Using the OLA protocol, all tidal volumes are delivered by a decelerating gas flow (pressure-controlled mode). As implemented, the OLA protocol consists of four parts and is based on the “open up the lung and keep it open” concept by Lachmann [
4]. First, a titration algorithm is used to search for an appropriate peak inspiratory pressure (PIP) to open recruitable areas of the lung (opening phase). During this opening phase, the PEEP is set to 20 cm H
2O before the recruitment procedure and a series of recruitment pressures is applied starting by setting the PIP to 45 cm H
2O for 3–5 breaths. Next, the SaO
2 is measured after 30 s, while the tidal volume is reduced to 6 ml/kg BW with the PEEP maintained at 20 cm H
2O. Only if the measured SaO
2 does not reach a value above 95%, the first recruitment pulse is followed by a second impulse using a PIP of 50 cm H
2O. If necessary, this stepwise increase of recruitment pressures is applied in steps of 5 cm H
2O until a maximum pressure of 70 cm H
2O is reached. Next, the critical PEEP below which lung derecruitment occurs is identified by titrating the PEEP downwards (from 20 cm H
2O) in steps of 1 cm H
2O (30 s intervals) until the measured SaO
2 falls below 90%. The PEEP value corresponding to this arterial saturation measurement is used as the critical derecruitment PEEP of the lung. Once the critical closing PEEP is identified, a reopening phase is used to reopen the lung (reopening phase) by applying the previously identified opening pressures and a PEEP value 2 cm H
2O above the critical closing PEEP value. All steps of the OLA protocol used to open up- and keep the lung open are performed fully automatically and equally to all animals. Next, a steady state ventilation phase is begun with I:E ratio fixed at 1:1 and the V
T and EtCO
2 regulated to 6 ml V
TBW and 35 mmHg respectively (ventilation phase). A so called “fuzzy” control of the system is used to achieve both, adjustment of V
T by appropriate PIP application and selection of RR based on continuous measurement of EtCO
2. Using a fuzzy control of the closed-loop system, it is not necessary to establish a precise mathematical model of how to achieve the target values (PIP, PEEP, V
T, EtCO
2). For a more detailed description of the deterministic finite automaton and fuzzy control of this closed-loop algorithm, a technical demonstration has previously been published based on the data of one animal [
7]. Figure
1b depicts the closed loop OLA protocol.