Postextrasystolic Contractility Normally Decays in Alternans in Canine In Situ Heart

We have reported that the postextrasystolic (PES) potentiation of left ventricular (LV) contractility usually decays in alternans at heart rates above 80-100beats/min in the canine excised, cross-circulated heart. We examined whether the PES contractility would also decay in alternans even in the canine in situ heart presumably more physiological than the excised heart. In anesthetized, ventilated, and open-chest mongrel dogs, we measured LV pressure and volume with a micromanometer and a conductance catheter cannulated into the LV and obtained LV end-systolic maximum elastance (E_max) as the reasonably load-independent contractility index. We inserted an extrasystole followed by a compensatory pause into steady-state regular beats at heart rates above 90beats/min and analyzed the PES decay pattern of E_max. We found that E_max potentiated in the first PES beat decayed in alternans within 5-6 PES beats. This indicates that PES contractility also decays in alternans in the normal canine in situ heart.