1. Electric Syncytium
We know from previous discussions that an electric current is associated with each heart beat, and that this current passes along and through adjacent heart cells. The cardiac cells form an "electric syncytium" i.e. they are a large multicellular conductor of electricity.
2. Transmembrane Potential
The basis of the current recorded by the electrodes of an ECG is the presence of a voltage difference between the outside and inside of the cell membrane. This is called the transmembrane potential (TMP).
The TMP exists in three states:
1. A baseline state in a resting (non stimulated) cardiac cell. This TMP is also called the resting membrane potential (RMP).
2. An inward flow of positive ions into the cardiac cell changes the TMP; this process is called depolarization, and the resulting change in membrane potential is the first part of the action potential. Local current flows at the junction of the depolarized area and the adjacent non-depolarized area allow the depolarization process to spread from one cell to another. A depolarization wave is thus produced that travels from one part of the heart to another part of the heart.
3. An outward flow of positive ions from the cell interior to the outside of the cell membrane (this process is called repolarization) returns the TMP to the RMP value, marking the end of the action potential.
3. V is for Vector
A vector is a concept used in mathematics and physics for 'things' that need both magnitude and direction to describe them completely. Mass and temperature are fully described by their magnitude (i.e. a numeric value), and do not have (or need) a description of their "direction". Mass and temperature are not vectors.
The flow of electric current has both magnitude and direction, and is a vector. A drawing of electric current requires a line whose length represents magnitude and whose orientation in space represents direction.
The inward flow of positive ions into a part of the heart produces a strip or zone of tissue that is depolarized. This depolarization strip is also called a dipole because it has a positive zone and a negative zone, like the poles of a magnet. This dipole wave moves in different directions through the heart i.e it is a vector. We therefore use the term "dipole vector". Some of the current associated with the dipole vector spreads from the heart into the tissues surrounding the heart. This current, and changes in this current, can be detected by measuring devices (electrodes) placed on the body surface.
4. Cartesian Coordinates: Where are You?
How do we describe the position of a point in a three dimensional space? We need a coordinate system. We begin by choosing a point in space for the origin of the coordinate system. In this case the point of origin is the electrical centre of the heart. The next step is to choose three perpendicular axes that pass through the point of origin. These are usually called x, y and z axes. Such a system of axes is called a Cartesian coordinate system. The use of Cartesian coordinates was first introduced by the French philosopher Rene Descartes (1590-1650).
The position of a point (P) in this Cartesian system is described by x, y and z coordinates, i.e. P(x, y, z), which give the perpendicular distance of P from the x, y and z planes that pass through the point of origin. If P is stationary then these coordinates do not change with time. When P moves we need to record time as well, so we end up with four coordinates (x, y, z and t [time]) that define a reference frame.
5. Electrodes and Leads
The current associated with the heart beat is detected by exploring (or measuring) electrodes placed on the body surface.
The electrodes are attached to/on specified/standard areas of the body. Lines drawn between some of the exploring electrodes, or between a exploring electrode and a derived electrode, form the leads (or lead axes) of the ECG. Each lead axis has a positive end and a negative end.
The lead axes can be rearranged to pass through the (electrical) centre of the heart. The lead axes thus form a three dimensional array of straight lines that are a reference frame for the dipole vector. Using x-y-z-t coordinates a moving dipole vector produces a three dimensional tracing called a vectorcardiogram.
The ECG we are familiar with is the projection of this vectorcardiogram onto two planes:
- a horizontal plane (with 6 precordial leads [or lead axes])
- a vertical plane (with 6 frontal leads [or lead axes])
The direction and length of a lead axis depend on the geometry of the body and on the varying electric impedances of the tissues in the torso i.e. the lead axes of the ECG are also lead vectors. The size of the dipole vector, and its orientation to/with the lead vector, determine the size, shape and direction of the cardiac complex seen on a monitor or printed by the ECG machine.
6. Twelve Lead Electrocardiograph
The ‘standard’ ECG has 12 leads or lead axes. All the leads have a positive (+) end and a negative (-) end. The terms “unipolar”” and “bipolar” should be avoided.
Frontal Leads: Six from Four
Four limb lead electrodes are placed on the wrists and the ankles.
Six frontal leads are derived from these four limb electrodes:
1. Three limb leads (leads I, II, and III)
2. Three augmented limb leads in which a derived electrode is paired with an exploring electrode to form leads aVR, aVL, and aVF
Precordial Leads: Six from six
Six electrodes are placed on the chest:
V1: fourth intercostal space at the right sternal border;
V2: fourth intercostal space at the left sternal border;
V3: midway between V2 and V4;
V4: fifth intercostal space in the mid-clavicular line;
V5: in the horizontal plane of V4 at the anterior axillary line, or if the anterior axillary line is ambiguous, midway between V4 and V6;
V6: in the horizontal plane of V4 at the mid-axillary line.
The position of precordial leads seen from the front is shown in the following figures.
Six precordial leads are formed by pairing a derived electrode with the exploring chest electrodes (Leads V1 to V6). In the horizontal plane these six lead axes form a array of leads that pass through the electrical centre of the heart and are called the horizontal hexaxial plane system.