Session P94.4

An Algorithm to Estimate the ST Segment Level in 24-Hour Ambulatory ECG Records

A Smrdel*, F Jager

University of Ljubljana
Ljubljana, Slovenia

We developed an algorithm to estimate the ST segment level and to construct the ST segment level functions in 24-hour ambulatory ECG records, a necessary step in automatic detection of transient ischemic ST segment episodes. The algorithm was developed and tested using the records of the Long-Term ST Database (LTST DB), which contains 86 two- and three-channel ambulatory records. The input to the algorithm were raw ECG data and ARISTOTLE's fiducial points (FP) of normal and non-noisy heart beats as determined during development of the LTST DB, and are available to the users of the LTST DB. The algorithm initially constructs an average heart beat for each individual normal and non-noisy heart beat using normal and non-noisy heart beats from the 16-second neighborhood of this current beat. The algorithm then searches for the position of the isoelectric level in each ECG lead of the average heart beat by searching for the "flattest" 20 ms part of the waveform from the FP backwards within 108 ms interval. The flattest part is defined as that which yields minimum sum of the mean absolute deviations taken from its own mean. The final unique position of the isoelectric level for the j-th average heart beat in all leads, I(j), is then defined as that isoelectric point from the leads, which yields the minimum sum of the mean absolute deviations taken over all leads. Next, the algorithm searches for the position of the J point in each lead by searching from the FP forward within 68 ms interval for the part of the waveform which starts to "flatten". The final unique J point in all leads, J(j), is the right most J point of the leads. The ST segment level function is then derived for each ECG lead. The j-th sample of the ST segment level function is derived as the amplitude measured at the point, J(j)+80 ms, of the average heart beat in that lead relative to the amplitude at the position of the isoelectric level, I(j). The aggregate average error between the positions of the J points for 7.832.400 clean and non-noisy heart beats of 86 records of the LTST DB as estimated automatically by the algorithm and those manually determined by human expert annotators was -3.20 ms (std. 5.16 ms). The aggregate average error between the samples of the automatically derived ST segment level functions and those derived on the basis of manually determined positions of the J points for 190 ECG leads of the LTST DB was 0.70 uV (std. 8.84 uV).

(Abstract Control Number: 17)