Determination of functional residual capacity during mechanical ventilation

Abstract

A modification of Laws' method for functional method for Functional residual capacity (FRC) determination by helium dilution is presented which has several advantages. First, FRC can be determined while the patient continues to be ventilated with the frequency and tidal volume used prior to the study or during mechanical ventilation with positive end expiratory pressure (PEEP). The patient is not disconnected from the ventilator for the measurement; the connection to the FRC circuit is made by turning two valves. FRC can be determined at any inspired oxygen concentration. In line analysis of this concentration allows for correction of the catharometer deflection produced by oxygen. Finally, the method does not require physiologic knowledge of ventilation or technical skills, and can be learned by inhalation therapists, nurses, and paramedical personnel.