From Ask Dr Wiki
Signal Averaged ECG (SAECG)
- Filtered ECG that is able to detect low amplitude potentials filtered out of standard ECGs.
- Myocardial scar (infarction, ARVD) creates zones of slow conduction that appear as low amplitude late potentials on SAECG. Areas of slow conduction are necessary components for reentry.
- Late potentials from within scar sometimes are not detected in SAECG
- Bundle branch block delays depolarization ipsilateral to the site of block. The delayed conduction may conceal late potentials on SAECG.
- The base of the left ventricle is the last area to depolarize when bundle branch block is not present. Inferior scar is easier to detect on SAECG than anterior scar because the inferior scar boarder zone is activated later than the anterior wall. Therefore, the late potentials are not concealed by depolarization in other areas of the ventricle.
Criteria for abnormal SAECG
- Root mean squared voltage of the terminal 40 msecs is less than 20 microvolts. This shows low voltage potentials late in ventricular depolarization and reflect depolarization in slowly conducting scar boarder zones
- Total QRS duration greater than 114 msec
- Duration of the low amplitude signal that is less than 40 microvolts is greater than 38 msec
Predictive value
- The negative predictive value of SAECG is 97%. The positive predictive value is only 20%.
- It is useful in that a negative test result makes VT as the cause of syncope unlikely.
