起死回生是否可能？

文/伊莱·麦金农 译/梁嘉琪 审订/张政

起死回生是否可能？

文/伊莱·麦金农 译/梁嘉琪 审订/张政

In 1999， a Swedish medical student named Anna Bagenholm lost control while skiing and landed head fi rst on a thin patch of ice covering a mountain stream. The surface gave way and she was pulled into the freezing current below; when her friends caught up with her minutes later， only her skis and ankles were visible above an 8-inch layer of ice.

[2] Bagenholm found an air pocket and struggled beneath the ice for 40 minutes as her friends tried to dislodge her. Then her heart stopped beating and she was still. Forty minutes after that，a rescue team arrived， cut her out of the ice and administered CPR as they helicoptered her to a hospital. At 10:15 p.m.， three hours and 55 minutes after her fall， her fi rst heartbeat was recorded.Since then， she has made a nearly full recovery.

[3] Bagenholm was the very de fi nition of clinically dead: Her circulatory and respiratory systems had gone quiet for just over three hours before she was brought back to life. But what was happening in her body on a cellular level during the hours she went without a heartbeat? Were her tissues dying along with her consciousness? And how much longer could she have gone with no blood circulation?

[4] Can scientists learn anything from cases like this that could help them revive people who have been “dead” for an even longer period?

[5] These are the types of questions that preoccupy the staff of the University of Pennsylvania’s Center for Resuscitation Science (CRS)， a team of scientists， clinicians and engineers that’s revolutionizing the way we treat cardiac arrest and nudging forward the line between life and death. It all starts by learning what’s going on at the cellular level. According to Dr. Honglin Zhou， an assistant professor of emergency medicine at the University of Pennsylvania and an associate director of the CRS， scientists generally agree that， unlike the larger organisms they compose， there are clear ways to tell whether an individual human cell is dead.

[6] Every cell has a tight outer membrane that serves to separate its own contents from its surroundings and filter out the molecules that are nonessential to its function or survival.As a cell nears the end of its life， thisprotective barrier will begin to weaken and， depending on the circumstances of a cell’s death， one of three things will happen: It will send an “eat me” signal to a specialized maintenance cell that will then devour and recycle the ailing cell’s contents; it will quarantine and consume itself in a kind of programmed altruistic suicide; or it will rupture abruptly and spill its contents into the surrounding tissue， causing severe in fl ammation and further tissue damage.

[7] In all cases， when the integrity of the outer membrane is compromised， a cell’s fate is sealed. “When the permeability of the membrane has increased to the point that the cellular contents are leaking out，you have reached a point of no return，”Zhou said.

[8] Because even a mad scientist can’t put Humpty Dumpty1是旧童谣中的蛋状人物，从墙上掉下来被摔碎。’s cells back together again， a real-life Frankenstein’s monster2英国作家玛丽·雪莱（Mary Shelley）在1818年创作的小说《弗兰肯斯坦》（Frankenstein）中的形象，疯狂的科学家弗兰肯斯坦用许多碎尸块拼接成一个“人”，并用电将其激活。is not a possibility in the foreseeable future. But， as it turns out， it can take some cells quite a long time to die.

[9] When human cells are abruptly cut off from the steady supply of oxygen，nutrients and cleaning services that blood fl ow normally provides them， they can hold out in their membranes for a surprisingly long time. In fact， the true survivalists in your body may not die for many days after you’ve lost circulation，consciousness and most of the other things most people consider integral parts of living. If doctors can get to the patient before these cells have crashed，reanimation is still a possibility.

[10] Unfortunately， the cells that are most sensitive to nutrient and oxygen deprivation are brain cells. Within five to 10 minutes of cardiac arrest， neuronal membranes will begin to rupture and irreparable brain damage will ensue.Making revival efforts more difficult， a surefire way to kill a cell that has been cut off from oxygen and nutrients for an extended period of time is to give it oxygen and nutrients. In a phenomenon called reperfusion injury， blood-starved cells that are abruptly reintroduced to a nutrient supply will quickly self-destruct.

[11] The exact mechanisms of this process are still not well-understood，but Zhou speculates that when cells lose blood supply they may go into a kind of metabolic hibernation， with the goal of self-preservation. When the cells are roused from this state by an onslaught of oxygen and panicking white blood cells in an environment where toxins have accumulated， they are overwhelmed with in fl ammatory signals and they respond with self-immolation.

[12] Though scientists don’t fully understand the causes of reperfusion injury， they know from experience that one thing that sti fl es its onset is to lower a patient’s body temperature. This is why Bagenholm， who arrived at the hospital with an internal body temperature of 56 degrees Fahrenheit (about 13 degrees Celsius)， was able to recover and why one of the primary areas of research for the CRS is the application of so-called“therapeutic hypothermia.”

[13] By rapidly lowering a patient’s body temperature to about 91 degrees F (33 degrees C) using an intravenous cooling solution or a kind of ice-pack bodysuit as soon as possible after a cardiac arrest， ER doctors have found they can greatly decrease the risk of reperfusion injury as they work to revive the patient. This process sometimes allows patients who have been clinically dead for tens of minutes to make full recoveries.

[14] Whether this kind of medical miracle qualifies as reanimating the dead is not the principal concern of doctors， but survivors of clinical death do seem to have reemerged from an interlude of profound mental absence.Said Zhou: “I’ve met with people who have recovered from cardiac arrest， and it was just totally blank in their brain what happened. The brain’s not dead，but they couldn’t retrieve anything during that cardiac arrest stage.” ■

1999年，瑞典医科学生安娜·巴根霍姆滑雪时失去控制，头部朝下撞上一层薄冰，冰层下面是山涧。冰层破碎，她跌入寒冷刺骨的水流中，朋友们几分钟后赶到时，只看到她的滑雪板和脚踝露在8英寸厚的冰层外。

[2]巴根霍姆在冰层下面找到气窝，坚持了40分钟，等待朋友设法救她出来。然而她的心脏还是停止了跳动，生命体征也几近消失。又过了40分钟，救援队赶到救出她，并在直升机送往医院途中实施心肺复苏术。当晚10时15分，也就是距离事故发生3小时55分钟后，巴根霍姆的心脏终于恢复了跳动。最终，她的身体几乎完全康复。

[3]巴根霍姆事发当时的状态，完全符合医学上临床死亡的界定——在抢救成功前，她的循环系统和呼吸系统停止工作三小时以上。但是，心脏停跳的三小时中，她体内的细胞究竟出现了什么情况？组织器官也随着她的意识一同死去吗？在没有血液循环的状况下，她还能再坚持多久？

[4]从类似的事例中，研究人员又是否能找到方法，救活那些“死亡”时间更长的人呢？

[5]这些问题也一直是宾夕法尼亚大学复苏科学研究中心研究的重点，该研究团队的科学家、医学家和工程师一直在探索治疗心脏骤停的新方法，将生与死的界限再向前推进一步，而这些都要从研究心脏骤停期间体内细胞活动开始。宾夕法尼亚大学急诊医学助理教授、复苏科学研究中心副主任周洪林（音）博士表示，科学家普遍认为，与人体那些较大的器官不同，判断细胞死亡有更明确的方法。

[6]每个细胞都有一层严密的外膜，将细胞内部与外界隔开，过滤掉细胞正常运转和维持生存所不需要的分子。细胞走到生命尽头的时候，这层屏障的作用开始减弱，且根据细胞死亡环境的不同，会出现以下三种情况中的一种：向一种特殊的维护细胞发出“吃我”的信号，维护细胞便赶来将濒死的细胞吞噬掉，回收利用；或自我隔离，以程序性利他型自杀的形式凋亡；或突然破裂，其内含物浸入周围组织，导致严重感染和更严重的组织损伤。

[7]然而不论哪种情况，外膜一旦遭到破坏，细胞便无法存活。“如果细胞外膜的膜透性增大至细胞内含物漏出，结果就不可逆了。”周说。

[8]即使异想天开的科学家也不可能将破碎的细胞拼合如初，在可预见的未来，真实的“科学怪人”只是幻想。但是这样的案例表明，有些细胞确实可以在缺血缺氧的情况下存活很久。

[9]通常由血液循环提供的氧气、养分供给和代谢被突然切断后，人体细胞在外膜的保护下存活时间之长实在出乎我们的意料。事实上，甚至在人全身循环停滞，意识丧失，失去大多数人认为维持生命必需的物质后，体内细胞仍有可能顽强存活数日。如果医生能在这些细胞死亡前采取措施，起死回生并不是不可能。

[10]遗憾的是，脑细胞对缺血缺氧最为敏感。心脏骤停5至10分钟内，神经元膜开始破裂，继而造成不可逆的脑损伤。给复苏治疗增大难度的是，给长时间处于缺血缺氧状态的细胞重新输送氧气和养分，会给细胞本身造成致命的伤害。有种现象被称为再灌注损伤，即缺血的细胞突然恢复养分供应后，反而很快死亡。

[11]人们目前尚不完全清楚这种现象背后的机理，但据周推测，细胞在缺血状态下为了自我保护可能会进入一种代谢休眠状态。然而当供血恢复，休眠的细胞在大量毒素沉积的环境中，突然接触到氧气和来势汹汹的白细胞，炎症信号过量涌入，细胞难以应对，只好选择同归于尽。

[12]尽管研究人员尚不完全了解再灌注损伤的原理，但经验告诉他们，避免这种损伤的方法之一是降低病人的体温。这就解释了为什么巴根霍姆能够从濒死状态恢复过来，她被送到医院时体温只有56华氏度（约13摄氏度），这也是所谓“低温治疗”的应用成为宾夕法尼亚大学复苏科学研究中心主要研究领域之一的原因。

[13]心脏骤停后，通过静脉注射冷却液或特制的冰衣冰敷，将病人体温迅速降低到大约91华氏度（33摄氏度），急诊医生发现这样可以在抢救病人时，大大降低再灌注损伤的风险。运用这一疗法，有时可使临床诊断死亡数十分钟的病人恢复如初。

[14]医生们最大的担忧不在于这种医学奇迹是否适合作为救治临床死亡患者的方法，而是这些从临床死亡状态苏醒的幸存者意识上似乎确实出现一段空白。周说:“我曾见过心脏骤停后恢复过来的病人，这段经历在他们的大脑中完全是一片空白。这就说明虽然大脑没有死亡，但在心脏骤停期间，大脑无法获取任何信息。”□

（译者单位：北京师范大学）

Is It Possible to Reanimate the Dead?

By Eli MacKinnon