肺保护性机械通气对脑损伤患者脑灌注压和脑氧代谢的影响

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

目的观察肺保护性机械通气对颅脑损伤患者脑灌注压（CPP）及脑氧代谢的影响。方法选择ICU需要机械通气的严重颅脑损伤伴呼吸衰竭患者40例，所有患者均行颅内压（ICP）监测、右侧颈内静脉逆行穿刺置管。将患者随机（随机数字法）分为①肺保护性通气组：潮气量为6～8mL／kg，初始吸氧体积分数40％，逐步提升呼气末正压（PEEP），PEEP与吸氧（FiO2）匹配同步升高，保持FiO2允许性低值；②常规通气组（对照组）：潮气量为8-12mL／kg，FiO2与PEEP匹配同步升高，保持PEEP允许性低值。监测桡动脉血气、平均动脉压（MAP）、颈静脉血氧饱和度（SjVO2），颈静脉血二氧化碳分压（P_jvC02），计算CPP=MAP-ICP；氧合指数PaO_2／FiO_2。结果肺保护性通气组PEEP（8．2±3．3）cmH_2O（1cmH_20=0．098kPa）、ICP（19．7±3．6）mmHg（1mmHg=0．133kPa）、PaCO_2（54±7．3mmHg）高于对照组，VT、FiO_2低于对照组，差异具有统计学意义；两组PaO_2／FiO_2、S_jVO_2、MAP、CPP差异无统计学意义。相关分析提示PaCO_2与CPP呈正相关（r=0．368，P=0．019），与ICP、Pa02、S_jVO_2、P_jvCO_2等并无相关性（P〉0．05）；PEEP与ICP呈正相关；PEEP分为≤5cmH20、6~10cmH20及〉10cmH_2O三组，各组间ICP两两比较差异有统计学意义；PEEP在0～10cmH：O上升，CPP变化不明显；PEEP〉10cmH_2O时与CPP呈明显负相关（r=-0．395，P=0．017），CPP（58．5±7．2）mmHg，低于PEEP0~5cmH_2O时的（69．1±9．7）mmHg，差异具有统计学意义；PEEP越高，氧合指数越低；不同的PEEP水平下MAP、S_jVO_2、P_jvCO_2无明显变化。将PaC02分为35～45mmHg和46~60mmHg组，后者的CPP高于前组者，差异具有统计学意义（P〈0．05）。S_jVO_2与PaO_2及P_jvCO-2相关，与PaCO_2、CPP、ICP、MAP及PEEP等均无相关性。结论肺保护性通气策略对颅脑损伤患者来说是相对安全的。适当的CO_2潴留联合较高的PEEP不影响脑灌注。肺保护性通气与常规通气相比S_jVO_2，差异无统计学意义。提示两种通气方式下脑氧代谢无变化。

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 FIO24O%, 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 O 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 （Piv 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 〈 O.O5 or P 〈 O. O1 ）. 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 weresignificantly correlated with CPP （r =O. 368, P =O. O19） while there was no correlation with ICP, PaO2, SjVO2, Pjv CO2 （ all P 〉 O. O5）. CPP （69. 7 ± 12. 3 mmHg） was higher in case of PaCO2 （46 ± 6OmmHg） than those （61.5 ±9. 1 mmHg） in case of PaCO2 （35 -45 mmHg）. There was correlation between PEEP and ICP （r = O. 436, P = O. OO5 ）. When PEEP was divided into three groups： ≤ 52 cmH2O , 6 -1O2 cmH=O and 〉 1O2 cmH=O , ICPs were different one another among three groups. When PEEP 〉 1O2 cmH2O , it had a distinguished negative correlation with CPP （r = -O. 395, P = O. O17 ）, while PEEP ±〈 1O2 cmH2O , CPP presented decreasing tendency. SjVOz correlated with PaO2 （r = O. 4O3, P = O. O14） and PjvCO2 （ r = - O. 5O2, P = O. OO1 ） 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 -6O mmHg） combined with higher PEEP （ 〈 1O2 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.