A leadless pacing system comprising: a leadless pacing device configured to sense an electric cardiac signal and configured to operate in a sensing without pacing mode and an atria-ventricular pacing mode; and a processing module configured to: receive the electrical cardiac signal sensed by a leadless pacing device while the leadless pacing device is in the sensing without pacing mode, detect, based on the electrical cardiac signal, a first atrial activation event, determine, based on the electrical cardiac signal, a ventricular sense event was not detected within a first ventricular event detection window that begins at the first atrial activation event, and characterized by the processing module being configured to: detect, based on the electrical cardiac signal, a second atrial activation event after determining a ventricular sense event was not detected within the first ventricular event detection window; determine, based on the electrical cardiac signal, a ventricular sense event was not detected within a second ventricular event detection window that begins at the second atrial activation event, and, subsequently, control the leadless pacing device to switch from the sensing without pacing mode to an atria-ventricular synchronous pacing mode based on the determination that the ventricular sense event was not detected within at least the second ventricular event detection window after determining a ventricular sense event was not detected within the first ventricular event detection window, wherein the processing module is configured to control the leadless pacing device to switch to the atrio-ventricular synchronous pacing mode by at least controlling the leadless pacing device to deliver a pacing pulse to the patient according to the atria-ventricular synchronous pacing mode, but wherein the leadless pacing device does not deliver a pacing pulse to the patient within a cardiac cycle that includes the first atrial activation event. a leadless pacing device configured to sense an electric cardiac signal and configured to operate in a sensing without pacing mode and an atria-ventricular pacing mode; and receive the electrical cardiac signal sensed by a leadless pacing device while the leadless pacing device is in the sensing without pacing mode, detect, based on the electrical cardiac signal, a first atrial activation event, determine, based on the electrical cardiac signal, a ventricular sense event was not detected within a first ventricular event detection window that begins at the first atrial activation event, and characterized by the processing module being configured to: detect, based on the electrical cardiac signal, a second atrial activation event after determining a ventricular sense event was not detected within the first ventricular event detection window; determine, based on the electrical cardiac signal, a ventricular sense event was not detected within a second ventricular event detection window that begins at the second atrial activation event, and, subsequently, control the leadless pacing device to switch from the sensing without pacing mode to an atria-ventricular synchronous pacing mode based on the determination that the ventricular sense event was not detected within at least the second ventricular event detection window after determining a ventricular sense event was not detected within the first ventricular event detection window, wherein the processing module is configured to control the leadless pacing device to switch to the atrio-ventricular synchronous pacing mode by at least controlling the leadless pacing device to deliver a pacing pulse to the patient according to the atria-ventricular synchronous pacing mode, but wherein the leadless pacing device does not deliver a pacing pulse to the patient within a cardiac cycle that includes the first atrial activation event. detect, based on the electrical cardiac signal, a second atrial activation event after determining a ventricular sense event was not detected within the first ventricular event detection window; determine, based on the electrical cardiac signal, a ventricular sense event was not detected within a second ventricular event detection window that begins at the second atrial activation event, and, subsequently, control the leadless pacing device to switch from the sensing without pacing mode to an atria-ventricular synchronous pacing mode based on the determination that the ventricular sense event was not detected within at least the second ventricular event detection window after determining a ventricular sense event was not detected within the first ventricular event detection window, wherein the processing module is configured to control the leadless pacing device to switch to the atrio-ventricular synchronous pacing mode by at least controlling the leadless pacing device to deliver a pacing pulse to the patient according to the atria-ventricular synchronous pacing mode, but wherein the leadless pacing device does not deliver a pacing pulse to the patient within a cardiac cycle that includes the first atrial activation event.