Modified limb lead ECG system effects on electrocardiographic wave amplitudes and frontal plane axis in sinus rhythm subjects.

OBJECTIVE: Modified Limb Lead (MLL) ECG system may be used during rest or exercise ECG, or atrial activity enhancement. Because of modification in the limb electrode placement, changes are likely to happen in ECG wave amplitudes and frontal plane axis, which may alter the clinical limits of normality and ECG diagnostic criteria. The present study investigated the effects of the modified limb electrode position on the electrocardiographic waveforms, ST segment amplitudes (STa) and frontal plane axis.

METHODS: The observational study included sixty sinus rhythm subjects of mean age 38.85±8.76 (SD) in the range 25 to 58 years. In addition to 12-lead ECG, MLL ECG was recorded with, the RA electrode placed in the 3rd right intercostal space to the right of the parasternal line, the LA electrode placed in the 5th right intercostal space to the right of the mid-clavicular line and the LL electrode placed in the 5th right intercostal space on the mid-clavicular line.

RESULTS: The modification produced profound changes in ECG wave amplitudes and STa amplitudes in frontal plane leads. The QRS and T wave axis shifted on the average by -17o and 41o, respectively, with considerable individual variation, which altered the diagnostic criteria.

CONCLUSION: The ECG amplitudes and STa changes produced by the MLL system showed that all remains within the clinical limits, except the R wave amplitude in the modified lead I. It is evident that the MLL system produced deviations in frontal plane QRS axis which altered the diagnostic interpretation.