A device comprising: at least one implantable cardiac contraction sensing circuit configured for providing a sensed contraction signal from an atrium and a sensed contraction signal from a ventricle; a timer circuit configured for providing a time duration of an atrial-atrial (A-A) interval between successive atrial contractions, a ventricular-ventricular (V-V) interval between successive ventricular contractions, and an atrial-ventricular (A-V) interval between an atrial contraction and a same cardiac cycle ventricular contraction; an electrical stimulation circuit configured to provide pacing electrical stimulation energy to at least one implantable electrode in the atrium and at least one implantable electrode in the ventricle; and a controller communicatively coupled to the cardiac contraction sensing circuit, the timer circuit, and the electrical stimulation circuit, wherein the controller includes: an event detection module configured for determining whether atrial-ventricular (A-V) block events are sustained over multiple cardiac cycles, based on ventricular-ventricular (V-V) intervals and a pacing module configured for: providing pacing therapy according to a primary pacing mode including: pacing at least one atrium when an A-A interval exceeds an atrial lower rate limit interval; and pacing at least one ventricle, without regard to any event occurring in the atrium, when a V-V interval exceeds a ventricular LRL interval, wherein the ventricular LRL interval is longer than the atrial LRL interval; and switching the pacing therapy to a secondary pacing mode if A-V block events are sustained over multiple cardiac cycles, wherein the secondary pacing mode includes pacing the at least one ventricle when an atrial-ventricular contraction (A-V) interval exceeds a first A-V delay interval. at least one implantable cardiac contraction sensing circuit configured for providing a sensed contraction signal from an atrium and a sensed contraction signal from a ventricle; a timer circuit configured for providing a time duration of an atrial-atrial (A-A) interval between successive atrial contractions, a ventricular-ventricular (V-V) interval between successive ventricular contractions, and an atrial-ventricular (A-V) interval between an atrial contraction and a same cardiac cycle ventricular contraction; an electrical stimulation circuit configured to provide pacing electrical stimulation energy to at least one implantable electrode in the atrium and at least one implantable electrode in the ventricle; and a controller communicatively coupled to the cardiac contraction sensing circuit, the timer circuit, and the electrical stimulation circuit, wherein the controller includes: an event detection module configured for determining whether atrial-ventricular (A-V) block events are sustained over multiple cardiac cycles, based on ventricular-ventricular (V-V) intervals and a pacing module configured for: providing pacing therapy according to a primary pacing mode including: pacing at least one atrium when an A-A interval exceeds an atrial lower rate limit interval; and pacing at least one ventricle, without regard to any event occurring in the atrium, when a V-V interval exceeds a ventricular LRL interval, wherein the ventricular LRL interval is longer than the atrial LRL interval; and switching the pacing therapy to a secondary pacing mode if A-V block events are sustained over multiple cardiac cycles, wherein the secondary pacing mode includes pacing the at least one ventricle when an atrial-ventricular contraction (A-V) interval exceeds a first A-V delay interval. an event detection module configured for determining whether atrial-ventricular (A-V) block events are sustained over multiple cardiac cycles, based on ventricular-ventricular (V-V) intervals and a pacing module configured for: providing pacing therapy according to a primary pacing mode including: pacing at least one atrium when an A-A interval exceeds an atrial lower rate limit interval; and pacing at least one ventricle, without regard to any event occurring in the atrium, when a V-V interval exceeds a ventricular LRL interval, wherein the ventricular LRL interval is longer than the atrial LRL interval; and switching the pacing therapy to a secondary pacing mode if A-V block events are sustained over multiple cardiac cycles, wherein the secondary pacing mode includes pacing the at least one ventricle when an atrial-ventricular contraction (A-V) interval exceeds a first A-V delay interval. providing pacing therapy according to a primary pacing mode including: pacing at least one atrium when an A-A interval exceeds an atrial lower rate limit interval; and pacing at least one ventricle, without regard to any event occurring in the atrium, when a V-V interval exceeds a ventricular LRL interval, wherein the ventricular LRL interval is longer than the atrial LRL interval; and pacing at least one atrium when an A-A interval exceeds an atrial lower rate limit interval; and pacing at least one ventricle, without regard to any event occurring in the atrium, when a V-V interval exceeds a ventricular LRL interval, wherein the ventricular LRL interval is longer than the atrial LRL interval; and switching the pacing therapy to a secondary pacing mode if A-V block events are sustained over multiple cardiac cycles, wherein the secondary pacing mode includes pacing the at least one ventricle when an atrial-ventricular contraction (A-V) interval exceeds a first A-V delay interval.