起死回生是否可能？

文/伊萊·麥金農(nóng) 譯/梁嘉琪 審訂/張政

起死回生是否可能？

文/伊萊·麥金農(nóng) 譯/梁嘉琪 審訂/張政

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是舊童謠中的蛋狀人物，從墻上掉下來被摔碎?！痵 cells back together again， a real-life Frankenstein’s monster2英國作家瑪麗·雪萊（Mary Shelley）在1818年創(chuàng)作的小說《弗蘭肯斯坦》（Frankenstein）中的形象，瘋狂的科學(xué)家弗蘭肯斯坦用許多碎尸塊拼接成一個(gè)“人”，并用電將其激活。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，瑞典醫(yī)科學(xué)生安娜·巴根霍姆滑雪時(shí)失去控制，頭部朝下撞上一層薄冰，冰層下面是山澗。冰層破碎，她跌入寒冷刺骨的水流中，朋友們幾分鐘后趕到時(shí)，只看到她的滑雪板和腳踝露在8英寸厚的冰層外。

[2]巴根霍姆在冰層下面找到氣窩，堅(jiān)持了40分鐘，等待朋友設(shè)法救她出來。然而她的心臟還是停止了跳動(dòng)，生命體征也幾近消失。又過了40分鐘，救援隊(duì)趕到救出她，并在直升機(jī)送往醫(yī)院途中實(shí)施心肺復(fù)蘇術(shù)。當(dāng)晚10時(shí)15分，也就是距離事故發(fā)生3小時(shí)55分鐘后，巴根霍姆的心臟終于恢復(fù)了跳動(dòng)。最終，她的身體幾乎完全康復(fù)。

[3]巴根霍姆事發(fā)當(dāng)時(shí)的狀態(tài)，完全符合醫(yī)學(xué)上臨床死亡的界定——在搶救成功前，她的循環(huán)系統(tǒng)和呼吸系統(tǒng)停止工作三小時(shí)以上。但是，心臟停跳的三小時(shí)中，她體內(nèi)的細(xì)胞究竟出現(xiàn)了什么情況？組織器官也隨著她的意識(shí)一同死去嗎？在沒有血液循環(huán)的狀況下，她還能再堅(jiān)持多久？

[4]從類似的事例中，研究人員又是否能找到方法，救活那些“死亡”時(shí)間更長的人呢？

[5]這些問題也一直是賓夕法尼亞大學(xué)復(fù)蘇科學(xué)研究中心研究的重點(diǎn)，該研究團(tuán)隊(duì)的科學(xué)家、醫(yī)學(xué)家和工程師一直在探索治療心臟驟停的新方法，將生與死的界限再向前推進(jìn)一步，而這些都要從研究心臟驟停期間體內(nèi)細(xì)胞活動(dòng)開始。賓夕法尼亞大學(xué)急診醫(yī)學(xué)助理教授、復(fù)蘇科學(xué)研究中心副主任周洪林（音）博士表示，科學(xué)家普遍認(rèn)為，與人體那些較大的器官不同，判斷細(xì)胞死亡有更明確的方法。

[6]每個(gè)細(xì)胞都有一層嚴(yán)密的外膜，將細(xì)胞內(nèi)部與外界隔開，過濾掉細(xì)胞正常運(yùn)轉(zhuǎn)和維持生存所不需要的分子。細(xì)胞走到生命盡頭的時(shí)候，這層屏障的作用開始減弱，且根據(jù)細(xì)胞死亡環(huán)境的不同，會(huì)出現(xiàn)以下三種情況中的一種：向一種特殊的維護(hù)細(xì)胞發(fā)出“吃我”的信號(hào)，維護(hù)細(xì)胞便趕來將瀕死的細(xì)胞吞噬掉，回收利用；或自我隔離，以程序性利他型自殺的形式凋亡；或突然破裂，其內(nèi)含物浸入周圍組織，導(dǎo)致嚴(yán)重感染和更嚴(yán)重的組織損傷。

[7]然而不論哪種情況，外膜一旦遭到破壞，細(xì)胞便無法存活。“如果細(xì)胞外膜的膜透性增大至細(xì)胞內(nèi)含物漏出，結(jié)果就不可逆了?！敝苷f。

[8]即使異想天開的科學(xué)家也不可能將破碎的細(xì)胞拼合如初，在可預(yù)見的未來，真實(shí)的“科學(xué)怪人”只是幻想。但是這樣的案例表明，有些細(xì)胞確實(shí)可以在缺血缺氧的情況下存活很久。

[9]通常由血液循環(huán)提供的氧氣、養(yǎng)分供給和代謝被突然切斷后，人體細(xì)胞在外膜的保護(hù)下存活時(shí)間之長實(shí)在出乎我們的意料。事實(shí)上，甚至在人全身循環(huán)停滯，意識(shí)喪失，失去大多數(shù)人認(rèn)為維持生命必需的物質(zhì)后，體內(nèi)細(xì)胞仍有可能頑強(qiáng)存活數(shù)日。如果醫(yī)生能在這些細(xì)胞死亡前采取措施，起死回生并不是不可能。

[10]遺憾的是，腦細(xì)胞對(duì)缺血缺氧最為敏感。心臟驟停5至10分鐘內(nèi)，神經(jīng)元膜開始破裂，繼而造成不可逆的腦損傷。給復(fù)蘇治療增大難度的是，給長時(shí)間處于缺血缺氧狀態(tài)的細(xì)胞重新輸送氧氣和養(yǎng)分，會(huì)給細(xì)胞本身造成致命的傷害。有種現(xiàn)象被稱為再灌注損傷，即缺血的細(xì)胞突然恢復(fù)養(yǎng)分供應(yīng)后，反而很快死亡。

[11]人們目前尚不完全清楚這種現(xiàn)象背后的機(jī)理，但據(jù)周推測，細(xì)胞在缺血狀態(tài)下為了自我保護(hù)可能會(huì)進(jìn)入一種代謝休眠狀態(tài)。然而當(dāng)供血恢復(fù)，休眠的細(xì)胞在大量毒素沉積的環(huán)境中，突然接觸到氧氣和來勢洶洶的白細(xì)胞，炎癥信號(hào)過量涌入，細(xì)胞難以應(yīng)對(duì)，只好選擇同歸于盡。

[12]盡管研究人員尚不完全了解再灌注損傷的原理，但經(jīng)驗(yàn)告訴他們，避免這種損傷的方法之一是降低病人的體溫。這就解釋了為什么巴根霍姆能夠從瀕死狀態(tài)恢復(fù)過來，她被送到醫(yī)院時(shí)體溫只有56華氏度（約13攝氏度），這也是所謂“低溫治療”的應(yīng)用成為賓夕法尼亞大學(xué)復(fù)蘇科學(xué)研究中心主要研究領(lǐng)域之一的原因。

[13]心臟驟停后，通過靜脈注射冷卻液或特制的冰衣冰敷，將病人體溫迅速降低到大約91華氏度（33攝氏度），急診醫(yī)生發(fā)現(xiàn)這樣可以在搶救病人時(shí)，大大降低再灌注損傷的風(fēng)險(xiǎn)。運(yùn)用這一療法，有時(shí)可使臨床診斷死亡數(shù)十分鐘的病人恢復(fù)如初。

[14]醫(yī)生們最大的擔(dān)憂不在于這種醫(yī)學(xué)奇跡是否適合作為救治臨床死亡患者的方法，而是這些從臨床死亡狀態(tài)蘇醒的幸存者意識(shí)上似乎確實(shí)出現(xiàn)一段空白。周說:“我曾見過心臟驟停后恢復(fù)過來的病人，這段經(jīng)歷在他們的大腦中完全是一片空白。這就說明雖然大腦沒有死亡，但在心臟驟停期間，大腦無法獲取任何信息?！薄?/p>

（譯者單位：北京師范大學(xué)）

Is It Possible to Reanimate the Dead?

By Eli MacKinnon