There is provided a method of determining a measure of the cardiac output, CO, of a patient, the method comprising obtaining measurements of one or more physiological characteristics of the patient, the physiological characteristics including at least the heart rate, HR, of the patient, the systolic blood pressure, S, of the patient and the diastolic blood pressure, D, of the patient; and processing the measurements to determine a measure of the cardiac output of the patient; wherein the measure of the cardiac output of the patient is derived using the relationship CO = K 1 ⁡ ( HR ⁢ S - D S + D ) , CO = K 2 ⁢ HR ⁡ ( S - D ) ⁢ ( PTT ) 2 , and ⁢ / ⁢ or ⁢ CO = K 2 ⁢ HR ⁡ ( S - D ) ⁢ ( PAT - PEP ) 2 where K1 and K2 are patient-specific calibration factors, PAT is the pulse arrival time, PEP is the pre-ejection period and PTT is the pulse transit time, the time taken for a pulse wave to travel between two points in the body of the patient.