The impact of lung-protective mechanical ventilation on cerebral perfusion pressure and cerebral oxygen metabolism in patients with severe cerebral injury combined with respiratory failure

Abstract

Objective To explore the impact of lung-protective mechanical ventilation (low tidal volume and optimal positive end-expiratory pressure (PEEP) on cerebral perfusion pressure (CPP) and cerebral oxygen metabolism.Methods Forty patients with severe cerebral injury along with respiratory failure were randomly assigned into two groups:lung-protective ventilation group A and conventional ventilation group B.Group A was planned to prescribe tidal volume 6 ～ 8 mL/kg,initial FiO240％,PEEP gradually increasing from 2 cmH2O to matched with FiO2 elevation,but the FiO2 was kept at permissive lower level.Group B was formulated with tidal volume 8 ～ 12 mL/kg,PEEP stepwise increasing from 0 2 cmH2O to match with FiO2 elevation,but PEEP was kept at permissive lower pressure.The intracranial pressure (ICP),mean arterial pressure (MAP),CPP,arterial and jugular venous blood gas were monitored.Results PEEP (8.2±3.32 cmH2O),ICP (19.7 ±3.6 mmHg),PaCO2 (54±7.3 mmHg),jugular venous carbon dioxide partial pressure (PjV CO2,56.7 ± 9.6 mmHg) in group A were higher than those (5.7±2.3 cmH2O,16.9±3.8 mmHg,41 ±5.2 mmHg,49.8 ±6.9 mmHg) in group B (P＜ 0.05 or P ＜ 0.01).VT,FiO2 in the group A were lower than those in the group B.There were no differences in PaO2/FiO2,jugular venous oxygen saturation (SjVO2),MAP,and CPP between two groups.PaCO2 were significantly correlated with CPP (r =0.368,P =0.019) while there was no correlation with ICP,PaO2,SjVO2,PjVCO2 (all P ＞0.05).CPP (69.7 ± 12.3 mmHg) was higher in case of PaCO2 (46 ～60mmHg) than those (61.5 ±9.1 mmHg) in case of PaCO2 (35 ～45 mmHg).There was correlation between PEEP and ICP (r =0.436,P =0.005).When PEEP was divided into three groups:≤52 cmH2O,6 ～ 102 cmH2O and ＞ 102 cmH2O,ICPs were different one another among three groups.When PEEP ＞ 102 cmH2O,it had a distinguished negative correlation with CPP (r =-0.395,P =0.017),while PEEP ≤ 102 cmH2O,CPP presented decreasing tendency.SjVO2 correlated with PaO2 (r =0.403,P =0.014) and PjVCO2 (r =-0.502,P =0.001) respectively.There were no significant relationships between SjVO2 and CPP,ICP,MAP,PEEP,respectively.Conclusions Lung-protective mechanical ventilation was relatively safer in patients with severe cerebral injury compared with conventional mechanical ventilation.Mild PaCO2 elevation (46 ～ 60 mmHg) combined with higher PEEP (＜ 102 cmH2O) did not decrease CPP.There was no difference in SjVO2 between the two modes of mechanical ventilation,suggesting no changes in cerebral metabolism occurred. Key words: Cerebral injury; Cerebral perfusion pressure; Jugular venous oxygen saturation; Respiratory failure ; Mechanical ventilation ; Acute lung injury ; Tidal volume ; Blood gas analysis ; Positive end-expiratory pressure