翻译：@DrWalker
关键词学习：ECG - 心电图；cardiac cycle - 心动周期；QT interval - QT间期；LQTS - 长QT间期综合征；QTc - 校正QT间期；Bazett's formula - Bazett公式；QRS complex - QRS复合波；Depolarize - 除极、去极化；Repolarize - 复极化；Cardiac Action Potential - 心肌细胞动作电位；membrane potential - 心肌细胞膜电位；L-type calcium channels - L型钙通道；Early Afterdepolarization (EAD) - 早后除极；premature ventricular contraction (PVC) - 室性期前收缩、室性早搏；reentrant tachycardia - 折返性心动过速；propagate - 传导；ventricular tachycardia - 室性心动过速；Torsade de Pointes (TdP) - 尖端扭曲型室速 Twisting of points - 150-250 beats/min；Polymorphic VT - 多形性室速；isoelectric line - 等电线；episodes - 间歇发作；Revert to a normal rhythm - 自行转复；palpitation - 心悸；syncope - 头晕、晕厥；sudden cardiac death - 心源性猝死；Class IA Antiarrhythmic Drugs - IA类抗心律失常药 - block Na & K channels；Class III antiarrhythmic drugs - III类抗心律失常药 - block K channels Quizlet：https://quizlet.com/_5i6pb6
在正常心电图上，存在P波、Q波、R波、S波和T波。QT间期为Q波开始到T波结束的范围。长QT综合征，也称作LQTS，是指一个人的QT间期比正常人长，一般小于心动周期的一半。比如心跳60次每分钟的人，QT间期一般对于男性大于440ms，女性大于460ms者，视为异常。如果你测心率为90次/分患者的QT间期，结果是400ms，你并不能直接把这个值和心率60次/分者比较，因为QT间期和心率相关：随着心率加快，QT间期也会缩短。
因此，我们引入了校正QT间期这一概念，也叫QTc，在心率不同时可以和心率为60次/分时相比较。尽管有很多校正公式，但Bazett公式是最为简单的，QTc等于QT间期(ms)除以RR间期(s)比上1s的平方根。
提醒一下，一般这个公式中的"比上1s"通常就省略不写了。但是机智的小伙伴会发现如果省略以后单位就不对了。其实原版公式确实有"比上1s"确保单位正确，但由于某些原因，在以前的某些论著中被省略掉了 ，所以没有"比上1s"的单位错误版本就从此流传下来啦！
那我们就拿一个QT间期400ms的男性举个简单例子，他的心率是90次/分。与心率60次/分的人相比，400ms不会是长QT间期，如果我们用之前的公式算一下，那么，把400ms和90次套进公式里，或者是0.66s每一跳(RR间期=60/心率)，那么我们得到了400ms的QT间期，再除以根号下(0.66s/1s)；第二步，计算400ms除以根号下0.81，这个是没有单位的，就得到一个校正的QT间期493ms，这个数值大于440ms，所以实际上，一个400ms心跳90次/分的QT间期是延长的。
既然QT间期有点儿长，那它长了又咋样呢？我们说，QRS复合波反映了心室的除极收缩过程。在去极化之后，心室肌需要复极化，这对应着T波。当一个人QT间期延长，这意味着他的部分心肌存在异常的复极化延长，但并不是所有心肌都复极化延长（这点很重要，敲黑板！），而是特指部分心肌相对于相邻心肌细胞而言，需要更多时间复极。部分心肌复极化延长，一般是由于通过离子通道的异常离子流引起的。
离子通道与心肌的除极和复极密切相关，每一次除极和复极叫做一次心肌细胞的动作电位，这个过程中有离子进出细胞，共分为4期，我们可以绘制一张心肌细胞膜电位随时间变化的图像来表示。在动作电位2期，钾通道开放，钾离子外流，使膜电位更负（复极化），但L型钙通道开放，使得钙离子内流入细胞，使膜电位更正（去极化），维持了平台期。
在动作电位3期，L型钙通道关闭，使得细胞复极化。所以，L型钙通道的功能紊乱是引起复极时间延长的原因之一。准确地说，这会使动作电位2期有更多钙离子内流，使得膜电位更正（去极化），导致早后除极，也称EAD。
其他机制与钠通道和钾通道相关，也可功能紊乱并引起早后除极。如果早后除极足够明显，兴奋会扩布并引起心室除极，造成室性期前收缩或室性早搏，也称PVC，通俗的说，就是异常的心室肌收缩。
此时，部分周围的细胞已经准备好下一次的除极，而有的细胞并没有做好准备，所以除极波便会激动做好准备的心肌细胞，但却被没有准备好的心肌细胞所中断。因此有时当可激动的细胞形成环路，除极波可以再次回头，可能会形成一个折返环路，引起折返型心动过速，表现为心率极快，原因是电激动会在环路中传导一次一次又一次地循环往复。
这是一种特殊类型的室速，与长QT间期综合征息息相关，称之为尖端扭转型室性心动过速，因为QRS复合波看起来围绕着等电位线扭转，而且由于QRS波形态上区别很大，所以很明确这也是一种多形性室速。尖端扭转型室速的心率一般在150-250次/分，波形的顶端看上去趋势很快。尽管这样的间歇发作可以自行转复为正常心律，但也可以后果很严重。尖端扭转型室速的患者可以感到心悸、眩晕、头晕，也可发生晕厥，甚至可能引起心源性猝死。
对于长QT综合征和尖端扭转型室速，离子通道的异常通常是先天性的，这意味着他们生来就有这种疾病，与某些遗传因素相关。有至少10种特定位点的基因突变与长QT综合征有关。比如LQT1，NLQT2等等，以上基因都影响着一个或多个离子通道。
除了先天性的原因，有些长QT间期还可能和特定的影响离子通道的药物有关。比如IA类抗心律失常药，阻滞了钠通道和钾通道，III类抗心律失常药阻滞钾通道。这些因素可引起QT间期延长，增加病人尖端扭转型室速的发生风险。
On a normal ECG, you’ve got the P, Q, R, S, and T waves. The QT interval spans from the start of the Q to the end of the T wave. Long QT syndrome, or LQTS, is when somebody’s QT interval is longer than normal, which should typically be less than half of a cardiac cycle. In fact, for a heart rate of 60 beats per minute, the QT interval’s generally considered to be abnormally long when it’s greater than 440 milliseconds in males or 460 milliseconds in females. If you measure someone’s QT interval at a different rate, say 90 beats per minute and it was 400 milliseconds, you can’t really use that to compare that to these value at 60 beats per minute, since the QT interval changes depending on the rate. As rate increases, the QT interval decreases.
So what we have to do is find the corrected QT interval, or QTc, at the different rate so that you can compare it to the QT interval at 60 beats per minute. Even though there are several formulas you can use, the Bazett’s formula’s probably the simplest, where the corrected QT interval equals the QT interval in milliseconds divided by the square root of the R to R interval in seconds divided by 1 second.
As a bit of a side-note, usually this formula’s expressed without the “divide by 1 second” bit, but the astute observer will notice that the units won’t work out if you do that. Interestingly, the originally formula did include dividing by 1 second to get the units to work out, but for some reason in a paper way back when that step wasn’t included, and basically the version without the 1 second, the sort of unit-incorrect version, has been used ever since!
Anyways, let’s do a quick example of a male with a 400 milliseconds QT interval at a rate of 90 beats per minute. Comparing to the values at 60 beats per minute, 400 milliseconds wouldn’t be considered a long QT, right? If we use our handy formula, though, we’ll plug in 400 for QT and 90 beats per minute or 0.66 seconds per beat. So we have a QT of 400 milliseconds divided by the square root of 0.66 seconds over 1 second, which is 400 milliseconds divided by 0.81, which is unitless, and we get a corrected QT interval of 493 milliseconds, which is greater than 440, so actually, a 400 milliseconds QT interval at 90 beats per minute is considered long.
Alright so the QT interval’s a little long, what’s wrong with that? Well, the QRS complex corresponds to the ventricles depolarizing and contracting. After they depolarize, they have to repolarize, and that’s captured by the T wave. When someone has a long QT interval, it means that they have an abnormally long repolarization of some of their heart cells, but not all of their heart cells - which is an important point to remember. Specifically some of the heart cells are taking longer than normal to repolarize compared to their neighboring heart cells.
Having some cells with an abnormally prolonged repolarization phase is thought to be caused by abnormalities in the movement of ions through ion channels, which is responsible for both depolarization and repolarization, and each time it depolarizes and repolarizes, it’s called a cardiac action potential, where ions flow in and out of the cell, and this happens in four phases, which we can plot on a graph of membrane potential over time.
During phase 2, potassium channels open and let potassium flow out, which tends to wanna make the membrane potential more negative, but L-type calcium channels open and let calcium flow into the cell, which tends to wanna make the cell more positive and maintains the “plateau” phase.
During phase 3, the potassium channels stay open, but now the L-type calcium channels close, which let’s the cell repolarize. It’s thought that dysfunction in the L-type calcium channels is one mechanism that can lead to a long repolarization phase. Specifically, they might let in more calcium during phase 2, making the membrane potential more positive and causing an early after-depolarization, or an EAD.
Other mechanisms involve sodium and potassium ion channels which can also malfunction and cause early after-depolarization well. If the EAD is large enough it might propagate out and depolarize the ventricles, causing a premature ventricular contraction, or a PVC, in other words an unexpected ventricular contraction. if at this point, some neighboring cells are ready for another depolarization and some aren’t, then the wave of depolarization will go through the ready cells but get blocked on the not-ready cells.
Then, at some point when the ready cells come around, the wave of depolarization can double back, potentially creating a reentrant circuit, which leads to reentrant tachycardia, which are super fast heart rates that happen because the signals might propagate around in a circular way, over and over and over again.
This becomes a special type of ventricular tachycardia or VT which is associated with Long QT syndrome called Torsade de Pointes, which means “the twisting of points”, because the QRS complexes seem to twist around the isoelectric line, and since the QRSs are different in shape, this is more specifically a type of polymorphic VT. Rates for Torsades range between 150 and 250 beats per minute, the upper end of which is really fast, right? So although these episodes can revert to a normal rhythm spontaneously, they can also be really serious and people with Torsades can feel palpitations, dizziness, syncope—or fainting, and it can even potentially lead to sudden cardiac death.
Alright so with long QT syndrome and Torsades, the cause of these abnormal ion channels is often congenital, meaning it’s present at birth and caused by some genetic abnormality. There’re at least 10 specific gene mutations that are known to be linked to Long QT syndrome, which are referred to as, for example, LQT1, LQT2, etc, all of which have some effect on one or more ion channels.
Aside from congenital causes though, sometimes the QT interval can be prolonged by certain medications which affect ion channels. For example, class IA antiarrhythmic drugs block sodium and potassium channels, while class III block potassium channels. These effects can lead to QT prolongation and an increased risk of Torsade de Pointes in some patients.
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