血壓為什麼會高?
要回答這個問題,要先問有哪些因素會影響血壓?其實和電的物理公式很像:
電壓,等於電流量乘於電線阻力。
血壓,等於血流量乘於血管阻力。
所以說,會影響血壓的種種因素,可以簡化成兩個:血流量、與血管阻力。而撇開複雜的流體力學計算來說,血流量可再進一步簡化成總血量,也就是全身血液的量。
你或許會問,為什麼血壓和全身血量有關係?我常用氣球裝水的多寡,來比喻身體裝血量的多寡給患者了解,看圖就了解:
那麼,血壓高,又如何呢?
會有東西爆炸啊!水裝在氣球裡裝的多,氣球就爆炸;血裝在血管裡裝的多,血管就爆炸!所以中風、心臟病樣樣來,超級恐怖。
當腎臟功能逐漸變差時,腎臟排除毒素的能力下降,而且排水的能力也下降,因此,腎臟衰竭的患者體內裝了一大堆水和毒素,得要靠固定透析才能把這些髒水排除。如果,透析沒有把水脫乾淨的時候,水積在體內,就會像上面的圖中脹大的氣球一樣,壓力上升,血壓自然就高了起來。
因此,當洗腎患者的血壓高,第一個想法是把體內的水量脫乾一點,讓氣球消風,自然血壓就會降低,血壓藥也就不一定需要吃了。
如果已經脫得很乾,血壓卻還是高,那就要來降低第二個影響血壓的因素,血管阻力了。甚麼叫做阻力?舉個例子,含著波霸奶茶的吸管吹氣,這叫阻力小,含著養樂多的吸管吹氣,這叫做阻力大。我們能夠藉由改變血管的管徑大小,來改善血管阻力,白話一點就是,把血管放鬆、血管管徑變寬,血流的道路變寬了,阻力就下降了。
因此,降低血管阻力的方法,就是使用血壓藥物,藥物能夠使得血管放鬆,進而降低血壓。
洗腎患者的建議血壓值是多少呢?
大家先要了解,為什麼會有建議的血壓值,是有研究證實血壓在這個族群中,超過多少或者低於多少,會使得死亡率增加,而根據這樣的研究,才能來訂出建議的血壓值,如果這個特定族群尚未有紮實的研究來佐證的話,大家只能莫衷一是,沒有結論。KDIGO最近的一次共識,是發表在2010年的Kidney International期刊中,針對洗腎患者血壓目標的討論中,是這麼寫的:
1. 比起洗腎前與洗腎後的血壓,患者在家中固定量測的血壓,較能代表患者的真正血壓,在家中量測的血壓,與心臟病(左心室肥大)關聯性比較強,但是洗腎前與洗腎後的血壓,關聯性卻比較弱。這告訴我們,雖然我們都有量測洗腎時的血壓,但是腎友們在家裡一定還是要規則測量自己的血壓,好知道是否需要改變治療。
2. 遙遠的KDOQI [2]告訴我們,洗腎前的血壓建議在 140/90 mmHg以下,洗腎後的血壓建議在 130/80 mmHg以下,不過這建議只是專家意見,證據其實很薄弱,再加上前一點已經告訴我們,其實是家中的血壓更為重要,所以家中的血壓應該保持在多少以內呢?目前沒有針對洗腎患者的研究,因此只能依循一般人高血壓的建議,應該要保持家中血壓 <140/90 mmHg。
大家一定要注意自己家中的血壓,保持在 140/90 mmHg 以內,身體才會好喔!如果超過標準,試著洗腎時脫乾一點,血壓也會變好喔!
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http://doi.org/10.1038/ki.2009.469 2. KDOQI Clinical Practice Guidelines for Cardiovascular Disease in Dialysis Patients Section II. Guidelines on management of cardiovascular risk factors