1. A method for monitoring a condition of a patient under anesthesia or sedation, the method comprising the steps of: acquiring at least a first signal representing a cardiovascular activity of the patient; deriving, from said first signal, at least a first and a second parameter value; applying a predetermined mathematical index for probability of patient comfort, in which index said at least first and second parameters are variables; calculating successively changing values of said mathematical index; and indicating said successive index values.