anodal stimulation, biventricular pacing, CRT devices
Clinical ProblemAn 84-year-old patient with history of nonischemic dilated cardiomyopathy with severe systolic dysfunction (left ventricular ejection fraction [LVEF] 30%), heart failure symptoms (New York Heart Association [NYHA] class III), sinus rhythm, and left bundle branch block (LBBB; QRS duration 140 ms) with a cardiac resynchronization therapy (CRT) pacemaker Anthem TM (St. Jude Medical, St. Paul, MN, USA) attended to scheduled device check-up (Fig. 1). Since CRT implantation 1 year before, the patient's clinical situation had mildly improved to NYHA classes II-III, and LVEF was 35%. Device interrogation showed a biventricular pacing >99%. Right atrial and right ventricular (RV) stimulation threshold tests were performed normally (0.75 V at 0.4 ms and 0.75 V at 0.5 ms, respectively). Automatic left ventricular (LV) stimulation threshold was 3.0 V at 0.5 ms in bipolar configuration (tip LV-proximal ring RV). In order to optimize LV output, stimulation threshold for both configurations (bipolar and unipolar) were assessed using simultaneous electrocardiogram recording. LV stimulation threshold test in unipolar configuration showed a threshold of 2.75 V at 0.5 ms. LV threshold test in bipolar configuration (tip LV-proximal ring RV) showed an interesting phenomenon: at the beginning of the test, LV stimulation produced narrow biventricular stimulation QRS morphology; with the decreasing output energy it changed to RV stimulation QRS morphology and finally it showed lost capture (Fig. 2).
DiscussionAnodal stimulation is a phenomenon of activation of the myocardium in proximity to the anode. Pectoral muscle stimulation is a classic example of anodal stimulation associated with unipolar pacing. Biventricular pacing systems that utilize a unipolar lead for LV pacing via a coronary vein may create RV anodal pacing.1 The tip electrode of the LV lead is the cathode, and the proximal electrode of the bipolar RV lead often provides the anode for LV pacing. This arrangement creates a common anode for RV and LV pacing. A high current at the common anode during biventricular pacing may cause anodal capture, manifested as a paced QRS complex with a somewhat different configuration from that derived from standard biventricular pacing. During LV pacing at a relatively high output, RV anodal capture produces a paced QRS complex identical to that registered with biventricular pacing (Fig. 3). As we documented in our case, occasionally this type of anodal capture prevents electrocardiographic documentation of pure LV pacing if the LV pacing threshold is higher than that of RV anodal stimulation. Such anodal stimulation may complicate threshold testing and should not be misinterpreted as pacemaker malfunction. Moreover, in our case, it showed that the real LV stimulation threshold was 3.75 V at 0.5 ms for bipolar configuration and the automatic stimulation threshold algorithm considered RV paced beats as LV captures during the test (Fig. 2). Since the device can not accurat...