Analysis of the ST-T complex of the electrocardiogram using the Karhunen-Loeve transform: adaptive monitoring and alternans detection

The Karhunen-Loeve transform (KLT) is applied to study the ventricular repo larisation period as reflected in the ST-T complex of the surface ECG. The KLT coefficients provide a sensitive means of quantitating ST-T shapes. A t raining set of ST-T complexes is used to derive a set of KLT basis vectors that permits representation of 90% of the signal energy using four KLT coef ficients. As a truncated KLT expansion tends to favor representation of the signal over any additive noise, a time series of KLT coefficients obtained from successive ST-T complexes is better suited for representation of both medium-term variations (such as ischemic changes) and short-term variation s (such as ST-T alternans) than discrete parameters such as the ST level or other local indices. For analysis of ischemic changes, an adaptive filter is described that can be used to estimate the KLT coefficient, yielding an increase in the signal-to-noise ratio of 10 dB (u = 0.1), with a convergenc e time of about three beats. A beat spectrum of the unfiltered KLT coeffici ent series is used for detection of ST-T alterans. These methods are illust rated with examples from the European ST-T Database. About 20% of records r evealed quasi-periodic salves of ischemic ST-T change episodes and another 20% exhibit repetitive, but not clearly periodic patterns of ST-T change ep isodes. About 5% of ischemic episodes were associated with ST-T alternans.