Scherlag and co-workers1 introduced a simple technique for recording the electrical activity of the His bundle in the human heart. The technique consisted of passing an electrode catheter to the area of the tricuspid valve. With proper catheter positioning, amplification, and appropriate frequency filtering, the bundle of His potential could be reliably recorded. The His bundle catheter records an atrial, His bundle (H), and ventricular electrogram (Fig 1).
Recording of H potentials allows subdivision of the PR interval into three subintervals. These are the following: PA, from the onset of the surface P wave to the onset of the low right atrial electrogram; AH, from the low right atrial electrogram to the onset of the first high frequency potential of the His bundle electrogram; and HV, from the H potential to the onset of surface QRS. The PA, AH, and HV intervals are, respectively, measures of intra-atrial (from high