心脏搭桥手术期间如何确定病人个性化动脉压下限

脑血流自动调节能力，是维持大脑持续恒定脑血流供应的主要功能之一，如经典的Lassen曲线所示（图1），当平均动脉压在一定范围内变动时，健康的脑自动调节能力可保证大脑在不同的血压范围内仍能维持稳定的血流供应 【1-4】；一旦平均动脉压超出这个稳定范围，即低于动脉压下限或者高于动脉压上限时，一点微小的动脉压变化，将会导致脑血流较大的波动，从而引发脑缺血或者脑溢血【5-9】。英国剑桥大学Marek Czosnyka教授团队，利用脑血流及全身动脉压的动态关系，提出可实时评估大脑自动调节能力的相关系数法，该系数越大，脑血流与动脉压的相关性越大，大脑自动调节能力越差；反之，若该系数为0或负值，意味着大脑有较强的自动调节能力。其中，脑血流可通过颅内压（intracranial pressure, ICP）、大脑中动脉经颅多普勒 (Transcranial Doppler ,TCD) 血流速度，近红外光谱（Near infrared spectroscopy, NIRS）血氧饱和度等进行监测，从而定义不同的相关参数，如压力反应指数pressure reactivity index (PRx)，平均血流指数mean flow index (Mx)，脑血氧指数 cerebral oximetry index (COx) 以及 血红蛋白容积指数hemoglobin volume index (HVx) 等 【6, 16, 18-21】。

图1. 平均动脉压与大脑中动脉脑血流速度构成的Lassen 曲线

（MAP: mean arterial blood pressure; TCD: transcranial doppler; CBFV: cerebral blood flow velocity; ULA: upper limit of autoregulation; LLA: lower limit of autoregulation. ）

在心脏搭桥手术期间，为了减少侧支流血、有一个清晰的手术视野，医生会尽可能降低病人手术期间的动脉压【10】，然而，一旦动脉压低于病人所能承受的动脉压下限，手术期间大脑可能会经历脑缺血。约翰霍普金斯大学麻醉重症系Charles H. Brown团队，经多年的研究指出，心脏搭桥手术期间病人长期经历低动脉压、低灌注，与术后精神错乱、急性肾衰竭、主要发病和死亡率（major morbidity and mortality ）显著相关【1, 10, 12】。其中的一个原因可能是，大脑是人体最重要的器官，在极度缺氧缺血情况下，大脑可能会牺牲其他的器官（如肝、肾等），保护大脑，从而导致其他的器官在术后出现相应症状。

因此，如何精准的为每一位心脏搭桥手术患者定位其可承受的最低血压下限，确保手术期间大脑有充足的血流供应，改善心脏搭桥手术的预后至关重要。目前，国际指南是‘One-to-All’ policy, 提议维持手术期间，平均动脉压在50 mmHg以上。但已有研究指出，由于个体差异性及生理、病理多样性，每位病人能承受的最低动脉压非常不同，介于40-90 mmHg 。

若将上述COx 或者Mx置于纵轴，平均动脉压置于横轴，会产生一个如图2所示的‘’U‘’型曲线，其最低点所对应的横轴的值即为大脑该时刻最佳自动调节能力所对应的动脉压，我们称之为“最佳动脉压”；若确定一个合适的阈值，比如 Mx=0.45, 那么Mx=0.45这条线与U型曲线的两个交叉点，定义为脑血流调节动脉压上、下限。而如何精准的确定该阈值，来改善病人的预后，是目前存在的一大难题[17,19,20,22-24]。

图2. 平均动脉压 (Mean Arterial Blood Pressure, MAP) 与平均血流指数（Mean Flow Index, Mx） 建模形成的U型曲线/平均动脉压与大脑中动脉脑血流速度构成的Lassen 曲线

（MAP: mean arterial blood pressure; TCD: transcranial doppler; CBFV: cerebral blood flow velocity; ABPopt: optimal arterial blood pressure.）

围绕上述问题，约翰霍普金斯大学的刘秀云及合作者，通过持续、同步记录心脏搭桥手术期间病人的近红外光谱NIRS、经颅多普勒TCD以及平均动脉压等多模态信号，提出确定心脏搭桥手术病人个性化动脉压下限的新方法，研究成果发表于Critical Care Medicine（题：Determining Thresholds for Three Indices of Autoregulation to Identify the Lower Limit of Autoregulation During Cardiac Surgery）。

基于传统的Lassen 曲线，其为该组心脏搭桥手术患者确定各自的最低血压下限。在226位病人中，有59位患者的数据可以产生完整的Lassen曲线。之后，研究人员又利用Mx-MAP (或COx-MAP, 或 HVx-MAP) 曲线，确定不同阈值（0.1~0.9）所计算出的动脉压下限。经对比，结果显示, ‘U’型曲线在Mx=0.45, COx=0.35, HVx=0.3 阈值处计算的动脉压下限，与金标准Lassen 曲线产生的动脉压下限最为接近。

随后，Liu等基于上述结论及个体动脉压下限，为每位患者计算其手术期间低灌注的程度（area under the curve），结果证明术后出现急性肾衰竭患者的术中低灌注的程度（时间久，动脉压低）显著超过术后未出现急性肾衰竭的患者。

该项研究为心脏搭桥手术期间，病人个性化动脉压管理提供了一个有借鉴意义、可操作的方法，为术中精准定位病人最低血压下限提供了可参考的参数及其相应阈值。作者指出，该研究用到的三个参数：Mx, COx 和HVx 分别通过大脑中动脉血流速度（Mx）以及前额脑血氧饱和度 (COx 和HVx) 计算的，他们可能与解剖结构、病人年龄、血管疾病等密切相关，这些参数应当在未来的研究中被考虑进去。

原文链接：https://journals.lww.com/ccmjournal/Abstract/2021/04000/Determining_Thresholds_for_Three_Indices_of.8.aspx

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