A system for detecting/quantifying the conductance of a right-to-left cardiac shunt includes a mouthpiece assembly, a solenoid-driven vacuum/pressurization assembly; a controller for operating the solenoid-driven vacuum/pressurization assembly; and a monitor for displaying the instructions from the controller. A microbubble counting cell and digital image sensor are combined with software-based image analysis to determine a number of microbubbles contained in a microbubble counting zone. A monitor enables operator specification of total volume of contrast agent to be injected into the patient. One or more first Doppler ultrasound transducer arrays are positioned adjacent to targeted intracranial arteries at one side of the skull or a pair of first Doppler ultrasound transducer arrays positioned adjacent to targeted intracranial arteries at both sides of the skull. A second Doppler ultrasound transducer is positioned on the precordium of the patient to detect the arrival of microbubble-containing contrast agent in the right atrium of the patient.