A patient-worn ambulatory cardiac monitoring device for monitoring a patient during a patient activity, comprising: at least one physiological sensor configured to detect signals indicative of cardiac activity, the at least one physiological sensor comprising an ECG sensor configured to monitor an ECG signal of the patient; an activity sensor and associated circuitry configured to monitor patient movements; a vibrational sensor configured to monitor a cardio-vibrational signal of the patient; and at least one processor in communication with the at least one physiological sensor, the activity sensor, and the vibrational sensor, and configured to determine initiation of the patient activity by detecting a predetermined elevation in the patient's heart rate derived from the ECG signal of the patient; measure, during the patient activity, a plurality of electromechanical time intervals between an ECG fiducial point in the ECG signal and a cardio-vibrational fiducial point in the cardio-vibrational signal during at least one cardiac cycle of the patient's heart; and calculate a representative electromechanical time interval value for the plurality of electromechanical time intervals based on at least one of: averaging the plurality of electromechanical time intervals, taking a median electromechanical time interval value, taking a mode electromechanical time interval value, and smoothing the plurality of electromechanical time intervals by ignoring outliers during the patient activity. at least one physiological sensor configured to detect signals indicative of cardiac activity, the at least one physiological sensor comprising an ECG sensor configured to monitor an ECG signal of the patient; an activity sensor and associated circuitry configured to monitor patient movements; a vibrational sensor configured to monitor a cardio-vibrational signal of the patient; and at least one processor in communication with the at least one physiological sensor, the activity sensor, and the vibrational sensor, and configured to determine initiation of the patient activity by detecting a predetermined elevation in the patient's heart rate derived from the ECG signal of the patient; measure, during the patient activity, a plurality of electromechanical time intervals between an ECG fiducial point in the ECG signal and a cardio-vibrational fiducial point in the cardio-vibrational signal during at least one cardiac cycle of the patient's heart; and calculate a representative electromechanical time interval value for the plurality of electromechanical time intervals based on at least one of: averaging the plurality of electromechanical time intervals, taking a median electromechanical time interval value, taking a mode electromechanical time interval value, and smoothing the plurality of electromechanical time intervals by ignoring outliers during the patient activity. determine initiation of the patient activity by detecting a predetermined elevation in the patient's heart rate derived from the ECG signal of the patient; measure, during the patient activity, a plurality of electromechanical time intervals between an ECG fiducial point in the ECG signal and a cardio-vibrational fiducial point in the cardio-vibrational signal during at least one cardiac cycle of the patient's heart; and calculate a representative electromechanical time interval value for the plurality of electromechanical time intervals based on at least one of: averaging the plurality of electromechanical time intervals, taking a median electromechanical time interval value, taking a mode electromechanical time interval value, and smoothing the plurality of electromechanical time intervals by ignoring outliers during the patient activity. <EOS>