An experimental randomized study of six different ventilatory modes in a piglet model with normal lungs

A randomized study of 6 ventilatory modes was made in 7 piglets with normal lungs. Using a Servo HFV 970 (prototype system) and a Servo ventilator 900 C the ventilatory modes examined were as follows: SV-20V, i.e. volume-controlled intermittent positive-pressure ventilation (IPPV); SV-20VIosc, i.e. volume-controlled ventilation (IPPV) with superimposed inspiratory oscillations; and SV-20VEf, i.e. volume-controlled ventilation (IPPV) with expiratory flush of fresh gas; HFV-60 denotes low-compressive high-frequency positive-pressure ventilation (HFPPV) and HVF-20 denotes low-compressive volume-controlled intermittent positive-pressure ventilation; and SV-20P denotes pressure-controlled intermittent positive-pressure ventilation. With all modes of ventilation a PEEP of 7.5 cm H₂O was used. In the abbreviations used, the number denotes the ventilatory frequency in breaths per minute (bpm). HFV indicates that all gas was delivered via the HFV 970 unit. The ventilatory modes described above were applied randomly for at least 30 min, aiming for a normoventilatory steady state. The HFV-60 and the HFV-20 modes gave lower peak airway pressures, 12–13 cm H₂O compared to approximately 17 cm H₂O for the other ventilatory modes. Also the mean airway pressures were lower with the HFV modes 8–9 cm H₂O compared to 11–14 cm H₂O for the other modes. The gas distibution was evaluated by N₂ washout and a modified lung clearance index. All modes showed N₂ wash-out according to a two-compartment model. The SV-20P mode had the fastest wash-out, but the HFV-60 and HFV-20 ventilatory modes also showed a faster N₂ wash-out than the others. Regarding the lung clearance index, the SV-20P, HFV-60 and HFV-20 modes showed better indices than the other modes. No relationship was found between the ventilatory mode and extravascular lung water, and there were no differences in the hemodynamic variables.This study was supported by the Swedish Medical Research Council (project 4252), Sweden