Figure 1.

Measurement of sinoatrial conduction time (SACT) by premature atrial stimulation through esophagus in vivo (from the young group). The stimuli were introduced earlier in atrial diastole when progressive shortening of the test cycle (A-AT) failed to result in reciprocal lengthening of the return cycle (AT-AR) but the return cycle duration remained nearly constant. The return cycle was no longer fully compensatory but was greater than one spontaneous sinus cycle (A-A). This change in return cycle response marked the transition from zone I to zone II. Then the difference between AT-AR and A-A reflects conduction time into and out of the SAN