous, manifested by QRS duration > 130 ms.² They differ from conventional permanent pacemakers by virtue of having a third lead that is inserted into the epicardial surface of the left ventricle via the coronary venous system. Pacing stimulation of the right and left ventricles by the biventricular pacemaker resynchronizes ventricular activation,² often with dramatic improvement in clinical CHF status. Anodal stimulation in biventricular pacemakers may negate the beneficial clinical outcome due to lack of left ventricular pacing. Anodal pacing makes it impossible to separate the timing of left ventricular and right ventricular pacing. Therefore, recognizing anodal stimulation is important in these cases. During echo optimization of biventricular pacing in particular, the knowledge of the chamber captured is essential. We present a case of amplitude-dependent anodal stimulation.

Case Report

A 74-year-old male with a past medical history of coronary artery disease presented to the emergency department (ED) with complaints of shortness of breath, fatigue, dizziness, a non-productive cough, and increased pedal edema. In the emergency department, the patient was found to be markedly bradycardiac with a heart rate of 30 beats per minute (bpm) and low blood pressure (81/64 mmHg). The ECG revealed third degree heart block and left bundle branch block.
In the ED, the patient was stabilized with a dopamine drip, and temporary transcutaneous pacer pads were applied. Moderate decrease in ejection fraction of 45% was identified, and the decision was made to implant a biventricular pacemaker. During implantation, a QRS pattern of left ventricular capture was obtained during isolated left ventricular pacing (Figure 1) when low amplitude stimulation was used. However, when higher amplitude pacing (10 mA) was used to assess for diaphragmatic stimulation during left ventricular pacing, an ECG pattern similar to that seen with right ventricular pacing was noted (Figure 2). We switched to unipolar pacing of the LV (skin to LV lead) at various amplitudes and noted the LV paced QRS configuration only (Figure 3). We established the diagnosis of amplitude-dependent anodal stimulation. Since the anodal stimulation occurred only at unduly high pacing output and LV pacing thresholds were very low, we left the leads in that position and completed the implant.

Discussion