Myocardial Ischemia
One method of detecting intraoperative myocardial ischemia is
automated ST-segment monitoring.[107]
Several computer
programs for online detection of ischemia and analysis of ST segments are available
commercially. Each manufacturer uses different analysis techniques, and not all
the algorithms are in the public domain. In one system (Marquette Electronics),
an ST learning phase begins by looking at the first 16 beats in all leads for the
dominant normal or paced shape. The shapes are correlated using a selected number
of points on each of the active valid lead waveforms. An algorithm looks for leads
in the fail or artifact mode to determine the number of valid leads used in the analysis.
The algorithm also makes all leads positive to enable totaling the sum of the points
on
Figure 34-28
Computerized template of the underlying normal QRS complex
(X) and an ectopic complex called a test beat (Y). Beat Y is matched to beat X by
the computer during the region of comparison by cross-correlation algorithms. (Adapted
from Morganroth J: Ambulatory Holter electrocardiography: Choice of technique and
clinical uses. Ann Intern Med 102:73, 1985.)
the valid leads. This sum is used in determining a peak or fiducial point. The
fiducial point is used as a point of reference on the QRS. A template is formed
from selected points around the fiducial point for each electrocardiographic lead.
As each beat is analyzed, its template is compared with templates of previous beats.
If the templates correlate within 75% of a previously stored shape, it is deemed
a match and is classified as an existing shape. If there is no match, it becomes
a new shape. On the 17th beat, the dominant normal QRS shape or paced shape is determined.
The algorithm then searches for an additional 16 beats that correlate with the dominant
template. With the 18th beat, a process called incremental
averaging is initiated. The incremental averaging is a method of tracking
positive or negative changes occurring on the waveform. These changes are tracked
for each of the valid leads. The changes may be physiologic, such as ST-segment
changes resulting from ischemia or related to artifact caused by high-frequency noise.
The tracking of the changes is achieved by only allowing a 0.1-mm adjustment, positive
or negative, from the prior shape of each beat. On the 32nd beat, the product of
the incrementally averaged templates becomes the learned ST templates. Until ST
is relearned, all changes in the QRS shape are tracked against this learned template.
The isoelectric point and ST points are determined during the learning phase and
are based on the width of the QRS shape. The isoelectric point is placed 40 msec
before the onset of the QRS, and the ST point is placed 60 msec past the offset of
the QRS measurement. The isoelectric point provides the point of reference for
determining the ST-segment measurement. The technique of incremental averaging is
well suited to a continuous input with slow changes. However, it links the speed
at which changes occur in the template to the heart rate.[108]
This system was evaluated in the intraoperative setting in patients
undergoing cardiac surgery.[109]
The device monitored
three selected leads and displayed the absolute values of the ST segment as a line.
Upward deflection of the trend line indicated worsening ischemia, whereas a downward
trend reflected a return of the ST segment toward the isoelectric line. It was concluded
that once the device was clinically accepted, the awareness for ischemic changes
was heightened among the participating anesthesiologists and therapeutic interventions
were more rapidly instituted, possibly leading to improved outcome.
A second ST-segment analysis system (Hewlett Packard) differs
from the foregoing in several ways. A period of 15 seconds is analyzed first, and
the ST displacement is determined on the basis of five "good" beats. These displacements
are ranked, and the median value is determined. This technique eliminates the influence
of occasional VPBs and ensures that a representative beat is selected. The objective
of this procedure is to obtain a representative beat, rather than an average template.
The measurement point for the ST segment can be selected as the R wave + 108 msec
(default) or the J point + 60 or 80 msec. ST values and representative complexes
are stored at 1-minute resolution for the most recent 30-minute trend and at 5-minute
resolution for the preceding 7.5-hour trend.
In a third system (Spacelabs), a composite ST-segment waveform
is developed every 30 seconds and is compared with a reference tracing acquired during
an initial learning period. The isoelectric and ST-segment points can be manually
adjusted to any location on the electrocardiographic tracing, or they may be automatically
set to predetermined values. Using selective ST-segment displacement on seven different
types of digitally simulated ECGs, London and Ahlstrom[110]
bench-tested a version of a Spacelabs automated ST-analysis device, the PC2 Bedside
Monitor. The device performed very well with five of the simulated ECGs, but it
had some difficulty with two because of improper placement of the isoelectric point.
Visual confirmation of ST-segment analyzer results was therefore advised.
The relative merits and shortcomings of the different ST-segment
analysis systems in the clinical setting have not been fully elucidated. The ability
of two automated ST-segment analysis systems to detect myocardial ischemia during
noncardiac surgery was compared with 8-lead printed ECG and transesophageal echocardiography
as reference standards.[111]
In this study of 44
patients, the automated ST-analysis systems showed only fair agreement with transesophageal
echocardiography or ECG in detecting ischemia. A different, brief, cautionary report
mentions a case in which automated ST-segment monitoring falsely signaled the presence
of intraoperative ischemia.[112]
