Objective: To test whether in cases of considerably non-homogenous gas distribution in acute pathological lung conditions it is possible to improve gas distribution into slow broncho-alveolar compartments while decreasing the volume load of the fast compartments, and to improve gas exchange in the lungs while sustaining the principles of “non-injurious ventilation“, by using tri-level (3LV) IPPV. Setting: Department of Anaesthesiology and Intensive Care Unit. Materials and methods: Authors applied 3LV ventilation to a group of 12 patients with non-homogenous lung injury. Tri-level ventilation is defined as a type (modification) of IPPV consisting of background ventilation using the CMV, PCV or PS (ASB) ventilation mode and an added level called “on-background ventilation“ consisting of multiple levels of PEEP: PEEP (constant) and PEEPh (PEEP high) with variable frequency and duration of transition between individual levels of PEEP. Results: The study population consisted of 12 patients with severe non-homogenous lung injury/disorder (atypical pneumonia and ARDS/ALI) who failed to achieve successful ventilation in the PCV mode after a recruitment manoeuvre (PaO2/FiO2 = 5–6). After the application of 3LV with respiratory rate of fPCV = 26 ± 4 b . min-1 and PEEPh with frequency of fpeeph = 7 ± 2 b . min-1 reaching minute ventilation (MV) of 12 ± 4 b . min-1, a considerable improvement in gas exchange was observed within 1–4 hours. Pulmonary shunt decreased from 50 ± 5% to approx. 30 ± 5%. Elimination of CO2 improved from 7.8 ± 0.5 kPa to less than 6.0 ± 0.3 kPa and PaO2 increased from 5.4 ± 0.4 kPa to 7.5 ± 1.2 kPa while FiO2 could be reduced to 0.8–0.4. Alveolar recruitment due to PEEP of 1.2 ± 0.4 kPa which was manifested by an increase in static compliance Cst from 0.18 ± 0.02 l/kPa to 0.3 ± 0.02 l/kPa and later on 0.38 ± 0.05 l/kPa helped to improve gas exchange. Airway resistance (Raw) decreased by more than 30%. The improved aeration of the lungs is considered to be a manifestation of improved gas distribution to the areas with a long time constant. Patients were weaned to pressure support ventilation in 5 ± 1 and later successfully weaned off the ventilator and transferred to a standard ward. Conclusions: The clinical results support the mathematical and physical simulation results of ventilation using 3LV. The authors conclude that 3LV improved pulmonary gas exchange compared to PCV in 2–4 hours. Tri-level ventilation could be a promising ventilatory mode for the lungs affected by a diffuse non-homogenous pathological process.

        Key words: IPPV – tri-level IPPV – ARDS – ALI