Correction to the paper ‘Delivery of tidal volume from four anaesthesia ventilators during volume-controlled ventilation: a bench study’

Abstract

Editor-We 1 have previously questioned the accuracy of the calculations and results of the study by Wallon and colleagues 2 from 2013 and had therefore anticipated that some corrective action would be taken by the authors. Nonetheless, no correction has been presented, and their paper has now been cited in further articles, 3-5 which is why this new letter is being submitted.Wallon and colleagues 2 investigated the accuracy of delivery of tidal volume by four anaesthesia ventilators (Aysis, FLOW-i, Primus, and Zeus) resulting from different ventilator settings, such as PEEP, tidal volume, inspiratory pause, and fresh gas flow, in combination with different lung mechanics parameters. The experiments were performed in a laboratory environment at room temperature and in dry circuit conditions. A Fleisch pneumotachograph was used as a reference to measure the inspired tidal volume. Gas volumes were normalized in body temperature and pressure when saturated with water vapour (BTPS) conditions for all ventilators and the reference system. Unfortunately, the calculations were based on incorrect premises, and the validity of the results and conclusions presented in the paper can therefore be questioned. We present the mistakes made in the original paper 2 and explain how to correct them. We also describe how this alters the conclusions of the study.First, saturated conditions were used when converting the measured delivered volumes from the experimental dry circuit conditions to the BTPS state; see page 1047 of the paper. 2 Second, an incorrect reference gas state was used for the FLOW-i ventilator; see Table 1 of Wallon and colleagues. 2 FLOW-i software version 1.02 reports volumes in AP21 (ambient pressure, 21°C, and the actual humidity of the gas mixture in the circuit), 6 not BTPD as stated by Wallon and colleagues. 2 The relative error of tidal volumes in the BTPS state is defined by:A positive value of Verror means that the ventilator in fact delivers a larger volume than expected. The K1 and K2 factors have here been introduced to represent the conversion to BTPS from any original state used by a reference system and a ventilator, respectively. The reference measurement system and the Aisys ventilator report Table 1 Summary of reported conditions and results, together with corrected conditions and results. AP21, Ambient Pressure 21°C, actual humidity of circuit

“Neonatal ventilation; What we don't know, can't hurt… right?”

“…There are a number of studies which demonstrate that the tidal volume displayed on the anesthetic machine is prone to error, particularly for small tidal volumes when ventilating infants . Moreover, it has been demonstrated that the accuracy between the set and delivered tidal volume may be different between ventilators and is highly dependent on the gas humidity and temperature at the Y piece …”