4)Normal and abnormal cardiac electrical axis (Fig.1-10) 正常和异常的心电轴（图1-10）

Average QRS axis: The frontal plane QRS axis represents the average direction of ventricular depolarization forces in the frontal plane. The QRS axis can be detected by the six individual frontal plain ECG leads. The diagram below shows that the normal range of QRS axis is shaded pink (Fig.1-10,-30°to +90°). For a healthy adult, left axis deviation (i.e., superior, leftward arrow) is defined from-30°to -90°, and right axis deviation (i.e., inferior, rightward arrow) is defined from +90°to±180°. From -90°to±180° is uncertain electrical axis and may be due to leading placement error.

The causes of left axis deviation: left ventricular hypertrophy, left anterior fascicular block,et al.

The causes of right axis deviation: right ventricular hypertrophy, left posterior fascicular block, et al.

平均QRS电轴：即心室激动过程中，QRS综合向量（电轴）在额面上的方位。利用额面六轴系统进行测量QRS电轴。正常成人电轴分布在-30°～+90°(图1-10，粉红色阴影标示)；电轴左偏（电轴朝上朝左）的范围在-30°～-90°；电轴右偏（电轴朝下朝右）的范围在+90°～±180°；电轴在-90°～±180°则为不确定电轴或者连接放置错误。

心电轴左偏的原因：左心室肥厚、左前分支阻滞等。

心电轴右偏的原因：右心室肥厚、左后分支阻滞等。

Einthoven’s Triangle: Each of the six frontal“limb”leads has a negative and positive pole (as indicated by the “+” and “-” signs), such as lead Ⅰ（right-to-left in direction）.

Einthoven三角：“+”和“-”代表额面6个导联的正负方向，比如Ⅰ导联方向是从右至左。

Fig.1-10　Normal QRS axis: -30°～ +90°

图1-10　正常QRS电轴：-30°～+90°

Examples of QRS axis deviation (Fig.1-11 and Fig.1-12) QRS电轴偏移举例（图1-11和图1-12）

Note that Lead Ⅰ is positive, LeadsⅡ and Ⅲ are mostly negative.

Ⅰ导联主波方向为正，Ⅱ和Ⅲ导联主波方向为负。

Fig.1-11　Left axis deviation（LAD）：-30°～-90°

图1-11　心电轴左偏：-30°～-90°

Note that Lead Ⅰ is mostly negative, Lead 　Ⅲ is mostly positive.

Ⅰ导联主波方向为负，Ⅲ导联主波方向为正。

Fig.1-12　Right axis deviation (RAD): +90°～+180°

图1-12　心电轴右偏：+90°～+180°