上一篇,描述了透析液中鈉離子濃度之歷史演進,這一篇,將試著把現在最新的概念,做個完整的闡述。
透析液中的鈉離子,會有何影響?
鈉,可以把它想成食物中的鹽,當吃比較鹹的食物之後,我們會感覺口渴,會想喝水,這是因為體內的鹽(鈉離子)變多了,需要水去把它稀釋,這樣子口渴的感覺才會消失。
為什麼濃度不能太高?
透析時,假如透析液中的鈉離子濃度高於體內,會導致鈉離子進入身體裡,就好像吃了一頓很鹹的大餐,透析之後會覺得口渴,進而喝比較多的水,使得水分攝取增加、體重增加過多,而導致下一次透析的時候,脫水量增加、脫水速度增加,如此一來,容易在透析過程中產生不適、掉血壓、抽筋等等問題。
咦?照上一段所說,鈉離子濃度高這麼糟糕,那濃度設定越低就越好囉?
從前一篇中(從歷史所得到的經驗)就知道,透析液中鈉離子設定的低,也容易掉血壓、產生不適的症狀,所以也不能夠太低!
為什麼濃度不能太低?
我們來看圖說故事,原本,血液裡和組織液裡的濃度是一樣的(假設都是140),而透析液的濃度比較低(假設為135)。
當血液到體外時,透析機器會在血液這一側施加壓力,把水擠出去,這個擠出去的總水量,就是我們在機器上設定的量,如:2公斤的水。另一方面,血液的鈉離子濃度,會逐漸接近於透析液的濃度,也就是濃度會下降。
當血液回到體內時,因為血液與組織液濃度差異,受到濃度差的影響,水會從濃度低的地方,被拉到濃度高的地方,這個量比較難計算,一般來說,濃度差的越大,水被拉的越多,為了解說方便,就假設為0.5公斤吧。
結果,血管內的水,除了被機器脫掉的2公斤之外,還會有0.5公斤跑到組織間去,使得血管內總共所減少的水量有2.5公斤,比實際機器所設定脫水的量還要多!!體內濃度差的影響,在透析開始之後,慢慢增加,一直要到透析結束幾個小時之後,隨著時間慢慢的減少(所以才註明是短時間),這個差異到最後會消失,而組織液中的水,會回到血液中,所以透析患者透析之後,需要一點時間休息,有部分原因於此。
所以透析液的鈉離子濃度如果設定的低,會容易掉血壓,容易產生透析中不適。
等……等一下
剛剛說鈉離子濃度高,會口渴、進而喝水、體重增加更多、脫水速度變快、也容易掉血壓……,這樣是說,濃度太低或濃度太高,都容易會掉血壓??
是的,你沒看錯,只不過兩種情況掉血壓的原因不同,
濃度太高的情況,是因為脫水速度太快,變成脫太快;
濃度太低的情況,是因為血管內減少的水量比機器設定的多,變成脫太乾(暫時)。
那麼該怎麼設定最好呢?
從上面的圖,您大概已經得知,濃度差異,會產生一些麻煩,這一點,我在鈣離子與鉀離子的文章中也都提到過:透析的過程當中,人體裡的各項數值變化,越小越好,鈣離子、鉀離子、鈉離子都是如此,所以,這篇文章裡提到,有越來越多的證據,支持個人化的設定,讓透析液的鈉離子濃度,和腎友血液中的鈉離子濃度,越接近越好。
要多接近?
建議把透析液中的濃度,設定小於血液中的濃度,而且把差距設定在 2 mEq/L,如:抽血報告是140,透析液使用138就好。這樣一來,濃度變化不會太大,能夠穩定透析中的症狀,濃度略小於血液,能夠減少身體鹽分的負擔,減少口渴症狀,減少體重增加,進而減少脫水速度,還能夠減輕高血壓。
但是,還是有例外的,如果腎友本身的鈉離子就偏低,小於 137 mEq/L者,本身不口渴、體重不會重太多、卻又容易掉血壓者,這種情況,選用略高的鈉離子濃度,比較合適。
總之,要如何決定最適合的鈉離子濃度?要綜合考慮:
透析前,血中鈉離子濃度 。
平常體重增加多少?
平常的血壓高嗎?
透析當中,狀況穩不穩定?
除了上述那些例外狀況,讓透析液濃度比血中低 2 mEq/L,可能是目前最好的答案。
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