The sinus node electrogram rediscovered.

In 1971 Cramer [1] reported the characteristics of unambiguous measurement of sino-atrial conduction time, unipolar electrograms recorded in vitro from the pacethe identification of pacemaker shifts and the extent to maker region of the rabbit sinus node and subsequently which atrial impulses penetrate the node and act on the published similar records from the sinus pacemaker of the pacemaker. Indeed the method should be applicable to the canine heart studied both in vitro and in situ [2]. Substudy of any site at which phase 4 depolarization leads to sequently several investigators employed both unipolar and impulse initiation and thus to clearly differentiate between remote bipolar catheter electrodes to demonstrate autoautomatic and reentrant rhythms. matic activity of the sinus pacemaker in the human heart. The findings when TTX was used to prevent excitation However, potentials from remote sites made identification of atrial muscle clearly add to our earlier findings with the and characterization of pacemaker potentials difficult and same intervention. In addition to providing sinus node uncertain and the recording technique never became electrograms free of extrinsic potentials, maps of excitation popular. sequence during exposure to TTX demonstrate a marked Now Yamamoto and colleagues have used very small, slowing of conduction within the node. It seems reasonable close bipolar electrodes to record pacemaker potentials to assume that this slowing results in large part from block from the sinus node region of the isolated rabbit right of sodium channels in latent pacemaker cells where both atrium and to map the spread of excitation from the sodium and calcium inward currents contribute to the primary pacemaker throughout the node and to the atrium action potential upstroke. However, the slowing also may [3]. Use of the small, close bipolar electrodes minimizes be due in part to the loss of rapid impulse spread in potentials arising at remote sites and thus provides undisperinodal atrial fibers which would exert some influence on torted and beautifully clear records of the sinus node nodal propagation. TTX had one additional effect: it electrograms. These records consistently identify pacecaused the disappearance of the second slow negative maker, latent pacemaker and non-pacemaker sites within deflection in the electrogram or its transformation into a the node and also permit quite precise mapping of the slow positive deflection. It is likely that the great slowing spread of activity. The records are in qualitative agreement of conduction so modified the sequence of repolarization with those obtained with unipolar leads in that they that the pacemaker region no longer was the last area to demonstrate the typical extracellular potential changes recover. This finding makes is more likely that the second associated with the latter part of phase 4 depolarization and slow negative deflection in control records is the |T wave the upstroke of the pacemaker action potential. A new of the pacemaker region. observation is the identification of a second slow negative When activity of the primary pacemaker was suppressed deflection, synchronous with repolarization of the domiby calcium channel block with nifedipine, the pacemaker nant pacemaker cells, that almost certainly results from shifted to the crista terminalis and the rate increased. The that repolarization. findings that the intrinsic cycle length of the latent One hopes that use of a similar recording method will pacemaker was shorter than that of the primary sinus provide equally clear records of pacemaker activity in the pacemaker is puzzling. The calcium channel block dehuman heart. The pacemaker electrogram can provide creased the duration of the action potentials but the information that otherwise is not available. The primary magnitude of this effect could not account for the large negativity resulting from pacemaker depolarization (not to abbreviation in cycle length. There was no indication that be confused with primary negativity of atrial muscle) the ectopic pacemaker was uncoupled from the atrium and precisely localizes the pacemaker region and permits indeed nifedipine is not known to have such an effect.