An apparatus, comprising: a probe having a plurality of electrodes (32; 33) and adapted for insertion into a heart of a living subject; and a processor, which is configured to receive an electrical signal from the electrodes and to perform the steps of: recording electrograms from the electrodes at respective locations in the heart; determining bipolar windows in the electrograms; annotating local activation times within the bipolar windows; determining from readings of a set of electrodes that a block point exists in a region of the set of electrodes; repositioning the annotated local activation time annotations responsively to the block point; determining revised windows that include respective local activation times; identifying primary slopes denoting activation at the respective locations, and secondary slopes denoting remote effects, in the electrograms; determining whether the electrodes are in contact with the heart; determining whether the primary slopes and the secondary slopes are coupled to one another; and determining lines of conduction block in the heart when the primary slope and the secondary slope are coupled to one another and the electrodes are in contact with the heart. a probe having a plurality of electrodes (32; 33) and adapted for insertion into a heart of a living subject; and a processor, which is configured to receive an electrical signal from the electrodes and to perform the steps of: recording electrograms from the electrodes at respective locations in the heart; determining bipolar windows in the electrograms; annotating local activation times within the bipolar windows; determining from readings of a set of electrodes that a block point exists in a region of the set of electrodes; repositioning the annotated local activation time annotations responsively to the block point; determining revised windows that include respective local activation times; identifying primary slopes denoting activation at the respective locations, and secondary slopes denoting remote effects, in the electrograms; determining whether the electrodes are in contact with the heart; determining whether the primary slopes and the secondary slopes are coupled to one another; and determining lines of conduction block in the heart when the primary slope and the secondary slope are coupled to one another and the electrodes are in contact with the heart. recording electrograms from the electrodes at respective locations in the heart; determining bipolar windows in the electrograms; annotating local activation times within the bipolar windows; determining from readings of a set of electrodes that a block point exists in a region of the set of electrodes; repositioning the annotated local activation time annotations responsively to the block point; determining revised windows that include respective local activation times; identifying primary slopes denoting activation at the respective locations, and secondary slopes denoting remote effects, in the electrograms; determining whether the electrodes are in contact with the heart; determining whether the primary slopes and the secondary slopes are coupled to one another; and determining lines of conduction block in the heart when the primary slope and the secondary slope are coupled to one another and the electrodes are in contact with the heart.