An unusual pattern of Para-Hisian pacing. The role of infra-Hisian conduction delay

A 26-year-old male athlete with a history of palpitations and intermittent preexcitation was referred to our center. The echocardiogram did not reveal any relevant abnormalities, while his basal 12-lead electrocardiogram showed sinus rhythm and incomplete left bundle branch block (LBBB). An electrophysiological study was performed, and multipolar diagnostic catheters were positioned at the His bundle (HB) region and coronary sinus. The AH and HV intervals were 60 milliseconds and 45 milliseconds, respectively. Retrograde conduction was concentric and decremental. During Para-Hisian pacing (PHP) maneuver, an interesting phenomenon was observed (Figure 1A). The loss of direct HB capture at a lower pacing-output led to delayed retrograde HB activation, with a subsequent significant and homogenous delay in atrial activation, consistent with a nodal response (beat 2, Figure 1A). The H-A interval was slightly shorter (∼10 milliseconds) at beat 2, likely because the loss of HB capture caused a marked prolongation of the H-H interval compared to the basal pacing cycle length favoring faster retrograde conduction over the AV node (ie, decremental conduction). However, the typical QRS widening was absent, and there was only a slight change in QRS morphology (earlier R/S transition in chest leads), while QRS duration remained nearly unchanged (∼120 milliseconds). Notably, the basal incomplete LBBB was observed to be more evident either spontaneously (Figure 1B), or at slightly faster rates of atrial pacing. Typically, direct HB capture at high pacing-output produces a narrower QRS because of the support of His-Purkinje system providing faster and more synchronous activation of both ventricles. In particular, the LBB should play a dominant role in this scenery since it supports the activation of the left ventricle (LV) that is far from the pacing site. In our case, the status of HB capture during PHP (direct capture vs. delayed retrograde activation) did not affect LV timing nor had a major effect on ventricular activation time (QRS duration), likely due to the basal conduction defect at the LBB level. In this study, no inducible tachycardia or accessory pathways were observed even at high-dose Isoproterenol and ablation was not performed. This case highlights an additional potential pitfall during PHP maneuver and how basal infra-Hisian conduction delay may affect the typical QRS changes that are essential criteria to recognize the status of HB capture. Multipolar HB recordings and the ability to detect retrograde HB potential, rather than relying on QRS changes, are the key for correct interpretation in such cases.