A system is provided including a cardiac output monitor configured to be operatively connected to a detection module that obtains electrocardiogram (ECG) signals from the patient. The monitor includes an axis analysis module and a cardiac output module. The axis analysis module is configured to obtain ECG axis information including information corresponding to at least one ECG axis of a patient. The axis analysis module is also configured to determine ECG axis change information corresponding to a change in the ECG axis information of the patient. The cardiac output analysis module is configured to determine a change in cardiac output using the ECG axis change information.