An implantable medical device system comprising: a therapy delivery circuit comprising a holding capacitor (162, 352, 354, 356, 358) and a charging circuit (154, 330) configured to charge the holding capacitor to a pacing voltage amplitude; a sensing circuit configured to receive a cardiac electrical signal from a patient's heart; and a control circuit coupled to the sensing circuit and the therapy delivery circuit and configured to: control the therapy delivery circuit to deliver a pacing pulse; start a first pacing interval corresponding to a pacing rate in response to the delivered pacing pulse; control the therapy delivery circuit to charge the holding capacitor during the first pacing interval according to a first charging mode; detect an increased intrinsic heart rate from the cardiac electrical signal that is at least a threshold rate faster than the pacing rate; switch from the first charging mode to a second charging mode in response to detecting the increased intrinsic heart rate; while operating in the second charging mode, start a second pacing interval in response to a first intrinsic cardiac event sensed from the cardiac electrical signal; and control the therapy delivery circuit to withhold charging of the holding capacitor for at least a portion of the second pacing interval according to the second charging mode. a therapy delivery circuit comprising a holding capacitor (162, 352, 354, 356, 358) and a charging circuit (154, 330) configured to charge the holding capacitor to a pacing voltage amplitude; a sensing circuit configured to receive a cardiac electrical signal from a patient's heart; and a control circuit coupled to the sensing circuit and the therapy delivery circuit and configured to: control the therapy delivery circuit to deliver a pacing pulse; start a first pacing interval corresponding to a pacing rate in response to the delivered pacing pulse; control the therapy delivery circuit to charge the holding capacitor during the first pacing interval according to a first charging mode; detect an increased intrinsic heart rate from the cardiac electrical signal that is at least a threshold rate faster than the pacing rate; switch from the first charging mode to a second charging mode in response to detecting the increased intrinsic heart rate; while operating in the second charging mode, start a second pacing interval in response to a first intrinsic cardiac event sensed from the cardiac electrical signal; and control the therapy delivery circuit to withhold charging of the holding capacitor for at least a portion of the second pacing interval according to the second charging mode. control the therapy delivery circuit to deliver a pacing pulse; start a first pacing interval corresponding to a pacing rate in response to the delivered pacing pulse; control the therapy delivery circuit to charge the holding capacitor during the first pacing interval according to a first charging mode; detect an increased intrinsic heart rate from the cardiac electrical signal that is at least a threshold rate faster than the pacing rate; switch from the first charging mode to a second charging mode in response to detecting the increased intrinsic heart rate; while operating in the second charging mode, start a second pacing interval in response to a first intrinsic cardiac event sensed from the cardiac electrical signal; and control the therapy delivery circuit to withhold charging of the holding capacitor for at least a portion of the second pacing interval according to the second charging mode.