A medical device system comprising a medical device comprising: a plurality of electrodes (17A-17D) configured to collect an electrogram signal of a patient, wherein the EGM signal includes a plurality of depolarizations; and an accelerometer configured to collect an accelerometer signal that indicates which posture of a set of postures that the patient is occupying; a wearable device configured to collect a photoplethysmography signal of the patient, wherein the PPG signal includes a plurality of PPG features indicating the occurrence of a cardiac pulse; and processing circuitry in communication with a memory, wherein the processing circuitry is configured to: determine a plurality of pulse transit time intervals, wherein a PTT interval of the plurality of PTT intervals corresponds to each depolarization of the plurality of depolarizations, wherein each PTT interval of the plurality of PTT intervals represents an amount of time between the respective depolarization of the set of depolarizations and a PPG feature of the plurality of PPG features that occurs after the respective depolarization and before a subsequent depolarization of the plurality of depolarizations; determine, based on the accelerometer signal, a posture of the patient from a plurality of postures corresponding to each PTT interval of the plurality of PTT intervals; classify each PTT interval of the plurality of PTT intervals based on the respective posture of the patient corresponding to the respective PTT interval; and monitor, based on the classified plurality of PTT intervals, a patient condition. a medical device comprising: a plurality of electrodes (17A-17D) configured to collect an electrogram signal of a patient, wherein the EGM signal includes a plurality of depolarizations; and an accelerometer configured to collect an accelerometer signal that indicates which posture of a set of postures that the patient is occupying; a wearable device configured to collect a photoplethysmography signal of the patient, wherein the PPG signal includes a plurality of PPG features indicating the occurrence of a cardiac pulse; and a plurality of electrodes (17A-17D) configured to collect an electrogram signal of a patient, wherein the EGM signal includes a plurality of depolarizations; and an accelerometer configured to collect an accelerometer signal that indicates which posture of a set of postures that the patient is occupying; a wearable device configured to collect a photoplethysmography signal of the patient, wherein the PPG signal includes a plurality of PPG features indicating the occurrence of a cardiac pulse; and processing circuitry in communication with a memory, wherein the processing circuitry is configured to: determine a plurality of pulse transit time intervals, wherein a PTT interval of the plurality of PTT intervals corresponds to each depolarization of the plurality of depolarizations, wherein each PTT interval of the plurality of PTT intervals represents an amount of time between the respective depolarization of the set of depolarizations and a PPG feature of the plurality of PPG features that occurs after the respective depolarization and before a subsequent depolarization of the plurality of depolarizations; determine, based on the accelerometer signal, a posture of the patient from a plurality of postures corresponding to each PTT interval of the plurality of PTT intervals; classify each PTT interval of the plurality of PTT intervals based on the respective posture of the patient corresponding to the respective PTT interval; and monitor, based on the classified plurality of PTT intervals, a patient condition. determine a plurality of pulse transit time intervals, wherein a PTT interval of the plurality of PTT intervals corresponds to each depolarization of the plurality of depolarizations, wherein each PTT interval of the plurality of PTT intervals represents an amount of time between the respective depolarization of the set of depolarizations and a PPG feature of the plurality of PPG features that occurs after the respective depolarization and before a subsequent depolarization of the plurality of depolarizations; determine, based on the accelerometer signal, a posture of the patient from a plurality of postures corresponding to each PTT interval of the plurality of PTT intervals; classify each PTT interval of the plurality of PTT intervals based on the respective posture of the patient corresponding to the respective PTT interval; and monitor, based on the classified plurality of PTT intervals, a patient condition.