an apparatus for determining the magnitude of heart muscle contractions has two ultrasonic transceivers , both of which are disposed in sonic alignment on selected points of the heart . a digital counter electrically interconnects the transceivers with a digital microprocessor . the counter determines a pulse transit time which corresponds to the difference in time between the transmission of a sonic pulse from the transmitter and the reception of the sonic pulse by the receiver . immediately after determining each pulse transit time , the counter sends the transit time signal to the microprocessor , which first filters the signals to reject any signal which indicates a heart size that is unreasonably large or small . after filtering , the transit times are averaged in groups of a predetermined number and then converted to corresponding heart size measurements for storage and display .