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A basic knowledge of the anatomy of the heart and conduction system will help you better understand the electrical forces from which the electrocardiogram (ECG) is derived.

First, let’s review some anatomical terminology. We are accustomed to looking at the body in three standard anatomical planes—the frontal (coronal), horizontal (transverse, cross-sectional, or axial), and sagittal (longitudinal) (Figure 2-1). We can use these planes to describe the direction of electrical forces that comprise the ECG as anterior or posterior (frontal), superior or inferior (horizontal), and right or left (sagittal). Remember that right and left always refer to the patient’s right and left, not to the direction viewed from your perspective. Interpreting ECGs would be a lot simpler if the heart conformed to these anatomical planes, but unfortunately, it doesn’t. Let’s take a closer look.

Figure 2-1.

Anatomical planes separate one half of the body from the other. The frontal plane separates anterior (front) from posterior (back), the horizontal plane separates superior (upper) from inferior (lower), and the sagittal plane separates right from left.


The midline sagittal plane that divides the body into two equal right and left sides is termed the median plane.

The heart is about the size of your fist and lies in the middle of the thorax. It sits behind the sternum, adjoining the third to sixth costal cartilages, between the lungs and above the diaphragm. The long and short axes of the heart are oriented obliquely compared with the rest of the body such that about two-thirds of the heart’s mass actually lies left of the midline (Figure 2-2). The geometry of the heart is described as a truncated ellipsoid, with the long axis, from base to apex, rotated 60 degrees leftward from the sagittal plane and 45 degrees inferior from the horizontal plane. “What the heck does that mean?” you ask.

Figure 2-2.

The axes of the heart are oriented obliquely with the rest of the body.

You can understand this orientation more easily by thinking of the heart as shaped like a foam football (Figure 2-3). If you were to cut a section off the back end, the intact nose of the football would represent the apex of the heart and the flat end the base. Hold the flat end of the football against your sternum with the nose away from you and the laces facing straight up to the ceiling. Turn the nose of the ball in an arc 60 degrees toward your left, still keeping the laces pointed up. Now tilt the nose of the football in an arc 45 degrees straight down toward your left foot. This is a reasonable approximation of the ...

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