ECG: basic electrocardiography

Principle

An electrocardiogram or ECG is a graphic representation of the electrical activity of the heart over time produced by an electrocardiograph. Understanding the various waves and normal vectors of depolarization and repolarization yields important diagnostic information.

Calibration

A typical electrocardiograph (or PC monitor) runs mostly at a paper speed of 25 mm/s. With a paper ECG, the finest division is a block of 1 mm² and 25 mm/s represents 1 mm/40 ms. A diagnostic quality 12-lead ECG is calibrated at 10 mm/mV.

Filter selection

Modern ECG monitors offer multiple filters for signal processing with a monitor mode and diagnostic mode. In monitor mode, the low frequency filter (also called the high-pass filter: signals above the threshold or cut-off frequency are allowed to pass) is set at either 0.5 Hz or 1 Hz. The high frequency filter (the low-pass filter) is set at 40 Hz (see Linear filters). The high-pass filter reduces wandering of the baseline and the low pass filter reduces high frequency noise and 50 or 60 Hz power line hum. (Hum can also be suppressed by a selective band pass filter, a T-filter). In diagnostic mode, the high pass filter is set at 0.05 Hz, allowing accurate ST segments. The high low pass filter is set to 40, 100, or 150 Hz. Consequently, the monitor mode ECG display is more filtered than diagnostic mode.

Waves and intervals of the ECG

Fig. 1 Schematic representation of normal ECG

The baseline voltage of the electrocardiogram is known as the isoelectric line. A typical ECG tracing of a normal heartbeat (or cardiac cycle) consists of a P wave, a QRS complex and a T wave. A small U wave is normally visible in 50 to 75% of ECGs. For a detailed description the reader is referred to the textbooks of clinical physiology.

Working principle of electrodes

Fig. 2 Relationship between positive electrodes (green circles) and the propagating depolarization wavefronts (at the right).

An ECG is obtained by measuring electrical potentials between various points of the body using a biomedical instrumentation amplifier. A lead, records the electrical signals of the heart from a particular combination of recording electrodes which are placed at specific points on the body.

When a depolarization wavefront (or electrical vector) moves toward and away a positive electrode, it creates a positive and negative deflection in the corresponding lead respectively as depicted in Fig. 1.

When a depolarization wavefront (or electrical vector) moves perpendicular to a positive electrode, it creates an equiphasic (or isoelectric) complex. It will be positive as the depolarization wavefront (or mean electrical vector) approaches (A), and then become negative as it passes by (B).

Standard ECG leads

Fig. 3 Lead II

The basic three bipolar limb leads Leads I, II (Fig. 3) and III are the so-called bipolar limb leads because electrodes are attached to the arms and legs forming the Einthoven's triangle. All three electrodes are ‘active’ and there is no reference electrode. Therefore the leads are bipolar. Einthoven, who discovered the ECG, placed legs and arms in buckets of salt water (Fig. 4). They are the first three leads of the modern 12 lead-ECG (see ECG: 12-lead ECG).

Fig. 4 ECG as done by Willem Einthoven based on electromechanical technology with a wire galvanometer as sensor.

The bipolar (standard) leads

The electrodes are attached as follows:

• lead I = left arm positive minus right arm negative (LA–RA)

• lead II = left leg positive minus right arm negative (LL–RA). • lead III = left leg positive minus left arm negative (LL–LA).

Application

· It is the gold standard for the evaluation of cardiac arrhythmias

· It guides therapy and risk stratification for patients with suspected acute myocardial infarction.

· It helps detect electrolyte disturbances (e.g. hyperkalemia and hypokalemia)

· It allows for the detection of conduction abnormalities (e.g. right and left bundle branch block)

· It is used as a screening tool for ischemic heart disease during a cardiac stress test

· It is occasionally helpful with non-cardiac diseases (e.g. pulmonary embolism or hypothermia)

However, the electrocardiogram does not assess the contractility of the heart.

More info

Lead II should be equal to the sum of leads I and III, so I + III = II. This is called Einthoven’s Law. It is written this way (instead of I + II + III = 0) because Einthoven reversed the polarity of lead II in Einthoven's triangle. Then, QRS complexes are upright.

The position from which the heart is viewed by each of these leads is shown in Figure 5.

Fig. 5 Diagram of the effective position of the bipolar (standard) leads. See text for interpretation.

In lead I the right arm electrode feeds in the negative input of the amplifier, so the vector obtains a negative sign (-ve). The diametrically position (‘-Right arm’) inverts the negative sign to positive. Now this vector can be added to +ve, the left arm vector. Finally the vector midway between these two results in the vector of lead I. Similar principles can be applied to derive the effective direction of the leads II and III.