12-Lead ECG Analysis Program (12SL
TM
)
227 490 02-C
Marquette Responder® 3000
47
Median Beat, Signal Averaging
Computer measurement for features within the
QRS complex are very susceptible to artifact. To
remove artifact, filtering may be used. Beyond
filtering, there is another method able to eliminate
noise from the QRS complex: signal averaging.
Instead of analyzing a single QRS complex, the 12
SL program generates a median complex. In other
words, all QRS complexes of the same shape are
aligned in time. Next, the algorithm generates a
representative QRS complex from the median
voltages that are found at each successive sample
time. This is more complicated than creating an
average. The method results in a cleaner signal
since it is able to disregard outliers.
Comparisons of all averaging against all non-
averaging programs yields a 70% higher detection
instability for the non-averaging programs and
worse performance in most tested measurements.
Measurements, Onsets and Offsets
All ECG computer programs are composed of two
parts: one which measures the waveforms and one
which does the analysis based upon those
measurements. The main task of computerized
measurement determines the location of major
reference points (onsets and offsets for P, QRS,
and T waves).
Consistent with the signal processing portion of
the program, the major wave onsets and offsets are
delineated by an analysis of the slopes in all 12
simultaneous leads. That is, QRS duration is
measured from the earliest onset in any lead to the
latest deflection in any lead. Similarly, the QT
interval is measured from the earliest detection of
depolarization in any lead to the latest detection of
repolarization in any lead.
After the onsets and offsets for P, QRS, and T
complexes have been demarcated, the waves
within each complex are then measured according
to published standards. These amplitudes and
durations result in a measurement matrix contain-
ing more than 1600 values. Measurements are then
passed onto the criteria portion of the program so
that it can generate an interpretation.
Literature
"Marquette Hellige 12SL
TM
ECG Analysis
Program",
PN MO 1417 DE 0, English
