[A study of the parameters of the delivered tidal volume. Ventilation on a lung model using the CICERO anesthetic ventilator]

Abstract

In many anaesthesia ventilators in common use, the tidal volume delivered is different from the tidal volume preset on the respirator. Tidal volume delivered by mechanical ventilation during anaesthesia may be influenced by fresh gas flow (FGF), the respiratory rate (RR) or the inspiratory: expiratory ratio (I:E). This may cause inadequate hypo- or hyperventilation in small children, especially in newborns and neonates. Using small tidal volumes from 20 to 100 ml preset on the respirator, we investigated in a lung model the tidal volumes delivered by the anaesthesia ventilator CICERO (Dräger, FRG) with variations of FGF, RR and I:E. MATERIAL AND METHODS. The anaesthesia ventilator CICERO (software version 4.16) was equipped with the low-compliance tubes of the "Ulmer Kinder-Set" (Rüsch Co.) and the regular CO2 canister (1500 ml) of the machine. The circuit was connected to a lung model consisting of a glass clyinder filled with copper wool with a compliance of 3.3 ml/mbar. To create a pressure-volume correlation of the entire system, i.e. the lung model, the anaesthesia circuit and the ventilator, calibrated glass syringes were used and the pressure increase in the test lung was measured. This pressure-volume correlation was linear. The pressure increase in the lung model caused by the tidal volume during ventilation therefore reflected the actual tidal volume delivered. The study was performed with small tidal volumes from 20 to 100 ml that could be adjusted exactly on the ventilator. Delivered tidal volumes were studied by varying the FGF from 1 to 6 l/min and the RR from 20 to 60/min (with I:E = 1:1.5) and by varying the RR from 20 to 60/min and the I:E from 2:1 to 1:3 (with FGF = 21/min). RESULTS. By varying FGF, RR and I:E no changes in delivered tidal volumes were noted. In all settings of the ventilator studied, the delivered tidal volume was similar to the desired tidal volume preset on the ventilator. The highest deviation from the delivered tidal volume to the tidal volume preset was 17.5% with a tidal volume of 20 ml. In preset tidal volumes 30-100 ml this deviation was lower than 10%. An intermittent "auto-PEEP" up to 5 mbar was noted during high respiratory rates (50 and 60/min) combined with an I:E at 2:1 and 1:1 or with a FGF at 4 or 6 l/min. The compliance of the ventilator equipped with the circuit was 4.2 ml/mbar. CONCLUSION. The findings in this study prove that with tidal volumes ranging from 20 to 100 ml the actual tidal volume delivered by the anaesthesia ventilator CICERO is equivalent to the tidal volume set on the machine regardless of the variation of FGF, RR and I:E. These findings are mainly based on two circumstances. Firstly, fresh gas flow is fed into a reservoir and not added to the volume delivered by the bellow during inspiration as in many other respirators. Secondly, the CICERO works with a compliance correction function integrated into the machine. Computed compressible volume from the circuit and the ventilator is added to the tidal volume preset on the ventilator; therefore, the volume delivered by the bellow consists of the volume set on the ventilator plus the compressible volume. With these characteristics the anaesthesia ventilator CICERO meets important requirements for a ventilator in paediatric anaesthesia. However, for final assessment further clinical studies are required.