A medical system, comprising: one or more electrodes (802; 951-956) for delivering pacing pulses to a patient's heart; an energy delivery and sensing unit coupled to the one or more electrodes (802; 951-956); a pulmonary artery pressure sensor capable of making pulmonary artery pressure measurements; a memory configured to store at least the pulmonary artery pressure measurements; and a controller coupled to the memory, pulmonary artery pressure sensor, and the energy delivery and sensing unit, the controller configured to: vary, for each repetition of an acute burst protocol, a parameter of pacing applied to the patient's heart for a specified number of cardiac cycles via the energy delivery and sensing unit during the acute burst protocol; provide, between each repetition of the acute burst protocol, an intervening period of sensing which involves no pacing or pacing at preestablished intrinsic values for a specified number of cardiac cycles; store the pulmonary artery pressure measurements (X R,A) made during the repetitions of the acute burst protocol; store the pulmonary artery pressure measurements (X B) made between repetitions of the acute burst protocol to determine a baseline pulmonary artery pressure; and provide pacing therapy using a value of the parameter that is selected based on an optimum ventricular preload that is determined via the stored pulmonary artery pressure measurements, wherein the optimum ventricular preload is determined by comparing the baseline pulmonary artery pressure with the pulmonary artery pressure measurements stored during the repetitions of the acute burst protocol. one or more electrodes (802; 951-956) for delivering pacing pulses to a patient's heart; an energy delivery and sensing unit coupled to the one or more electrodes (802; 951-956); a pulmonary artery pressure sensor capable of making pulmonary artery pressure measurements; a memory configured to store at least the pulmonary artery pressure measurements; and a controller coupled to the memory, pulmonary artery pressure sensor, and the energy delivery and sensing unit, the controller configured to: vary, for each repetition of an acute burst protocol, a parameter of pacing applied to the patient's heart for a specified number of cardiac cycles via the energy delivery and sensing unit during the acute burst protocol; provide, between each repetition of the acute burst protocol, an intervening period of sensing which involves no pacing or pacing at preestablished intrinsic values for a specified number of cardiac cycles; store the pulmonary artery pressure measurements (X R,A) made during the repetitions of the acute burst protocol; store the pulmonary artery pressure measurements (X B) made between repetitions of the acute burst protocol to determine a baseline pulmonary artery pressure; and provide pacing therapy using a value of the parameter that is selected based on an optimum ventricular preload that is determined via the stored pulmonary artery pressure measurements, wherein the optimum ventricular preload is determined by comparing the baseline pulmonary artery pressure with the pulmonary artery pressure measurements stored during the repetitions of the acute burst protocol. vary, for each repetition of an acute burst protocol, a parameter of pacing applied to the patient's heart for a specified number of cardiac cycles via the energy delivery and sensing unit during the acute burst protocol; provide, between each repetition of the acute burst protocol, an intervening period of sensing which involves no pacing or pacing at preestablished intrinsic values for a specified number of cardiac cycles; store the pulmonary artery pressure measurements (X R,A) made during the repetitions of the acute burst protocol; store the pulmonary artery pressure measurements (X B) made between repetitions of the acute burst protocol to determine a baseline pulmonary artery pressure; and provide pacing therapy using a value of the parameter that is selected based on an optimum ventricular preload that is determined via the stored pulmonary artery pressure measurements, wherein the optimum ventricular preload is determined by comparing the baseline pulmonary artery pressure with the pulmonary artery pressure measurements stored during the repetitions of the acute burst protocol.