Abstract: There is provided a dual site pacing system, either bi-ventricular or bi-atrial, wherein the pacemaker looks for a signal sense during the refractory period following delivery of the pulse pair. If the threshold of either heart chamber has risen above the level of the delivered pulses, that chamber will not be captured, and will not have an inherent refractory period following delivery of the pulse pair. Under these circumstances, and where the patient has conduction delay from one chamber to the other, e.g., LBBB or RBBB, the excitation signal from the other chamber will be sensed in the non-captured chamber during the pacemaker refractory period. Such a sensing during the pacemaker refractory period is recognized to result from loss of capture, and is utilized to automatically increase pulse output back up to a safe level above threshold.