## Saturday, 26 March 2011

### ECG graph paper

The output of an ECG recorder is a graph (or sometimes several graphs, representing each of the leads) with time represented on the x-axis and voltage represented on the y-axis. A dedicated ECG machine would usually print onto graph paper which has a background pattern of 1mm squares (often in red or green), with bold divisions every 5mm in both vertical and horizontal directions. It is possible to change the output of most ECG devices but it is standard to represent each mV on the y axis as 1 cm and each second as 25mm on the x-axis (that is a paper speed of 25mm/s). Faster paper speeds can be used - for example to resolve finer detail in the ECG. At a paper speed of 25 mm/s, one small block of ECG paper translates into 40 ms. Five small blocks make up one large block, which translates into 200 ms. Hence, there are five large blocks per second. A calibration signal may be included with a record. A standard signal of 1 mV must move the stylus vertically 1 cm, that is two large squares on ECG paper.
Layout

By definition a 12-lead ECG will show a short segment of the recording of each of the 12-leads. This is often arranged in a grid of 4 columns by three rows, the first columns being the limb leads (I,II and III), the second column the augmented limb leads (aVR, aVL and aVF) and the last two columns being the chest leads (V1-V6). It is usually possible to change this layout so it is vital to check the labels to see which lead is represented. Each column will usually record the same moment in time for the three leads and then the recording will switch to the next column which will record the heart beats after that point. It is possible for the heart rhythm to change between the columns of leads.

Each of these segments is short, perhaps 1-3 heart beats only, depending on the heart rate and it can be difficult to analyse any heart rhythm that shows changes between heart beats. To help with the analysis it is common to print one or two "rhythm strips" as well. This will usually be lead II (which shows the electrical signal from the atrium, the P-wave, well) and shows the rhythm for the whole time the ECG was recorded (usually 5–6 seconds). Some ECG machines will print a second lead II along the very bottom of the paper in addition to the output described above. This printing of Lead II is continuous from start to finish of the process.

The term "rhythm strip" may also refer to the whole printout from a continuous monitoring system which may show only one lead and is either initiated by a clinician or in response to an alarm or event.

The term "lead" in electrocardiography causes much confusion because it is used to refer to two different things. In accordance with common parlance the word lead may be used to refer to the electrical cable attaching the electrodes to the ECG recorder. As such it may be acceptable to refer to the "left arm lead" as the electrode (and its cable) that should be attached at or near the left arm. There are usually ten of these electrodes in a standard "12-lead" ECG.

Alternatively (and some would say properly, in the context of electrocardiography) the word lead may refer to the tracing of the voltage difference between two of the electrodes and is what is actually produced by the ECG recorder. Each will have a specific name. For example "Lead I" (lead one) is the voltage between the right arm electrode and the left arm electrode, whereas "Lead II" (lead two) is the voltage between the right limb and the feet. (This rapidly becomes more complex as one of the "electrodes" may in fact be a composite of the electrical signal from a combination of the other electrodes (see later). Twelve of this type of lead form a "12-lead" ECG

To cause additional confusion the term "limb leads" usually refers to the tracings from leads I, II and III rather than the electrodes attached to the limbs.
Placement of electrodes

Ten electrodes are used for a 12-lead ECG. The electrodes usually consist of a conducting gel, embedded in the middle of a self-adhesive pad onto which cables clip. Sometimes the gel also forms the adhesive.[12] They are labeled and placed on the patient's body as follows:[13][14]
Proper placement of the limb electrodes, color coded as recommended by the American Heart Association (a different colour scheme is used in Europe). Note that the limb electrodes can be far down on the limbs or close to the hips/shoulders, but they must be even (left vs right).[15]
Electrode label (in the USA)     Electrode placement
RA     On the right arm, avoiding thick muscle.
LA     In the same location that RA was placed, but on the left arm this time.
RL     On the right leg, lateral calf muscle
LL     In the same location that RL was placed, but on the left leg this time.
V1     In the fourth intercostal space (between ribs 4 & 5) just to the right of the sternum (breastbone).
V2     In the fourth intercostal space (between ribs 4 & 5) just to the left of the sternum.
V3     Between leads V2 and V4.
V4     In the fifth intercostal space (between ribs 5 & 6) in the mid-clavicular line (the imaginary line that extends down from the midpoint of the clavicle (collarbone)).
V5     Horizontally even with V4, but in the anterior axillary line. (The anterior axillary line is the imaginary line that runs down from the point midway between the middle of the clavicle and the lateral end of the clavicle; the lateral end of the collarbone is the end closer to the arm.)
V6     Horizontally even with V4 and V5 in the midaxillary line. (The midaxillary line is the imaginary line that extends down from the middle of the patient's armpit.)

The classical 12-lead ECG can be extended in a number of ways in an attempt to improve its sensitivity in detecting myocardial infarction involving territories not normally "seen" well. This includes an rV4 lead which uses the equivalent landmarks to the V4 but on the right side of the chest wall and extending the chest leads onto the back with a V7, V8 and V9.

In both the 5- and 12-lead configuration, leads I, II and III are called limb leads. The electrodes that form these signals are located on the limbs—one on each arm and one on the left leg.[16][17][18] The limb leads form the points of what is known as Einthoven's triangle.[19]

* Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode:

I = LA − RA.

* Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA) electrode:

II = LL − RA.

* Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA) electrode:

III = LL − LA.

Simplified electrocardiograph sensors designed for teaching purposes at e.g. high school level are generally limited to three arm electrodes serving similar purposes. [20]

There are two types of leads: unipolar and bipolar. Bipolar leads have one positive and one negative pole.[21] In a 12-lead ECG, the limb leads (I, II and III) are bipolar leads. Unipolar leads also have two poles, as a voltage is measured; however, the negative pole is a composite pole (Wilson's central terminal, or WCT) made up of signals from lots of other electrodes.[22] In a 12-lead ECG, all leads besides the limb leads are unipolar (aVR, aVL, aVF, V1, V2, V3, V4, V5, and V6).

Wilson's central terminal VW is produced by connecting the electrodes, RA; LA; and LL, together, via a simple resistive network, to give an average potential across the body, which approximates the potential at infinity (i.e. zero):

V_W = \frac{1}{3}(RA+LA+LL).

Leads aVR, aVL, and aVF are augmented limb leads (after their inventor Dr. Emanuel Goldberger known collectively as the Goldberger's leads). They are derived from the same three electrodes as leads I, II, and III. However, they view the heart from different angles (or vectors) because the negative electrode for these leads is a modification of Wilson's central terminal. This zeroes out the negative electrode and allows the positive electrode to become the "exploring electrode". This is possible because Einthoven's Law states that I + (−II) + III = 0. The equation can also be written I + III = II. It is written this way (instead of I − II + III = 0) because Einthoven reversed the polarity of lead II in Einthoven's triangle, possibly because he liked to view upright QRS complexes. Wilson's central terminal paved the way for the development of the augmented limb leads aVR, aVL, aVF and the precordial leads V1, V2, V3, V4, V5 and V6.

* Lead augmented vector right (aVR) has the positive electrode (white) on the right arm. The negative electrode is a combination of the left arm (black) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the right arm:

aVR = RA - \frac{1}{2} (LA + LL).

* Lead augmented vector left (aVL) has the positive (black) electrode on the left arm. The negative electrode is a combination of the right arm (white) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the left arm:

aVL = LA - \frac{1}{2} (RA + LL).

* Lead augmented vector foot (aVF) has the positive (red) electrode on the left leg. The negative electrode is a combination of the right arm (white) electrode and the left arm (black) electrode, which "augments" the signal of the positive electrode on the left leg:

aVF = LL - \frac{1}{2} (RA + LA).

The augmented limb leads aVR, aVL, and aVF are amplified in this way because the signal is too small to be useful when the negative electrode is Wilson's central terminal. Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system, which is used to calculate the heart's electrical axis in the frontal plane. The aVR, aVL, and aVF leads can also be represented using the I and II limb leads:

\begin{align} aVR &= -\frac{I + II}{2}\\ aVL &= I - \frac{II}{2}\\ aVF &= II - \frac{I}{2} \end{align}