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ECG monitoring leads are different from the usual leads used for recording a 12 lead ECG. This is because monitoring leads are used mainly for rhythm analysis and sometimes for ST segment monitoring to look for ongoing ischemia. Any lead which can show the P waves, QRS complexes and T waves well can be used for ECG monitoring. Often the monitoring lead will have to be tailored for each individual. It may be worthwhile to look at the 12 lead ECG and decide on which lead is likely to give the maximum required information for that particular person. Most intensive care units monitor only one lead while some resort to two lead monitoring for better arrhythmia analysis.
All 12 leads are monitored during an electrophysiology study as well as during a treadmill exercise test. Several types of lead modifications are used depending on the situation. In the coronary care unit, modified lead II is usually used. Modification is that the limb electrodes are placed on the proximal parts of the limbs or adjacent torso than over the distal part of the extremity in order to reduce artefacts due to limb movements. Most modern monitors have the option to use chest leads in addition to limb leads. When chest lead monitoring is needed, additional electrodes are placed on the chest.
The location of the leads depends on the type of patient. While anterior locations are available in the coronary care patient, a surgical patient may require different locations depending on the location of the surgical incision and dressings. Esophageal leads may be used in some rare situations to decipher difficult arrhythmias.
Mason-Likar modification of 12 lead ECG is most popular during treadmill exercise test. It can also be used in the electrophysiology laboratory. In this lead system, limb electrodes are placed on the nearest location on the torso to prevent movement artefacts during exercise. Chest electrodes are placed in the conventional positions. Difference in pattern of the modified 12 lead ECG mandates caution while trying to interpret Q waves and other abnormalities on a Mason-Likar modification. The QRS axis has a rightward shift due to upward shift of the lower limb electrodes. But there is no difficulty in interpreting ST segment shift and arrhythmias during a treadmill test with this lead system.
Lewis lead is a modified lead to enhance the amplitude of P waves and thereby enable better arrhythmia analysis. Lewis lead is a modified lead I in which the right arm electrode is placed in the second right intercostal space close to sternum and left arm lead is placed in the fourth right intercostal space close to sternum (V1 lead position). This lead system is useful in detecting P waves during a wide QRS tachycardia and helps in differentiating between ventricular and supraventricular tachycardia.