Figure 3.

Dual V-Ca_i²⁺ optical snap shots at the onset of spontaneous AF initiated with tyramine (5 μM) in an atrium isolated from a dog with chronic LVMI. Panel A shows dual V-Ca_i²⁺ snap shots during the last 2 sinus beats (beats #1 and 2) that propagate over the left atrial appendage (LAA). A focal activity originates 78 milliseconds after the second sinus beat from the LSPV (a in panels A and D) triggering the onset of AF shown in panel B and C. The time during the first sinus beat is arbitrarily chosen as 0 milliseconds and the white arrows indicate the direction of propagation. Panel B shows simultaneous dual V-Ca_i²⁺ signals at pixel labeled a located at the LSVP (shown in panel C). Notice that initially the site a manifests a small (subthreshold) EAD that fails to trigger (downward pointing black arrow, in panel B); however, 3 beats later the late EAD (upward pointing black arrow, sinus beat #2 in panel B) succeeds in causing triggered activity that degenerates to AF. Notice the longer duration of the Ca transient signal compared to the optical action potentials both in panels B and C and that the origination of the EAD (upward pointing black arrow in B) occurs when the underlying Ca_i²⁺ level is 75% of the peak systolic calcium transient amplitude (dashed vertical line and downward pointing red arrow in panel B). Panel C shows 5 consecutive simultaneous V-Ca_i²⁺ recordings (labeled a to b) located at sites shown in panel D. Notice that V-Ca_i²⁺ activation during the sinus rhythm is almost simultaneous (double headed arrow top of panel C) which then becomes sequential from a to b (downward pointing black arrows in panel C and labeled 1 to 3) as the focal activity originating from the LSPV and propagates toward the LAA. The 2 double-headed black at the bottom of C indicate the mapped period shown in panel A.