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。’s 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，瘋狂的科學家弗蘭肯斯坦用許多碎尸塊拼接成一個“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)科學生安娜·巴根霍姆滑雪時失去控制，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，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，堅持了40分鐘，等待朋友設法救她出來。然而她的心臟還是停止了跳動，生命體征也幾近消失。又過了40分鐘，救援隊趕到救出她，并在直升機送往醫(yī)院途中實施心肺復蘇術。當晚10時15分，也就是距離事故發(fā)生3小時55分鐘后，巴根霍姆的心臟終于恢復了跳動。最終，她的身體幾乎完全康復。

[3]巴根霍姆事發(fā)當時的狀態(tài)，完全符合醫(yī)學上臨床死亡的界定——在搶救成功前，她的循環(huán)系統(tǒng)和呼吸系統(tǒng)停止工作三小時以上。但是，心臟停跳的三小時中，她體內(nèi)的細胞究竟出現(xiàn)了什么情況？組織器官也隨著她的意識一同死去嗎？在沒有血液循環(huán)的狀況下，她還能再堅持多久？

[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”時間更長的人呢？

[5]這些問題也一直是賓夕法尼亞大學復蘇科學研究中心研究的重點，該研究團隊的科學家、醫(yī)學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一步，而這些都要從研究心臟驟停期間體內(nèi)細胞活動開始。賓夕法尼亞大學急診醫(yī)學助理教授、復蘇科學研究中心副主任周洪林（音）博士表示，科學家普遍認為，與人體那些較大的器官不同，判斷細胞死亡有更明確的方法。

[6]每個細胞都有一層嚴密的外膜，將細胞內(nèi)部與外界隔開，過濾掉細胞正常運轉(zhuǎn)和維持生存所不需要的分子。細胞走到生命盡頭的時候，這層屏障的作用開始減弱，且根據(jù)細胞死亡環(huán)境的不同，會出現(xiàn)以下三種情況中的一種：向一種特殊的維護細胞發(fā)出“吃我”的信號，維護細胞便趕來將瀕死的細胞吞噬掉，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，導致嚴重感染和更嚴重的組織損傷。

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

[8]即使異想天開的科學家也不可能將破碎的細胞拼合如初，在可預見的未來，真實的“科學怪人”只是幻想。但是這樣的案例表明，有些細胞確實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，人體細胞在外膜的保護下存活時間之長實在出乎我們的意料。事實上，甚至在人全身循環(huán)停滯，意識喪失，失去大多數(shù)人認為維持生命必需的物質(zhì)后，體內(nèi)細胞仍有可能頑強存活數(shù)日。如果醫(yī)生能在這些細胞死亡前采取措施，起死回生并不是不可能。

[10]遺憾的是，腦細胞對缺血缺氧最為敏感。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。給復蘇治療增大難度的是，給長時間處于缺血缺氧狀態(tài)的細胞重新輸送氧氣和養(yǎng)分，會給細胞本身造成致命的傷害。有種現(xiàn)象被稱為再灌注損傷，即缺血的細胞突然恢復養(yǎng)分供應后，反而很快死亡。

[11]人們目前尚不完全清楚這種現(xiàn)象背后的機理，但據(jù)周推測，細胞在缺血狀態(tài)下為了自我保護可能會進入一種代謝休眠狀態(tài)。然而當供血恢復，休眠的細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胞，炎癥信號過量涌入，細胞難以應對，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)恢復過來，她被送到醫(yī)院時體溫只有56華氏度（約13攝氏度），這也是所謂“低溫治療”的應用成為賓夕法尼亞大學復蘇科學研究中心主要研究領域之一的原因。

[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，大大降低再灌注損傷的風險。運用這一療法，有時可使臨床診斷死亡數(shù)十分鐘的病人恢復如初。

[14]醫(yī)生們最大的擔憂不在于這種醫(yī)學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)蘇醒的幸存者意識上似乎確實出現(xiàn)一段空白。周說:“我曾見過心臟驟停后恢復過來的病人，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。”□

（譯者單位：北京師范大學）

Is It Possible to Reanimate the Dead?

By Eli MacKinnon