Effects Of HFOV On Gas Distribution In Swine With ARDS

Part Ⅰ Effects of conventional setting of high-frequency oscillatory ventilation on regional gas distribution in a porcine model of acute respiratory distress syndromeObjective:To explore effects of conventional setting of high-frequency oscillatory ventilation (HFOV) on regional gas distribution in a porcine model of ARDS.Methods:ARDS was induced by repeated bronchoalveolar lavage in ten healthy male swines until the PaO2/FiO2 decreased to less than 100 mmHg and remained stable for 30min. The baseline state was defined as Control group. Optimal positive end-expiratory pressure (PEEP) was titrated according to PaO2/FiO2 ratio. The swines received HFOV or conventional mechanical ventilation (CMV) randomly one after another. (1) CMV group:the swines were ventilated with low tidal volume (6 ml/kg) and optimal PEEP. (2) HFOV group:the mean airway pressure (mPaw) was set 5cmH2O above mean airway pressure of CMV. After 5min’s stablization, the relative impedance change monitored by electrical impedance tomography (EIT) was recorded to represent the gas distribution and lung homogeneity. Meanwhile, PaO2/FiO2 and hemodynamics of each group were recorded.Results:The model of ARDS was successfully induced by repeated bronchoalveolar lavage. (1) settings of CMV and HFOV were as below:the setting of CMV was PEEP 14.4±4.6 cmH2O, VT 6ml/kg, f 30 breaths/min, I:E 1:2, FiO2 100%. The setting of mPaw 24.1±1.9cmH2O, f 9Hz, ΔP 70cmH2O, FiO2 100%,1% 33%, Bias Flow 30L/min. (2) Effects of HFOV on gas exchange:the PaO2/FiO2 ratios decreased significantly after the model was successfully induced (Control group:435.6±117.1 mmHg, ARDS model group:65.6±43.2 mmHg). During the ventilation, the PaO2/FiO2 ratio of CMV group and HFOV group were significantly higher than that of ARDS model group, but there was no significant difference of oxygenation between two groups (p=0.989). The PaCO2 of HFOV group was significantly higher than that of Control group, ARDS group and CMV group. The pH of HFOV group was lower than that of Control group, but it was still in the range of permissive hypercapnia. (3) Effects of HFOV on hemodynamics:there was no significant difference in heart rate (HR), mean arterial pressure (MAP), central venous pressure (CVP) and pulmonary arterial wedge pressure (PAWP) among CMV group and HFOV group. But MAP of two groups was lower than that of ARDS model group, CVP was higher than that of Control group. (4) Effects of HFOV on regional gas distribution:during the ventilation, the ΔZ% of HFOV group significantly improved than ARDS group. The ΔZ% of ROI 1 of HFOV group was significantly lower than that of Control group and CMV group (p<0.001). There was no significant difference in ROI 2 and ROI 3 between two groups. The AZ% of ROI 4 of HFOV group and CMV group was significantly lower than that of Control group, but no difference was found between two groups.Conclusions:Conventional mPaw setting of HFOV could not improve gas distribution and inhomogeneity of lung injury.Part II Effects of mean airway pressure on regional gas distribution during high-frequency oscillatory ventilation in a porcine model of acute respiratory distress syndromeObjective:To compare different levels of mPaw on regional gas distribution during HFOV in a porcine model of ARDS.Methods:ARDS was induced by repeated bronchoalveolar lavage in ten healthy male swines until the PaO2/FiO2 decreased to less than 100 mmHg and remained stable for 30 min. The baseline state was defined as Control group. After RM, stepwise PEEP decrements were performed to titrate optimal PEEP during CMV. Then animal was switched to HFOV, and randomly assigned to four groups of which ventilation settings were as follows:(1) Pmean+5 cmH2O group:Pmean was acquired by analysis of respiratory mechanics of CMV. The mPaw of HFOV was set 5 cmH2O above the Pmean. (2) Pplat+5 cmH20 group:the mPaw was set 5 cmH2O above the Pplat under the optimal PEEP of CMV according to OSCAR Trial. (3) mPaw 30 cmH2O group: the mPaw was set 30 cmH20 according to OSCILLATE Trial. (4) Optimal oxygenation group:the mPaw was set on the level titrated by oxygenation. After 5min’s stablization, the relative impedance change and pixels monitored by electrical impedance tomography (EIT) was recorded to represent the gas distribution and lung homogeneity. Meanwhile, PaO2/FiO2 and hemodynamics of each group were recorded.Results:The model of ARDS was successfully induced by repeated bronchoalveolar lavage. (1) mPaws in each group were determined as follows:mPaw 30 cmH20 group: 30.0±0.0 cmH20, Pplat+5 cmH20 group:27.2±3.0 cmH2O, Pmean+5 cmH2O group: 24.1±1.9 cmH20, Optimal oxygenation group:20.4±3.1 cmH20. The other settings were f 9Hz, Δ P 70cmH2O, FiO2 100%,1% 33%, Bias Flow 30L/min. (2) Effects of different mPaws on gas exchange:the PaO2/TiO2 of ARDS model decreased significantly after the model was successfully induced. The PaO2/FiO2 of each group improved significantly during ventilation by CMV and HFOV (p<0.001). No significant difference was found on oxygenation among four groups. The PaCO2 of each group was higher than that of Control group while the pH of each group was lower, but it was still in the range of permissive hypercapnia. (3) Effects of different mPaws on hemodynamics:There was no significant difference on HR, MAP, CVP and PAWP among four groups, but CVP and PAWP were higher than that of Control group and MAP was lower than that of Control group. (4) Effects of different mPaws on gas distribution of non-dependent and dependent lung regions:there was no significant difference on gas distribution of non-dependent and dependent lung regions among each group (p>0.05). (5) Effects of different mPaws on overdistention regions:the overdistended pixels of different mPaws were as below:mPaw 30 cmH20 group:181.2±72.4, Pplat+5 cmH20 group:128.4±53.5, Pmean+5 cmH20 group: 137.2±38.7, optimal oxygenation group:36.9±15.4. The overdistended pixels of optimal oxygenation group was significantly lower than those of Pmean+5 cmH20 group, Pplat+5 cmH20 group and mPaw30 cmH20 group (p<0.001). (6) Effects of different mPaws on recruited regions:there was no significant difference on recruited pixels in four groups (p=0.884).Conclusions:Setting mPaw according to oxygenation during HFOV helps recruit the collapsed alveolar and reduce overdistention.