Abstract: A method and apparatus (10) for determining cardiac output by directing airflow or gas flow (fluid) to a fluid containment structure (12) coupled to an inspiratory limb (16) of a ventilator circuit instead of the patient and enabling the patient to directly inhale fluid from the fluid containment structure and preventing the patient from inhaling through an expiratory limb (18) of the ventilator-circuit. The method and apparatus also prevents exhalation by the patient into the fluid containment structure. The method and apparatus also comprises an aid helps the patient become consistent in their breathing patterns. When the fluid containment structure fills with a volume of fluid, the aid provides an indication that the patient should inhale; when the fluid containment structure empties of fluid, the aid provides a signal that the patient should exhale. Thus, the aid provides feedback to the patient to help the patient control their ventilation pattern.