Cardiac electrophysiology: Membrane potential, action potential, automaticity and electrical vectors
心脏电生理学：膜电位、动作电位、自律性及电向量

Video lecture  视频讲座

The following video lecture summarises this chapter.
以下视频讲座总结了本章内容。

The action potential includes a depolarization (activation) followed by a repolarization (recovery). The action potential occurs in all cardiac cells but its appearance varies depending on cell type. During de- and repolarization ions (Na⁺ [sodium], K⁺ [potassium] and Ca²⁺ [calcium]) flow back and forth across the cell membrane. Because ions are electrically charged, their movement generates an electrical current. This means that the propagation (spread) of the action potential is equal to the spread of an electrical current.
动作电位包括去极化（激活）和随后复极化（恢复）。动作电位发生在所有心脏细胞中，但其表现因细胞类型而异。在去极化和复极化过程中，离子（Na ⁺ [钠]、K ⁺ [钾] 和 Ca ²⁺ [钙]）在细胞膜两侧来回流动。由于离子带有电荷，它们的运动会产生电流。这意味着动作电位的传播等同于电流的传播。

All tissues and fluids surrounding the heart have an abundance of ions, enabling them to function as electrical conductors. As a result, the electrical currents produced by the myocardium are transmitted through the body to the skin, where they can be detected using electrodes. The electrocardiograph (ECG machine) records and processes these electrical currents and presents them as the electrocardiogram (ECG). As mentioned earlier, the electrical potentials of the conduction system are much too small to be detected by skin electrodes; the ECG presents the electrical activity of the atrial and ventricular myocardium.
心脏周围的所有组织和液体都富含离子，使其能够作为电导体发挥作用。因此，心肌产生的电流通过身体传导到皮肤表面，可以通过电极检测到。心电图机（ECG 机）记录并处理这些电流，并将其呈现为心电图（ECG）。如前所述，传导系统的电位过小，无法被皮肤电极检测到；心电图显示的是心房和心室心肌的电活动。

Automaticity of pacemaker cells
起搏细胞的自律性

The automaticity of the cells in the sinoatrial node is explained by the fact that these cells start leaking sodium (Na⁺) into the cell as soon as they return to their resting state (Figure 1). As sodium leaks into the cell, the membrane becomes more positive. When the membrane potential reaches its threshold –40 mV, the action potential is triggered and the cell depolarizes. At –40 mV voltage-gated calcium (Ca²⁺) channels open so that calcium flows into the cell and causes depolarization. Subsequently, outward-directed potassium (K⁺) channels open which results in cell repolarization. The cycle then repeats itself (Figure 1). Note that the leakage of sodium during the resting phase is called pacemaker potential.
窦房结细胞的自律性可以通过这样一个事实来解释：这些细胞一旦恢复到静息状态，就开始向细胞内泄漏钠离子（Na⁰）（见图 1）。随着钠离子泄漏进入细胞，膜电位变得更加正向。当膜电位达到阈值-40 mV 时，动作电位被触发，细胞去极化。在-40 mV 时，电压门控钙离子（Ca¹⁺）通道打开，钙离子流入细胞，引起去极化。随后，向外导向的钾离子（K²⁺）通道打开，导致细胞复极化。该循环随后重复进行（见图 1）。注意，静息期钠离子的泄漏称为起搏电位。

The depolarization spreads from the sinoatrial node to the atrial and ventricular myocardium. Propagation of the action potential is possible because all cardiac cells are electrically interconnected by gap junctions (Figure 1). Gap junctions are protein channels that connect the cell membranes of adjacent cells and enable the flow of ions between cells. This means that the action potential spreads from one cell to another via gap junctions. The density of gap junctions within the Purkinje network is very high, which explains the rapid impulse transmission in the network. Cells of the atrioventricular node, on the contrary, have a low density of gap junctions, explaining the slow impulse conduction through the atrioventricular node. Transmission of the action potential between contractile myocardial cells is also slow, owing to the scarcity of gap junctions between them.
去极化从窦房结传播到心房和心室心肌。动作电位的传播之所以可能，是因为所有心脏细胞通过缝隙连接电性相互连接（图 1）。缝隙连接是连接相邻细胞膜的蛋白通道，允许离子在细胞间流动。这意味着动作电位通过缝隙连接从一个细胞传播到另一个细胞。浦肯野网络内缝隙连接的密度非常高，这解释了网络中冲动传导的快速性。相反，房室结细胞的缝隙连接密度较低，解释了冲动通过房室结时的传导缓慢。收缩性心肌细胞之间动作电位的传导也较慢，这是由于它们之间缝隙连接稀少所致。

The action potential in contractile cells
收缩性细胞中的动作电位

The contractile cells, unlike cells of the sinoatrial node, display a true resting potential (phase 4), which is around –90 mV. These cells must be stimulated in order to evoke an action potential. Upon stimulation, sodium (Na+) channels open which causes a rapid influx of sodium and depolarizes the cell. Contractile cells start to contract a few milliseconds after the start of the depolarization and they start relaxing a few milliseconds after the repolarization is completed. The duration of the action potential is approximately 0.20 seconds in atrial myocardium and 0.3 seconds in the ventricular myocardium (Figure 1).
收缩细胞与窦房结细胞不同，表现出真正的静息电位（第 4 期），约为–90 mV。这些细胞必须受到刺激才能引发动作电位。受到刺激后，钠离子（Na+）通道开放，导致钠离子迅速内流，使细胞去极化。收缩细胞在去极化开始后几毫秒开始收缩，在复极完成后几毫秒开始松弛。动作电位的持续时间在心房心肌约为 0.20 秒，在心室心肌约为 0.3 秒（图 1）。

The phases are also illustrated in Figure 1 (above).
各个阶段也在图 1（上方）中有所示意。

• Phase 4 (resting phase): Only potassium (K+) channels are open during the resting phase and efflux (outward flow) of potassium establishes a negative resting membrane potential (approximately –90 mV).
  第 4 期（静息期）：在静息期仅有钾（K+）通道开放，钾的外流（向外流动）建立了负的静息膜电位（约–90 mV）。
• Phase 0 (depolarization): Upon stimulation, rapid depolarization occurs via influx (inward flow) of sodium (Na+) and the cell becomes positively charged (approximately 20 mV).
  第 0 期（去极化）：受到刺激后，通过钠（Na+）的内流（向内流动）发生快速去极化，细胞变为正电位（约 20 mV）。
• Phase 1 (early repolarization): During this phase, another type of potassium (K+) channels opens and a brief efflux of potassium repolarizes the cell slightly.
  第 1 期（早期复极化）：在此阶段，另一种钾（K+）通道开放，钾的短暂外流使细胞略微复极化。
• Phase 2 (plateau phase): Almost simultaneous with the opening of potassium channels in phase 1, persistent calcium (Ca2+) channels open whereby calcium flows into the cell. The influx of calcium is steady and gives rise to the plateau phase whose long duration explains why the vast majority of the ventricular myocardium contracts simultaneously (at some point during the contraction).
  第 2 期（平台期）：几乎与第 1 期钾通道开放同时，持续的钙（Ca2+）通道开放，钙流入细胞。钙的内流是稳定的，产生平台期，其长时间持续解释了为何大部分心室心肌在收缩过程中某一时刻能同时收缩。
• Phase 3 (repolarization): Calcium (Ca2+) channels close and potassium (K+) channels open again and the efflux of potassium repolarizes the cell.
  第 3 期（复极）：钙离子（Ca2+）通道关闭，钾离子（K+）通道再次开放，钾离子的外流使细胞复极。

Absolute and relative refractory periods during the action potential
动作电位期间的绝对不应期和相对不应期

During the greater part of the action potential, the myocardial cell is absolutely refractory to stimulation, meaning that an additional stimulus cannot trigger a new action potential, regardless of the intensity of the stimulus. The absolute refractory period is followed by a relative refractory period, during which a strong stimulation may trigger a new action potential. The absolute and relative refractory periods are displayed in Figure 1 (above) and Figure 2 (below).
在动作电位的大部分时间内，心肌细胞处于绝对不应期，意味着无论刺激强度多大，额外的刺激都无法触发新的动作电位。绝对不应期之后是相对不应期，在此期间，强烈的刺激可能触发新的动作电位。绝对不应期和相对不应期分别显示在图 1（上方）和图 2（下方）。

As seen in Figure 2, the relative refractory period coincides with the T-wave apex. This phase has traditionally been described as a vulnerable phase during the cardiac cycle, because electrical stimulation during this phase may evoke another action potential which can lead to potentially life-threatening ventricular arrhythmias (ventricular fibrillation). This occurs occasionally in clinical practice. The stimulus is typically a premature ventricular beat (i.e. an ectopic beat from within the ventricles) or inappropriate stimulation from an artificial pacemaker. When such a ventricular depolarization is superimposed on the T-wave, it is said that an R-on-T phenomenon has occurred. However, it should be noted that R-on-T phenomena are very common and the risk of ventricular fibrillation is small unless there is electrical instability in the ventricles (such as during acute myocardial infarction, long QT syndrome, etc.). Figure 3 (below) shows two cases of R-on-T phenomenon.
如图 2 所示，相对不应期与 T 波峰值重合。传统上，这一阶段被描述为心动周期中的易损期，因为在此阶段的电刺激可能引发另一次动作电位，进而导致潜在的危及生命的室性心律失常（室颤）。这种情况在临床实践中偶尔发生。刺激通常是早发性室性搏动（即来自心室内的异位搏动）或人工起搏器的不适当刺激。当这种室性去极化叠加在 T 波上时，称为 R-on-T 现象。然而，应注意的是，R-on-T 现象非常常见，除非心室存在电不稳定（如急性心肌梗死、长 QT 综合征等），否则发生室颤的风险较小。下图 3 显示了两例 R-on-T 现象。

What does the ECG show?
心电图显示了什么？

Figure 4 shows the classical ECG waveforms. The first deflection (henceforth called wave) is the P-wave, which represents atrial activation (depolarization). The repolarization of the atria is usually not visible because it occurs simultaneously with ventricular activation (depolarization), which generates substantially larger electrical potentials and therefore dominates that moment in the cardiac cycle. Ventricular depolarization is visible as the QRS complex. The QRS complex consists of three waves: Q, R and S. Differences in the direction of these waves are due to changes in the direction of the electrical impulse during ventricular depolarization. The T-wave represents the repolarization of the ventricles.
图 4 显示了经典的心电图波形。第一个偏转（以下简称为波）是 P 波，代表心房激活（去极化）。心房复极通常不可见，因为它与心室激活（去极化）同时发生，心室激活产生的电位明显更大，因此在心动周期中占主导地位。心室去极化表现为 QRS 波群。QRS 波群由三波组成：Q 波、R 波和 S 波。这些波方向的差异是由于心室去极化过程中电冲动方向的变化。T 波代表心室复极。

Note the straight line between the P-wave and the QRS complex (Figure 4). This line is referred to as the PR segment and it represents the impulse delay in the atrioventricular node.
注意 P 波和 QRS 波群之间的直线（图 4）。这条线称为 PR 段，代表房室结中的冲动延迟。

It is fundamental to know the genesis of these waveforms and this requires knowledge of the electrical vectors that exist during the cardiac cycle. Vector theory is a rather complex matter, which is why the discussion below will exclude overly complicated aspects and focus on the main electrical vectors and how they impact the ECG curve.
了解这些波形的起源是基础，这需要了解心动周期中存在的电向量。向量理论相当复杂，因此以下讨论将排除过于复杂的内容，重点介绍主要的电向量及其如何影响心电图曲线。

Electrical vectors  电向量

A vector is a physical quantity that has both magnitude and direction in space. The movement of electrically charged particles – which occurs during the spread of the cardiac action potential – generates an electrical vector. The depolarization propagates through the myocardium similar to the spread of a wavefront in water. At any given instant in time there are numerous small depolarizing wave fronts that propagate through the myocardium (Figure 5, left-hand side). The average of all individual wavefronts, at any given instant in time, represents the main electrical vector (Figure 5, right-hand side). Thus the electrical vector is the average direction of the impulse. The ECG waveforms displayed in Figure 5 actually represent the electrical vectors of the cardiac cycle.
向量是一种在空间中具有大小和方向的物理量。带电粒子的运动——即心脏动作电位传播时发生的运动——产生了电向量。去极化通过心肌传播，类似于水中波前的扩散。在任何特定时刻，心肌中都有许多小的去极化波前传播（图 5，左侧）。在任何特定时刻，所有单个波前的平均值代表主要电向量（图 5，右侧）。因此，电向量是冲动的平均方向。图 5 中显示的心电图波形实际上代表了心动周期的电向量。

In order to understand how an electrical vector generates a wave on the ECG curve, it is crucial to understand how ECG leads are constructed. Note that the ECG leads will be discussed in detail in the next chapter; here, we only mention aspects relevant to electrical vectors. The electrocardiograph (ECG machine) uses two electrodes to calculate one ECG curve (Figure 6). This is done by comparing the electrical potentials detected by each of the electrodes. One electrode is defined as positive (also called exploring electrode) and the other is negative (also called reference electrode). The electrocardiograph compares the electrical potentials detected by the exploring electrode and the reference electrode. The machine is constructed such that a vector heading toward the exploring electrode yields a positive deflection (wave) on the ECG curve. A vector heading away from the exploring electrode yields a negative wave (Figure 6).
为了理解电向量如何在心电图曲线上产生波形，关键是要了解心电图导联的构造。请注意，心电图导联将在下一章中详细讨论；这里仅提及与电向量相关的方面。心电图机使用两个电极来计算一条心电图曲线（图 6）。这是通过比较每个电极检测到的电位来完成的。一个电极被定义为正极（也称为探测电极），另一个为负极（也称为参考电极）。心电图机比较探测电极和参考电极检测到的电位。机器的构造使得指向探测电极的向量在心电图曲线上产生正向偏移（波形）。远离探测电极的向量则产生负向波形（图 6）。

It is traditionally taught that the exploring electrode is the one “viewing” the heart, and this notion facilitates ECG interpretation. The physiological rationale behind this is explained below.
传统教学中认为探测电极是“观察”心脏的电极，这一概念有助于心电图的解读。其生理学原理如下所述。

Now that the relation between electrical vectors and ECG waves has been clarified, it is time to study the main vectors of the heart and how they give rise to the classical ECG curve. Leads V1 and V5 have been selected for pedagogical reasons. V1 and V5 primarily detect vectors traveling in the horizontal plane. This is due to the placement of the exploring electrode and the reference. The exploring electrode is placed anteriorly on the chest wall. The reference is slightly more complicated because it is derived by taking the average of the potentials recorded by the limb electrodes (right arm, left arm, left leg) and this yields a reference located in the chest (Figure 7). This will be discussed in greater detail in the next chapter, but for the current discussion, it is sufficient to note that the exploring electrode is located anteriorly on the chest and the reference point is located inside the chest (Figure 7).  We shall now examine the main electrical vectors of the heart and how they are reflected in V1 and V5. Study this figure carefully, as it explains the genesis and appearance of the P, Q, R and S waves on the ECG curve.
既然已经阐明了电向量与心电图波形之间的关系，现在是时候研究心脏的主要向量及其如何产生经典的心电图曲线了。出于教学目的，选择了导联 V1 和 V5。V1 和 V5 主要检测在水平面内传播的向量。这是由于探测电极和参考电极的位置所致。探测电极置于胸壁前方。参考电极则稍微复杂一些，因为它是通过取肢体电极（右臂、左臂、左腿）记录的电位的平均值而得出，这使得参考点位于胸腔内（图 7）。这一点将在下一章中详细讨论，但在当前讨论中，只需注意探测电极位于胸前，参考点位于胸腔内（图 7）。现在我们将考察心脏的主要电向量及其如何反映在 V1 和 V5 导联中。请仔细研究此图，因为它解释了心电图曲线上 P 波、Q 波、R 波和 S 波的起源及其表现。

The first vector: the atria
第一个矢量：心房

The first vector originates from atrial depolarization. The depolarization starts in the sinoatrial node, from where it spreads through the right atrium and subsequently to the left atrium. During activation of the right atrium, the vector is directed anteriorly and to the left (and downwards). The vector turns left and somewhat backward as the depolarization heads toward the left atrium. Thus, the atrial vector is slightly curved (Figure 7). Lead V1 detects the initial vector heading towards it and displays a positive deflection, the P-wave. V1 may also detect that the vector heads away from it when the left atrium is activated and this might yield a small negative deflection on the terminal portion of the P-wave (Figure 7). Lead V5 on the other hand, only notes vectors heading towards it (albeit with varying angles) throughout the course of atrial activation and therefore displays a uniformly positive P-wave.
第一个矢量起源于心房去极化。去极化始于窦房结，从那里传播到右心房，随后传至左心房。在右心房激活期间，矢量指向前方和左侧（以及向下）。当去极化向左心房传播时，矢量向左并略微向后转动。因此，心房矢量呈轻微弯曲状（图 7）。导联 V1 检测到朝向它的初始矢量，并显示为正向偏移，即 P 波。V1 也可能检测到当左心房被激活时矢量远离它，这可能导致 P 波末端出现小的负向偏移（图 7）。另一方面，导联 V5 在整个心房激活过程中仅记录朝向它的矢量（尽管角度不同），因此显示出均匀的正向 P 波。

The second vector: the ventricular (interventricular) septum
第二个矢量：心室（室间）隔

The ventricular septum receives Purkinje fibers from the left bundle branch and therefore depolarization proceeds from its left side towards its right side. The vector is directed forward and to the right. The ventricular septum is relatively small, which is why V1 displays a small positive wave (r-wave) and V5 displays a small negative wave (q-wave). Thus, it is the same electrical vector that results in an r-wave in V1 and a q-wave in V5.
室间隔接受来自左束支的浦肯野纤维，因此去极化从其左侧向右侧推进。矢量指向前方和右侧。室间隔相对较小，这就是为什么 V1 显示小的正波（r 波），而 V5 显示小的负波（q 波）。因此，正是同一电矢量导致 V1 出现 r 波，V5 出现 q 波。

The third vector: the ventricular free wall
第三个矢量：心室游离壁

The vectors resulting from the activation of the ventricular free walls are directed to the left and downwards (Figure 7). The explanation for this is as follows:
由心室游离壁激活产生的矢量指向左侧和向下（图 7）。其解释如下：

1. The vector resulting from activation of the right ventricle does not come to expression, because it is drowned by the many times larger vector generated by the left ventricle. Thus, the vector during activation of the ventricular free walls is actually the vector generated by the left ventricle.
   右心室激活产生的矢量未能表现出来，因为它被左心室产生的矢量所淹没，后者的幅度大数倍。因此，心室游离壁激活时的矢量实际上是由左心室产生的矢量。
2. Activation of the ventricular free wall proceeds from the endocardium to the epicardium. This is because the Purkinje fibers run through the endocardium, where they deliver the action potential to contractile cells. The subsequent spread of the action potential occurs from one contractile cell to another, starting in the endocardium and heading toward the epicardium.
   心室游离壁的激活从心内膜向心外膜进行。这是因为浦肯野纤维分布于心内膜，通过它们将动作电位传递给收缩细胞。随后，动作电位从一个收缩细胞传播到另一个，起始于心内膜，向心外膜扩散。

As evident from Figure 7, the vector of the ventricular free wall is directed to the left (and downwards). Lead V5 detects a very large vector heading towards it and therefore displays a large R-wave. Lead V1 records the opposite and therefore displays a large negative wave called S-wave.
如图 7 所示，心室游离壁的矢量指向左侧（并向下）。V5 导联检测到一个非常大的指向它的矢量，因此显示出一个大的 R 波。V1 导联记录到相反方向的矢量，因此显示出一个称为 S 波的大负波。

The fourth vector: basal parts of the ventricles
第四个矢量：心室基底部

The final vector stems from the activation of the basal parts of the ventricles. The vector is directed backward and upwards. It heads away from V5 which records a negative wave (s-wave). Lead V1 does not detect this vector.
最后一个矢量源自心室基底部的激活。该矢量指向后方和上方。它远离 V5，V5 记录到一个负波（s 波）。V1 导联未检测到该矢量。

The vector of the T-wave
T 波的矢量

The T-wave represents the rapid repolarization phase (phase 2). The T-wave should be concordant with the QRS complex, meaning that it should have the same direction as the net direction of the QRS complex. A QRS complex that is net negative should be followed by a negative T-wave, whereas a QRS complex that is net positive should be followed by a positive T-wave. If the QRS complex and T-wave have opposite directions, it is said that the T-wave is discordant (Figure 8).
T 波代表快速复极相（第 2 相）。T 波应与 QRS 波群一致，意味着其方向应与 QRS 波群的净方向相同。净负的 QRS 波群应跟随负向 T 波，而净正的 QRS 波群应跟随正向 T 波。如果 QRS 波群和 T 波方向相反，则称 T 波为不一致（图 8）。

It may seem illogical that the QRS complex and the T-wave should have the same direction, given that the ion flows during de- and repolarization are opposite. It seems more logical that de- and repolarization should have opposite directions. Evidently, this is not the case because not only are the ion flows opposite, but so is the direction of the electrical vector. Recall that depolarization of the ventricular free wall proceeds from endocardium to epicardium. Repolarization, on the other hand, starts in the epicardium and is directed toward the endocardium (Figure 9). This is explained by the fact that epicardial cells have shorter action potentials and therefore begin repolarizing earlier than endocardial cells. Refer to Figure 9 for an illustrated explanation.
QRS 波群和 T 波方向相同似乎不合逻辑，因为去极化和复极化期间的离子流方向是相反的。看起来去极化和复极化应该具有相反的方向更为合理。显然，事实并非如此，因为不仅离子流方向相反，电向量的方向也相反。回想一下，心室游离壁的去极化是从心内膜向心外膜进行的。另一方面，复极化始于心外膜，方向朝向心内膜（图 9）。这是因为心外膜细胞的动作电位较短，因此比心内膜细胞更早开始复极化。详见图 9 的示意说明。

To conclude, since both (1) ion flows and (2) the direction of the vector are opposite during repolarization, there is no net effect on the ECG curve and the T-wave will be concordant with the QRS complex.
总之，由于复极化期间（1）离子流和（2）向量方向均相反，因此对心电图曲线没有净效应，T 波将与 QRS 波群方向一致。

The T-wave vector is normally directed forward and slightly to the left and downwards. However, children and adolescents may have a T-wave vector directed more to the left and backward, which yields negative T-waves in the right-sided chest leads (V1–V4). These become normalized (i.e. positive) during puberty. Note, however, that a negative T-wave in lead V1 is a common finding and it is concordant with the QRS complex, which is generally negative in V1 (these aspects are discussed later).
T 波矢量通常指向前方，略偏左下方。然而，儿童和青少年的 T 波矢量可能更多地指向左后方，这会导致右侧胸导联（V1–V4）出现负 T 波。这些在青春期期间会恢复正常（即变为正向）。但需注意，V1 导联出现负 T 波是常见现象，并且与 QRS 波群一致，QRS 波群在 V1 导联通常为负（这些内容将在后文讨论）。

Finally, note that the P-wave and T-wave are smooth waves, whereas the QRS complex has sharp spikes. This is due to the fact that P- and T-waves are generated by low-frequency signals, whereas the QRS complex has a much higher frequency.
最后，注意 P 波和 T 波是平滑波形，而 QRS 波群则具有尖锐的峰值。这是因为 P 波和 T 波由低频信号产生，而 QRS 波群的频率则高得多。