Lead systems for ECG monitoring

Simple 3-electrode bipolar lead monitoring
Leads that can be monitored using this system are:
Lead I
Lead II
Lead III
Modified chest lead such as MCL1 in – which the indifferent electrode (RL) is placed in the region of left shoulder and active electrodes are placed in the locations of V1 (LA) and V6 (RA).
“True” V1 lead is not available in this simple 3 electrode system. This simple system is used in Used in portable monitor defibrillators. It is useful to track heart rate, detect R waves for synchronized DC shock and to detect ventricular fibrillation. But it is inadequate for sophisticated arrhythmia monitoring and ST-segment monitoring.
5-electrode lead system
All the four limb electrodes are placed on the torso and an additional electrode is placed in one of the chest lead positions. This system can record any of the 6 limb leads + 1 precordial lead. “True” V1 lead is available. Lead V1 is considered the best lead for diagnosing RBBB and LBBB, which is useful to confirm proper right ventricular location of the electrode in temporary pacing and to distinguish ventricular tachycardia from supraventricular tachycardia with aberrancy. But V 1 insensitive for detecting acute myocardial ischemia. In this lead system, more than one precordial lead cannot be displayed at the same time.
10-electrode Mason-Likar 12-lead ECG system
Four limb electrodes are placed on the torso and 6 chest electrodes are placed in the conventional v1 to v6 positions. In this system, more than one chest leads can be displayed simultaneously. In the Mason-Likar 12-lead ECG system, limb lead QRS is slightly different in amplitude and axis from the conventional ECG because the electrodes are on torso. Precordial leads also may vary slightly because of alteration in the Wilson central terminal with the change in the location of the limb electrodes. Hence there should be caution while comparing with a standard 12 lead.
Major advantage of Mason-Likar 12-lead ECG system
ST-segment monitoring software analyzes all 12 leads and sounds alarm for ST changes, even if all leads are not being displayed.
Disadvantages of Mason-Likar 12-lead ECG system
6 precordial electrodes often interfere with diagnostic procedures like echocardiograms and chest x ray and emergency procedures like defibrillation. Precordial sites are difficult to maintain in men with hairy chests and women with large breasts.
EASI monitoring lead system
12-lead ECG is derived from 5 electrodes (EASI + Ground electrode)
Lderived = a(A – I) + b(E – S) + c(A – S)
EASI electrodes are placed on the upper and lower ends of sternum and both sides of the chest. The derived ECG is comparable to standard 12-lead ECG for diagnosis of wide QRS tachycardias and acute myocardial ischemia. It detects ischemia missed by routine leads (V1 & II). Caution for serial comparison with standard 12 lead ECG is needed because the QRS, ST, and T waves may differ between the 2 systems.