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<p>
An initial set of beat labels was produced by a slope-sensitive QRS detector,
which marked each detected event as a normal beat. Each two-hour, two-channel
ECG record was printed out in full disclosure format, each page two minutes
in duration, with the addition of QRS detection marks, trend plots of ST
segment displacement and T-wave amplitude (measured for each beat), and boxes
for checking annotation operations. For each record, two cardiologists
(neither of whom was a member of the research group which had submitted the
record) were given copies of the full-disclosure printout, trend plots of
mean heart rate and ST-T parameters at 10-second intervals, and record-specific
transparent plastic rulers for measuring time intervals and ECG signal
displacements. A heart rate scale and a two-channel
<a href=#reference-beat>reference QRST complex</a>
(taken from the first 30 seconds of each record) were printed on each ruler.</p>
<p>
Working independently, the cardiologist-annotators visually checked the
computer-generated beat labels on the full-disclosure printouts and manually
corrected them, and inserted annotations indicating changes in ST and T
morphology, rhythm, and signal quality. Annotations from the two cardiologists
were compared and the differences were resolved by a cardiologist of the
coordinating group. This method assumes that the third cardiologist is able
to make a more reliable judgement since he knows both sets of annotations.</p>
<h2>Definition of ST and T episodes</h2>
<p>
The cardiologists participating in the project jointly defined and followed
a set of rules for locating <i>ST episodes</i> and <i>T episodes</i> (i.e.,
intervals during which the ECG exhibits significant ST segment or T-wave
changes). To identify and annotate an ST episode, these criteria were
applied:</p>
<ul>
<li> <i>ST segment deviations</i> are measured relative to a reference
waveform for each subject (usually selected from the first 30 seconds of
each record). Measurements of ST segment deviation are taken 80 milliseconds
after the J point if the heart rate does not exceed 120 bpm, and 60
milliseconds after the J point otherwise.</li>
<li> <i>ST episodes</i> must contain an interval of at least 30 seconds during
which the absolute value of the ST deviation is no less than 0.1 millivolt
(mV).</li>
<li> The <i>beginning</i> of each ST episode is annotated. The beginning is
located by searching backward from the time at which the absolute ST deviation
first exceeds 0.1 mV. The search continues until a beat is found for which
the absolute ST deviation is less than 0.05 mV, and for which the absolute
ST deviation is less than 0.1 mV throughout the previous 30 seconds. An
ST change annotation which indicates the beginning of the episode is placed
immediately following this beat.</li>
<li> The <i>peak</i> (i.e., the greatest deviation, positive or negative) of
each ST episode is annotated. An ST change annotation is placed before the
beat judged to exhibit the greatest ST deviation; this annotation contains
a manual measurement of the peak ST deviation.</li>
<li> The <i>end</i> of each ST episode is annotated. The end is
located by searching forward from the time at which the absolute ST deviation
last exceeds 0.1 mV. The search continues until a beat is found for which
the absolute ST deviation is less than 0.05 mV, and for which the absolute
ST deviation is less than 0.1 mV throughout the following 30 seconds. An
ST change annotation which indicates the end of the episode is placed
immediately before this beat.</li>
</ul>
<p>To identify and annotate a T episode, similar criteria were applied:</p>
<ul>
<li> <i>T deviations</i> are measured relative to the same reference
waveform which is used for measuring ST deviations. The quantity
<i>A<sub><font size=-1>T</font></sub></i> is defined as the amplitude
of the dominant phase of the T-wave, measured relative to baseline (at
the PQ junction); if the T-wave is inverted, or if the dominant phase
of a biphasic T-wave is below the baseline, <i>A<sub><font
size=-1>T</font></sub></i> is negative. The T deviation is defined as
the difference (positive or negative) between the values of
<i>A<sub><font size=-1>T</font></sub></i> for the current waveform and
for the reference waveform.</li>
<li> <i>T episodes</i> must contain an interval of at least 30 seconds during
which the absolute value of the T deviation is no less than 0.2 mV.</li>
<li> The <i>beginning</i> of each T episode is annotated. The beginning is
located by searching backward from the time at which the absolute T deviation
first exceeds 0.2 mV. When an interval of at least 30 seconds is found in
which the absolute T deviation does not exceed 0.2 mV, the end of that interval
defines the beginning of the episode. A T change annotation is placed before
the first beat of the episode.</li>
<li> The <i>peak</i> (i.e., the greatest deviation, positive or negative) of
each T episode is annotated. A T change annotation is placed before the
beat judged to exhibit the greatest T deviation; this annotation contains
a manual measurement of the peak T deviation.</li>
<li> The <i>end</i> of each T episode is annotated. The end is
located by searching forward from the time at which the absolute T
deviation last exceeds 0.2 mV. When an interval of at least 30
seconds is found in which the absolute T deviation does not exceed 0.2
mV, the beginning of that interval defines the end of the episode. A
T change annotation is placed after the last beat of the episode.</li>
<li> Within T episodes which contain absolute T deviations exceeding 0.4 mV,
additional T change annotations are placed whenever the absolute T deviation
crosses the 0.4 mV threshold value which defines <i>extreme T deviations</i>.
These additional T change annotations indicate the beginning and end of each
such interval of extreme T deviation.</li>
</ul>
<p>
These rules were applied to each of the two signals independently; for this
reason, each ST and T change annotation indicates the signal to which it
applies.</p>
<p>
Each ST and T change annotation contains a text field which describes its
significance. The text field contains characters which identify the episode
type ('ST' or 'T'), the signal number ('0' or '1'), and the direction of the
deviation ('+' or '-'; extreme T deviations are signified by '++' and '--').
The text field of an annotation which marks the beginning of an episode
contains a '(' prefix. For an annotation which marks the end of an episode,
there is a prefixed 'A' and an appended 3- or 4-digit decimal number which
expresses the magnitude of the peak deviation in microvolts. An annotation
which marks the end of an episode has a ')' appended to the end of its text
field. For example, an episode of ST depression in signal 0 with a peak
(absolute) deviation of 200 microvolts would be marked by three annotations,
with text fields of '(ST0-', 'AST0-200', and 'ST0-)'.</li>
<p>
In six records (e0161, e0509, e0601, e0611, e0613, and e0615), axis shifts
resulting from positional change give the appearance of real ST or T changes.
These axis shifts are annotated using comment annotations. The text fields
of these annotations are constructed in the same way as for ST and T change
annotations, except that lower-case characters are used in order to make it
easier to distinguish these axis shift episodes from real ST or T change
episodes. For example, an axis shift in signal 1 which gives the appearance
of a peak T deviation of 350 microvolts would be marked by three annotations,
with text fields of '(t1+', 'at1+350', and 't1+)'.</li>
<h3>Annotation types</h3>
<p>
The following types of annotations appear in the European ST-T Database
reference (<tt>.atr</tt>) annotation files. The <b>Code</b> column shows
the symbols defined in
<a href="/physiotools/wfdb/lib/ecgcodes.h">ecgcodes.h</a>, and the
<b>Mnemonic</b> column indicates how these annotations are displayed by
WFDB applications such as <a href="/physiotools/dbag/wave-1.htm">WAVE</a>,
<a href="/physiotools/dbag/wview-1.htm">WVIEW</a>, and
<a href="/physiotools/dbag/pschar-1.htm">pschart</a>.</p>
<div class="edbtable">
<table>
<th class="headerrow">Code</th><th>Mnemonic</th>
<th>Meaning</th></tr>
<tr><td>NORMAL</td><td>• [bullet]</td><td>Normal beat</td></tr>
<tr><td>ABERR</td><td>a</td><td>Aberrated atrial premature beat</td></tr>
<tr><td>NPC</td><td>J</td><td>Nodal (junctional) premature beat</td></tr>
<tr><td>SVPB</td><td>S</td><td>Supraventricular premature or ectopic beat
(atrial or nodal)</td></tr>
<tr><td>PVC</td><td>V</td><td>Premature ventricular contraction</td></tr>
<tr><td>FUSION</td><td>F</td><td>Fusion of ventricular and normal beat</td>
</tr>
<tr><td>UNKNOWN</td><td>Q</td><td>Unclassifiable beat</td></tr>
<tr><td>ARFCT</td><td>|</td><td>Isolated QRS-like artifact</td></tr>
<tr><td valign=top>RHYTHM</td><td></td>
<td>Rhythm change, specified by text field:<br>
<table>
<tr><td>(AB</td><td>Atrial bigeminy</td></tr>
<tr><td>(AFIB</td><td>Atrial fibrillation</td></tr>
<tr><td>(B</td><td>Ventricular bigeminy</td></tr>
<tr><td>(B3</td><td>Third degree heart block</td></tr>
<tr><td>(N</td><td>Normal sinus rhythm</td></tr>
<tr><td>(SAB</td><td>Sino-atrial block</td></tr>
<tr><td>(SBR</td><td>Sinus bradycardia</td></tr>
<tr><td>(SVTA</td><td>Supraventricular tachyarrhythmia</td></tr>
<tr><td>(T</td><td>Ventricular trigeminy</td></tr>
<tr><td>(VT</td><td>Ventricular tachycardia</td></tr>
</table></td></tr>
<tr><td valign=top>STCH</td><td></td>
<td>ST change, specified by text field:<br>
<table>
<tr><td>(ST...</td><td>Beginning of ST episode</td></tr>
<tr><td>AST...</td><td>Peak of ST episode</td></tr>
<tr><td>ST...)</td><td>End of ST episode</td></tr>
</table></td></tr>
<tr><td valign=top>TCH</td><td></td>
<td>T change, specified by text field:<br>
<table>
<tr><td>(T...</td><td>Beginning of T episode</td></tr>
<tr><td>AT...</td><td>Peak of T episode</td></tr>
<tr><td>T...)</td><td>End of T episode</td></tr>
</table></td></tr>
<tr><td valign=top>NOTE</td><td></td>
<td>Comment annotation, specified by text field:<br>
<table>
<tr><td>(st...</td>
<td>Beginning of ST deviation resulting from axis shift</td></tr>
<tr><td>ast...</td>
<td>Peak of ST deviation resulting from axis shift</td></tr>
<tr><td>st...)</td>
<td>End of ST deviation resulting from axis shift</td></tr>
<tr><td>(t...</td>
<td>Beginning of T deviation resulting from axis shift</td></tr>
<tr><td>at...</td>
<td>Peak of T deviation resulting from axis shift</td></tr>
<tr><td>t...)</td>
<td>End of T deviation resulting from axis shift</td></tr>
<tr><td>BUTTON</td>
<td>Patient-activated event button pressed</td></tr>
<tr><td>TS</td>
<td>Tape slippage</td></tr>
</table></td></tr>
<tr><td valign=top>NOISE</td><td valign=top><i>qq</i></td>
<td>Signal quality change: the first character ('c', 'n', or 'u')
indicates the quality of signal 0 (clean, noisy, or unreadable),
and the second character indicates the quality of signal 1.
The <tt>subtyp</tt> field of the annotation encodes these
characters:<br>
<table>
<tr><td>0x00</td><td>cc</td>
<tr><td>0x01</td><td>nc</td>
<tr><td>0x02</td><td>cn</td>
<tr><td>0x03</td><td>nn</td>
<tr><td>0x11</td><td>uc</td>
<tr><td>0x12</td><td>un</td>
<tr><td>0x20</td><td>cu</td>
<tr><td>0x21</td><td>nu</td>
<tr><td>0x33</td><td>uu</td>
</table></td></tr>
</table>
</div> <!-- end edbtable -->
<h3 id="reference-beat">Which was the reference beat in each record?</h3>
<p>
As noted above, the expert annotators were given a clear plastic
template on which had been printed a reference waveform. The position
of this waveform was not recorded, however, and the original plastic
templates no longer exist. The only available information about the
choice of reference beat is that the waveform was taken from the first
30 seconds of the record being annotated. One may assume that the
waveform was typical of those within the 30-second interval, and that
if the amount of noise varied significantly within the interval, the
reference was one of the cleaner waveforms.</p>
<p>
As a practical matter for evaluation of an algorithm for automated ST
analysis using this database, this question need not be an issue.
Some of the patients represented in the database had prior myocardial
infarctions with consequent fixed ST elevation or depression. The ST
annotations in this database mark transient ST changes that are
superimposed on any fixed elevation or depression. The important
point is that this database's annotations provide samples not of
the <em>ST level</em> function (the difference, for any given time,
between the ECG amplitudes of the nearest beat during the ST segment
and at the isoelectric point), but of the <em>ST deviation</em>
function (the difference between the ST level function measured at any
given time and during the first 30 seconds of the record).
Put another way, the ST level function is the sum of the fixed
elevation or depression (the reference ST level) and the transient
changes in ST level (the ST deviation function).</p>
<p>
To use this database to evaluate an ST analysis algorithm, the
algorithm needs to estimate the ST deviation function, a
task that requires determining its own reference ST level (using any
desired method; a median of its ST level measurements made during the
first 30 seconds is a commonly used approach). The algorithm's ST
deviation function is the difference between its ST level function and its
reference ST level. See <a href="/physiotools/wag/epicmp-1.htm">epicmp</a>
for details on how to record an algorithm's ST deviation function in an
annotation file, and how to use standard software to measure how well
an algorithm's ST deviation measurements match those provided with the
database.</p>
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