Use of the Concept of Ventilator Compliance in the Determination of Static Total Compliance

Abstract

Static total compliance (Cst) provides a noninvasive means of determining the mechanical properties of the respiratory system at the bedside. Together with certain other indices of cardiorespiratory function, Cst is often useful in determining the best level of positive end-expiratory pressure (so-called "optimum PEEP") to employ in the treatment of a particular patient. The accurate and precise measurement of Cst, in turn, requires the correct technique for measuring tidal volume and the simultaneous change in the static pressure that accompanied the delivery of that volume. Consequently, for patients being mechanically ventilated, the ventilator's compressible volume must be determined and substracted from the total volume exhaled through a spirometer during a single breathing cycle. The change in pressure, measured statically, that is divided into the tidal volume figure is defined as the "plateau pressure" minus the end-expiratory pressure. This presentation consists of a discussion of physiologic concepts, practical hints, and actual clinical examples relat-ing to the measurement of Cst.