[Evaluation of a method for the measurement of static total compliance (Siemens Elema LMC 940 calculator)]

The aim of the present study was to validate the method for the measurement of total compliance proposed by Siemens Elema. The pressure and flow signals of the Servo 900 respirator are treated by an LMC-940 calculator. 1 - A study with a pulmonary model was used to evaluate the applicability of the measuring techniques used by the calculator. Tele-inspiratory alveolar pressure is accurately assessed for successive measurements of inspiratory resistance if the interruption of inspiratory flow is at least 0.3 seconds. Tele-expiratory alveolar pressure is accurately interpreted even if flow at the end of expiration is not nil, as long as airway resistance remains constant. The error in measurement of tidal volume depends in part upon the compression relaxation effect of gases in the circuit. The device used to compensate for this effect is effective up to a tele-inspiratory pressure of 1.96 kPa (20 cm H2O). Beyond this pressure, tidal volume is overestimated. The error is small, if the time after which the flow is integrated is 130 ms. For shorter periods, the correction is less satisfactory. 2 - A study in eight patients was used to assess the validity of 38 measurements of Ctot on the respirator in comparison with contemporary measurements made under static conditions using a syringe. A highly significant linear correlation was seen between values of Ctot obtained by the two methods: Ctot syringe = 0.94 X Ctot calculator = 0.55 ml.cmH2O-1. (n = 38; r = 0.95; p less than 0.001). The mean error of the calculator for a given reference value was 0.099 l.kPa-1 (9.65 ml.cmH2O-1). There was a closer relation for variations in Ctot recorded by the two methods. Variations were always in the same sense, and their correlation was highly significant (n = 19; r = 0.98; p less than 0.001). Such a method for the surveillance of compliance is extremely easy. The accuracy of absolute values for Ctot is limited but it is a reliable method for following variations in compliance and hence the effects of treatment.