A system including, or for use with, an atrial leadless pacemaker, aLP, and a ventricular leadless pacemaker, vLP, that are configured to communicate with one another, wherein the aLP and the vLP are configured to collectively provide DDD operation when an atrial-to-ventricular, a2v, message transmitted by the aLP is successfully received by the vLP, and a ventricular-to-atrial, v2a, message transmitted by the vLP is successfully received by the aLP, during a cardiac cycle, and wherein the aLP and the vLP are configured to collectively provide at least one of VDD operation, DDI operation or VDI operation at least some times when at least one of an a2v message transmitted by the aLP is not successfully received by the vLP, and/or a v2a message transmitted by the vLP is not successfully received by the aLP, during a further cardiac cycle, the system comprising one or more processors (112, 1320), characterized in that the one or more processors (112, 1320) is/are configured to: obtain a2v throughput, v2a throughput, and event sequence information, for a period of time during which the aLP and the vLP were configured to collectively provide the DDD operation, wherein the a2v throughput specifies how often a2v messages transmitted by the aLP were successfully received by the vLP during the period of time during which the aLP and the vLP were configured to collectively provide the DDD operation, and wherein the v2a throughput specifies how often v2a messages transmitted by the vLP were successfully received by the aLP during the period of time; determine, based on the a2v throughput and the v2a throughput, an estimated DDD metric, an estimated VDD metric, an estimated DDI metric, and an estimated VDI metric, for the period of time; and determine an atrio-ventricular, AV, synchrony metric for the period of time based on the estimated DDD metric, the estimated VDD metric, the estimated DDI metric, the estimated VDI metric, and the event sequence information, wherein the AV synchrony metric specifies an estimate of how often AV synchrony was achieved during the period of time. wherein the aLP and the vLP are configured to collectively provide DDD operation when an atrial-to-ventricular, a2v, message transmitted by the aLP is successfully received by the vLP, and a ventricular-to-atrial, v2a, message transmitted by the vLP is successfully received by the aLP, during a cardiac cycle, and wherein the aLP and the vLP are configured to collectively provide at least one of VDD operation, DDI operation or VDI operation at least some times when at least one of an a2v message transmitted by the aLP is not successfully received by the vLP, and/or a v2a message transmitted by the vLP is not successfully received by the aLP, during a further cardiac cycle, the system comprising one or more processors (112, 1320), characterized in that the one or more processors (112, 1320) is/are configured to: obtain a2v throughput, v2a throughput, and event sequence information, for a period of time during which the aLP and the vLP were configured to collectively provide the DDD operation, wherein the a2v throughput specifies how often a2v messages transmitted by the aLP were successfully received by the vLP during the period of time during which the aLP and the vLP were configured to collectively provide the DDD operation, and wherein the v2a throughput specifies how often v2a messages transmitted by the vLP were successfully received by the aLP during the period of time; determine, based on the a2v throughput and the v2a throughput, an estimated DDD metric, an estimated VDD metric, an estimated DDI metric, and an estimated VDI metric, for the period of time; and determine an atrio-ventricular, AV, synchrony metric for the period of time based on the estimated DDD metric, the estimated VDD metric, the estimated DDI metric, the estimated VDI metric, and the event sequence information, wherein the AV synchrony metric specifies an estimate of how often AV synchrony was achieved during the period of time. obtain a2v throughput, v2a throughput, and event sequence information, for a period of time during which the aLP and the vLP were configured to collectively provide the DDD operation, wherein the a2v throughput specifies how often a2v messages transmitted by the aLP were successfully received by the vLP during the period of time during which the aLP and the vLP were configured to collectively provide the DDD operation, and wherein the v2a throughput specifies how often v2a messages transmitted by the vLP were successfully received by the aLP during the period of time; determine, based on the a2v throughput and the v2a throughput, an estimated DDD metric, an estimated VDD metric, an estimated DDI metric, and an estimated VDI metric, for the period of time; and determine an atrio-ventricular, AV, synchrony metric for the period of time based on the estimated DDD metric, the estimated VDD metric, the estimated DDI metric, the estimated VDI metric, and the event sequence information, wherein the AV synchrony metric specifies an estimate of how often AV synchrony was achieved during the period of time.