Re: [申覆] 醫學(二) 10
※ 引述《ejackylate (小飛機)》之銘言：
: 10.下列何者與心室顫動(ventricular fibrillation)的成因較無關?
: (A)心臟肥厚(hypertrophy) (B)心肌缺血(ischemia)
: (C)血鉀過高(hyperkalemia) (D)Purkinje system傳導阻滯
: 答案：A
: 但查了E-medicine: http://emedicine.medscape.com/article/158712-overview
: 在下面的Causes中有 Hypertrophic cardiomyopathy
: 故本題無答案 建議送分
這則paper開頭就有說
http://findarticles.com/p/articles/mi_6875/is_4_6/ai_n28400194/
Hypertrophic cardiomyopathy (HCM) is associated with an increased incidence
of supraventricular and ventricular arrhythmias. Atrial fibrillation (AF) is
the most common arrhythmia in HCM with a prevalence of 20% and an annual
incidence of two percent per year.
再來看這是Hypertrophic cardiomyopathy with atrial fibrillation的心電圖
以及相關解釋的內文我覺得蠻有力的
http://cardiophile.org/2009/01/hypertrophic-cardiomyopathy-with-atrial-fibrillation.html
Atrial activity is seen as irregular fibrillary waves suggesting atrial
fibrillation. The QRS complexes have a large amplitude in chest leads
overlapping between the leads. Tall R waves in lateral leads and deep S waves
in anterior leads along with gross ST segment depression with T wave
inversion in lateral leads are suggestive of severe left ventricular
hypertrophy. The QRS width is also increased to about 120 msec mimicking left
bundle branch block. The ECG is from a case of advanced hypertrophic
cardiomyopathy with atrial fibrillation. Development of atrial fibrillation
leads to cardiac decompensation in hypertrophic cardiomyopathy due to loss of
atrial kick. In a hypertrophied ventricle the booster effect of atrial
contraction is very important for diastolic filling. Even though the
contribution of atrial contraction to ventricular filling in a normal person
is about 15 – 20 %, it may be over 30% in an individual with diastolic
dysfunction as in hypertrophic cardiomyopathy. When the ventricular filling
decreases, it reduces the cardiac output as well as an increase in the left
ventricular outflow obstruction. This is because the severity of left
ventricular outflow tract obstruction increases when the ventricular cavity
size decreases.
再來就是美國心臟協會的journal(被引用次數是181次)
http://content.onlinejacc.org/cgi/content/abstract/15/6/1279
內容是有關Atrial fibrillation in hypertrophic cardiomyopathy的長期研究
最後
重點是要試著從教科書上的敘述來跟申覆委員講了
guyton中文版第十一版第152頁的解釋中
首先ventricular fibrillation是心臟衝動在心室肌肉間爆衝所引發
衝動先刺激一部份心室肌肉，再換其他部分
導致心肌永遠無法一起做協調性的收縮
然而心肌協調性的收縮卻是心臟做幫浦循環所必須的
ventrcular fibrillation下儘管有大量刺激訊息流過心室，心室卻不擴張也不收縮
只維持部分收縮的不穩定狀態，幾乎無法抽送出血液
另一方面guyton說特別會引起fibrillation的原因有二
1. 心臟受到突然的電流刺激
2.心肌或特化的傳遞系統局部缺血
以上是GUYTON對ventricular fibrillation的敘述
讓我們看上面fibrillation的原因的第二點
接下來要證明的
就是hypertrophy會造成所謂傳導系統的局部缺血
使得傳導路徑出問題
hypertrophic cardiomyopathy的feature中
Myocardial hypertrophy impinging on the left ventricular (LV) cavity,
narrowing the LV outflow tract during systole, can cause dynamic obstruction
也就是心臟肥大會侵犯到左心室的腔室容量
使心收縮期的左心室outflow受阻
也就是體循環的動脈血出不去
ok,guyton中文版第十一版第153頁有說
ventricular fibrillation的傳導路徑出問題分為兩種
一種是傳導路徑的延長：其發生在心臟擴張時
一種是傳導速率減慢：可能是因為purkinje fiber傳導受阻
肌肉局部缺血
血鉀過高
以及許多其他原因
一種是心室不反應期的縮短：比較常出現在對藥物的反應
注意關鍵字："許多其他原因"
ventricular fibrillation目前的理論依據
是所謂的"circus movement"
也就是說當心臟衝動傳遍整個心臟到達心房時
整個心室肌肉還是處於不反應期
衝動無法繼續傳遞
因而中止
直到SA node產生下一個動作電位時，心臟才有所反應
今天hypertrophy造成左心室outflow受阻
產生局部缺血
進而影響到ventricular fibrilation
http://content.onlinejacc.org/cgi/content/abstract/15/6/1279
這也是以上美國心臟協會這篇journal說明hypertrophy會造成
ventricular fibrillation 的原因
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