這個問題,是很多人的疑問,確實也不容易回答
如果我每次洗腎洗超過 4個小時,身體會不會更好?
這個問題的本質是:如果我一次洗得更久,或者更頻繁的透析,理論上會洗得更乾淨,那我會不會更健康?
從之前的文章“KDOQI新版血液透析治療指引(2015最新資料)總整理”中提到,目前的證據告訴我們,更頻繁的、或更長時間的透析,可以有以下兩大好處:
1. 改善血壓的控制 [達到更乾的乾體重、進而改善心臟肥大問題]
2. 可降低血中磷濃度 [減少磷結合劑的用量]
後面括號的部分,只是描述的角度不同,但其實道理是一樣的。
1. 因為透析時間拉長了,所以脫水速度自然比較慢;脫水速度比較慢,就比較能夠脫得更乾;脫水脫的越乾,血壓就越容易控制,而且體內越乾,心臟也會比較小。
2. 因為透析時間拉長了,躲在細胞內的磷才有更多時間跑到細胞外,才洗得掉,所以可以增加磷的移除;磷能夠移除得越多,血磷的數值當然就可以越低;血磷數值越低,當然就不需要吃那麼多磷結合劑囉。
但是,細心的大家,有沒有發現缺了一件極度重要的事情?也就是所有人最在意的“死亡率”呢?
目前所知道的是,延長透析時間的患者,死亡率在某些研究確實有下降。不過,這些研究難免被質疑有採樣誤差(selection bias),意思是“能夠選擇”延長透析的患者,通常也代表他們原本身體狀況就比較好,自然會比那些“因為身體狀況沒有很好,所以只能選擇正常透析”的患者,來的更有生存優勢。因此,到底延長透析時間,是否能夠降低死亡率,目前還有爭議。
或許研究的時間還不夠長,還看不出頻繁長時間透析的優勢。或許只單單改善了磷濃度與血壓控制,還不足以讓死亡率降低。不過最近有一項研究,觀察到了可能的原因…
一項為期52周的觀察性研究,找了 33位腎友,將一周三次的透析時間延長為 7-8小時,作為實驗組;另外 20位腎友,維持標準一周三次,一次四小時,當作對照組。兩組在 52周之後,抽析前的血來比較看看結果如何。
這個研究特別的地方在於,不只是看血磷、白蛋白等常見的數值而已,他們還用了液相層析串聯式質譜儀(LCMS),分析了體內高達 164種代謝物,來看兩組變化的差異如何。
這 164種代謝物中,其中 25種被認為與尿毒相關的毒素(如:p-Cresol sulfate、Indoxyl sulfate、Trimethylamine-N-oxide等等),在延長透析時間的這一組受試者的結果,竟然!沒有顯著的下降!
而胺基酸等營養素的小分子,卻因為延長透析時間,而有顯著的下降…,例如之前文章所提過的肉鹼(L-carnitine),就因為延長透析時間而顯著的下降了。
為什麼?
這有點像念書考試,假設每天花一個小時念書,可以得到 65分,而如果每天增加念書時間變成兩個小時,分數就會變成 100分嗎?我想大部分人的經驗都知道,努力加倍,獲得的結果往往不成比例。
延長透析時間,尿毒素濃度沒有明顯下降的其中一個原因,就是即使延長兩倍的時間,也多不了多少清除效果。反過來說,針對這些尿毒素,原本的透析時間就夠了,能洗得掉的,在這四小時中就幾乎洗掉了。
另一個原因,也有研究顯示,清除掉越多尿毒素,它產生回來的速度也會增加,因此導致析前的濃度差不多。
總合以上所述,延長透析時間的明顯好處,就是 1. 磷的清除會增加,2. 血壓能夠更容易控制。至於能不能得到更好的健康,目前還有爭議,畢竟析前的尿毒素濃度並沒有顯著下降,而有些營養的胺基酸卻下降了。所以,要延長透析時間當然可以,不過,要知道這樣能夠得到甚麼好處,才不會產生錯誤的期待喔。
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