Indirectly estimated sinoatrial conduction time by the atrial premature stimulus technique: patterns of error and the degree of associated inaccuracy as assessed by direct sinus node electrography.

The atrial premature stimulus method for estimating sinoatrial conduction time (SACT) is commonly used. When the stimulated atrial premature depolarization (APD) does not appear to affect sinus node automaticity or conduction, the indirectly estimated SACT (SACT1) is quite accurate. That is, SACT1 correlates quite highly with SACT measured directly (SACTD) on sinus node electrograms (SNE). In this study we used direct SNE recordings in 17 patients to assess SACT1 when factors thought to produce inaccuracy in SACT1 were present. Three patients had sinoatrial entrance block, which might make some expect sinoatrial exit delay to be present. However, SACTD was normal in two (60 and 70 msec) and prolonged (130 msec) only in the one who had other evidence of sinus node dysfunction. Therefore, sinoatrial entrance block does not necessarily indicate sinoatrial exit delay. Thirteen patients had apparent depression of sinus node automaticity by the induced APD (A3A4 greater than A1A1). In all 13, SACT1 overestimated SACTD. One patient had apparent sinoatrial conduction delay induced by the APD and/or vagal transmitter release induced by the APD. In this patient, too, SACT1 exceeded SACTD. Thus when sinoatrial automaticity or conduction are depressed by the stimulated APD, SACT1 will overestimate SACTD. If SACT1 is normal, SACTD will be normal; however, if SACT1 is prolonged, SACTD may or may not be prolonged.