A device for continuously monitoring the output of a heart in a surgical patient includes a probe head attached at one end to a catheter adapted to be inserted into the patient. The probe head includes a probe and a pair of flexible flat projections extending outwardly from opposite sides of the probe for securing the probe head in the transverse sinus of the patient. At least one transducer is positioned in the probe for detecting a condition of the heart. A hollow needle is adapted for receiving the probe head prior to its installation in the patient, such that the probe head can be installed in the patient immediately after mediansternotomy and division of the patient's pericardium by inserting the needle through the patient's chest wall. A method for continuously monitoring the condition of the heart in a surgical patient includes the steps of providing a probe head responsive to heart function; positioning the probe head in the transverse sinus of the patient proximal to the aortic valve and behind the ascending aorta; and displaying an output of the probe head reflective of the heart condition.